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July 1991 ° £1.50 


CGTRONICS 


CIENCE AND TECHNOLOGY 





Satellites com 
down to Eart 


Electronics wit 
intelligence 


Simple designs 
for beginners 


Practical 


Components 


Everything abou 
transistors 


Data Sheet 


The New Hitachi 
H8/330 


01 


(700 





Take the Sensible Route! 





Boaramaker is a powerful software tool which provides a 
convenient and fast method of designing printed circuit 
boards. Engineers worldwide have discovered that it provides 
an unparalleled price performance advantage over other 
PC-based and dedicated design systems by integrating 
sophisticated graphical editors and CAM outputs at an 
affordable price. 





In the new version V2.40, full consideration has been given to 
allow designers to continue using their existing schematic 
capture package as a front end to BoardMaker. Even 
oowerful facilities such as Top Down Modification, 
Component renumber and Back Annotation have been 
accomodated to provide overall design integrity between 
your schematic package and BoardMaker. Equally, powerful 


R +7.866, +6.3131N Track 1/86 45° Zoom3gl Layer=1 Width= 3015) Node 1 of 1 


Full analogue, digital and SM support - ground and 
power planes - 45 degree, arced and any angle 
tracks with full net-based Design Rule Checking. 





features are included to ensure that users who do not have Lead nets 
schematic capture software can still take full advantage of Clear nets 
BoardMaker’s net capabilities. ae POM acc. 
BoardMaker V2.40 is a remarkable £295.00 (ex. carriage & nw | 


VAT) and includes 3 months FREE software updates and full EEE eae Renunber 
telephone technical support. ee ee ee 





BoardkRouter is a new integrated gridless autoroute module 
which overcomes the limitations normally associated with 
autorouting. YOU specify the track width, via size and design 
rules for individual nets, BoardRouter then routes the board 
based on these settings in the same way you would route it 
yourself manually. 


Optimized placement by deplaying ratsnest per 
component. Lines indicate the unrouted nets. 
This ability allows you to autoroute mixed technology designs 
(SMD, analogue, digital, power switching etc)in ONE PASS 
while respecting ALL design rules. 





No worrying about whether tracks will fit between pins. If the 
track widths and clearances allow, BoardRouter will 
automatically place 1, 2 or even 3 tracks between pins. 





You can freely pre-route any tracks manually using 
BoardMaker prior to autorouting. Whilst autorouting you can 
pan and zoom to inspect the routes placed, interrupt it, 
manually modify the layout and resume autorouting. 


BoardRouter is priced at £295.00, which includes 3 months 
FREE software updates and full telephone technical support. 
BoardMaker and BoardRouter can be bought together for 
only £495.00. (ex. carriage & VAT) 








Tsien (UK) Limited | Don’t just take our word for it. Call us 
Cambridge Research Laboratories foday for a FREE Evaluation Pack and 
181A Huntingdon Road judge for yourself. 


Cambridge CB3 ODJ UK 
Tel 0223 277777 


" Voter -3-4-) Sa 
{S1EN. Fax 0223 277747 ~*~ _ 
| 





All trademarks acknowledged 


Cm mmm g <<< July 1991 © Volume 27 No. 7 










Due to a lack of communication 

petvrear iycolt anil Hlkica ne A Low Hertz Meter... en ee ssosneeeeeeennnnteeessniieesnanieeente 25 
H8/330 did not get published as More essential test equipment to build and use. 

advertised on the cover last month. 

Farlnaiclv- esc itlon hasitesk PE Chronos............. coeeesneenentensaien seseeneensavensnensaeessnensarenan 3/ 
sorted out and the chip is featured in The final part of the Universal Counter Timer. 

the Data Sheet section this month. LOGIC ANAIVSEM oo... eeeecescesecessesscsesessssscescsesuecsuseeeseneees 47 






Judging by the number of phone 
Calls to PE's offices, many readers are 
missing the PCB service. 
Unfortunately, this had to be 
withdrawn for economic reasons - it 
has since been farmed out to a third 
party. 

As anyone who has done it will tell 
you, it is quite easy to build your own 
PCBs and the materials are widely 
available. To prove the point | set 
about making a board the results of 
which can be seen on page 43 — to my 
shame, | have to admit that this is the 






A digital code convertor for the workshop. 









The Satellite Story oo... cccccssscesscssssssssescssecssesssnsenes 10 
The Earth is caught up in an invisible web of telecommunications. 
Since the launch of TelStar, satellites have played an important role. 
Neural Networking ........c cece ceecssesecsssesessssesssesscssessssseesees 16 
Computers modelled on the human brain may offer startling new 
processing capabilities. 













first PCB | have ever made (degree Innovation and Alienation ..........cccccccsccccssccccesssceeeessseeeeeees 22 
level electronics courses are heavy on | Technology affects our values in subtle ways 
the theory but can be light on the nee 

? : Printed Circuit COnStruction .......cccsccssccsecssecseeseessecseeseeseenee. 43 






practice). 





Kenn Garroch, Editor} Almost all designs use PCBs. Learn how to build them yourselt. 


Wavelength... ccccccccccccssssccscessssescesesesesseeeessessreeeseeessnnees 5 
The great debate over resistor symbols rumbles on plus 
modificationbs to the PE Tele-Snap. 








PIVOT ONS ssanactntssaecicencenantacesttuceeeterntdustetecessenacteneetencastann 6 
Microcircuitry, macroeconomics, meters and much more. 
DECC IRs) 1 :1:) ae 31 


Full specifications and diagrams of the new Hitachi H8/330 super 
powered microcontroller. 

Practical COMPONENES.............ceccceesesesseceneesenseeeesteeserseesens 34 
The transistor is the key component in modern logic circuit 
technology. 

TOCHINIQUES .....c cece ccccceccccccccessssseeeeeeeeeesseesssnteeeeeneeeeeeeeesens 58 
A variety of problems solved by our technical expert, including 
teperature sensors and guitar pickups. 

Barry FOX ......ccecccccccccsseeeeecesseeeessesseeseesessuesesecessueveseseeseas 63 


Remote sensing... Charge-Coupled Once again the hydra-like problem of radio spectrum bandwidth 


Devices have enormously extended the allocation rears its ugly heads. 
reach of modern telescopes. IRIS, the 

latest IR device, has probed structures 

millions of light years away. 









Editor: Kenn Garroch Technical Editor: Andrew Armstrong Technical Illustrator: Derek Gooding Advertisement Manager David Bonner Production Manager: Richard Milner Production Assistant: Michael Sullivan Office Manager: Laura 
Esterman Office Secretary: Wendy Rhodes Publisher: Angelo Zgorelec ¢ Practical Electronics Intra House 193 Uxbridge Road London W12 9RA Tel: 081-743 8888 Fax: 081-743 3062 Telecom Gold: 87: SQQ567 « 
Advertisements The Publishers of PE take reasonable precautions to ensure that advertisements published in the magazine are genuine, but cannot take any responsibility in respect of statements or claims made by advertisers. The 
Publishers also cannot accept any liability in respect of goods not being delivered or not working properly. ¢ © Intra Press 1991. Copyright in all drawings, photographs and articles published in PRACTICAL ELECTRONICS is fully protected, 
and reproduction or imitations in whole or in part are expressly forbidden. All reasonable precautions are taken by PRACTICAL ELECTRONICS to ensure that the advice and data given to readers is reliable. We cannot, however, guarantee it, 
and we cannot accept legal responsibility for it. Prices quoted are those current as we go to press. All material is accepted for publication on the express understanding that the contributor has the authority to permit us to do so. ¢ Practical 
Electronics is typeset at Intra Press on Macintosh computers using Quark Xpress. Reproduction by Tetracolour Ltd. Printing by Andover Press, St Ives plc. Distribution by Seymour Press ¢ ISSN 0032-6372 © 








July 1991 Practical Electronics 3 


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8” 100 WATT C8100GPM GEN PURPOSE, LEAD GUITAR, EXCELLENT MID . DISCO ESErORMEC OS ponee te idee’ 
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10” 100 WATT C10100GP GUITAR, VOICE, ORGAN, KEYBOARD, DISCO, EXCELLENJ MID FREQ. RANGE 45H7-20KH? BOTH MONITO' INDICATOR 
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12” 200 WATT C12200B HIGH POWER BASS. KEYBOARDS. DISCO, PA : 99 PER PAIR Pete aan aee 
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12” 300 WATT C12300GP HIGH POWER B«SS LEAD GUITAR, KEYBOARDS, DISCO, ETC OMP 12-200 (200W 102dB) PRICE £209.99 PER PAIR Siicks...anerensas 
RES. FREQ, 45Hz FREQ, RESP, TOSKHz SENS, 100dB PRICE £85.79 + £3.50 P&P. — 
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TYPE ‘B’ (KSN1005a) 3'>" super horn. For general 
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TYPE ‘C’ (KSN6016A) 2” « 5" wide dispersion horn. For 
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TYPE ‘D’ (KSN1025A) 2’«6' wide dispersion horn. 
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TYPE ‘E’ (KSN1038A) 3°." horn tweeter with attractive 
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12” 100 WATT EB12-100 BASS. STUDIO. HI-FI, EXCELLENT DISCO 

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4 Practical Electronics July 1991 





Wavelengths 


If you have any comments, suggestions, subjects you think shouled be aired, write to PE 


found the Tele-Snap project in 
[es May issue of PE interesting 

until the software part came into 
view. 

It seems a pity that GW-Basic 
was used as there are so many good 
versions of Basic available for the 
PC these days. However, once I saw 
Locomotive Basic 2 mentioned, this 
signalled the Amstrad PC1512 
computer. 

If Mr. Becker was using a 
PC1512 he is correct when he says 
he cannot use 16 colours from GW- 
basic, but if he was using an IBM 
compatible with EGA graphics then 
my version of GW-Basic will allow 
the use of 16 colours. 

I was an Amstrad PC1512 owner 
and I quickly got fed up with Basic 
2 so I taught myself to program in 
C, Borland Turbo C in fact, the 
effort being well worth it I would 
say. 
Now to the best part; because I 
wanted to use the unique 16 colour 
mode of the Amstrad PC1512 I 
wrote a graphics library in C which 
allows the programmer to access 
the 16 colours just as easily as if 
using a proper EGA card - the 
library is available from me at the 
fully inclusive price of £10.95 and is 
compatible with Turbo C v1.5 and 
Turbo C v2.0. 

Not content with this I then 
rewrote the library in Turbo Pascal 
and this is also available from me 
for £10.95 and is compatible with 
Turbo Pascal v4.0, v5.0 and v5.5.. 

Both disks contain a demo 
program and several example 
programs and are available on 
5.25in or 3.5in disks, please state 
size when ordering. 

If, like me, you already have an 
expansion card fitted to your PC 
which uses address space at 300H 
then the following modification 
may be of use. 

The PC allocates I/O memory 
space to a prototype card from 
300H to 31FH, its official title is 
“prototype card I/O map” but this 
really means the area has not been 
reserved by IBM for its own use 
and is available for the user safe in 


the knowledge that the computer 
will not try to access it for any 
internal use. 

As the card already installed in 
my computer uses I/O addresses 
300H to 303H I would need to 
change the Tele-Snap I/O address 
to 310H using the following 
method. 





By cutting a few tracks on the 
PCB and soldering a few links on, 
the unused gate in IC3 can be used 
to invert A4 which now makes the 
base address of the Tele-Snap 310H 

If a clearer modification is 
required then an IC socket can be 
used for IC1 and a small board 
made up which plugs into this 
using a header plug —- an extra 
74HC04 would then be required. 
Douglas Fisher 
Highfield 
Braithwaite 
Workingham 
Cumbria 
CA14 4TG 


More Squiggles 

In May 1991, John Bilson 
commented that he thought that the 
‘squiggle’ resistor was the standard 
symbol. However, I understand 
from my GCSE and A level studies 
that the rectangle was now 
standard. I remember in the second 
year I wrote down a squiggle and 
was told not to do it again! 

On the subject of the new design 
of Practical Electronics, I would like 
to ask what has become of Space 
Watch? I have been an intermittent 
read of the magazine over the past 
twelve months and the main reason 
that made me decide to start 
reading it regularly was the space 
item that set it apart from other 


Letters 


magazines. I read with interest the 
so called ‘science feature’ on 
superconductors in the May 
edition. Is this going to be a regular 
feature, perhaps taking in some of 
the items from space watch? 

Your series named ‘How It 
Works’ is very good. I like the cut 
away pictures that have been 
included. Talking of ‘How It 
Works’, are you going to publish 
follow up articles on Nicam Stereo. 
It seems a pity that ,after showing 
technical details, this is not backed 
up with a practical project to show 
the system, perhaps a small project 
that would fit into an existing TV? 

The article on computer 
interfacing (April 1991) was 
illuminating. Here I am typing this 
letter into my QL with all its slots 
and plug holes and not a mention 
in the articles. Am I the only reader 
with such a computer? I doubt it as 
it has been described by some 
people as a good hacking machine 
or good for experimenting with add 
on electronics. In addition to having 
256k put aside for add-ons, the 
computer can automatically check 
for such devices and link in any 
required extensions to the operating 
system, if the hardware is 
accompanied by a small ROM. 

Overall I very much like your 
magazine. I nearly entered the 
loyalty bonus contest last month 
but, to my consternation, I find for 
this and other months that the slip 
we have to send in is on the other 
side of some pretty informative 
articles. Is it not possible to arrange 
that the slip is with the other 
adverts, or even on a page of its 
own? 

James Gibson 
Newbury 
Berks 

Alas, Space Watch is no more but 
there will be other articles covering 
similar ground in the future. I'll have 
to look into the Nicam question — 
nothing is planned at the moment. As 
for the QL, I'm afraid that there wasn't 
space, perhaps in a future issue. Turn 
to page 53 for the solution to your 
tearing problem. a 


July 1991 Practical Electronics 5 


Innovations 


Telepoint Tax Peril 


he new tax on 
mobile telephones 
may apply to 


telepoint phones as well, 
because the Treasury is 
apparently unaware of 
the difference between 
telepoint and cellular 
phones. Telepoint phones 
are cheaper but can only 
make outgoing calls and 
must be in range of a base 
station, thus cannot be 
used to annoy customers 
in restaurants. With only 
about 5000 subscribers the 
system is struggling to 
stay alive. Telepoint 
operators are keeping 
their heads down hoping 
the tax will not be levied 
on their service. 


TV interference 


he proposed 
Channel 5 TV 
service is liable to 


clash with the frequencies 
used by millions of home 
video units in the UK. The 
new service will use two 
UHF frequencies called 
channels 35 and 37 — these 
are currently used by 
radio microphones and 
airport radar. The 
channels will be freed for 
TV use by moving radio 
mics off the TV waveband 
and by transferring radar 
to channel 36. 

Unfortunately a lot of 
domestic video 
equipment already uses 
these frequencies, albeit at 
low power, to connect to 
the aerial sockets of TV 
sets, and often suffers 
from interference as a 
result. For instance in the 
area around Heathrow 
airport, which uses 
channel 36 radar, many 
video recorders must be 
retuned to channel 37 
from the usual channel 36 
setting. 

Under the terms of the 


n 


broadcasting Act 1990, 
franchisees of Channel 5 
TV must pay for any 
modifications to 
equipment necessary 
because of interference. 
Taking into account the 
numbers of video sets, 
satellite receivers and 
game machines involved, 
the cost is likely to run 
into many millions of 
pounds. 


Funding Doom? 


he annual State of 
the Universities 
report from the 


6 Practical Electronics July 1991 





Committee of Vice- 
Chancellors and 
Principals suggests that 
increases in research 
grants from industry and 
medical charities have 
more than made up for a 
relative lack of 
government funding over 
the past decade. The cash 
available for each tenured 
researcher has increased 
by 35% since 1984, and 
there are now more 
researchers doing more 
research. But since the 
new funds are often 
applied to short-term, 
applied research with a 
quick payoff - rather 





than basic science, which 
is regarded as_ the 
government’s respon- 
sibility - long-term 
research projects have 
suffered. 


Protectionism 


uropean Com- 
munity policy on 
support for the 


home electronics industry 
may be in disarray after a 
recent move by France. 
When the European 
Commission, which 
controls Brussels-based 
aid to electronics 
companies, announced a 
reorientation of funding 
away from basic research 
and towards market-lead 
developments, the French 
called for an agency 
modelled on the ESA to 
support electronics 
without interference from 
Brussels and announced 
£1 billion of support for 
Bull and Thomson, the 
computer giants which 
made record losses last 
year. The Commission 
replied that the last 
attempt at such an 
agency, the  French- 
sponsored Eureka plan, 
failed to gain significant 
support from industry. 
Furthermore the 
Commission has 
promised to investigate 
the French aid as an 
unfair subsidy and a 
distortion of free trade. 

A report in The 
Economist (20-26 April) 
claims that state subsidies 
for European computer 
companies, in the form of 
grants and large 
government contracts, 
have merely enabled 
manufacturers to survive 
in the face of superior 
international competition, 
and are thus counter- 
productive. 





Instruments 


PCB failures can now be 
tracked down to 
component level without 
powering up the boards 
and without circuit 
documentation using 
T3000, a new trouble- 
shooting instrument for 
fault-finding on analog or 
digital printed circuit 
boards. Designed by 
Polar and distributed in 
the UK by Whingate, this 
portable instrument uses 
the powerful impedance 
signature technique to 
address PCB repair issues. 

The T3000 tests a PCB 
On a component by 
component basis. It 
works by applying an AC 
signal which is both 
voltage and _ current 
limited to the component 
under test and displaying 
the resulting dynamic 
“impedence signature” on 
its built in screen. 
Different types of 
component present 
different forms _ of 
signature and it is easy to 
spot when one has failed. 
This technique easily 
identifies the vast 
majority of service-related 
problems; catastrophic 
component failures and 
open or shorts. In its 
“compare” mode it allows 
the signature of a good 
board to be displayed 
alongside that of the 
faulty board under test 
and it is also possible to 
spot a large proportion of 
complex parametric 
failure modes. 

The T3000 checks any 
kind of device technology 
from discrete resistors, 
capacitors, inductors, 
diodes and transistors to 
analog or digital ICs. It is 
also possible for non- 
electronics specialists to 
operate as it has built in 
easy to use features such 
as the autoranger which 
auto selects the best drive 





Innovations 


for the device under test. 
The cost is £2,450 and it is 
available from Whingate 
Test Services Ltd 0202 
605239. 


When measuring far 
away from any mains 
power, the new HP 8590B 
rugged spectrum analyser 
run from the Hewlett 
Packard 85901A AC 
power source could prove 
useful. It covers a 
frequency of 9kHz to 
1.8GHz and an amplitude 
range from -115dBm to 
+30dBm. The new 
spectrum analyser also 
has 32 kbytes of 
nonvolatile program 
memory and a built in 
clock/calendar which can 
be used to stamp time and 
date information onto test 
results. Rates are £281 + 
VAT for one week’s rental 
which can be obtained 
from Hewlett Packard 
Rental Department on 
0753 580000. 


Testing TVs can 
sometimes be confusing 
since it is important to 
know whether it’s the box 
or the remote control 
which is to blame. A 
remote control tester, now 
available from Maplin in 
kit form, checks whether 





the remote control unit is 
transmitting an infra-red 
carrier or data stream. 
This tester ignores any 
ambient infra-red energy, 
so it can be used in full 
sunlight. It works by 
detecting changes in the 
ambient infra-red level 


July 1991 Practical Electronics 7 


Innovations 


and converts these into 
visible light by means of 
an amplifier and red LED. 
The kit costs £7.10 inc. 
VAT and is available from 
Maplin on 0702 554155. 


Logic Probes designed for 
use with TTL, DTL and 
CMOS IC’s are now 
available from Maplin for 
£15.28 (plus postage). 
Using its memory 
function to detect the 
occurrence of single 
pulses or logic level, the 
probe can recognise 
pulses from 30ms up to 
500ms. It has 3 coloured 
LRDs showing high logic 
level (red), low logic level 
(green) and pulse(yellow). 


Radio buffs are set for an 
exciting time as with the 
deregulation of the 
airwaves as 200 or more 
new independent local 
radio station are planned. 
To enable listeners to take 
advantage of the variety 
of stations, the Maplin 
Radio Rod has been 
launched. This rod is 


Omni-directional and 
designed to receive 
national and local 


transmissions from any 
direction in the range 88- 
108 MHz. It can be fitted 
in the loft or on the 
outside of the house 
together with a coax plug 
or coupler. Cost £14.25 
inc. VAT (plus postage). 


General 


Miniature lamps 
designed for building into 
electronic and electrical 
equipment are now 
available at quite 
reasonable prices. The 
range consists of 3 basic 
types: signal filament 6- 
70V current ratings 20- 
200mA; signal neon and 
green fluorescent lamps 
60-220V and current 
rating 0.3-3.5mA; multi 
LED lamps in red, green 


or yellow 6-48 VDC 
current rating 10-35mA. 
The lamps have a life 
expectancy of between 
3000 and 100,000 hours 
and are available from 
Electroustic Ltd on 0254 
333664. 


Ion imbalances in the 
home can be restored 
using the new ioniser kit 
from Maplin. The ioniser 
called Breeze, produces 
negative ions 
electronically to 
counteract the excessive 
positive ions generated by 
electronic apparatus such 
as TVs. The kit costs 
£33.67 inc. VAT and is 
aimed at the experienced 
constructor as the ioniser 
generates very high 
internal voltage and 
needs to be built with 
care. For more 
information contact Vic 
Sutton of Maplin on 0702 
554155. 





8 Practical Electronics July 1991 





Computers 
The Oak VGA card from 
Southern Peripherals, 
keenly priced at £42, 
enables users to get 
graphics display on IBM 
PC’s AT’s and XTs ata 
low cost. It claims to have 
more functions than a 
standard IBM VGA and 
as it is fully compatible 
with the IBM Bios, it is 
also downwards 
compatible with MDA, 
monochrome Hercules, 
CGA and EGA standards. 
One of the other key 
features is that the Oak 
VGA can be used with a 
variety of packages 
including Windows 3.0, 
Lotus 1-2-3 version 2, 
Framework II and III, 
Wordperfect 5.0 and 5.1, 
Ventura VI.I and 
Autocad. The card also 
supports 132 column text 
mode applications such as 
Wordstar version 3.3 and 
4.2 and Wordperfect 
version 4.2. The card gas a 
single ship VGA graphics 
controller which provides 
fast host access to video 
memory, allowing 32bit 
access to the video RAM. 





It operates with a memory 
configuration of 
256kbyte. It can be bought 
from Southern 
Peripherals in Hampshire 
0256 819 221. 


Bigger Bytes are in from 
Mitsumi with their 3.5 
inch floppy disk drives in 
anticipation of the move 
to higher capacity 4Mbyte 
design as the next market 
standard. This 1 inc high 
drive is compatible with 
1Mbyte, 2DD, 2Mbyte 
and 2HD media and is 
powered from signel 5V 
supplies. It is distributed 
by Southern Peripherals 
and enquiries should be 
directed to their sales 
office on 0256 819221. 


Events 


ussex Amateur Radio 
and Computer Fair is on 
14th July at the Brighton 
Racecourse, Brighton. It 
starts at 10.30 and 
purports to bring together 
the best of computer, 
amateur radio and 
component specialists 
under one roof. Contact: 
Ron Bray (0273) 415654. 


Fourth International 16- 
Bit Computer Show, 12th- 
14th July,The Novotel, 
London W6. Forum for 
the latest products and 
services for the Atari ST, 
Commodore Amiga and 
PC. Contact: Tim Collins 
(081) 549 3444. 


Reference 
Readers may find the 
Crydom Solid State Relay 
Handbook at £11.87 a 
usefully comprehensive 
reference book. It covers 
over 233 pages detailing 
the use and application of 
devices in the range 0.7A 
to 90A;120 to 480 AC 4 to 
15V DC. Sales enquiries to 
Alan Coulling, Unitel 
0438 321393. a 


| mr 
SQ) Murtifesters 


y ¥ 4 \ eo TALKING METER 


£79.95 2:16: 


[A] Talking Multitester. Press 
a button on the probe and 
the meter will call out it’s 
reading in clear English. The 
reading is also shown on the 
units large, easy to read LCD 
display. Features autoranging, 
autopolarity, continuity 
sounder, diode-check and 
over-range indicators. 
Measures to 1000 VDC, 750 
VAC, 300mA AC/DC, 30 
megohms resistance. 
Requires 4 “AA” batteries. 


Digital Multimeter. Full 

autorange or manual range 

control, selectable by a 

switch. Easy to read LCD 

display. Ideal for use in the 

field, lab, shop, bench or 

home. Fold-out stand allows DiC METER 
you to adjust position for 

better visibility or to hang 

unit. Features continuity £39 , 95 22-184 
check, autopolarity, diode- 

check and low battery 

indicator. Measures to 1000 

VDC, 750 VAC, 200 mA AC/DC, 

20 megohms resistance. 

Requires 2 “AA” batteries. 


andy 


ALL THE ACTION 
AS IT HAPPENS! 


InterTAN U.K. Ltd., 
Tandy Centre, 
Leamore Lane, Walsall, 
West Midlands. WS2 7PS 
Tel: 0922 710000 
July 1991 Practical Electronics 9 








Satellite Feature 


Talking fo 





The Skies 





Satellite TV, once a dream, is now an accepted part of everyday entertainment. John Brook 
switches on tunes in and wigs out to find out what it is all about. 


s well as being useful for 
A reviational purposes (they 

always point south), TV 
satellite dishes are bringing more 
and more news and entertainment 
into our homes. After getting off on 
a fairly rocky start, satellite TV is 
now an established part of the UK 
entertainment scene. 

There are 16 major satellites in 
geosynchronous or Clarke orbits, 
visible from the UK. All transmit 
TV signals in one form or another 
and represent a colossal investment 
in the future of communications 
and direct broadcast TV. 
Unfortunately, there are a number 
of different standards in use for 
encoding the picture as well as a 
few systems that use scramblers as 
well. 

By far the most common formats 
are PAL as used in the majority of 
TV sets in the UK and Germany 
and SECAM, used by French TV. 
The other system used in satellite 
broadcasting in MAC (Multiplexed 


10 Practical Electronics July 1991 


Intelsat Ill in a Clarke orbit covers one face of cai 


Analogue Components) in two 
formats - D MAC and D2 MAC. 

D MAC is an amplitude 
modulated form of the FM MAC 
standard and in its normal format, 
as used in the UK, it can 
accommodate eight data or digital 
audio channels alongside the video. 
The European format, D2 MAC 
only supports four channels and, 
consequently, has a narrower 
bandwidth. Both have to be 
converted into PAL or SECAM by a 
decoder before being fed into 
“normal” TV sets and have been 
adopted by the European 
Commission as the standard for all 
new direct TV _ broadcasting 
systems. 

The future of MAC lies with 
high definition TV (HD MAC) with 
its proposed 1250 lines and aspect 
ratio of 16:9 - much better than the 
current 4:3 used on standard 625 
line TV. The drawback is the need 
for complicated video displays and 
cameras that can cope with the 


id 


™ Postmaster General. 





greater number of lines and the 
increased screen width. On the plus 
side, however, the signals will still 
be compatible with 625 line TVs 
and D MAC decoders allowing the 
technology to be introduced before 
any demand actually exists. 


Moving The Data 


The majority of UK direct broadcast 
services operate in cooperation with 
British Telecom’s London Teleport 
situated in the Docklands. Data is 
beamed up to orbit from the 13m 
antennae to the various satellites, 
Eutelsat II F1, Intelsat VA F11 and 
Astra and, as well as providing 
transmissions for such popular 
programs as CNN and Sky, it also 
serves the nearby _ business 
community with high speed data 
links and teleconferencing. 
Communicating with a satellite 
uses two main channels, the up-link 
which transmits all of the control 
and data information and the 


Satellites from the UK 


Name position TV 
channels 






Intelsat VB F15 60°E 5 
DFS 1 Kopernicus  23.5°E 8 
Astra 1A 19.2°E 17 
Eutelsat II F1 13°E 9 
Eutelsat 1 F5 10°E - 
Eutelsat 1 F2 TE 5 
Tele X 5°E 2 
Intelsat VA F12 1°W = 
Telecom 1C 5°W 6 
Telecom 1A 8°W 1 
Olympus 1 3.8°W 3 
TDF 1A 19°W 3 
TV Sat 2 19.2°W 4 
Intelsat VA F11 eiow 7 
Marcopolo 1 31°W 4, 
Panamsat 1 F1 45°W 1 








Getting Into Orbit 


Getting an object into earth orbit is a 
Simple enough idea as can be seen in 
the illustration on the right. The basic 
task is to give the object enough speed 
away from the surface so that it doesn’t 
fall back down to Earth. The top picture 
shows the object fired of at a relatively 
Slow speed at a tangent to the surface. 
After moving away for a short distance, 
it is eventually attracted back due to the 
force of gravity. Giving the object more 
speed allows it to travel further but it is 
still attracted back toward the ground. 
The centre diagram shows that the 
attraction causes the object to curve 
around with the surface for some way 
until it hits again. The next obvious step 
from this situation is to give it more 
Speed enabling it to go into orbit - 
lower diagram. The velocity required to 
get an object into an orbit is called 
escape velocity and for the earth it is 
3075m/s (around 17000mph). 


Unfortunately, the pictures don’t take 
into account atmospheric drag, and 
perturbations in the earth’s gravity 
field. The solution to the atmospheric 
problem is to lift the object out of the 
atmosphere before giving it orbital 
speed. The minimum height at which 
drag is zero is around 240km. Once 
there it will stay in orbit long enough to 
allow ground control to manouver it so 
that gravitic irregularities can be 
compensated for. 

At 240km the satellite circles the earth 






Earth's spin 


Polar orbits 


down-link. In the Astra system, 
used, among other things, to 
transmit Sky TV, there are two 
down-link antennae, one using 
horizontal polarisation and the 
other vertical. A total of 16 channels 
are available ranging from 
11200MHz to 11450MHz. and the 


every 1.5 hours or so. This is fine for 
some uses — spy satellites are placed at 
this height to get good pictures (it is 
just about possible to photograph a car 
number plate) — but for others, such as 
communications, it makes life very 
difficult. The ground based antennas 
would have to track across the sky at 
quite a speed to keep up with the 
Satellite and it would only be in view for 
a short while. Since satellites in low 
orbits circle faster than the earth’s day, 
moving them further away, although 
increasing their speed, causes them to 
be overhead for longer periods. 
Eventually a point is reached where the 
Satellite is moving around the earth at 
the same rate at which the earth is 
spinning — every 24 hours. This makes 
it appear to be stationary above a single 
spot and is ideal for communications 
since any antennae can be pointed at 
the same spot all of the time. 


For satellites that are used for Earth 
sensing, meteorology, crop analysis 
and so on, another type of orbit can be 
used. Once above the atmosphere they 
are given a velocity at right angles to 
the earth’s rotation. This puts them into 
a polar orbit allowing virtually every 
spot on the planet to be observed 
(lower left). The problem with this is 
that it takes a lot more fuel to get the 
Satellite in position. Normal orbits 
travel in the same direction as the Earth 
spins since even before the rocket 
takes off it is travelling at 1667 km/h 
(along with everything else at the 
surface of the Earth). Trying to cancel 
out this speed would take a lot more 
fuel as would manouvering to different 
orbits — the Space Shuttle is launched 
from Cape Canaveral which is 28° 
north of the equator and since it must 
go into a circular orbit before it 
launches any satellites it is more 
expensive in terms of fuel than the 
European launch vehicle, Ariane, which 
takes off from Kourou which is only 5° 
north of the equator. 


system is designed mainly to 
operate with direct broadcast TV. 
The two technological 
developments that make direct 
broadcasting of TV into homes are 
the travelling wave tube (TWT) and 
the low noise block (LNB). The first 
determines the power that the 


Satellites 








Getting there. 


satellite is able to broadcast. Low 
power systems require larger 
receiving dishes whereas higher 
power systems can beam images 
directly into the home. Low power 
TWTs are generally capable of 
transmitting up to 25W and need a 
dish of around 3m in diameter on 
the ground. They are used for 
intercontinental telephone traffic 
and cable TV programs. In the 
medium range are the TWTs used 
in Astra running at about 55W. 
These produce enough power to 
transmit directly into 60cm dishes. 
At the top end are the high power 
TWTs used in Marcopolo 1. These 
are able to produce enough power 


July 1991 Practical Electronics 11 





Satellites 


Gravity 


The force F between two bodies, one to mass times accellaration), this gives 
having mass M, and the other Mo, is the acceleration g with which bodies 
given by the following formula: will fall to Earth: 


F=-GM,M,/d, g=GM/R2 











G is the universal constant of gravity where M is the mass of the earth 








(6.670x10"''Nm2kg"2) and d is the (5.98x10"4 kg) and R is the radius 
distance between the bodies. In the (6367.5 km) and results in a figure of 
case of the earth and according to 9.83 ms? which corresponds closely 






Newton (who defined force to be equal _—‘ to experimental results. 





























for dishes of 30cm and, in 
particular, the squarial. 

To develop more power from a 
medium sized satellite, a number of 
TWTs can be operated in parallel. 
Two 55W transmitters can produce _ | ) 
a signal in the region of 110W : 
which is about the same as a high 
power system. 

It may appear that the signal 
from a direct broadcast satellite can 
be received at any point on the face 
of the Earth where the satellite is in 
direct sight. In practice, only a 
predetermined area or footprint is 
available. At its centre the dishes 
used to receive the signal would be 
the smallest. Moving away from 
this area, larger antennae are 
needed. For example, the Astra V1 
beam is centred on the south of 
England and most of France. In 
these areas the conventional 60cm 
dish can be used. By the time the 
signal reaches Norway, mid Italy, 
south Spain and Iceland, a large 
1.5m dish is needed. 


Earth Remote Sensing 















By the time you read this there should Earth for analysis that will lead to a (frequency change) and the received 
be (at least) one new satellite in orbit clearer understanding of the way in echo compressed to generate high 
around the earth. ERS-1 or European which the planet operates and the resolution pictures which depend upon 
Remote Sensing Satellite number 1 is effects we are having on it. 3 the roughness, dielectric properties and 
set to be launched from Kourou in range of the target. Due to power 
French Guyana on the 3rd May 1991. As __ By far the most sophisticated piece of limitations, the SAR can only be 
its name suggests it will be used to equipment on ERS-1 is the Synthetic Operated in image mode for 10 minutes 
gather more information about the state Aperture Radar or SAR. This is part of per orbit and because of the large 
of the planet and its environment. the active microwave instrument and is § amount of information ¢ :“erated, it can 
able to obtain high resolution pictures only be used when in sigit:_:f a ground 
Placed in a near polar orbit at an of strips of the earth’s surface 100km __ station which can record the data. 
altitude of 785km ERS-1 has on board a __ wide. Its 10m long antenna is aligned The second operating mode for the SAR 
number of sophisticated instruments parallel to the orbital path of the satellite is wave mode. This allows 5kmx5km 
able to make detailed height and directs a narrow radar beam onto pictures every 200km vcvered and can 






measurements, resolve images down to _ the surface of the earth, using the delay measure the directiona and length of 
25m, measure the temperature of the and strength of the returned signals to sea waves from 100 to 1000m. 

sea and the speed of the winds. All of build up an image. The transmitted : 

this information is transmitted back to signal is phase coded with a linear chirp 










12 Practical Electronics July 1991 


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July 1991 Practical Electronics 13 


Satellites 


The First Communications Satellites — 


The idea of putting an object into 
orbit was first put forward in the 
1920s but, unfortunately, the 
technology was inadequate and it 
was not until after World War I! that 
practical rockets became 
available. The development of the A4 
or V2 by the Germans was taken up 
by the USA and USSR and the first 
Satellite was placed into orbit in 
October 1957. Sputnik | put the 
Soviet Union into the lead in what 
soon became known as the space 
race. Other countries quickly became 
involved with France, the UK, West 
Germany, Japan, China and Italy all 
getting devices into orbit at some 
Stage of the game. 


To begin with, satellites were used 
purely for research purposes with 
experiments to see what “outer 
Space” was actually like. However, in 
1945 one A C Clarke published an 
article which described how artificial 
Satellites could be used to transmit 
radio waves over the horizon and, by 
selecting the correct orbit, the 


ee aedee i d 
a we 





Satellite could be kept over the same 
spot on earth all the time. This 
geostationary orbit was calculated to 
be at 36000km (22300 miles) and is 
now known as a Clarke Orbit. 


In 1955 a paper was published by Dr 
John Robinson Peirce who was the 
director of electronics research at 
Bell labs and, incidentally came up 
with the name “transistor”. Entitled 
Orbital Radio Relays it started off 
with the Clarke Orbit idea and went 
on to describe all of the technical 
details needed to actually build a 
communications satellite. A few years 
later he persuaded NASA to launch 
Echo, a large passive metalised 
balloon, which was used to test and 
prove the basic principles — it was 
also a very bright sky object and was 
seen by a large number of people. In 
1962 the same team launched the 
world’s first real telecommunications 
Satellite, Telstar, and the age of global 
communications had begun. 


om « 
— 


Ground \ 
. station ‘ 


'*| Satellite 





Ground ‘ 


ae Clarke orbit ao 


"==.  S6G00kM 


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14 Practical Electronics July 1991 


= 





The Down Side 

At the other end of the link, the 
LNB down convertor forms a low 
noise amplifier that boosts the 
signal as it is received. It is 
positioned at the focus of the dish 
and is the most important item in 
the receiving system. 

The LNB receives the weak 
signals directly from the satellite 
transmitter 36000km away and 
amplifies them. It also converts 
them to lower frequencies which 
can then be fed down a waveguide 
to the rest of the decoding 
equipment by the TV set. Without 
modern electronic advances in very 
low noise high frequency 
amplifiers, the LNB and satellite TV 
would be impossible. 


At The TV 


When the signal has be put through 
the LNB, it is fed into the satellite 
receiver. Here it may be either in 
plain (clear) format or scrambled. In 
the first case, the signal is converted 
into a UHF signal which can be fed 
directly into a TV aerial socket in a 
similar way to a VCR. If the signal 
is encrypted, the user must supply 
some sort of identification in the 
form of a password before the 
picture can be seen. 

The Videocrypt system operated 
by Sky TV uses Public Key 
cryptography to code up the 
pictures. As the image lines are 
transmitted, they are cut at 
randomly selected points and 
transposed using rotation to 
produce a jumbled result. Without a 
descrambling key, it is impossible 
for the user to watch the program. 

The encryption algorithm is 
transmitted as part of the TV 
picture during the vertical blanking 
period. Decoding the signal 
requires a smart card with a 
microprocessor and memory built 
onto it. When this is placed in the 
decoder it provides the key for the 
algorithm and the picture becomes 
clear. In the event of a security 
breach, the data sent with the 
program can easily be changed and 
new cards dispatched to the users 
by post. An interesting aspect of the 
system is that the audio signal is not 
scrambled, presumably to tempt 
people to buy decoders and smart 
cards for the channels that sound 
good. g 





Practical Electronic 


Technology Feature 





Neural Networks 


The average human brain has 10"' neurons (the same as the number of stars in the galaxy) 
and 10"4 interconnections. Douglas Clarkson examines the latest electronics substitutes. 


fter five decades of rapid 


improvement in_ the 
capabilities of digital 
computers, neural network 


developers are now able to utilise 
these powerful systems to develop 
both their theoretical models and 
working ideas. There has been an 
awareness since the early 1940's of 
the potentialities of networks of 
interconnected logical units or 
neurons. Unfortunately, a lack of 
understanding of how to configure 
them to perform useful work has 


forced them to the academic 
sidelines. 

The general level of interest in 
neural networks ebbed 
considerably when in 1969 Minsky 
and Papert published ‘Perceptrons’ 
showing that useful tasks could 
only be undertaken by neural 
networks with architectures which, 
at the time, could not be usefully 
trained. 

Fortunately for the science, a 
publication by Rosenberg around 
1985 of a ‘back propagation’ 





method for training networks to 
perform useful tasks provided a 
new avenue for research. This 
changed the emphasis from finding 
out how to make neural networks 
perform useful tasks to using them 
to solve problems. Almost every 
department of Computer Science 
has by now have spawned a neural 
network group. 

The simple arithmetic 
operations which neural networks 
tend to utilise can readily be 
implemented by conventional 
electronic components such as 
operational amplifiers, 
potentiometers and so. on. 
However, before looking at the 
details, it is a good idea to look at 
such networks are designed. 


Structure 


Figure 1 shows a simple neural unit 
with three inputs [1, 12 and [3 each 
with relative weighting W1, W2 
and W3. The neural unit can be 
described as having a threshold 


value T. Assuming the inputs I1, [2 


and I3 can assume either a logical 1 
or 0, the summed value of the 
inputs modified by the respective 
weighting values is I1*W1 + I2*W2 
+ 13*W3. If this value is greater than 
the value T, then the neural unit 
will ‘fire’ and its output will be 1. 
The weights W1, W2 and W3 can be 
both positive or negative values. 
Figure 2 shows how such a 
circuit can be implemented from 
conventional analogue circuitry. 
The weighting components of each 
input are determined by adjusting 
analogue potentiometers between 
reference voltages +V and -V. When 
an input is logic 0, the input value 
is set to 0 volts by, for example, a 
CMOS analogue switch. When the 


July 1991 Practical Electronics 15 





Neural Networks 


NT404 NISP 


The world’s first dedicated neural 
instruction set processor or NISP was 
recently announced by Neural 
Technologies in conjunction with Micro 
Circuit Engineering and Recognition 
Research. 

Based upon a novel RISC (Reduced 
Instruction Set Computer) architecture 
and optimised for high performance 
neural network tasks, the NT404 is 
aimed at applications where real time 
intelligent processing is required — 
pattern recognition such as speech, 
graphics, finger prints and optical 
character recognition should all be 
possible. | 

The chip is designed to overcome 
the limitations and deficiencies inherent 
in software simulated solutions; 


conventional processor based hardware 


and existing neural network chips. It 
operates in real time and unlike many 
current systems can be configured for 
virtually any network size or topology. 
Its flexible I/O architecture and and 
standard microprocessor interface 
ensure ease of integration for the 
system designer. An advanced neural 
development environment (NDEV) is 
also available. 

The NDEV runs on an IBM-PC 
system and comes complete with 
software and plug in expansion card 
containing the NT404, NISP 1/0 


input is set to logic 1, the input 
weighting voltage is input directly 
to the current summing amplifier. 
The output of the summing 
amplifier is connected to a 
comparator with threshold voltage 
T. Where a positive w righting value 
is required, for example +V1, then 
this is in fact implemented by 
establishing a voltage -V1 due to 
the inverting nature of the 
amplifier. 

When the threshold value Vref is 
exceeded, the output of the voltage 
comparator will rise close to its 
positive power rail and a logical 1 
output will be available at the zener 
diode output. This output can in 
turn be fed to the input stages of 
other neural units. 

The network will need to be 
trained using a simulated computer 
model and the resulting solution 
parameters incorporated into the 
hardware. The weighting voltages 
and the voltage reference could, 
however, be implemented using 
A/D converters though this would 


16 Practical Electronics July 1991 


b Research Liaited 


fal Meteerk Application Sgster 


a ret fers. (ach of t 
aag PAT 88H registers & 


te GOEREL GY amd EAPaRLIGH registers ar 
Hig, the software will display their 

Hit eot Re able to moe inte these Fie | 
COnEeREs . : 


"The NISP software and PC board. / 





interface and bread-board area for 
prototyping. 

The software suite comprises a 
powerful design system software 
package, the DSS, and a comprehensive 
run time system. Based upon 
Recognition Research’s Autonet, it 
accepts a variety of data formats and 
automatically generates trained NT404 
compatible network files with no 
programming or prior neural network 
knowledge. 


be a costly option. 

While demonstration neural 
network elements can_ be 
constructed using building block 
units such as discrete operational 
amplifiers, practical 
implementations are _ being 
developed which utilise VLSI 
fabrication techniques in order to 
implement the function of 
hundreds or even thousands of 
neuron elements in a single silicon 








! , | Wet Le Sal v 

Operating at 40MHz the NT404 
supports up to 8000 neurons, 57,000 
synapses and any number of layers. It 
has 16-bit data I/O and a simple 
handshaking system for real time data 
transfer up to 20MHz. Neuron transfer 
or threshold functions are fully 
programmable and operation through 
the network is maintained with 16-bit 
accuracy. Both 8 and 16-bit interfaces 
are Supported. 





device. 

Even the very simple network 
shown in Fig. 3 where four inputs 
are fed forward to a middle layer 
and then to the single output, 
would require a relatively large 
number of components to 
implement using conventional 
analogue circuitry, assuming that 
relevant training values for the 
voltage weight values could be 
determined. 


od “Fig. 3. Forward feed neural network. 








Fig. 


The weights in a network are 
usually initially established as a 
series of random values distributed 
about zero. A training set of data 
consists usually of a series of inputs 
to the network and a corresponding 
set of expected outputs. The process 
of training operates by adjusting the 
values of individual weight values 
until there is minimum error 
between the expected and observed 
output for a specific input pattern. 
Even relatively small neural 
networks can require many hours 
of computer time to implement a 
training process. 

While much of the use of neural 
networks gives the appearance of a 
‘turn the handle’ approach, a 
significant amount of skill lies in 
matching a specific problem and 
training set to an appropriate 
network topology. A _ poorly 
designed network may always 
perform poorly no matter the extent 
of the training data available. 

The training of neural networks 
using such back propagation 
methods is essentially a software 
process. Once, however, the weight 
spread of a specific problem has 
been determined, that solution can 
be embedded into hardware which 
will then undertake the function of 
the network trained previously. 


Solutions in Silicon 


While it is certainly easier to design, 
train and simulate neural networks 
using conventional computer 
systems, applications increasingly 
require a ‘black box’ approach 
where the network is implemented 
in hardware. A range of methods 
are being utilised in order to build 
neural networks onto chips. 

It is of course possible to 


4. AWISARD discriminator. 





download weight values into an 
EPROM which can be read by a 
conventional digital processor and 
numerous systems have been 
developed which demonstrate this 
facility. Interest in hardware 
implementations of neural 
networks has been directed at a 
more fundamental level in order to 
develop systems which have 
networks directly built on silicon. 

In the search for hardware 
solutions, the options available are 
for analogue, digital or hybrid. 

Specific implementations using 
conventional analogue VLSI 
circuitry have been implemented. 
These have incorporated fixed 
weight values and cannot be 
adapted to cope with new training 
scenarios. One approach to increase 
the flexibility is to use digital 
switches to select various weight 
values. The high overhead of such 
digital switches limits the 
usefulness of such designs. 





Neural Networks 


Chipping In 

AT&T is developing a neural 
computing chip with 256 neurons 
and more than 100,000 synapses 
using fixed resistor values. Such 
devices could be used to compress 
the bandwidth of video images so 
that they can be sent down phone 
lines in real time. 

With all the families of silicon 
technology available, it is not 
surprising that a broad range of 
researchers are using a range of 
approaches to achieve neural 
networks. While digital systems 
offer the advantage of speed and 
arbitrary accuracy, natural neural 
network systems cope very well 
with much poorer levels of 
accuracy. This argument is often 
used by researchers implementing 
neural networks using analogue 
devices. 

Often systems which are 
described as being digital VLSI 
neural networks are not strictly so. 
This could be argued for the 
WISARD (Wilkie, Stoneham and 
Aleksander’s Recognition Device) 
system. This functions by 
implementing a_ series’ of 
discriminator units are shown in 
Fig. 4. 

Specific address lines based on 
data bits set in the sampled pattern 
relate to specific bits set in 
associated RAM. Thus 0000 maps to 
top left bit in the figure and 1111 
maps to bottom right. The 
presentation of multiple training 
patterns writes or overwrites a 
logical 1 in the appropriate RAM 
element. The system functions by 


July 1991 Practical Electronics 17 


Neural Networks 





learning on a training set during 
which RAM space is updated and 
then compared against specific 
problem cases. A real system would 
consist of a series of such 
discriminator units, each of which 
seeks to pattern match against a 
previous training set. The system is 
perhaps closer to a _ nearest 
neighbour classifier than a 
conventional multilayer perceptron. 

The advantage of the WISARD 
system in common with that of an 
orthodox neural network, is that the 
result of a comparison of a test 
pattern against a store of trained 
information takes place rapidly 
without the need for conventional 
software programming. 

A significant amount of work in 
hardware implementations of 
neural networks’ has_ been 
undertaken by Carver Mead at 
Caltech in the USA. Extensive use 
has been made of analogue 
MOSFET technology to build 
systems for investigation of sensory 
processing systems such as an 
artificial eyes and ears. In general 
the senses are by no means as 
simple as they would appear. Even 
the most basic sensory systems use 
complex natural neural 
preprocessing networks’ to 
unscramble data before passing it 
on to the brain for processing at a 
higher level. The key feature of such 
processes is that they are 
undertaken locally, close to the site 
of the initial sensory stimuli. Thus 
these sensory systems are a 
valuable source of information of 
how to use artificial neural 
networks to copy nature’s example. 


Figure 5 shows a typical circuit 


18 Practical Electronics July 1991 


Vswitch 


Current pulse 


used by Mead in the form of a four 
quadrant Gilbert Multiplier circuit 
where the output current [,,, is 
proportional to (V,-V,) * (V3-V,). 
This allows neural networks to be 
implemented using compact 
analogue multiplier units, allowing 
higher densities of on-chip neurons 
than could be implemented using 
digital systems. 

The technique of the so called 
pulse stream arithmetic is being 
developed in the UK, primarily at 
Edinburgh University, to use 
analogue circuit elements for neural 
network implementations. Fig. 6 
indicates a pulse stream synapse 
circuit. The weight of the synapse 
T;, is stored as a voltage on a 
capacitance. An input voltage pulse 
V. of width D, at a specific 
frequency is input into the synapse. 
This causes an RC discharge as 
shown where the fall time during 
discharge is much longer than the 
rise time at the end of the input 
pulse. The inverter has a switching 
threshold Vswitch which 
determines the width of the 
subsequent pulse, so that this is the 
stage where multiplication is taking 
place. Vi, is in fact proportional 
to the initial synaptic weight Tj. 
The transistors T6/T7 are designed 
to either sink or source current 
respectively depending on the 
value of T;.. 

For T,, > 2 V, an increase in 
potential proportional to (T;, - 2V) 
will occur and for T;, < 2V. a 
decrease in potential proportional 
to (2V - T;.) will occur. 

Thus for synaptic weight values 
close to 2V, the synapse will be 
relatively insensitive to input clock 
pulses. The greater the deviation 
the more rapidly will the synapse 
be driven to saturation voltages, 








either positive or negative. Such 
circuits require typically around 11 
MOSFET transistors per synapse. 

MOSFET circuits can _ be 
designed which utilise so called 
transconductance multiplier effects, 
where the drain/source current is 
proportional to the term V,. * V4, 
(gs is gate to source and ds, drain to 
source). Such circuits can more 
directly implement current pulse 
switching at the output to simulate 
synaptic multiplication. Such a 
design which utilises 4 MOSFET 
elements is shown in Fig. 7. It is 
estimated, however, that using such 
a transconductance approach 
around 15,000 synapses could be 
implemented on an 8 mm by 8 mm 
die of Silicon using 3 um CMOS 
fabrication technology. 

While digital systems offer 
options for both precision and 
accuracy, there is the anticipation in 
some quarters that it will be 
analogue systems which will allow 


high density hardware 
implementations of neural 
networks. 


The Neural Bit Slice 


One approach of implementing 
digital neural networks in silicon 
has been the development of the 
Neural Bit Slice (NBS) computing 
element by Micro Devices in the 
USA. The device is constructed in 
CMOS technology and packaged in 
a 68 pin PLCC. It comprises eight 
independent neurons, each of 
which can have eight external 
digital inputs. In addition, there is 
the facility for routing back to a 
specific neuron the outputs from 
the other seven neural units. The 
block diagram of the MD1220 is 
shown in Fig. 8. 

Sets of 16-bit wide synaptic 
weights are held in external weight 
memory. Each computational cycle 
is called a frame and consists of 
calculating a bit wise multiplied 


Vfixed 
Current 


JUUUL 





product from the input data and the 
appropriate weight data. The dot 
product register exists as two 
separate 8-bit registers. An extra 
register can be accessed to check for 
accumulator overflows. The digital 
nature of the implementation 
avoids errors of drift and offset 
which may be present with an 
analogue implementation. 

Each neuron has _ reserved 
weight space for 15 synaptic weight 
values, eight for external inputs and 
seven for internal inputs derived 
from outputs of other seven neuron 
elements. 

In calculating the value of the 
output in the serial mode, time 
multiplexed inputs unique for each 
neuron are applied and multiplexed 
with the appropriate weight value. 
In the parallel mode, each of the 
synaptic inputs SI(j) is applied to 
each of the neurons during each 
weight cycle. 

The calculated value can be 
compared against a threshold 
selection function as outlined in 
Fig.9. In the single threshold case, 
for example, the output of the 
neuron will be 1 when the summed 
value is greater than 0 and zero 
otherwise. 


some Real Uses 

The MD1220 neural bit slice device 
is designed for use as a building 
block in real time neural network 
systems. The design of the device 
allows expansion along rows and 
columns. The processing time 
through a single NBS is 7.2 uS, 
which is claimed to be equivalent to 
a 55 MIPS processor. The 16-bit 
weight space provides excellent 
resolution. It would be undesirable, 
for example, to have too course a 
resolution (say 8-bit) since this 
would lead to difficulties in 
successful training of the network. 


About zero 





8 
2 
= 
| 
a 
oC 
= 
J€ 
_& 
8 
a. 


Neural Networks 


Weignt data 


To and from 
CPU 


Fig. 8. Block diagram of MD1220. 


Chips such as the MD1220 are 
used with a development system 
which can configure a specific 
network topology and determine 
the weight values necessary to 
perform specific functions. The 
MD1220 is supplied in the UK by 
Neural Technologies of Petersfield 
as an evaluation board which can 
be operated with the 
BRAINMAKER neural network 
software package on an IBM PC. 
Using such a development system, 
designs can be implemented to 
provide high performance real time 
neural network systems. 

While the MD1220 device does 
not provide a hardware solution for 
neural networks with hundreds of 
neurons, it does allow for less 
demanding tasks in embedded 
control to be implemented cost 
effectively. 

Neural Technologies have 
recently announced a dedicated 
Neural Instruction Set Processor 
(NiSP - see box) and National 





Semiconductor is offering the 
NU32, a device with 32 neurons 
which connects to a weight chip 
which can implement 1024 weight 
values. Intel is providing the N64, a 
neural network chip with 3 layers, 
each with up to 64 neurons. Weight 
values are stored in programmable, 
read only memory. 

Often however, it is the 
availability of support systems such 
as that available for the MD1220 
which allow such hardware devices 
to be evaluated effectively for 
specific applications. Thus the 
systems which tend to be the most 
popular need tend to be those for 
which the most technical support is 
provided. 

Neural networks are destined to 
play an important part in future 
smart interfaces simulating sensory 
functions. The role of hardware 
implementations will be important 
to provide both the speed of 
response and the sophistication of 
function required. a 


July 1991 Practical Electronics 19 


HIGH POWER AMPLIFIER For your car, it has 150 watts output. 
Frequency response 20HZ to 20 KHZ and a signal to noise ratio 
better than 60db. Has builtin short circuit protection and adjustable 
input level to suit youe existing car stereo, so needs no pre-amp. 
Works into speakers ref 30P7 described below. A real bargain atonly 
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HIGH POWER CAR SPEAKERS. Stereo pair output 100w each. 
4ohm impedance and consisting of 6 1/2" woofer 2" mid range and 
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PERSONAL STEREOS Customer returns but complete with a 
aair of stereo headphones very good value at £3.00 ref 3P83. 
2KV 500 WATT TRANSFORMERS Suitabie for high voitage 
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MICROWAVE CONTROL PANEL. Mains operated, with touch 
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outputs one for power and one for pulsed power (programmable). 
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FIBRE OPTIC CABLE. Stranded optical fibres sheathed in black 
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12V SOLAR CELL.200mA output ideal fortrickle charging etc. 300 
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PASSIVE INFRA-RED MOTION SENSOR. ah. 
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VIDEO SENDER UNIT Transmit both audio 
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FM TRANSMITTER housed in a standard working 13A adapter 
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WHISPER 2000 LISTENING AID.Enables you to hear sounds 
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CAR STEREO AND FM RADIOLow cost stereo system giving 
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LOW COST WALIKIE TALKIES. Pair of battery 

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7 CHANNEL GRAPHIC EQUALIZERPblus a 60 watt ¢:  - 
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1991 CATALOGUE AVAILABLE NOW IF YOU DO NOT 
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GEIGER COUNTER KIT.Complete with tube, PCB and all compo- 
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FM BUG KIT.New design with PCB embedded coil. Transmits to 
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FM BUG Built and tested superior 9v operation £14.00 ref 14P3 
COMPOSITE VIDEO KITS. These convert composite video into 
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ELECTRONIC SPEED CONTROL KITiorcS motor. 
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ACORN DATA RECORDER ALF503 Made for BBC computer 
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CAR IONIZER KIT Improve the air in your car! clears smoke and 
helps to reduce fatigue. Case required. £12.00 ref 12P8. 

6V 10AH LEAD ACIDsealed battery by yuasha ex equipment but 
in excellent condition now only 2 for £10.00 ref 10P95. 

12 TO 220V INVERTER KITAs supplied it will handle up to about 
15 wat 220v but with a larger transformer it will handle 80 watts. Basic 
kit £12.00 ref 12P17. Larger transformer £12.00 ref 12P41. 

VERO EASI WIRE PROTOTYPING SYSTEMideal for design- 
ing projects on etc. Complete with tools, wire and reusable board. 
Our price £6.00 ref 6P33. 

MICROWAVE TURNTABLE MOTORS. Ideal for window dis- 
plays etc. £5.00 ref 5P165. 

STC SWITCHED MODE POWER SUPPLY 220v or 110v input 
giving Sv at 2A, +24v at 0.25A, +12v at 0.15A and +90v at 0.4A £6.00 
ref 6PS9 

TELEPHONE AUTODIALLERS. These units, when triggered will 
automatically dial any telephone number. Onginally made for alarm 
panels. BT approved. £12.00 ref 12P23 (please state telephone no 
req'd). 

25 WATT STEREO AMPLIFIERc. STK043. With the addition of 
a handful of components you can build a 25 watt amplifier. £4.00 ref 
4P69 (Circuit dia included). 

LINEAR POWER SUPPLY Brand new 220v input +5 at 3A, +12 
at 1A, -12 at 1A. Short circuit protected. £12.00 ref 12P21. 

MAINS FANS. Snail type construction. Approx 4"x5" mounted on a 
metal plate for easy fixing. New £5.00 SP166. 

POWERFUL IONIZER KIT.Generates 10 times more ions than 
commercial units! Complete kit including case £18.00 ref 18P2. 
MINI RADIO MODULE Only 2" square with ferrite aerial and tuner. 
Superhet. Req's PP3 battery. £1.00 ref BD716. 

HIGH RESOLUTION MONITOR.9" black and white Phillips tube 


in chassis made for OPD computer but may be suitable for others. 
£20.00 ret ZEP2E 


BARGAIN NICADS AAA SIZE 200MAH 1.2V PACK OF 10 


£4.00 REF 4P92, PACK OF 100 £30.00 REF 30P16 





CB CONVERTORS.Converts a car radio into an AM CB receiver. 
Cased with circuit diagram. £4.00 ref 4P48. 

FLOPPY DISCS. Pack of 15 31/2" DSDD £10.00 ref 10P88. Pack 
of 10 51/4" DSDD £5.00 ref 5P168. 

SONIC CONTROLLED MOTOR One click to start, two click to 
reverse direction, 3 click to stop! £3.00 each ref 3P137. 
FRESNEL MAGNIFYING LENS 83 x 52mm £1.00 ref BD827. 


SOME OF OUR PRODUCTS MAY BE UNLICENSABLE IN THE UK 





20 Practical Electronics July 1991 





LCD DISPLAY. 4 1/2 digits supplied with connection data £3.00 ref 
3P77 or 5S for £10.00 ref 10P78. 

ALARM TRANSMITTERS. No data avaliable but nicely made 
complex transmitters 9v operation. £4.00 each ref 4P81. 

100M REEL OF WHITE BELL WIREfigure 8 pattern ideal for 
intercoms, door bells etc £3.00 a reel ref 3P107. 
TRANSMITTER RECEIVER SYSTEMoriginally made for nurse 
call systems they consist of a pendant style transmitter and a 
receiver with telescopic aenal 12v. 80 different channels. £12.00 ref 
12P26 

CLAP LIGHT. This device turns on a lamp at a finger ‘snap’ etc. 
nicely cased with builtin battery operated light. Ideal badeide light ate 
£4.00 each ref 4P82. 

ELECTRONIC DIPSTICK KIT.Contains all you need to build an 
electronic device to give a 10 level liquid indicator. £5.00 (ex case) 
ref SP194. 

UNIVERSAL BATTERY CHARGER.Takes AA's, C's, D's and 
PP3 nicads. Holds up to 5 batteries at once. New and cased, mains 
operated. £6.00 ref 6P36. 

ONE THOUSAND CABLE TIES!75mm x 2.4mm white nylon 
cable ties only £5.00 ref 5P181. 

PC MODEMS 1200/75 baud modems designed to plug into a 
PC complete with manual but no software £18.00 ref 18P12 
ASTEC SWITCHED MODE POWER SUPPLYs80mm x 165mm 
(PCB size) gives +5 at 3.75A, +12 at 1.5A, -12 at 0.4A. Brand new 
£12.00 ref 12P39. 

VENTILATED CASE FOR ABOVE PSwUnith IEC filtered socket 
and power switch £5.00 ref SP190. 

IN CAR POWER SUPPLY. Plugs into cigar socket and gives 
3,4,5,6,7.5,9, and 12v outputs at 800mA. Complete with universal 
spider plug. £5.00 ref 5P167. 

CUSTOMER RETURNEDswitched mode power supplies. Mixed 
type, good for spares or repair. £2.00 each ref 2P292. 

DRILL OPERATED PUMP. Fits any drill and is self priming. £3.00 
ref 3P140. 

PERSONAL ATTACK ALARM.Complete with built in torch and 
vanity mirror. Pocket sized, req's 3 AA batteries. £3.00 ref 3P 135 
POWERFUL SOLAR CELL 1AMP .45 VOLTbnly £5.00 ref 
5P192 (other sizes avaliable in catalogue). 

SOLAR PROJECT KIT.Consists of a solar cell, special DC motor, 
plastic fan and turntables etc plus a 20 page book on solar energy! 
Price is £8.00 ref 8P51. 

RESISTOR PACK.10 x 50 values (500 resistors) all 1/4 watt 2% 
metal film. £5.00 ref SP170. 

CAPACITOR PACK 1.100 assorted non electrolytic capacitors 
£2.00 ref 2P286. 

CAPACITOR PACK 2. 40 assorted electrolytic capacitors £2.00 
ref 2P287. 

QUICK CUPPA? 12v immersion heater with lead and cigar lighter 
plug £3.00 ref 3P92. 

LED PACK .50 red leds, 50 green leds and 50 yellow leds all 5mm 
£8.00 ref 8P52 

FERRARI TESTAROSSA. A true 2 channel radio controlled car 
with forward, reverse, 2 gears plus turbo. Working headlights. 
£22.00 ref 22P6. 

ULTRASONIC WIRELESS ALARM SYSTEMTwo units, one 
a sensor which plugs into a 13A socket in the area you wish to 
protect. The other, a central alarm unit plugs into any other socket 
elsewere in the building. When the sensor is tnggered (by body 
movement etc) the alarm sounds. Adjustable sensitivity. Price per 
pair £20.00 ref 20P34. Additional sensors (max 5 per alarm unit) 
£11.00 ref 11P6. 

TOP QUALITY MICROPHONE. Unidirectional electret con- 
denser mic 600 ohm sensitivity 16-18khz built in chime complete 
with magnetic microphone stand and mic clip. £12.00 ref 12P42. 
WASHING MACHINE PUMP.Mains operated new pump. Notself 
priming £5.00 ref 5P18. 

IBM PRINTER LEAD. (D25 to centronics plug) 2 metre parallel. 

£5.00 ref SP186. 

COPPER CLAD STRIP BOARD 17" x 4" of .1" pitch "vero" board. 
£4.00 a sheet ref 4P62 or 2 sheets for £7.00 ref 7P22. 

STRIP BOARD CUTTING TOOL.£2.00 ref 2P352. 

3 1/2" disc drive. 720K capacity made by NEC £60.00 ref 60P2 
TV LOUDSPEAKERS.5 watt magnetically screened 4 ohm 55 x 
125mm. £3.00 a pair ref 3P109. 

TV LOUDSPEAKERS. 3 watt 8 ohm magnetically screened 70 x 
50mm. £3.00 a pair ref 3P108. 

BBC TRACKBALLS Once again in stock only £4.00 ref 4P86 
CROSS OVER NETWORKS 8 ohm 3 way Japanese made units 
Excellent units avaliable at only £2.00 for a pair! ref 2P363 
SPEAKER GRILLS set of 3 matching grills of different diameters. 
2 packs for £2.00 (6 grills) ref 2P364 

50 METRES OF MAINS CABLE £3.00 2 core black precut in 
convenient 2 m lengths. Ideal for repairs and projects. ref 3P91 

4 CORE SCREENED AUDIO CABLE 24 METRES £2.00 
Precut into convenient 1.2 m lengths. Ref 2P365 

TWEETERS 2 1/4” DIA8 ohm mounted on a smart metal plate for 
easy fixing £2.00 ref 2P366 

COMPUTER MICE Originally made for Future PC's but can be 
adapted for other machines. Swiss made £8.00 ref 8P57. Atan ST 
conversion kit £2.00 ref 2P362. 

6 1/2" 20 WATT SPEAKER Built in tweeter 4 ohm£5.00 ref SP205 
5" X 3" 16 OHM SPEAKER 3 for £1.00!! ref CD213 
ADJUSTABLE SPEAKER BRACKETS ldeal for mounting 
speakers on internal or external corners, uneven surfaces etc. 2 for 
£5.00 ref 5P207 

PIR LIGHT SWITCH Replaces a standard light switch in seconds 
light operates when anybody comes within detection range (4m) and 
stays on for an adjustable time (15 secs to 15 mins). Complete with 
daylight sensor. Unit also functions as a dimmer switch! 200 watt 
max Not suitable for flourescents. £14.00 ref 14P10 

2 MEG DISC DRIVES 3 1/2" disc drives made by Sony housed in 
a 5 1/4" frame 1.2 meg formatted. £66.00 ref 66P1. 

360K 3 1/2" DISC DRIVES 1/2 height £25.00 ref 25P26 

40 CHANNEL TRANSCEIVER 4 WATT OUTPUT, 
HANDHELD SQUELCH CONTROL ETC £70.00 
EACH REF 70P1 

OR AVALIABLE AS A PAIR WITH NICAD 
BATTERY PACKS FOR £150.00 REF 150P1 
Illuminated channel display, 10 section aenal, 
Hi-Low power switch, external aerial socket, 

DC charger socket, 12v DC power socket, 
carrying strap and owners manual. 

















Personal View 


Does office automation 
mean alienation? 


Technology is changing our lives, but is it also destroying our values? — Tom Ivall ponders 
the effect of technology on life as we know it. 


he fact that some consumer 
technologies fail to make the 
intended mass markets — like 
the video disc and the Prestel 
viewdata system - tells us 
something about the relationship 
between technology and human 
beings. Without such failures it 
might seem that consumers are 
mere functional components of the 
market mechanism, permanently 
open mouths for whatever goodies 
designers, manufacturers and 
marketeers decide to push into 
them. In fact, of course, consumers 
are individuals with particular 
wishes, interests, tastes and values. 
In relation to consumer 
technologies our wishes, interests 
and tastes are often transitory and 
subject to the whims of fashion. But 
values — such as truth, beauty, 


justice, charity, respect for life —- 
though still subject to modification, 
are more enduring in the way they 
affect our actions. So it seems to me 
important that we should try to 
understand the interaction between 
values and technology as well as 
we can. 

This interaction is important to 
the people in industry who design 
the technology and its products 
because the success of their work 
may well depend on these 
transcendental factors. It’s 
important to all of us who live in 
industrialised societies because the 
technology of our world, by 
affecting the material way we live, 
may be changing the values we 
hold, even though we may be 
largely unconscious of them. 

“Technology is changing our 


There is a continuous 
loop of action linking 


Human 


problems 


technology and human 
values, within which 
causes and effects 


Technological 
advance 


can't be distinguished 
from eachother" 





Revised 


lives” is a familiar platitude that 
doesn’t get us very far and is only 
part of the truth. In conducting our 
lives we are also changing the 
technology — by what we choose to 
accept or reject. To consider 
technology either simply as a cause 
or simply as an effect is a sterile 
exercise. This is crude determinism, 
appropriate to machines but not 
human affairs. 

In one direction it would assume 
that technical innovation is free to 
act autonomously, which it is not, 
and in the other direction that there 
is some immutable human essence 
— human nature - controlling 
everything, which also is not true. 

The reality is much more 
complex. When you study what 
actually happens you find there is a 
continuous loop of action linking 
technology and human values, 
within which causes and effects 
can’t be distinguished from each 
other — rather as in an electronic 
feedback system. Certainly you 
can’t pin down a primary cause and 
an ultimate effect in this loop. 

Let's break into the loop 
arbitrarily at, say, the technology 
point and assume for the moment 
it’s computer-based automation. 
You find that this particular 
technology changes’ values 
connected with labour, skill and the 
formal relationships between 
managers and workers. In turn the 
modified values influence the social 
forces which control long-term 
research and industrial planning. 
Social forces are in turn influenced 
by political and economic change 
and human values are therefore 
subject to the combined impact of 
political, economic, social and 
technological change. In the end the 
modified research and planning 


July 1991 Practical Electronics 21 


Personal View 





goals have their effect on the future 
direction of the technology - 
completing the cycle. 

This continuous interaction 
produces a social pattern, but not 
through a single force. It results 
from a complex interplay of 
economic and political power, 
historical accident, ownership, size 
of organisations and so on. These 
factors don’t control anything 
directly but exert various pressures 
and apply various limits which 
combine to shape the emerging 
social pattern. 

The values involved are 
extremely complex in themselves. 
We know them to be abstract 
principles which we hold in high 
regard. But where do they come 
from? How do they get into our 
minds? Sciences like anthropology 
and sociology can describe in detail 
what is actually valued in various 
societies but can’t explain why. For 
example, is something valued 
because it is desired? Or is it 
desired because it is valued? Some 
values are considered intrinsic, like 
those listed above, while others are 
instrumental values — means to an 
end — like thrift or diligence. 

Electronics is contributing to at 
least three main areas where 
technology is interacting strongly 
with values. One is transport and 
communications. Here, private 
transport, the telephone/telecoms 
systems and electronic home 
entertainment are tending to 


22 Practical Electronics July 1991 


promote the instrumental values of 
flexibility and mobility over the 
traditional ones of security and 
stability. 

In the area of medicine and 
pharmacology, technology has 
influenced society by offering 
greater control over life; prolonging 
it, supporting it when it would 
otherwise cease, and preventing it 
altogether through contraception. 
Suffering and misfortune are being 
subtly revalued as_ technical 
problems which can be fixed. 
Together with genetic engineering, 
these perceptions are tending to 
diminish our sense of the 
uniqueness of human life. 

The third main area is office 





automation and computing. Here 
the interaction occurs mainly 
through the value systems of the 
workers involved. The benefits of 
greater productivity and easier, 
cleaner working conditions have to 
be paid for by loss of direct sensory 
experience and physical contact 
with the work, a feeling of 
remoteness or alienation. The work, 
whether in a factory or an office, 
becomes more abstract and 
intellectualised, experienced largely 
through the medium of symbols on 
VDU screens or computer print- 
outs. 

When these conditions are 
accepted as inevitable they become 
the basis of new instrumental 
values; order, uniformity, precision, 
efficiency. Some social critics find 
this a worrying trend. They feel that 
man is becoming preoccupied with 
his own tool-making, that efficiency 
and technique are being worshiped 
for their own sake and getting 
higher in the scale of values than 
the supposed object of all this 
striving — man himself. There is a 
fear that we may be devaluing the 
principle that life is an end in itself. 

However, it’s not fair to blame 
automation and computing alone 
for this distortion, if in fact it is 
occurring. They are just one 
component of industrialisation 
which has concentrated populations 
in towns and changed living habits, 
thereby modifying the culture and 
its values. One sad result is a 
narrowing of our ideas of what 
constitutes normal human 
behaviour. We are in danger of 
losing our mystics, eccentrics and 
originals. ; z 











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July 1991 Practical Electronics 23 


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24 Practical Electronics July 1991 





At The Lower 
End Of The Range 





Measurement Project 





Making measurements of low frequencies is not as straightforward as it may seem. This easy 
to build meter from Owen Bishop provides a simple way to measure from 0.5Hz to 20Hz. 


he two most commonly used 

instruments for measuring 

frequency are the oscilloscope 
and the frequency counter. With the 
oscilloscope, you display the 
waveform and use the grid on the 
screen to measure the time for one 
complete oscillation. The reciprocal 
of this gives the frequency. Those 
who have a frequency counter can 
measure frequency with much 
more precision but, for most 


amateurs, the expense of a 
frequency counter is rarely 
warranted. 


For very low frequencies a 
simple technique is to arrange for 
the signal to flash an LED and 
count the number of flashes 
observed during a given period of 
time, say 30 seconds. You then 
calculate the frequency. A related 
technique is to display the signal on 
an oscilloscope and count the rises 
and falls of the horizontal trace in a 
given time. 

A difficulty arises when you try 
to measure a frequency in the range 


0.5Hz to 10Hz. It is too fast for the 
LED flashing technique and too 
slow for the oscilloscope or 
frequency counter. Asa 
consequence of Murphy’s Law, I 
often find myself with a signal in 
this range and no means of 
measuring it. 


The Requirements 


The low frequency meter (LFM) is 
sensitive to an input signal with an 
amplitude of about 1mV peak-to- 
peak or greater. The input 
impedance is relatively high, being 
approximately 1MQ, input is 
protected up to +100V. The display 
is a moving-coil meter and 
although a digital panel meter 
could be used with this circuit, a 
much more comprehensible 
readout is obtained with a pointer 
moving over a scale. There are two 
switched ranges, 0-2Hz and 0-20Hz. 
The circuit also provides an output 
signal running at 100 times the 
frequency of the input signal. If you 


already have a frequency counter, 
this output can be fed to it and you 
simply divide the reading on the 
frequency counter by 100. 


Circuit Operation 


There are two main ways of 
determining frequency, measuring 
the period or counting the number 
of waves in a given time. Neither of 
these methods gives a rapid result 
with signals of low frequency. 
When measuring the period it is 
necessary to average out readings 
taken for a number of consecutive 
waves. With wave-counting, if the 
timing period is 10s and the 
frequency is 1Hz, for example, only 
10 waves are counted. The precision 
is only 10% and even less for lower 
frequencies. 

This circuit uses a technique that 
involves generating a signal with a 
frequency which is 100 times 
greater than the input signal 
frequency. This simplifies the 
measurement process and also 





July 1991 Practical Electronics 25 


Low Frequencies 


(provided that the frequency is not 
changing too rapidly) allows a more 
rapid measurement to be taken. 


Phase Locked Loop 


The initial stage (Fig.1) is an 
amplifier for detecting small 
signals. This is followed by a 
comparator to convert’ the 
waveform into a square wave with 
rapid rise and fall times suitable for 
driving to the next stage. 
Frequency multiplication is 
achieved by making use of a phase- 
locked loop (PLL). As Fig 1 shows 
this consists of 3 parts, a phase 
detector, a voltage-controlled 
oscillator (VCO) and a divider. The 
detector receives the low-frequency 
signal (call this the input signal) 
and compares it with another low- 
frequency signal (call this the 





"Fig. 3. Circuit diagram. 


26 Practical Electronics July 1991 


feedback signal) coming from the 
divider. The output of the detector 
depends on to what extent the two 
signals are in phase with each other. 
For them to be in phase and to 
remain in phase also implies that 
they are identical in frequency. 

If the frequency of the input 
signal is higher than that of the 
feedback signal the voltage at the 
output of the detector is high 
(+12V) causing the VCO frequency 
to rise to its maximum. If the 
frequency of the input signal is 
lower than that of the feedback 
signal, the voltage falls to zero, 
VCO frequency falls to its minimum 
value — in this circuit, to zero. If the 
two frequencies are equal and in 
phase, the output from the phase 
detector is open-circuit and the loop 
is broken. However, the phase 
detector produces a pulse at any 





time the signals begin to go out of 
phase (Fig.2). The pulse may be 
positive or negative. If the feedback 
signal starts to lag behind the input 
signal, the pulse is positive. This 
adds a little extra charge to a 
capacitor in the loop, causing the 
voltage supplied to the VCO to 
increase. This tends to increase its 
frequency, so that its output (or 
rather, the output of the divider) 
catches up with the input signal. 
The length of the pulse depends on 
how far out of phase the two signals 
are, so the rate of catching up is 
greater if there is a larger phase 
difference. 

Conversely, if the feedback 
signal goes ahead of the input 
signal, a negative pulse from the 
phase detector reduces the charge 
of the capacitor, causing the VCO to 
oscillate more slowly, until the 











feedback signal has fallen into step 
with the input signal. 

If the signals are both the same 
frequency and both exactly in 
phase, the charge on the capacitor is 
constant. The VCO oscillates at a 
constant frequency and the loop is 
stable. In practice, any capacitor has 
a small leakage current, so the VCO 
rate falls very slowly until the phase 
detector detects the phase 
discrepancy and emits a further 
positive pulse to replenish the 
charge. 

The system becomes stable when 
the input and feedback signal match 
— the system has locked on to the 
input signal. The feedback signal is 
the VCO frequency divided by 100 
and so the system locks on when 
the VCO frequency is 100 times that 
of the input signal. If the input 
signal subsequently changes in 
frequency the phase difference is 
detected immediately, the VCO 
frequency is altered accordingly 
and the system remains locked on. 

The output from the VCO is 
suitable for driving a conventional 
frequency counter so, if such an 
instrument is available, no further 
stages are required. For a self- 
contained frequency meter a stage 
is needed to measure the frequency 
of the VCO signal. In the LFM this 
is a tachometer IC specially 
designed for converting frequency 
to voltage. 

The signal arriving at the 
tachometer goes to a sub-circuit 
known as a charge pump. As each 
pulse arrives, a unit of charge is 


transferred to a capacitor. This is 
another storage capacitor, not the 
capacitor referred to in the 
description of the PLL. The higher 
the frequency, the more pulses 
arrive in a given time and the more 
rapidly the charge accumulates on 
the capacitor. The charge leaks 
away Slowly through a resistor and 





Low Frequencies 





thus the voltage across the capacitor 
varies with the frequency of the 
signal. This voltage is fed to a 
voltmeter which is calibrated to 
display the frequency of the 
incoming signal. 


Practical Considerations 


Input protection is provided by D1 
and D2 (Fig.3). When the input 
voltage exceeds +0.6V one or other 
of the diodes conducts. The diodes 
are rated to withstand a peak 
inverse voltage of 100V. R1 limits 
the current passing through the 
diodes to a safe value. The diodes 
are rated to withstand a peak 
inverse voltage of 100V so limiting 
the maximum allowable input 
swing to +100V. 

The signal passes through C1 to 
the amplifier (IC1), an operational 
amplifier connected in the non- 
inverting mode. R4 and R5 set the 
gain of the amplifier to 100. The 
amplifier signal goes to IC2, another 
operational amplifier but connected 
as a comparator. R6 and R7 hold the 
(+) input close to 6V and one signal 
input terminal is connected to this 
line. R8 and R9 hold the (-) input at 
6V too, but signals arriving from 


July 1991 Practical Electronics 27 


Low Frequencies 


+12V (via $1) 


Inputs 


VOZEZEOKFAY IO MMOIYW>Y 


From board A 


I35 


+12V via S1 


IC1 through C2 cause the voltage at 
the (-) input to rise and fall, 
producing sharp upward and 
downward sweeps of the output of 
IC2. The signal is thus converted 
into a square wave ready for the 
PLL. 

The phase comparator and VCO 
are contained in a single IC, IC3. 
R10 and C3 set the maximum 
frequency of the VCO to a little 
more than 2kHz, — just over 100 
times the maximum signal 
frequency. The signal goes to one 
input of the phase comparator at 
pin 14. 

R12, R13 and C4 comprise the 
loop filter. It is on C4 that the 
output pulses from the phase 
detector are stored, pin 13 being the 
control input of the VCO. The 
values have been selected so as to 
make the loop lock on reliably to 
low-frequency signals, though this 
is to a certain extent at the expense 
of inability to follow rapid changes 
of frequency. C4 is a tantalum bead 
capacitor; an electrolytic capacitor 
has too high a leakage current to be 
satisfactory for charge storage in a 
PLL. 


28 Practical Electronics July 1991 





The output of the VCO (pin 4) 
goes to the input of a divide-by-10 
counter in IC4. The output of this 
counter at pin 6 goes to a second 
divide-by-10 counter in the same 
IC. The final frequency, being the 
VCO frequency, divided by 100 
appears at pin 14. This is fed back to 
the phase comparator at pin 3 of 
IC3. The VCO output also goes to 
the tachometer, IC5. 

The charge pump circuit of the 
tachometer delivers a unit of charge 
to C6 as each pulse arrives from the 
VCO. The amount of charge 
delivered, and hence the voltage 
across C4 for any given frequency, 
depends on the values of C5 and 
the resistance between pin 3 and the 
OV rail. It also depends on the 
standard voltage used within the 
IC, which is set to 7.56V by an 
internal reference. The voltage 
reference makes the output 
independent of battery level above 
about 10V. Thus the output voltage 
at pin 5 is given by: 


Veu = 7.56 x RC 


R is the total value of the fixed 





Board A — cut strips at the following 
locations: 

G10, D16, G16-J16, G24-J24, G31- 
J31 


To board B 


Di2 


Board B — cut strips at the following 
locations: 

B7-17, B13-H13, 114, K15, D15, 
B17-117, 121-K21, B22-H22, 021, 
N26, B28-E28, G28, H28, K28 


Fig. 4. Stripboard layouts. 


resistor (R16 or R17) and the 
variable resistor (VR1 or VR2) 
selected by the range switch S2. To 
obtain a maximum output of 6.2V at 
2Hz, R16 and VR1 must total 410k 
. For the 20Hz range the total of R17 
and VR2 must be 41K . 

We use a moving-coil meter 
with a series resistor to measure the 
output voltage. The resistance 
required is make up of R18 and 
VR3, and is calculated from: 


R=V../I-r 


where I is the full-scale current 
of the meter and r is the resistance 
of its coil. in the prototype we used 


a 100UA meter with coil resistance 
940 ,sSo: 


R = 6.2/(100 x 10-6) - 940 = 61k 


This is obtained by using the 
fixed and variable resistors shown. 


Putting It In The Box 


The circuit requires only 30mA, so it 
is conveniently powered by two 


Continued on page 61 » 


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July 1991 Practical Electronics 29 


Rewarding Loyalt 


You can be a winner with our Reader 
Loyalty Bonus. 


This month's top ten readers: 
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Electronic Components refunded. 


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Number One Systems. 


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Greenweld. 


G.L. Marshall of Southport has the £99.95 spent with Maplin refunded. 
M. Smith of Edinburgh traded with Maplin and has £91.99 refunded. 
J. Thomas of London W11 gets back the £44.95 he spent with Tandy. 
S.N. Burn of Whitley Bay claims his £28.37 which he spent with Bull 


Electrical. 


H. Levin of Brighton secures his £21.35 for purchasing goods from 


Greenweld. 


A. Reekie from Liverpool is reimbursed £14.95 for trading with 


Cambridge Computer Science. 


Leon Volauseh of Pontypridd has £14.25 repaid for his purchase from 


Maplin. 


See page 53 for entry details! 















150ns pulls 1.00 
4164 100ns 1.45 
41256 120ns 1.40 
41256 100ns 1.55 
41256 80ns 1.65 
41464 100ns 2.20 
414256 80ns 4.45 
411000 80ns 4.45 
1Megx9 SIMM 80ns 37.50 
1Megx9 SIPP 80ns 39.50 






Motherboards 

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12Mhz £79 16Mhz £95 20MHz £112 
80386SX 16MHz £255 20MHz £290 
80386DX 25MHz £390 

80386DX 33Hz Cache £675 


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120Mb Internal Tape Streamer £215 
102 Key Tactile Keyboard £30 


Video 

8 Bit VGA card 256K 800x600 £37.50 
16 Bit VGA 512K 1024x768 £69 
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6264 100ns LP 2.45 
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Disk drives 

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30 Practical Electronics July 1991 







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Hitachi H8/330 


Data Sheet 


Perhaps the most sophisticated micro-controller available, this chip has everything built in, 
from 57 I/0 pins and ADC to DPRAM and EPROM. 


he new Hitachi H8/330 is a 

micro-controller with an 

exceptionally large number of 
features. It is able to operate at up 
to 1OMHz from an internal clock 
and offers 16k bytes of EPROM or 
one time programmable (OTP) 
ROM, 512 bytes of RAM, 15 bytes of 
dual port RAM (DPRAM), eight 
channels of 8-bit resolution 
analogue to digital conversion 
(ADC), full duplex synchronous 
and asynchronous serial 
communications interfaces, a 16-bit 
free running timer, two 8-bit multi- 
function timers, two 8-bit PWM 
modules, 57 parallel input/output 
lines and 9 input only lines. It has a 
low power consumption, being 
fabricated using CMOS technology 
and consumes only 12mA when 
operating at 6MHz. Further power 
savings can be made by putting 
internal modules to 'sleep' when 
they are not needed. 

The central processing unit 
(CPU) section of the chip is based 
around eight 16-bit registers, each 
of which can be divided into two. 
There is no distinction between 
data and address registers except 
that when they are being used for 
addresses, all of the 16-bits are 
used. Additional control registers 
are the 16-bit program counter and 
the condition code register (CCR) 
which holds all of the status flags — 
interrupt, half carry, negative, zero, 
overflow, carry and two flags for 
the user status. The stack pointer 
uses register R7 as a 16-bit pointer 
to the current stack position. 

A number of data formats are 
catered for, one bit, 8-bit bytes, 16- 
bit words and packed BCD. The 
arrangement of the working 
registers (RO-R7) is such that data 
sizes less than 16-bits can occupy 
either the low or high portion with 
no overhead such as extra 
instructions. 

The H8/330 instruction set 
supports eight addressing modes 
and has 150 instructions made up 
from seven basic instruction types. 


A large number of these are 
devoted to bit manipulations and 
allow data being moved to and 
from the I/O ports to be dealt with 
efficiently. 

The CPU is able to communicate 
with the facilities available on the 
chip via direct addressing. Most of 
the devices are able to contribute to 
the system's 30 interrupt vectors, 
nine of which are externa. How the 
internal and external memory are 
accessed depends on the operating 
mode selected. There are three of 
these allowing various memory and 
I/O configurations. Programs can 
be either from external ROM, a mix 
of external and internal or just 
from 16k bytes of internal — the 
external memory could also be 
RAM. The on chip registers are 
placed at the top end of the 


K/IRQ5 


g |_| P84/CTxD/IRQs 
¢ |_| PsvRsii0s 


10] | P8s/CRxD/IRO4 


11] | P8e/csc 
5 |__| P80/RSo/E 


P9s/RDY/AS| | 
P94/OE/WR| | 2 
PoxCS/RAD | | 2 


P9o/IRQZ/ADTRG| | 3 


P66/FTOB/IRQs a 


3 |__| P37/DDB7/D7 


memory occupying 112 bytes, just 
above the 512 bytes of internal 
RAM. 

An interesting feature of the 
H8/330 is the dual port RAM. This 
consists of 15 bytes of memory 
which can be accessed by the on 
chip CPU or by an external device. 
An interrupt is available so that the 
on board CPU can be notified of 
external accesses. 

Other on-board systems are the 
16-bit free running timer (FRT) 
which provides a host of useful 
timing facilities. Counting can be 
derived from the internal clock 
divided by 4, 8 or 32, or from an 
external source. Two comparators 
allow the output of independent 
waveforms and interrupts can be 
generated for seven different 
situations. 


P36/DDBe6/De 
P35/DDBs5/D5 
P34/DDB4/D4 
P33/DDB3/D3 
P32/DDB2/D2 
P31/DDB1/D1 
P30/DDBo/Do 


P14/A4 
P15/A5 
P16/A6 
P17/A7 
Vss 
P20/A8 
P21/A9 
P22/A10 
P23/A11 
P24/A12 
Vss 
P25/A13 
P26/A14 
P27/A15 
Vec 
P47/PW1 
P46/PWo 
P45/TMRI1 
P44/TMO1 
P43/TMCl1 
P42/TMRIo 





July 1991 Practical Electronics 31 


Data Sheet 





















































Port 9 


ee Power supply In 
Vss Ground In 
XTAL _ Crystal In 
EXTAL External crystal In 
“) System clock Out 
2 Enable clock Out 
10S. Input/output select Out 
RES Reset In 
STBY Standby In 
Ao-Ais Address bus Out 
Do-D7 Data bus In and out 
WAIT Wait In 
RD Read Out 
WR Write Out 
AS _ Address strobe Out 
NMI Non-maskable interrupt In 
IRQo-7 _ Interrupt request 0 to 7 In 
MDo- Operating mode select In 
FTCI Free run timer clock In 
FTOA Free run timer compare A Out 
FTOB Free run timer compare B Out 
FTIA Free run timer capture A In 
FTIB Free run timer capture B In 
FTIC Free run timer capture C In 
FTID Free run timer capture D In 
TMo-1 8-bit timer clock Out 
TMClo-I: 8-bit timer clock In 
TMRIlo-l: 8-bit timer counter reset In 
PWMo- PWMtimer — . Out 
ATxD Asynchronous transmit data Out 
ARxD Receive data In 
ASCK serialciock i aesti‘(C In and out 
CTxD Synchronous transmit data Out 
CRxD Receive data In 
CSCK Serial clock | In and out 
ANo-7 ADC analogue in In 
ADTRG ADC start conversion ext In 
AVcc Analogue power supply In 
AVss Analogue ground In 
DDBo7 Dual port RAM (DPRAM) bus In and out 
CS Chip select In 
RSo- DPRAM register select In 
OE Output enable ~ In 
WE_ Write enable — In 
RDY Ready Out 
Pto7 Port 1 In and out 
P207 Port 2 In and out 
P30 Port 3 In and out 
P4o-7 Port 4 In and out 
P5o-2 Port 5 In and out 
P60-7 Port 6 In and out 
P7o-7 Port 7 In and out 
Port 6 In and out 
In and out 





Other timing facilities are provided by the 8-bit timer 
which has two independent channels. These can be derived 
from the system clock divided by 8, 64, 1024 or an external 
source. As with the 16-bit timer, compare and interrupts are 
provided. 

The third system is the pulse width modulation (PWM) 
module which, again, has two independent channels. Each 
includes an 8-bit duty register which can be used to give 
pulses with any duty ratio from 0 to 100%. 

Serial communications are supported in both 
synchronous and asynchronous format. Data rates are 


32 Practical Electronics July 1991 


P10/Ao 
P11/A1 
P12/A2 
P13/A3 
P14/A4 
P15/A5 
P16/A6 
P17/A7 


P20/A8 
P21/A9 
P22/A10<—> 

P23/A11<—> 
P24/A12<—> 
P25/A13<—> 
P26/A14<—> 
P27/A15<—> 





P60/FTC| <—> 
P61/FTOA <—> | 
P62/FTIA <—> | 
P63/FTIB <—> 
P64/FTIC <—> 
P6s/FTID <—> 
P66/FTOB/IRQ6 <—> 
P67/IRQ7 <—> 





~~ 





P41/TMOo <—»> 


JDAnRSTAATIR 


P40/TMClo <—»>} 


determined by a built in baud rate generator or an exter 
source. Double buffering allows data to be sent a 
received continuously and overrun, frame and parity err 
are automatically detected. 

A single ADC (analogue to digital convertor) can 
programmed to sample up to eight channels with 8- 
resolution at 1OMHz. Conversions can be triggered eit! 
internally or externally and the results transferred to d 








Data Sheet 








<—> P90/IRQ2/ADTRG 
<> P91/IRQ1 

<> P92/IRQo 

<—> P93/CS/RD 
<—> P94/OE/WR 
<—> P95/RDY/AS 
<> P2/o 


<> P97/WE/WAIT 


<—» P30/DDBo/Do 
<> P31/DDB1/D1 
<— > P32/DDB2/D2 
<— > P33/DDB3/D3 
<> P34/DDB4/D4 
<> P35/DDB5/D5 
<— > P36/DDBé/De 
<—> P37/DDB7/D7 


<> P80/RS0/E 
<> P81/RS1/IOS 
<> P82/RS2 
<> P83/RS3 
<> P84/CTxD/IRQ3 
<—> P85/CRxD/IRQ4 
<—> P86/CSCK/IRO5 








AVcc 
AVss 


P47/PW1 <—>} a 


P46/PWo <—> 
P70/ANo <——> 
P71/AN1 <—> 
P72/AN2 <—>} 
P73/AN3 <—>I 
P74/AN4 <—> 
P75/AN5 <—> - 
P76/ANs <—>f JN | 
P77/AN7 <—> | 
P50/ATxD <—>} 
P51/ARxD <—>} 
P52/ASCK <—> 


P45/TMR1 


gisters corresponding to each channel. 
_ The most important part of the chip is its input/output 
ipabilities. There are 57 I/O pins, each of which can be 
rogrammed for input or output, and nine input only pins. 
lepending upon the operating mode, some of the pins will 
e used for external addressing and data bus functions. 
thers provide interrupt functions and register selection for 
ne DPRAM. a 





July 1991 Practical Electronics 33 





Component Feature 





Practical Components: 


— The Bipolar transistor 





Following on from last month's examination of diodes, Steve Knight BSc., looks at the most 
famous semiconductor of them all. 


he name transistor is a 

combination of two words, 

‘transfer’ and ‘resistor’. This is 
because transistor action depends 
upon the fact that a current 
generated in a circuit of low 
resistance can be transferred to a 
circuit of high resistance, so 
enabling a power gain to be 
acquired. The precise, detailed 
behaviour of a transistor will be 
found in advanced textbooks 
dealing with the mathematics of 
solid state physics and quantum 
theory. A simple explanation can, 
however, be made using the basic 
facts already obtained about diode 
operation. 


At The Junction 


Junction transistors consist of two 
PN diodes formed close together in 
one single crystal of germanium or 
silicon, so that either a PNP or an 
NPN configuration is produced. 
Fig.1 shows what is known as a 
diffused alloy junction construction. 
Here, two N-type regions are 
alloyed under heat to a thin P-type 
wafer and cooled sufficiently 
slowly for the P-type crystal lattice 
to grow out into the N-regions. 
There are then two PN junctions 
separating three distinct regions. 
Connecting leads are taken from 
each of these regions. There are 
then two PN junctions separating 
three distinct regions. Connecting 
leads are taken from each of these 
regions and the transistor is 
suitably encapsulated. It is an NPN 
device since it is formed of a 
sandwich in that order. A PNP 
transistor can be fabricated in 
exactly the same way. Modern 
transistors are made by a more 
sophisticated technique than the 


34 Practical Electronics July 1991 


alloying method and much closer 
control of the transistor 
characteristics is possible, but the 
basic principle is unchanged. 

It follows from the above that a 
transistor is two junction diodes 
connected back to back. Fig. 2 
shows the situation for both NPN 
and PNF transistors, and for each of 
these arrangements the external 
connections are known as the 
emitter, base and _ collector 
terminals. The arrowheads on the 
emitters indicate for both types of 
device the flow of conventional 
current in the base-emitter diode -— 
this symbolism immediately 
identifies the type of transistor 
concerned. 


Starting Up 


To get the transistor operational, 
sources of DC voltage are now 
applied to the terminals as shown 
in Fig. 2. A steady bias voltage VC 
is applied between the collector and 
base with a polarity such that the 


diode is reverse biased and appears 
as a very large resistance. A second 
steady bias voltage V, is then 
applied across the emitter and base 
such that this diode is forward 
biased and appears as a very low 
resistance. As a consequence, the 
emitter injects its majority carriers 
into the base region. Notice that the 
polarity of these carriers is opposite 
to the polarity of the collector 
region. The carriers accordingly 
find themselves in a narrow region 
faced with an attractive field. 
Looking at this from the point of 
view of an NPN device, Fig.3 
illustrates what happens: the 
majority carriers in the N-type 
emitter are electrons and these are 
injected into the P-type base under 
the influence of the forward bias 
Vz. Since the base is P-type and 
electrons exist there only as 
minority carriers, once they reach 
the depletion layer existing 
between the base and collector 
regions, they have an unrestricted 
run through the potential barrier 











and are rapidly swept into the 
collector region. Those electrons 
arriving at the collector, then, are 
derived almost entirely from the 
emitter. 

Almost? Yes, because in the base 
region a small proportion of the 
electrons recombine with holes and 
so effectually disappear from the 
active process. This loss of charge 
upsets the charge equilibrium 
because the base has captured a 
number of negative charges; this 
unbalance is corrected by the base 
battery supplying holes to the base 
region, which it does by an outward 
flow of electrons. Thus a small base 
current I, is established. The base 
region is made very narrow to 
reduce this loss of majority carriers, 
since the time spent by the electrons 
crossing the base is then short and 
the chance of recombination 
correspondingly reduced. 

As a result some 99.5% or more 
of the carriers reach the collector 
where they provide the collector 
current I.. This process is the 
fundamental, though simplified, 
mechanism whereby current 
amplification takes place in a 
transistor. 

Seen from the point of view of 
the external circuit, a relatively 
large current flows into the emitter 
terminal (I,), a small flow of 
electrons leave the base (I,), and 
from the collector a current (I,) 
flows which is almost equal to the 


emitter current flowing in. Clearly, 
the sum of the base and collector 
currents must equal the emitter 
current, so I, = I, + I.. For a small 
general purpose transistor we 
might find such typical values as I, 
= 2mA, I, = 50uA, so that I, would 
be 195m A. For power Ha alors: 
emitter currents can be a matter of 
several amperes. 

The physical behaviour of a PNP 
transistor can be deduced in the 
same way: the majority carriers in 
the emitter are now holes and these 
flow into the base when the base is 
biased negatively. Again, a small 
proportion of these holes will 
recombine with the majority 
electrons in the base and give rise to 
a small inward flow of current into 
the base to restore the charge 
equilibrium. Relative to the NPN 


Bipolar Transistor 


transistor, the PNP circuit simply 
reverses the battery connections 
and the external currents direction 
of flow. 


Hooking Up 

A transistor may be connected in 
any one of three possible ways into 
a practical circuit, and _ its 
amplifying or control characteristics 
depend upon which of these 
configurations is used. In Fig. 4a the 
common-base connection is shown; 
here the input signal is applied 
between emitter and base and the 
output signal is taken between 
collector and base, that is, the base 
is the common electrode. Fig. 4b 
shows the common-emitter (the 
most usual of the configurations) 
where the input is applied between 





s are true electronic flow. 


July 1991 Practical Electronics 35 


Bipolar Transistor 


ig. 4. The Three possible circuit 
configurations. 





base and emitter and the output is 
obtained between collector and 
emitter. Fig.4c shows the common- 
collector circuit (also known as the 
emitter-follower) where the input 
goes between base and collector 
and the output comes between 
emitter and collector. 

Since the internal operation of a 
transistor is independent of the 
configuration used, the relationship 
between the input and output 
currents is readily deduced. Taking 
the static gain off a circuit as the 
ratio: output current/input current, 
for the common base connection the 
gain (designated h,,) is I./I, and 
this must always be less than 1. For 
the common-emitter the gain 
(designated h,,) is expressed as 
I./I, and this is always much 
greater than 1. h,, can lie typically 
within the range 0.950 to 0.998, with 
hpp in the range 50 to 500. The gain 
of the emitter-follower, h,,, is for all 
intents and purposes equal to h,,. 


36 Practical Electronics July 1991 





It's All In The Gain 


Because both electrons and holes 
are carriers, the form of transistor 
described above is known as 
‘bipolar’. Field-effect transistors 
operate with only one type of 
carrier, electrons or holes and these 
are known as ‘unipolar’. 

The gain of a bipolar transistor 
used in a practical circuit where AC 
signals are superimposed on the 
static DC conditions is known as the 
dynamic gain. In general terms, this 
dynamic gain is always rather less 
than the static gain figure but not 
greatly so. Apart from their purpose 
of establishing the static conditions, 
the battery supplies are 
fundamentally irrelevant. 

A typical basic amplifier stage, 
considering an NPN common- 
emitter connection, is shown in 
Fig.5. This differs from the static 
circuit in that a load resistor R, is 
included in the collector circuit, and 
tbe base-bias voltage (formerly the 
battery V,) is derived from the 
single source by way of base 
resistor R,. 

If this circuit is properly to 
amplify anAC signal (supposing 
this to be a small sine wave for 
simplicity) then the base-emitter 
diode must be forward biased even 
when there is no signal input. If it is 
not, then the negative half cycles of 
the input will produce no response. 
Hence the value of R, is chosen so 
that a mean or quiescent base 
current flows which can be varied 
in a linear fashion on either side by 
the positive and _ negative 
alternations of the base signal. 
Typically, for a small signal 
transistor, the quiescent current 
would be, say, 10 - 100A. 

When an input signal voltage is 
applied to the base, it varies the 
base current alternately above and 





below the mean value and then 
changes the collector current and so 
the voltage developed across R,. 
We then have a device which is a 
current amplifier, but when used in 
the manner shown amplifies a 
voltage. Suppose, by way of an 
illustrative example, the h,, of the 
transistor shown to be 75, R, to be 
1K , and the mean I, to be 100A. 
Since 100U.A (= 0.1mA) is the steady 
base current, the collector current 
will be h,,I, = 75x0.1 = 7.5mA. This 
current flows through R, and 
develops a voltage across R, equal 
to 7.5x10-3x103 = 7.5V. Hence, for 
the 12V supply, 5.5V is the 
quiescent voltage at the collector. 

In any amplifier it is necessary 
first to establish, both at the base 
and at the collector, these quiescent 
conditions before the AC signal is 
applied. Unless there values are 
correct (though they are not usually 
extremely critical) the amplifier will 
produce a distorted output. The 
relevant point is that, when the 
signal is zero, the transistor must 
draw sufficient collector current to 
ensure that the collector voltage 
stands closely to the midway value 
between the supply level and the 
common earth rail, ready to swing 
in either direction relative to the 
polarity of the input signal. From 
the example, the collector can swing 
upwards from 5.5V towards the 
supply rail voltage (+12V) when the 
input goes fully negative, and 
downwards towards the earth rail 
(OV) for a positive-going input 
cycle. In practice, neither of these 
extremes (known respectively as 
‘saturation’ and ‘cut-off’) should be 
reached if the signal is to remain 
undistorted. Notice that there is a 
180° phase reversal in the output 
voltage relevant to the input 
voltage. x 














PE Chronos: 
Putting On Weight 


Measurement Project 





This month the Anthony H. Smith gets down to some real hardware and describes the input 
circuitry complete with capacitative compensation and ODR detection. 


o matter how stable and 
accurate a universal counter 
timer’s frequency reference 
may be, the performance of the 
instrument is often dictated by the 
quality of the input circuits. The 
reasoning is simple. If the 
instrument can’t handle a particular 
input signal and gives no reading, 
or - even worse - if it mistriggers 
and gives an erroneous reading, the 
frequency reference might just as 
well be a grandfather clock. 
Consequently, to justify the use 
of a high accuracy standard, and to 
facilitate truly universal 
measurements, the Chronos has 
two, fully comprehensive input 
conditioning channels. Channel A is 
identical to channel B, and is shown 
in the schematic of Fig.1 (overleaf). 


Frequency Breaks 


The position of Sw1 determines 
whether the input signal at Skt1 is 
DC or AC coupled via C1. Note that 
Cl effectively forms a high pass 
filter with the input resistance of 
the channel, namely 1M. The break 
frequency of this filter is: 


f. = 1/(2mCR) 
= 1/(2x0.1 x 10-6 x 1 x 106) 
= 16H. 


Sw2 is three-position slide 
switch mounted on the PCB (to 
minimise interconnections and 
noise), and is used to select either 
all pass (AP), high pass (HP), or 
low pass (LP) filtering. 

Selecting either LP or HP feeds 
the input signal through a first- 
order RC filter. The LP has a 3dB 
break at f, = 55.6kHz and the HP at 
f, = 1.2kHz. Note that the input 
resistance of the circuitry following 







the filters is 1M and is included in 
the calculation to take account of its 
shunting effect when determining 
the HP break frequency. 

The precision of the filters’ break 
frequencies depends mainly on the 
values of R1, C3 and R2, C2 so close 
tolerance components are used. 
Note, also, that being at the very 
input to the channel, the filters 
must be capable of withstanding 
overload so C2 and C3 are high 
voltage capacitors. 


Cutting Down The Signal 


Sw3 is used to select either x1, x10, 
or x100 attenuation of the input 
signal. In the x1 position, the signal 
is fed directly from the filters to the 
amplifier and undergoes no 
attenuation. In positions x10 and 
x100 the signal amplitude must be 
reduced by a factor of ten and a 
hundred, respectively, before 
reaching the amplifier. 
Unfortunately, there is an added 
complication, Fig.2a shows an 
attenuator formed from a simple 
resistive potential divider. At its 
output, there will always be some 
shunt capacitance Cs, due to strays 
and to the capacitance of the 
following circuitry. At low 
frequencies, Cs has negligible effect 


Distorted 
output 


cy Fig. 2b. Signal distortion caused by Cs. 


on the attenuator’s operation. At 
high frequencies, however, the low 
reactance of Cs adds unwanted 
attenuation to sinusoidal signals, 
and severely distorts pulse and 
rectangular inputs - Fig.2b. 
Neutralisation of the effect of Cs 
is provided by the ‘compensated 
attenuator’ of Fig.3. This can be 
regarded in two ways. Firstly, 
assume Cb represents the stray 
capacitance whose reactance grows 
smaller at higher frequencies. 
Normally, this would cause 
unwanted attenuation. However, 
the presence of Ca - whose 
reactance also, of course, reduces at 
high frequencies - has a balancing 
effect, cancelling out the 
attenuation introduced by Cb. 


Ca+Cb 


Ra+Rb 





mp sated attenuation. 


July 1991 Practical Electronics 37 


PE Chronos 





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38 Practical Electronics July 1991 


Alternatively, the compensated 
attenuator can be viewed as a 
resistive divider in parallel with a 
capacitive divider. If the attenuation 
due to each divider is the same: 


Rb = XCb = Aa 
Ra + Rb XCa + XCb Ca + Cb 


then there will be no frequency- 
dependent attenuation and no pulse 
distortion. In practice, Ca is made 
variable such that it can be trimmed 
to balance out exactly the effects of 
stray capacitance. 

Consider, for example, the x10 
attenuator: 


ATTEN= _Rb_ = —Ca_=0.1 
Ra+Rb Ca+Cb 


thus, Ca = 0.111Cb. 

If Cb is due to stray and 
unwanted shunt capacitance it will 
have a typical value of about 10pF. 
But since Ca=0.111Cb, Ca must be a 
variable capacitor with a mid-range 
value of aboue 1pF - obviously not 
very practical. 

The solution, ridiculous as it 
may seem, is to increase the stray 
capacitance. For example say 
Cb=82pF, then Ca=0.111 x (82pF + 
STRAYS) which equals 
approximately 10pF. Thus, a 2-22pF 
trimmer will do perfectly for Ca. 
This is then adjusted to give 
optimum pulse shape at the 
attenuator output for a given input 
pulse. Typical output pulse shapes 
obtained during the trimming 
procedure are shown in Fig.4. 





As well as getting the 
capacitance ratio just right the 
resistances values must also be 
correct. This is not as simple as it 
may seem. Not only must the 
attenuator’s total resistance equal 
1M , but account must be taken of 
the shunting effect caused by the 
protection circuitry on _ the 
attenuator output. In other words, 
Rb will be shunted by 1M . 

Resistors with 1% tolerance are 
used in the attenuators and 
although even closer 
approximations could be made by 
using, say, 0.1% resistors, these are 
usually expensive, hard to come by, 
and yield little significant 
improvement in performance. 


Amplifier Protection 


All of the circuitry preceding the 
amplifier is protected simply by 
using components rated to 
withstand the specified overload 
voltage (250V RMS). The amplifier, 
however, is a low voltage device 
which, on its own, cannot tolerate 
high voltages. An LH0032 op-amp 
(IC1) is used — this is a specialised 
device superior to most op-amps, 
and is one of the most expensive 
items used in the Chronos. 
Consequently, adequate overload 
protection is absolutely essential. 

The amplifier is operated on +8V 
supply rails (this allows the output 
to swing to +7V maximum, 
ensuring a dynamic range of +5V). 
Unfortunately, the LH0032 will be 
damaged if the input voltage 
exceeds the supply rail voltage so 
the diodes D1 and D2 are used to 
clamp excessive inputs. Normally, 
these diodes are reverse biased and 
have no effect on circuit operation. 
However, should the voltage at IC1 
input (pin 6) rise above +8V or fall 
below -8V, either D1 or D2, 
respectively, will conduct, 
preventing the input voltage 
increasing any further. 

As well as the clamp diodes, 
some current limiting is required. 
R9 limits the input current to a safe 
2.3mA RMS when the input is 
overloaded by 250V RMS. Under 
these circumstances, practically the 
entire 250V is dropped across R9. 
This means that is has to have a 
wattage rating of at least 0.57 watts 
—a0.6W type is ideal. 

The LH0032 has an extremely 
nigh input resistance, typically 
10°*Q (This high resistance is one of 


PE Chronos 








f uration (simplified). 





the things that makes the LH0032 so 
suitable for use in any application 
requiring a high resistance, low 
bias-current amplifier). However, 
since the Chronos must have an 
input resistance of 1M, it is 
necessary to shunt the op-amp 
input by R10 (910k), such that the 
resistance looking into the 
protection network is R9 + R10 = 
110k + 910k, approximately 1M . 


Compensation 
Unfortunately, R9 and R10 form a 
potential divider, which, just like 
the attenuators, has unwanted 
capacitance at its output. This 
capacitance is comprised of the 
input capacitance of the LH0032 
(typically 5pF), and the parallel 
combination of D1 and D2. It is 
minimised by using low 
capacitance devices for D1 and D2, 
but cannot totally be eliminated. 

Because of this the presence of 
C6 in parallel with VC3 across R9 is 
required to compensate the 
protection network. VC3 must be 
adjusted in the same way as VC1 
and VC2 to obtain optimum pulse 
response. 


High Performance 


In addition to its high input 
impedance, the LH0032 has several 
other features which are essential 
for optimum performance of the 
Chronos’s input circuits. In 
particular, its wide bandwidth 
(7OMHz at unity gain) and high 
slew rate (typically 500V/us) allow 
it to cope with high frequency 
signals, even those having large 
amplitude and fast rise times. 

The device is configured as a 
non-inverting amplifier with a 
nominal gain of ten (the actual gain 
is slightly higher to compensate for 
the attenuation caused by R9 and 
R10). 


July 1991 Practical Electronics 39 


PE Chronos 





As well as_ providing 
amplification, the op-amp must also 
be configured to allow a variable, 
DC trigger level voltage to be added 
to the output: the arrangement used 
is shown, simplified, in FIG.22. 

The output voltage is given by: 


Vo = Vi (Ra+ Ra+1)- Ra.VT 
Rc Rb Re 


For simplicity, let Ra = Rc such 
that: 


Vo=Vi.(2+Ra \)-VT. 
Rb 


This is the arrangement we 
require; Vo contains the input 
voltage Vi, multiplied by the factor 
(2 + Ra/Rb), along with the DC 
trigger offset V-. 

V; can be any value in the range 
-5V to +5V, and is derived from 
potentiometer VR2 buffered by 
unity gain voltage follower [C2 
(buffering is required to reduce the 
loading on VR2 so that it has a 
linear span, and also ensure that the 
set trigger level remains constant 
with changes in the amplifier input 
voltage). 

VR2 has an integral switch (Sw4) 
which provides a simple means of 
rapidly and precisely setting V+ to 
zero such that the trigger level 
offset is removed from the op-amp 
output: thus, the output will swing 
symmetrically about zero volts, 
which is usually the optimum 
setting for most frequency 
measurements. 

Note that V7 can be read on a 
voltmeter connected to Skt2 and 
Skt3. R14 is provided to safeguard 
against damage caused either by a 
short circuit between Skt2 and Skt3, 
or by an accidental overload which 
will be clamped by D3 and D4. C9 


40 Practical Electronics July 1991 





and C12 decouple any noise from 
the trigger voltage: if C12 is rated at 
100V, and R14 at 0.6W, an overload 
voltage up to 100V can be tolerated 
at the trigger level output. Note, 
however, that the presence of R14 
means that the trigger voltage must 
be read using a voltmeter with high 
input resistance (at least 1M );a 
low resistance meter will attenuate 
the output voltage, causing errors. 


Gain And Nulling 

By relating Fig.5 to Fig.6, Ra 
corresponds to R12, Rb to R13, and 
Rc to R11. Consequently, the closed 
loop op-amp gain is: 


Vo/Vi = (2 + 3600/390) = 11.23 


the overall gain of the amplifier 
stage is 


GAIN=11.23xR10=11:23x0.89=9.995, 
RO + R10 


which is as close an approximation 
to the required gain of ten as we are 
likely to get. 

The LH0032 has provision to 
zero its input offset voltage which 
would otherwise appear as an 
undesirable offset at the output. 
This is achieved using PR1, a 10k , 
multi-turn trimmer. 

Like many other op-amps of its 
kind, the LH0032 requires ‘external 
compensation’. This simply means 
the addition of one or two external 
capacitors (C7 and C8) which tailor 
the frequency response of the 
device. Without compensation, the 
op-amp would be likely to burst 
into parasitic oscillation under no- 
input conditions. 








Signal Monitoring 

The output of ICI is fed toa 
signal buffer built around TR1, TR2, 
TR3 and TR4. These transistors 
constitute a pair of complementary 
emitter followers, combined to 
create a unity-gain follower whose 
output at Skt4 is a replica of the op- 
amp output signal. 

The circuit’s input impedance at 
the bases of TR1 and TR2 is very 
high (around 350k ) so as not to 
load IC1 excessively. The output is 
a low impedance source, with R20 
and R21 providing short circuit 
protection. 

Tests on the prototype buffer 
showed that it has excellent fidelity 
and almost ideal gain. The output 
does, however, have a slight DC 
offset, and so the buffer should not 
be used to establish trigger levels 
(always use the trigger level voltage 
output at Skt2 and Skt3). 


Schmitt Trigger 


Hysteresis is essential when 
converting a noisy input signal to a 
clean digital waveform. In the 
Chronos input circuitry, this is 
accomplished by the Schmitt trigger 
built around IC4. The LM361 
comparator was chosen for its fast 
response time (20ns max.), and low 
input offset voltage (typically 1mV). 
It features complementary TTL 
outputs at pins 9 and 11 but, 
unfortunately, has the disadvantage 
that it cannot tolerate input voltages 
greater than +5V. Since the LH0032 
output can swing to +7V when 
saturated, it must be attenuated by 
5/7 in order to protect the LM361. 
this attenuation is provided by R15 
and VR1. 


tput signals. 








VR1 itself is provided to act as a 
variable, fine attenuation control. 
The use of variable attenuation will 
usually only be required when 
making frequency measurements 
and some period measurements. 
For the majority of time 
measurements it will usually be 
best to introduce no variable 
attenuation at all. 

The principles underlying 
Schmitt trigger operation are 
illustrated in the basic circuit of 
Fig.23. Assume, initially, that the 
input voltage Vi is zero, and that 
the comparator output is in positive 
saturation V+. 

The voltage fed back to the non- 
inverting input of the comparator 
will also be positive: 


Ve = Vt x Ry 
Rx + Ry 


If Vi now starts to rise above 
zero, there will come a point when 
the inverting input is more positive 
than the non-inverting input, at this 
point, the output will fall rapidly 
from positive saturation into 
negative saturation. As it does so, 
Vy will also go more negative, 
reinforcing the voltage difference 
between the inputs. 

When Vo is fully in negative 
saturation, V-, V_ will also be at its 
maximum negative value given by: 


Consequently, it will be 
necessary for Vi to fall and go more 
negative than the above value of V; 
before the non-inverting input 
becomes more positive than the 
inverting input, at which point the 
output will change stage again, and 
go into positive saturation. 

The actual transition at the 
output only takes an instant, and is 
enhanced by the positive feedback 
typical to all Schmitt triggers. The 
positive and negative values of V;, 
given above correspond to the 
upper and lower threshold voltages 
Vry and V+, respectively, which 
define the limits of the trigger 
window as described last month. 
Also, if the positive and negative 
saturation levels have equal 
magnitude, then V+; and V7, will 
also be equal in magnitude such 
that the trigger window is 
symmetrical about zero volts. 











This is exactly what we require 
for the Chronos Schmitt trigger. 
However, as the LM361 output(s) 
does not swing between positive 
and negative saturation levels, but 
instead has only positive logic 
levels, how do we obtain the 
required threshold levels? The 
solution is provided by the level 
shifting network formed around 
diodes D5 and D6 (Fig.1). 


Level Shifting 

The LM361 output at pin 11, “+ 
out”, corresponds to the output of 
the comparator in the example 
circuit of Fig.6. Thus, when the 
inverting input (“-in”, pin 4) is 
more positive than the non- 
inverting input (“+ in”, pin 3), pin 
11 goes low (typically to about 
0.3V). On the other hand, when the 
inverting input is more negative 
than the non-inverting input, pin 11 
goes high (to around 3.3V). 

Now, in order to obtain the 
required positive and negative 
threshold levels, Vry and Vy, we 
make use of the level shifting 
network comprising D5, D6, R27, 
R28, R29, R30 and R31. 

Diodes are used as _ low- 
capacitance voltage references and 
the nominal potential at the 
junction of D6 and R31 is always - 
0.65V. The values of R27, R28 and 





PE Chronos 





R29 are chosen such that when pin 
11 is high, the voltage at pin 3 of the 
comparator is nominally +70mV; 
when pin 11 goes low, however, the 
potential at pin 3 is pulled down to 
-70mV. Thus, the hysteresis levels 
are set at Vr; = +70mV and V+, = - 
70mV. Note that the relatively 
constant voltage drops across D5 
and D6 ensure that the threshold 
voltages are little affected by 
variations in supply rail voltages. 
The result of the Schmitt trigger 
action is that the signal at pin 11 of 


-IC4 is an “inverted”, digital version 


of the analogue input at pin 4. The 
output signal is then processed by 
the digital Schmitt triggers of IC5. 
This device is a 74HCT14, whose 
inputs are directly compatible with 
TTL voltage levels, such as those 
output by the IM361 (a 74HC14 
should not be used). 


Indications 

The digital outputs of IC5b and 
IC5c are the required conditioned 
signals fed to the main board via 
Sw5 which determines whether the 
Chronos triggers on the positive or 
negative signal slope. 

The output of IC5b is further 
inverted by IC5e whose output is 
thus an inverted, digital version of 
the input signal. This is input to the 
trigger LED drive circuit. 





Input voltage 


July 1991 Practical Electronics 41 


PE Chronos 


D10 is a tri-state LED. With its F 
(Flash) input high, it remains 
continuously illuminated; take F 
low and the LED blinks at a 
constant rate (about 3 flashes per 
second). When the input voltage 
remains continuously below the 
lower Schmitt threshold V7, or 
when the trigger level has been set 
so positive that the entire input 
signal is below the trigger window, 
the LED must be extinguished. 

When the input is continuously 
above the upper Schmitt threshold 
Vry, or when the trigger level has 
been set so negative that the entire 
signal is above the trigger window, 
the LED must be continuously on. 

Finally, when the input signal is 
crossing both hysteresis thresholds 
and correctly triggering the Schmitt, 
the LED must flash. 

The required driving signals are 
derived from IC5d and IC5f. Using 
the input signal to [C5e as a 
reference the way in which outputs 
of IC5d and IC5f respond to 
changes in the input signal can be 
seen — Fig.8. 


The time constants of R32-C15 


and R33-C16 have been set long 


enough to ensure correct flashing | 


operation when the input is 
pulsing, but not so long as to result 
in sluggish response of the trigger 
LED when the input signal reverts 
to a non-pulsing, high or low state. 


ODR Detection 


The circuitry built around [C3 
constantly monitors the nature of 
the signal output from the op-amp, 
and warns the user whenever this 
signal has exceeded the +5V 
dynamic range. The’ two 


42 Practical Electronics July 1991 


Small signal wit 
: dynamic rane i but 
| forced ODR 

| excessive n “ative 
: fe) prevents triggering. 


comparators of IC3 are entcared 
as a “window detector” with the 
boundaries of the window 
corresponding to the +5V limits of 
the dynamic range. 

Each comparator is fast and has 
an open collector output allowing 
both outputs to be tied together. 

The input to the window 
detector is actually derived from the 
signal monitor buffer — this is 
simply to reduce loading on the op- 
amp’s output. R22 and R23 
attenuate the output signal by a 
factor of 25. Thus, whenever the 
potential at the junction of R22 and 
R23 exceeds +200mV, the amplifier 
output is outside the t5V dynamic 
range. (+5V/25 = +200mV). The 
overall response of the circuit is 
shown in Fig.9. Under ODR 
conditions, the low voltage applied 
to the input of [C5a causes its 
output to go high, illuminating the 
ODR LED, D9. The value of R41 
depends on the type of LED used, 
and should be chosen to limit the 
LED current to 10mA, or less: a 
value in the region of 200 to 300 


will probably suffice. 


The ODR circuit operates very 
well over the entire DC to 1OMHz 
frequency range. When dealing 
with alternating signals outside the 
dynamic range (as opposed to just 
static voltage levels), the window 
detector output will, of course, be 
pulsed: hence, C21 is required to 
integrate these pulses. Without this 
capacitor, the LED would appear to 
flicker when indicating pulsed ODR 
conditions, rather than glowing 
continuously. 

The ODR feature is unique to the 
Chronos and both it and the trigger 
indicators are useful and reliable 





range but excessive 


positive tri igaer level 
pushes it ODR and 





aids in setting up the input controls 
for the best triggering conditions. 

In certain cases, it is possible to 
get away with a measurement, even 
though conditions have forced the 
signal outside the dynamic range — 
the second waveform of Fig.10 is a 
typical example. 


Power Sources 

The +8V, -8V and +5V rails are 
provided by three on-board 
regulators (IC6, IC7 and IC8, 
respectively) fed by unregulated 
voltages from the PSU section of the 
main board. Each channel has its 
own three regulators (there are six 
on the board in total) — necessary in 
order to isolate the two channels 
from each other. 

Without this power supply 
isolation, excessive crosstalk 
between channels was found to 
occur under certain conditions. 
Fortunately, providing each channel 
with its own regulator drastically 
reduces the interference. Even 
under worst case conditions, the 
channel separation is a minimum of 
SOdB. 

In order to allow for precise 
matching of the +8V rails, a variable 
voltage circuit is built around [C7 
(79LOSA). By adjusting the preset 
variable resistor PR2, the 
magnitude of the negative rail 
voltage can be made to equal the 
magnitude of the +8V rail. 

Matching the positive and 
negative 8V rails in this way 
ensures that the trigger level control 
(VR2) has a perfectly balanced span 
and also ensures that the threshold 
levels for the ODR circuit have 
precisely the same magnitude. & 

















D.LY. 






Printed Circuit Boards 


Hardware Project 





Shametully the editor had to admit to never having made a PCB, until now. After a little 
encouragement he rolled up his sleeves and gave it a go — what follows is the result. 


ome may think that making 
printed circuit boards is a 
difficult, messy, expensive and 
complicated process. I certainly 
suspected so before doing the 
practical parts of this article. The 
reality is somewhat differen 
howevert. The basic principles are 
very straightforward and all of the 
materials needed are easily 
available. 
There are two main approaches 
to PCBs, ‘draw your own’ or lift a 
pattern from a magazine. PE 
publishes a number of board 
designs every month so it seems 
natural to use this approach. I used 
the Frost Alarm design by John 
Becker published in the February 
issue of PE. 


Getting The Right image 

At first sight, the most difficult part 
of making up a PCB is getting the 
track pattern from magazine into a 
form which can be transferred to 
the board. The first step is to get 


hold of a can of transparentiser. 
This is a useful chemical which 
makes paper virtually transparent. 
Spraying a cut-out of a photocopy 
of the page makes the white parts 
of the paper resemble tracing paper 
while the black parts remain 
opaque. This can be used as a 


The track layout. 





The materials needed to make aPCB. 





positive image which can be 
impressed onto the board. 

Copper clad board comes in a 
number of sizes and there are two 
main types, SRBP and fibre glass. 
Of the two, the first is cheaper and 
the second is tougher. Which is 
used depends on the environmental 
conditions in which the board must 
operate. 

After cutting the board to size, 
cleaning it to remove any grease 
and stains — the best way to do this 
is with a pan scrub and some 
scouring powder (Vim or Flash or 
whatever) — it is dried and then 
sprayed with photo-resist. After 
cleaning it is a good idea handle the 
board with kitchen paper so that it 
doesn’t acquire any accidental 
thumbprints. Kitchen towel also 
comes in useful for wiping 
implements, catching unwanted 
spray and cleaning up any nasty 
messes — have plenty on hand. 

A photographic process is used 
to transfer the track pattern (also 
known as the foil pattern) to the 


July 1991 Practical Electronics 43 


PCBs 


Step By Step 


Get design from mag and photocopy 

(a few times just to make sure). 
Coat photocopy with transparentiser. 
Cut board to size. 





scourer and then rinse thoroughly 
and dry. 


— either by leaving it for 24 hours or 
placing it in front of a fan heater or in 
an electric oven at 80° C for 15 
minutes 

@While waiting for board and paper to 
dry make up the developer and 
etching solutions according to the 
manufacturer's instructions. 

Place design on board making sure it 

is the right way around and hold in 


Bel 


, | After exposure and development. . 


44 Practical Electronics July 1991 


Polish copper board with Vim or other 


@Spray board with photo resist and dry 








place with a piece of glass. 

Expose under a UV, sun-ray lamp or 
in sunlight for the correct time — 
around 10 mins seems to do the trick. 

Place in developer and wait for 
pattern to emerge, green on a copper 
coloured background. 

Once the pattern is clear, wash the 
board in cold water to stop the 
development and then dry it. 

@Check the design to make sure it is 
correct. If not, make corrections 
either by cleaning the board, 
respraying, re-exposing and 
redeveloping. Alternatively use an 
etch resist pen to make the correction 
marks. 

Place board in the etchant and wait 





board. The copper layer is coated in 
UV sensitive paint (photo-resist) 
which, when exposed and 
developed, resists the etching fluid 
leaving the copper tracks behind. It 
is possible to buy pre-coated board 
but this has the disadvantage that 
any mistakes that occur up to the 
etching stage cannot be easily 
rectified. Buying a spray can of 
photo-resist, although very smelly, 
does give the option of scrubbing it 
off and starting again from scratch. 

After spraying the board it 
should normally be left for 24 hours 
in a dark dust free area to dry. For 
those who don’t have the patience 
for this, an electric fan heater will 
do the job in around 15 minutes — 
just spray the board, wait until it 
goes touch dry and then place in 
front of the heater for a while. Bear 
in mind that the photo-resist is light 
sensitive so exposing it to bright 
lights before necessary is not a good 
idea. 

The next step is to place the 
transparentised track pattern onto 
the photo-resist covered board. 
Placing a piece of glass over it will 
allow the UV light through and 
keep the pattern in close contact 
with the photo-resist and copper. 
One thing to make sure of at this 
point is that the pattern is the right 
way around. Remember that the 
copper tracks go on the underside 
(copper side) of the PCB with the 
component leads coming through 
from the top. From the non-copper 
side of the board, this means that 
the tracks should be reversed. In 
practice, magazines usually print 


for the copper to clear away from the 
tracks — if all is going well the copper 
goes a rather pretty pink colour and 
the resist covered tracks appear 
bright green (depending on the type 
of photo-resist used). Swirl the 
mixture around to allow the 
precipitate to move off the board. The 
process takes around 45 minutes. 

@Wash the board to clear away the 
etchant and then scrub the tracks 
with scouring powder until they are a 
bright shiny copper colour. 

@With a fine drill (4mm) make all of the 
holes from the copper side using the 
pad holes as markers. 

insert the components and solder into 
place 






















the copper track pattern in reversed 
form just to make sure. 

The exposure time for the photo- 
resist depends upon where the UV 
light comes from. A specially made 
exposure box can cost around £50 
or more, so alternatives are useful. 
The next best method is a sun-ray 
lamp since this emits UV light at the 
right wavelengths (350 to 400nm). 
Placing the board, pattern and glass 
cover about 10 inches from the 
lamp and leaving it on for 10 
minutes or so gives a perfect result. 
Shorter exposure times mean that 
large areas of photo-resist are left 
where they shouldn’t be and over 
exposure will give no resist at all. 
The next step down from a sun-ray 
lamp is to use the sun itself. 
Exposure time will need to be a 
little longer and will depend upon 
the brightness of the sun — a little 
experimentation may be needed to 
obtain good results but using spray- 
on photo-resist means that the same 
piece of board can be used 
repeatedly. 








iinet a a PCBs 


Once the photo-resist has been 
exposed it is developed to remove 
the unwanted areas that need to be 
etched. This is normally done with 
a weak solution of sodium 
hydroxide in water. It can either be 
bought in concentrated liquid form 
or as solid crystals which are 
dissolved in water before use. The 
only real difference between the 
two methods is that, generally, the 
crystals must all be made up in one 
go. The concentrated liquid is easier 
to measure out so small quantities 
can be made as required. 

Placing the exposed board into 
the developer starts the process and 
a small black cloud can be seen to 
form over the board as the photo- 
resist comes away. The 


development is finished when the 
tracks and the copper are markedly 
different colours. The board is then 
washed and placed into the etching 
fluid. 

Ferric chloride is the chemical 





used to remove unwanted copper 
from PCBs. The iron in the solution 
is replaced by the copper to form 
copper chloride with the iron being 
precipitated. Fresh Ferric chloride is 
a yellowish colour with a tinge of 
deep red. Copper chloride is blue so 
when the etching solution becomes 
blue, it is used up and should be 
disposed of — be sure to add 
developer to it to make it liquid 
before pouring it down the drain. 
Either plastic or glass bowls can 
be used for the developing and 
etching — metal bowls will probably 
corrode with the ferric chloride. 
Etching takes 45 minutes to an 
hour and the liquid should be 
agitated from time to time to 
dislodge any debris that collects on 
the board surface. When finished, 
washing and scrubbing removes the 
rest of the etch resist and leaves the 
finished product. All that remains is 
to drill the holes and tidy up. A 
sharp 1mm bit is usually good 





enough to fit most components, 
placed in the centre of a pad where 
the copper has been etched away, it 
soon goes through SRBP - fibre 
glass is a little tougher. 

Care should be taken when 
using the developing and etching 
chemicals. Although the developer 
is dilute enough not to be corrosive, 
this is not the case when it is in 
concentrated form. If any is spilled 
on the skin it should be washed off 
with copious amounts of cold 
water. The same applied to ferric 
chloride which has the added 
disadvantage of leaving very 
yellow stains when spilled. 

The whole process from cutting 
the board to finished product takes 
a few hours, most of which is spent 
waiting for developments. The costs 
are relatively low since the 
chemicals will make quite a number 
of boards. a 


Component list 


Copper clad board — approx £0.75 ° 
for 75x100mm 

Positive photo-resist spray — 
£2.95 can covers 2.5m2 

Photo-resist developer— £2.75 
bottle gives 1It 

Ferric chloride crystals— £2.25 
enough to etch .23m2 

Etch resist pen — £1.98 useful for 
correcting mistakes or making 
modifications 

Drill 1mm — £1.50 

All of the above are available from 
good component suppliers 

Transparentiser— £9.95 

This is available from a number of 
office supply retailers or by mail 
order from | : 

Cannon and Wrinn, — 

68 High St, 

Chislehurst, 

Kent 

081 467 0935 





































Miscellaneous ; 

UV exposure box or sun lamp or 
sunshine — 

Fan heater for those in a hurry 

Roll of kitchen paper 

Two glass bowls 

Access to a photocopier 

Hacksaw 

Drill 

Pan scrub and scouring powder 


July 1991 Practical Electronics 45 


Electronic Components 


LOWEST PRICES ANYWHERE! 


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5mm Red & Grenn LEDs.... 
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Bruel & Kjaer Sound Level Meter 
2206- 1/2" mic. 
with carrying case £160 each 
other B&K equipment available 


Marconi TF2358/2357 Level Osc/Meter 20MHz 

HP 8620C Sweeper Main Frame 10-1300MHz 

HP Spectrum Analyser System with 8554 & IF Plug-in, 
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HP Spectrum Analyser System with 8554 & IF Plug-in, 
500KHz—-1250MHz 


We have the widest choice of used 
oscilloscopes in the country 


Tektronix 2445 Four Trace 150MHz Dual TB 

Tektronix 485 Dual Trace 350MHz 

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Tektronix 475 Dual Trace 200MHz Delay Sweep 

Tektronix 465 Dual Trace 100MHz Delay Sweep 
Schlumberger-Enertec 5218 Three Trace 200MHz Delay 





HP Spectrum Analyser 182 with 8558 0.1-1500MHz 
Demrtron TA120 with VP3 Vibrator 

Uninterruptable PSU. Unused 

Tektronix Waveformer Monitor type 528 

Racal Instrumentation Recorders store 4D and Store 7D 


Philips PM3217 Dual Trace 50MHz Delay Sweep 

Hitachi V650F Dual Trace GOMKz Delay Sweep 

Hitachi V352F Dual Trace 35MHz 

Hitachi V212 Dual Trace 20MHz 

Leader 1020 Dual Trace 20MHz 

Trio CS1022 Dual Trace 20MHz 

Gould 0S!100 Dual Trace 10MHz TV Trig 

Gould 0S300 20MHz Dual Trace 2mV/cm. Small, lightweight 


Keithley 224 Programmable Current Source 

Ferrograph RTS2 Recorder Test Set 

Thurlby PSU PL320MD 30V 2A Quad Mod Digital 

Thom Bench PSU 0-40V 0-30V Metered 

Farnell PSU H30/100 0-30V 0-100A 

Gould K40 Logic Analyser 32 Channel 

Telequipment CT71 Curve TRacer 

Marconi TF2700 Universal LCR Bridge, Battery......from £125 
Marconi TF2337A Automatic Distortion Meter 400H2/1Khz 


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Gould 1421 Digital Storage Dual Trace 20MHz 
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Marconi MOD Meters TF2300, TF2300B, TF2203.from £100 
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Racal/Dana Synthesized Sig Gen 10KH2-104MHz 

Racal/Dana (AIM) LCR Databridge 9341 

Wayne Kerr LCR Meter 4210 

Wayne Kerr Automatic Component Bridge B605 

Wayne Kerr Universal RF Bridge B602 

Wayne Kerr Component Tester B424 

Farnell Synthesized Oscillator DSG1 0.0001Hz— 99.99KHz 

Famell Automatic Mod Meter AMM 

Bradley Oscilloscope Calibrator 192 with Cal Cert 

Marconi TF2008 AM/FM 10KHz-310MHz Sig Gen 

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Feedback SFG606 Sweep Fun Gen. Sine/Sq/Tri 
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Feedback FG600 Fun Gen. Sine/Sq/Tri 0.01Hz—100KHz£55 
Avo Valve Charcteristic Meter MKIV 

Leader LMV186a Two Ch MV Meter 5Hz-S00KHz, 


Solartron 7045 Multimeter 4.5 Digit 30 Ranges, Auto/Man£95 
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46 Practical Electronics July 1991 





Components for Success 


D.V.M. Module Brand new full spec 3 1/2 Digit Voltmeter Module. Large 0.56 LED Display, FSR +/- 200mV. 
Supply required from 8 to 18V. Size 8cm x 5.5cm. Circuit supplied for multimeter 

MEGAPROM Programs 2764/128/256/5 120/010/020 (1 Meg/2 Meg 28/32 pin) HMOS, CMOS, NMOS type 
EPROMs. Attractively cased and supplied with power unit. Powerful operating software includes full screen 
Editor, extensive file handling (Intel Hex, Binary, ASCII Hex), Byte/block or complete ROM 
program/verification/editing. Fast algorithm 1K x 8 Bit per 1.9 secs. Runs on IBM or compats via RS232 port 
(works on laptops) £128.95 
Auto Tracks V6 PCB Designer Produce professional printed cirsuit boards quickly and easily on you IBM 
PC/XT/AT or Amstrad 1640. Single/Double sided. Auto routing, rats nest or point to point entry, Zoom Editing, 
Various line/pad sizes. Library & User defined parts. Printer options for 9/24 pin include 1:1, 2:1, Solder mask & 
Text. Too many features to list here, SAE for further details. Minimum system requirements IBM PC/XT with CGA 
graphics, Epson printer, Mouse optional 

Micro Engine Development board for MCS8031/51. Contains PLCC 8031, 24 pin EPROM socket, 6 Meg 
Xtal, address latch IC and 18 pin IDC header output Ports 1 & 3. Ideal for stand alone control applications. 
Size 7cn x 4.5cm approx. Full screen Editor assembler for IBM PC/XT/AT (downloads code to printer port or 


EPROM/ROM Emulator Works on ANY computer with Centronics printer port. Emulates all ROMs from 1K 
(2758) to 32K (27256) over 50 types supported. Any file sent to printer port appears as ROM on target 
machine. Fast download. Ideal for MICRO ENGINE development 


All prices inclusive, SAE for further details 


John Morrison (Micros) Dept PE 
4 Rein Gardens, Tingley, 
West Yorkshire WF3 1JR 
Telephone: 0532 537 507 



























T, 
i 





















S 
= 
Ndd-VORZ 














£17.50 

0 0 + 50p P&P 
Ot P 

ET) oof PIE] + VAT 


A computer doesn't have to look like you'd expect a computer to look. 
It doesn't have to have a keyboard and a screen and floppy disks and 
so on. 


The SBC-1 has the bare minimum of chips a Z80 computer can have 
and still be a computer: A 4 MHz Z80-CPU chip, an EPROM chip (up 
to 32K), a static RAM chip (up to 32K) and a pair of 8255A I/O (input 
output) chips giving 48 individual lines to waggle up and down. There 
are one or two additional “glue” chips included, but these are simple 
“74LS” or “HC” parts. 


A star feature is that no special or custom chips (ie PALs, ULAs, ASICs 
etc) are used — and thus there are no secrets. The Z80A is the fastest 
and best established of all the 8-bit microprocessors — possibly the 
cheapest too! 


Although no serial interface is included, it is easy for a Z80A to waggle 
one bit up or down at the appropriate rate — the cost is a few pence 
worth of code in the program: why buy hardware when software will do? 


Applications already identified include: Magnetic Card reader, mini 
printer interface, printer buffer, push button keypad, LCD alphanumeric 
panel interface, 40-zone security interface for auto sending of security 
alarms, code converter (eg IBM PC keyboard codes to regular ASCII), 
real time clock (with plug in module), automatic horticultural irrigation 
controller. 


By disabling the on-board Z80A-CPU this card will plug into our Interak 
1 CP/M Plus disk-based development system, so if you don’t fancy 
hand-assembling Z80 machine code you don't have to! 

The idea is (if you are a manufacturer) you buy just one development 
system and then turn out the cheap SBC-1 systems by the hundred. If 
you are really lazy we can write the program for you and assemble the 
SBC-1 cards so you can get on with manufacturing your product, leavin 
all your control problems to us. 


Greenbank 


For more details write or phone us: 
460 New Chester Road, Rock 
Tel: 051-645 3391. 


Greenbank Electronics, Dept PEO? 
Ferry, Birkenhead, Merseyside. L42 2AE. 











Looking At 
it Logically 











Test Equipment Project 


Complex digital circuit construction can be a difficult proposition. Simplify things with John 
Becker's versatile visual data code converter. 


his project has been designed 

| to assist in the testing and 
development of digital 
electronic circuits. It provides a 
visual readout of monitored logic 
codes, displaying them in a choice 
of binary, hex or decimal formats. 
Data may be input as 8-bit parallel 
or serial blocks. Frequency and 
pulse counting modes are also 
included. Binary codes are 
displayed on eight light emitting 
diodes (LEDs). A 4-digit liquid 
crystal device (LCD) displays hex 
and decimal codes. The text also 
takes a closer look at the functions 
of the 4534 multiplexed BCD 
(binary coded decimal) counter. 
The block diagram for the Logic 
Reader is shown in Fig.1. Most of 
the functions are performed by 
single dedicated chips, as detailed 
in the main circuit diagram of Fig.2. 


One After The Other 


8-bit parallel data is input to the 
octal tri-state non-inverting 
transparent latch gate IC7. Latching 
may be triggered either 
synchronously from an external 
clock, or asynchronously from the 
internal clock generator around 
ICl1d. S3 selects the sync source. 
The internal clock rate is set by C4 
and varied by the panel-mounted 
control VR2, with D3 forcing a 
rapid reset from the positive phase. 
IC7 inputs data to its register when 
the clock pulse is high. Data 
remains latched during low clock 
pulse phases. 

The latch output is enabled by 
an active low logic on IC7 pin 1 and 
is controlled by S5 and R8. With S5 
open-circuit to IC7, R8 holds pin 1 
high, setting the latch output into a 
high impedance state. 


Serial data is input to the 8-bit 
SIPO (serial in, parallel out) register 
IC3. Data bits presented to pin 14 
are shifted in on the positive-going 
edges of pulses received on pin 11. 
As with the parallel register, 
clocking may be_ triggered 
synchronously from an external 
source or asynchronously from the 
generator around [Cld. 

Data latching occurs on the 
negative-going edges of pulses 
received on IC3 pin 12. 54 provides 
for latching pulses to be sourced 
internally or externally. Since IC3 is 
an 8-bit register, latching is 
required following receipt of the 
eighth bit of any data block. 
Internal latching is controlled by 
the 7-stage binary ripple counter 
IC5. The counter is clocked by the 
same source as IC3 and its Q2 
output goes high on receipt of the 
eighth pulse following a previous 
reset. This transition is first delayed 
fractionally by the action of R6 and 
C5. When C5 has charged 


BINARY 
DECIMAL 
CLOCK 


FREQUENCY 
COUNTER 
CLOCK 


LOGIC 


“pyre 
g COUNTER 


Ss ay 
SERIAL 

REGISTER 
poses PARALLEL 


SELECTORS 


REGISTER 


Fig.1 Block diagram 


MULT I- 
PLEXED 


ATA : 


COUNTER 


sufficiently, it resets IC5. The 
resulting negative-going change of 
Q2 causes C5 to discharge and thus 
latch data into IC3. 

Converted serial-to-parallel data 
is available on IC3’s QA-QH 
outputs when output enable pin 13 
is held low via S5. With the 
opposite setting of S5, pin 13 is held 
high via R7, forcing the outputs into 
a high impedance state. 

As selected by 55, the eight 
outputs of the serial and parallel 
registers are taken to a common 
data bus. The binary code of the 
data on the bus is displayed by 
eight LEDs, one on each data line. 
Current through the LEDs is 
limited by their respective resistors 
R9-R16. For compactness, the 
printed circuit board has been 
designed to accept LEDs and 
resistors in modular packages, LM1 
and RM1 respectively. Individual 
components may be substituted if 
preferred. 

For subsequent readout in a hex 


MULT I- 
PLEXED 


ADDRESS 


DRIVER 


IC1@ 
fb ex 
en DECIMAL 
BLE Lcp 


NIB 
care READOUT 


IC4 


CHECKER 


COUNTER 


ics ICG 
LM 


BINARY | 





READOUT 





July 1991 Practical Electronics 47 


Logic Reader 


Ri 18@k 


$18 





ber peer 
/LATCH 


-88b 


PARALLEL IN 
a 
& 
: 


SERTAL 


$Sa 


Fig.2 Control circuit diagram. 


format, the register’s 8-bit output 
data byte is first split by IC4 into 
two nibbles each of four bits. IC4 is 
a quad 2-input data selector with 
tri-state outputs. Under control of 
the logic level on pin 1, data from 
either inputs AO-A3 or BO-B3 is 
routed to outputs YO-Y3. A low 
level on pin 1 selects the A-Y block 
path. The outputs are only enabled 
when a low logic level is on pin 15, 
they are otherwise in a high 


48 Practical Electronics July 1991 







: : Bes 


p PARALLEL 


3 12 





Ss 
OS ENABLE 
p—1DS RESET 
. DS CLK 
D2 
1N4148 


IC} 4893 
GNB PIN 7 
UCC PIN 14 
te eee 
GND PIN 8 
UCC PIN 16 


LCP Pa neCS TS 


impedance state. When selected by 
mode switch S1 in position 1, 
control of IC4’s routing and 
enabling is derived from IC2, the 
function of which will described 
shortly. 


Right Reading 
From IC4, the nibbles of output data 


are fed to the data inputs of the 
multiplexed 4-digit LCD driver 


EERLEEE 


COUNT © 


DECIMAL @ 











C6 Cc? C8 oe 
220 190m 188m 1080 © 





{C8 ¢4HCees 











CLED 1-8) 










RM1 
CRO-RI6> 
470R 


IC10. Full details of the connections 
between IC10 and the LCD are 
shown in Fig.3. Data is latched into 
IC10 and routed to the required 
LCD digit in response to the logic 
on pins 31-34, a high level selecting 
the respective digit register. 
Reading the LCD from left to right, 
the digits are controlled in order of 
pins 34-31, designated as MSD 
(most significant digit), NMSD 
(next MSD), NMSD, and LSD (least 


SD). IC10 decodes the 4-bit input 
data into a 7-segment alphanumeric 
output, i.e. 0-9, A, b, C, d, E, F (note 
that b and d are displayed in the 
equivalent of lower case). The chip 
is designed specifically to drive 
LCDs, toggling the output segment 
lines up and down in the correct 
phase against the LCD’s backplane 
drive clock. The backplane 
frequency is preset within the chip 
and, although this could be slowed 
by the connection of a capacitor 
between the OSC pin and OV, free- 
runs at about 125 Hz. 

When input data to IC10 is 
routed as alternating 4-bit nibbles 
from IC4, the LCD displays the 
equivalent hex codes in the two 
right hand digits. The full 
representation of all 256 hex codes 
implicit in an 8-bit data byte, from 
00 to FF, is available. 


Base Convertor 


Conversion of the serial and parallel 
input data from binary to its 
equivalent 0-255 decimal readout is 
done using a count-and-compare 
technique. This involves the 
synchronous clocking of binary and 
BCD counters, IC6 and IC2, and the 
use of an 8-bit equality comparator, 
IC8. Although the conversion could 
be achieved using arithmetic 
techniques, the count comparison 
method requires fewer components. 
Position 2 of switch S1 selects this 
mode. 

IC8 has two sets of 8-bit inputs. 
It compares the data on both inputs, 
and if they are found to be equal, 
output pin 19 goes low. If non- 
equality is detected, the output 
remains high. Data from the serial 
or parallel registers is routed to 
IC8’s BO-B7 inputs. Comparison 
data to the AO-A7 inputs is supplied 
by eight outputs of the 12-stage 
binary ripple counter IC6. The 
counter is clocked by a signal 
originating from the oscillator 
around I[Cla, gated via IC1b and 
inverted by IC9a. Resetting of the 
counter is triggered by the data 
clock source in parallel mode, and 
by the latch clock source in serial 
mode, as selected via S5b. It is reset 
via its pin 11 by the positive clock 
phase. When pin 11 goes low, the 
counter logic is enabled and counts 
the clock pulses on pin 10. IC8 
compares IC6’s count output 
against the data from the register 
output bus. When the count reaches 


- Fig.3 LCD and driver circuit diagram. 


the point at which the two data 
blocks are equal, IC8 pin 19 goes 
low, so closing gate IC1b and 
stopping the pulse count. The clock 
output of [Cla is gated and 
synchronised by the register 
latching pulses inverted by IC9f. 

Synchronously with [C6, counter 
IC2 is reset, clocked and stopped by 
the same signal sources. This chip 
comprises five cascaded BCD 
counters and multiplexed control 
logic. The functional block diagram 
in Fig.4 clarifies its operation. 
Whereas the output of counter [C6 
is in binary format, the Q0-Q3 
outputs of [C2 are in the BCD 
format required for producing a 
decimal readout on the LCD. With 
both counters under common 
control, binary codes from IC3 and 
IC7 are thus indirectly translated 
into decimal. 

There are two slight differences 
in the reset and clock controls for 
the two counters. IC2 is clocked on 
positive-going pulse edges, whereas 
IC6 is clocked on the negative 
edges, hence the inclusion of IC9a 
to invert the clock phase for IC6. 
Resetting of IC6 is dependent upon 
the immediate level of the control 
signal selected by S5b. [C2 is reset 
by a pulse generated across C3, R3 
and D2 by the transition of the 
selected control signal from low to 
high. 

The clocking rate generated by 
ICla has been set to allow the 
decimal conversion to be carried 
out almost instantaneously 
following the registering of fresh 
serial or parallel input data. 


On The Bus 


Logic Reader 


IC2/1C9+ 


TOMOODD NOM 
LOW WWW WWNNN DN 


ICi@ ICM?211 


S 
li 





The internal decade counters within 
IC2 have their contents presented to 
the common outputs Q0-Q3 under 
control of a digit select (DS) clock. 
In this circuit, the LCD backplane 
clock from IC10 is used as the DS 
clock. This triggers a five-stage 
counter, each stage of which selects 
the relevant decade of the main 
counter, routing it onto the output 
bus. The DS counter also has its 
own outputs, DS1-DS5, the first 
four of which are used to 
simultaneously select the LCD digit 
to which the count data is routed. 
Output DS5, although active, is not 
used. 

Outputs DS1 and DS2 also 
control the enabling of the outputs 
of both IC2 and IC4. When hex 
display mode is selected, DS1 
alternates IC4’s path switching 
between A-Y and B-Y. However, on 
its own, this simple control would 
cause the 2-digit hex data to be 
displayed as two identical pairs on 
the four LCD digits. The circuit 
around IC9b-e restricts the hex data 
pair to display only on digits one 
and two. DS1 and DS2 are taken to 
the NOR gate [C9d, the output of 
which is inverted by IC9c to become 
one input control for the NAND 
gate IC9e. In hex mode, the second 
input of IC9e is held high via S1d. If 
either DS1 or DS2 is high, 
controlling digits one and two, the 
output of IC9a will be taken low, 
enabling the output of IC4 and, 
inverted by IC9b, disabling the QO- 
Q3 outputs of [C2. If either DS2 or 
DS3 is high, the opposite enabling 


July 1991 Practical Electronics 49 


Logic Reader 








TO CAPACITORS 













ouTeuT 
CONTROL 










10 gg 
SCANNER CLOCK =—s_—“$s(<tésT«s>/ TnI: 
oe —/| DIGIT 
| SELECT 


Fig.4 4534 BCD counter block details. 



















_ BS eNeece 





condition occurs, routing [C2’s Q0- 
Q3 outputs to LCD digits three and 
four. Since the Q clock to IC2 via 
IC1b (driving the CLK A input) is 
disabled in this mode and the Q 
counter is reset by each data 
register latch pulse, LCD digits 
three and four are forced to display 
zeroes. The hex display will thus be 
in the range 0000-00FF. 


Through The Gate 


In mode three as selected by S1, 
frequency and pulse counting 
facilities are available, with the 
resulting 4-digit displays in decimal 
format. The source signal to be 
counted is switched via S1 and IC1b 
to the Q input of IC2. IC1b can now 
be held permanently open for pulse 
counting, or opened and closed at a 
fixed rate for frequency counting 
purposes. The control clock for the 
latter is generated by the circuit 
around IC1c. This is configured as a 
wide mark-space ratio pulse 
generator, in which C2 sets the 
basic clocking rate. Between them, 
VR1 and D1 set the relative 
duration of the positive pulse. The 
negative period is lengthened by 
the relatively high value of R2. On 
the positive-going transition of the 
output from ICIc, IC2 is reset by the 
pulse generated across C3. For the 
remainder of the positive phase of 
IClc, gate IC1b is held open, 
allowing external pulses to be 


50 Practical Electronics July 1991 


CLOCK A ee ye 
a oe fos — 
pu ee 
| | ERROR aut . 
| DETECTOR 





UCC = PIN 24 © 


















CARRY OUT 





Q ENABLE 5] 





counted. When ICIc’s output goes 
low, the gate closes and the LCD 
displays the number of pulses 
counted during the period. 

VRI1 is adjusted to produce a 
timing period of 0.1 seconds 
following the end of the reset pulse. 
Conversion of the count to kHz is 
simply performed by activating the 
second decimal point of the LCD 
readout. The point is switched on 
by S2b which routes the IC1c signal 
to it as its active clock. With S52 
switched to pulse count mode, the 
decimal point is triggered in phase 
with the backplane frequency, so 
turning it off. The usable frequency 
display range is 00.01 kHz to 99.99 
kHz. Greater accuracy in the timing 
of frequencies below 10kHz can be 
achieved by lengthening the 
sampling period to exactly one 
second and disabling the decimal 
point. This produces a display 
range of 0001 Hz to 9999 Hz. 
Reversing the polarity of D1 will 


our 
AND DISPLAY | 


INPUT CLOCK 


_| AT 4 to 5 TRANSITION 
_OF FIRST STAGE 





| SECOND STAGE 


‘AT 9 TO @ TRANSITION 
OF FIRST STAGE 


AT 7 to 8 TRANSITION 
OF FIRST STAGE 


extend the duration of the on phase 
whilst shortening the off phase. The 
value of C2 may need increasing to 
achieve the slower sampling rate. 

To allow pulse counting for an 
unspecified period, gate IC1b is 
held permanently open by taking its 
input pin 2 high via S2a. The LCD 
decimal point is inactive in this 
mode. Briefly switching 52 back to 
run-mode resets the pulse count to 
Zero. 


COUNTER OPTIONS 


Readers wishing to explore the 
merits of the multiplexed counter 
type 4534 in other situations will be 
interested by brief details of other 
facilities the chip offers. 

Fig.5 shows the mode control 
truth table for different selections of 
the Mode A and Mode B inputs. 

Capable of operating from a 
power supply up to 18V DC, the 
maximum clock input frequency is 
determined by the supply voltage. 
The figures are typically 1MHz at 
SV to SMHz at 15V. 

On reset of the DS counter, 
output DS5 and its Q-count 
multiplexer are selected. The DS 
counter is decremented by each DS 
clock pulse, stepping immediately 
back to 5 following 1. 

Fig.6 shows how detection of Q- 
counter clocking errors can be 
achieved by jointly clocking inputs 
CLK A and CLK B. In this mode, 
capacitors are connected to ground 
from both pin 1 and pin 22 
providing a clock slew rate 
comparison. The skew is the time 
difference between the low to high 
transition of CLK A to the high to 
low transition of CLK B, and vice- 
versa. The minimum useful 
capacitor value is around 100pF. A 
pulse error detection facility is 
especially useful when the primary 
clock source is generated, for 
example, by a manually operated 
keypad. 


APPLICATION 


S-DIGIT COUNTER 


TEGP RODE COCR DIREC ILY 
INITIO SIAGES 1, 2 AND 4 


4-DIGIT COUNTER WITH 1/12 
AND ROUNDOFF AT FRONT END 





 4@-DIGIT COUNTER WITH 
1/2 PENCE CAPABILITY 


h table for 4534 BCD counter. 





Board Building 

There are two printed circuit 
boards, one for the main circuit, the 
other for the LCD and its driver 
IC10, Figs.7 and 8 show their 
details, including the control wiring 
schematics. 

The LCD is mounted above 
IC10, increasing its height by 
plugging it into a second IC socket 
above the first, board-mounted, 
socket. 40-pin sockets of the width 
required for the LCD seem 
unobtainable but an alternative is to 
cut standard 40-pin sockets in half 
lengthwise to become two 20-pin 
SIL (single in line) strips. 

Terminations to the external 
input data and control leads are 
left to the reader’s discretion. In a 
practical workshop situation, 
soldering them as flying leads to 
test points of the circuit under 
examination is probably one of the 
most useful of options. 
Alternatively, they may be 
terminated in miniature probe 
clips suitable for attaching to 
individual component leads or IC 
pins. Also for practical reasons, the 
controls and PCBs of the prototype 
were mounted in a lidless box with 
integral front panel, so allowing 
direct access to all signal points. 
This housing method additionally 
removed the need to provide 
specific viewing slots for the LED 
and LCD displays. 

The unit requires a regulated 
+5V power supply. The circuit 
draws less than 1mA with the LEDs 
off, rising to 5}0mA with eight of 
them on. (Two of the LEDs in the 
10-way display are not used.) 

Take care that, with the inputs 
wired as shown, no external control 
or data signal level exceeds +5V, or 
falls below -0.5V. The unit may, 
however, be easily modified to 
accept signal levels above +5V by 
the inclusion of resistors in series 
with each input line. In most 
normal workshop situations, where 
perhaps CMOS logic circuits 
powered by 9V supplies may be 
under test, resistor values of 
between 1k and 10k should provide 
adequate protection for the chip 
inputs. 

Other than adjusting the 
frequency counter clock control 
preset VRI, no setting up is 
required. To adjust the clock pulse 
length, connect the unit to a digital 
signal source of a known frequency, 





15kHz foe pone switch S1 to 
mode three and adjust VR1 until the 
LCD shows the right answer, in this 
instance 15.00. 





Checking the correct operation 
of serial and parallel, hex and 
decimal readouts can be assisted 
using a computer with a parallel 
output port. The listing in Fig.9 







LCD PCB BP 


M 
RATE 
DECIMAL 


e 
HEX e@ e COUNT 
S1 | MODE 


PULSE INT 


foe fos 
FREQ EXT 
COUNT CLOCK 


INT SERIAL 












LATCH CLOCK 
RATE 




















reser [ss ss 





EXT PARALLEL 





PANEL LEGENDS 





(NOT TO SCALE) 





PRRALLEL IN 























INTERIOR FRONT PANEL TOP EDGE al 


Fig. 7. Control PCB layout and wiring details. 


Logic Reader 


_ 4 GOOD PULSE ,;ERROR pERROR 





shows two example routines 
written in Basic. They were written 
for a Commodore 3032 computer 
but are readily translatable for other 
machines having 8-bit parallel data 
ports with associated handshake 
lines. In particular, it will probably 
be necessary to change the register 
address codings seen in Lines 110 
and 120. DRT and AT are the 
variables holding the address 
codes for data direction and ATN 
(attention) registers respectively. 
The variables labelled OUT, UP 
and DN are self explanatory. As 
listed, the serial input and reset 
lines are controlled via computer 
lines DO and D7, and clocking is 
triggered via the ATN line. 

To access the unit via a PC- 
compatible computer, an address- 
decoding interface board will be 
needed. Examples of typical 
decoding circuits, sections of which 
can be suitably modified, have been 





SERIAL IN 






5 ela 


S3 
FREQ/PULSE INT/EXT INT/EXT 
COUNT CLOCK |RESET/LAT al 
ro 2 ae 


EXT RESET 





COUNT INPUT LCD DP2 EXT CLOCK 


July 1991 Practical Electronics 51 










Logic Reader 


[, LOGIC READER LCD 318 | 


irl 


LCD PCB tracks. 


can be suitably modified, have been 
published in recent issues of PE. 
The first section of the listing 
outputs decimal numbers 
incrementing from 0 to 255 as 8-bit 
parallel binary codes. The loop in 
line 170 slows down the output 


COMPONENTS 


Resistors | 
R1,R3-R8 100k 
HUE aca he 


R9-R16 470R (module of 8 
-commoned resistors) — 
All0.25W 5% carbon film or better. 


Capacitors 
C1, C3, C5 1n polystyrene 

C2, C4,C6 22u 16V electrolytic 
C7-C9 — _100n polyester 


Potentiometers 
VRi 


100k horiz skel preset 


VR2 1M lin rotary 


52 Practical Electronics July 1991 





coe 


rate. In the serial output 
section, decimal 
numbers incrementing 
from 0 to 255 are each 
decoded into their eight 
separate binary bits. 
Each bit is sent and 
clocked individually. 
The  latching/reset 
command is sent after 
each eighth bit. A 
keyboard pause 
command is included in 
line 260. 

If the computer used 
for testing the circuit is 
fast enough with its 
output commands, the 
frequency counter can 
be checked via looped triggering of 
the ATN line coupled to S1 position 
9. For example: 

280 POQOKE 
AT,DN:GOTO 280 

Repetitive pulse counting can 
also be checked out inthis way. MH 


JOTSAIbAL 


a 


AT,UP:POKE 


















Semiconductors 
1N4148 (3 off) 
4093 


4534 
74HC595 
74HC257 
4024 
4040 
74HC573 
74HC688 
IC9 4572 
C10 ICM7211 


Switches 


o1 4P3W rotary 
92, 90 


DPDT min toggle 





REM LOGIC READER TEST 
DRT=59459:A1T=59468: 
OUT=59471 
UP=237:DN=205:POKE 
DRT, 255 

REM GOTO 190 

REM PARALLEL QUTPUT 
FOR 2=) 10 255: POKE 
OUT Z:PRINT Z:; 

POKE Al UP: POKE Al DON 
FOR X=l TO 1000:NEXT 
NEXT:PRINT:GOTO 150 
REM SERIAL OUTPUT 

FOR Z=0 10 
255:PRINIZ::D=Z 

FOR [=/ [0 O SITEP- 
1:A=2*1:W=D/A 

B=0:I1F W>=1l THEN D=D- 
A:B=1 

POKE OU! ,B:POKE 
Al,UP:POKE AT ,DN 
PRIN] 6B: : NEXT: PRINT 
POKE QUI, 128:PUKE OUT. 
GEL 23: $=" THEN 


260 
NEXT:GOTO 200 
























Cc) 










Fig. 9. Basic test listing. 











co—«>) 
oe GS 2 = 
== sso o-3_, \S=s 






ir 












Control board. 


$3, 54 


DIL sockets 
14-pin (2 off), 16-pin (4 off), 20-pin (3 
off), 40-pin (3 off, see text). 


SPDT min toggle 


_ Miscellaneous 


10-way LED DIL module, 4-digit LCD, 
knobs (2 off), printed circuit boards, 


PCB supports (8 off), box to suit (see 
text). 


Constructors note 

The components used in the Author's 
unit were all purchased from 
RS/Electromail. 


£FLLELELELELELEL READER LOYALTY BONUS E£f£ELELELELELELE 









Practical Electronics is giving away 


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issue (including all products from our 
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The rules are simple: 
Buy any product from any advertiser and we will 
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FRASER ELECTRONICS 


42 ELM GROVE * SOUTHSEA * HANTS 
Telephone: 0705-815584 


EDINBURGH 
OMNI ELECTRONICS 


stock a wide range of 
electronic components at 
174 Dalkeith Road 
Edinburgh EH16 5DX 


Tel: 031 667 2611 
Open Mon-Fri 9am-6pm 
Sat. Yam-5am 
NEW CATALOGUE OUT NOW | 


Price is just £1.50 ~ 
Contact Omni Electronics 
Cooke International 
“We stock used scopes, signal generators, power 


_ now for details _ 
supplies, power meters, DVMs, oscillators, 
attenuators, test equipment. Much more available, 
visit our S/room and bargain store. 
Prices from £5 upwards!” 





































Contact: Cooke International, Unit Four, 
Fordingbridge Site, Main Road, Barnham, Bognor 
Regis, West Sussex PO22 OEB. 
Tel: 0243 545111 Fax: 0243 542457 
“Open: Mon — Fri 9am — 5pm or phone” 


CRICKLEWOOD 
ELECTRONICS 







One of the largest ranges 
of components in the UK! 


Send for our brochure - without obligation or 


telephone us on 06267 79398 (Ref: PE7/91) 


40 Cricklewood Broadway, 
LONDON NW2 3ET 
Tel: 081-450 0995/452 0161 


Radio & Telecommunications 
Correspondence School, 
12 Moor View Drive, Teignmouth, 
Devon TQ14 9U 





Practical Electronics July 1991 





SURPLUS/REDUNDANT ELECTRONICS 
COMPONENTS WANTED 


ICs - Tuners - Transistors - Valves - Diodes etc - any quantity 
considered -immediate payment. 


ADM ELECTRONICS SUPPLIES 
Tel 0827 873311 


Fax: 0827 874835 


TURN YOUR SURPLUS 


ICS transistors etc into cash, immediate 
settlement. WE welcome the opportunity to 
quote for complete factory clearance. 
Contact: 
COLES-HARDING & CO., 

103 South Brink, Wisbech, Cambs. 
ESTABLISHED 15 YEARS 
Tel: 0945 584188 - Fax: 0945 588844 


Components 


CAMBRIDGE COMPUTER SCIENCE LIMITED 


5.25" Disk Drives, 80Tk, DSDD £34.00 each 
5.25" Disks, DSDD, 48tpi, DoxeS Of 10 oo... ecceeeeteetecteeseeeeeeseeneeneens £3.00/box 
40W PSU 5V 3.75A, 12V 1.5A, -12V 0.4A, cased with on/off switch.£10.00 each 
Apricot Disk drive PSU £10.00 each 
Bare switch mode PSU 5V 2.5A, 12V 2A -12V 0.1A.......cccccceseeteeteee £7.00 each 
Oe EN he Nh sce aaa dae iss sv sui eqes usadscubacvasteesdpevatesbiaa eastenatonessieteaevensvess £4.80 each 
DV BOA os ascabcsacage tices dasa weleuaviesesatsijscauasbinteskaasaepiavivaeaites tieasbetseis £6.40 each 
68000 CPUs (The first order get 10MHZ Chips) ............c:cccceseeseeneeeeeeees £3.50 each 
BUCO GPU COG ass stip eiste diss ceecaiyseenuasdhsiccssey aues sen sasuadsaueteravacaebtuveactis teat: £2.00 each 
Z80A CPU, CTC PIO £1.20 each; DMA £2.00........ ccc cceceeseeteeteeeeeneens £4.50 all 4 
27128 EPROMS (Ex equipment) .............ccecseeeseeceeeteeetees £1.20 each or £5.00/5 
Fees, Ree FAI csc eS iuniosasate ctrecusduseas vusstiduaasukaeitaveseracisbarsuaseisassiciesishens £2.50 each 
ESL IE OS CE TOMNS eoseics i locecsstvoresisisivseettins tiivetes atau. £3.50 each 
256KByte DRAM Modules, removed from equipment ...............0:ce £6.00 each 
BTS CID VORA oissscsesdeaissciennselsavsacansineisesssvescienassnsnssvanedasitevneseasoees £1.10 each 
BZOG2 Te SEVIS COREA cgeasicacetsiesavsistavacsssveciadss¥iasiderssvsiascssncvasvantes sitions £3.80 each 
G2256-10 SARBY1O SRAM cis vcscssdscdeseysndscsscdesesoecteacst ccelarediestioenlatesiessees £5.00 each 
O5256 SAAB yte: RAMS. siiccasiicicscssiscliseisasiicanserteriorssivscasnessvnrearcaeeanes £4.00 each 
Smoke detectors, Small SiZ@ ...........cccscsscscesesssessescescsecsssessaceseseesesaeseees £7.00 each 
Smoke detectors, large with escape light and pause feature £13.00 each 


Prices include postage. Add 50p (plus VAT) to orders below £5.00 
All items new unless stated. Add 17.5% VAT to all prices. 
Send an SAE for our latest list or for more info. 

Dept PE, 374 Milton Road, Cambridge, CB4 1SU 
Tel: 0228 424602 or 0861 430496 (Please note mail order only) 








CATALOGUE £1.00 + 25P&P 


Resistor Pack 85 different E12 values + zero ohm link total content 
ERR MU MORIN 28 (chet ctl feacatessi lain Rea eadestanasibtaicieconivassieessantanate £8.95 
LEDs 3mm or Smm red/green .....sssssscsssseeeeeee 6p each. Yellow 1 Ip each, 
Cable ties 1p each £5.95/1,000 £49.50 per 10,000 

Stepping motor 4 phase 12V 7.5' step 50 OHMS. ......ssssesssssssssseeeeeesen £8.95 
SAA1027 stepping motor driver Chip .o.c..sscccsssssssseseseesssssesssssssssevees £3.95 
FM Transmitter kit good quality SOUNd .........ssssssssssssescssccessssssseesees £8.60 
High quality photo resist copper clad epoxy glass boards 





















Dimensions _ single sided double sided 
3x4 inches £ 0.95 £1.07 
4x8 inches £2.40 £2.68 
6x12 inches £5.37 - 
12x12inches _— £10.66 - 

Special Offers 
Computer Grade Capacitors with screw terminals 38000uf 20V.... £2.50 
8700uf LOV £1.95, 68000uf 15V £2.95, 10000 UF 16V.........ccceceees £1.50 
7 segment Common anode led display 12MM .......c.ssessessseeseeeseesneens £0.45 
LM2931ATS.0 Low drop out SV regulator T0220 package ..........+« £0.85 






BS250 P channel MOSFET £0.45, BCSS9 transistor ......... £3.95 per 100 
T4LS05 hex invertor £10.00 per 100, used 8748 Microcontroller ...£3.50 
Stereo LW/MW/FM Tuner pre-amp assembly complete with volume/tone 
controls and tuning scale Brand new in maker's carton £6.95, faulty £3.50 
Circuit etc. for above £0.50. 

Hour counter (used) 7 digit mains 240V AC 50HZ u.....scscsssesssseeeeoee £1.45 
LCD display 16 digit 7x5 dots dot matrix .....ccccccssssssssssssssesseesssseees £2.50 
Qwerty keyboard 58 key good quality switches MEW .......ssesssssssseees £5.00 
Qwerty keyboard with serial output, no data .......cccsssssseesessssseeeseen 










Wide range of CMOS TTL 74HC 74F Linear transistors kits 
capacitors, resistors tools etc always in stock. 
Please add 95p towards P&P - VAT Included 


JPG Electronics 276 - 278 Chatsworth Road 
hesterfield $40 2B 
Access/Visa orders (0246) 211202. Callers welcome 







Components 


High Stability Resistors 

Carbon Film Resistors 0.56R to 10MO E24 series 1/4W - 1p 

100 off per value - 75p. 1000 off in even hundreds per value - £6.00 

Metal Film 1/4W 10RO to 1MO E12 series 5% - 2p. 1% E24 series - 3p 
Mixed metal/carbon film 1/2W E24 series 1RO to 10MO - 1 1/2p 

Mixed metal/carbon film 1W 4R7 to 10MO E12 series - 5p 

BC107/8/9 - 12p. BC182, BC 183, BC184, BC212 & L versions - 10p 
BFY50,51 & 52 - 20p. 2N3055 - 50p. TIP31A,32A - 25p. TIP, 41,42 - 40p 


Tantalum bead electrolytics subminiature (Mfds/Volts) 

0-1/35, 0.22/35, 0.47/35, 1.0/35, 2.2/35, 3.3/16, 3.3/20, 4.7/16 — 14p. 3.3/35, 4.7/25, 
4.7/35 — 15p. 6.8/16, 6.8/35 — 16p. 10/16, 20/25 — 20p. 10/35, 22/16, 33/10, 47/10 — 30p. 
100/6 — 30p. 100/16 — 50p. 220/6 — 60p 

Aluminium Electrolytics (Mfds/Volts) 

1/50, 2.2/50, 4.7/50, 10/16, 10/25, 10/50 - 5p. 22/16, 22/25, 22/50, 47/16, 47/25, 47/50 - 
6p. 100/16, 100/25 - 7p. 100/50 - 12p. 220/16 - 8p. 220/25, 220/50 - 10p. 470/16, 
470/25 - 11p. 1000/16 - 15p. 1000/25 - 18p. 2200/25 - 22p 


Miniature Polyester Capacitors 250V Wkg. Vertical Mounting 
.01, .015, .022, .033, .047, .068 - 4p. 0.1 - 5p. 0.15, 22 - 6p 


Mylar Capacitors 100V Wkg. Vertical Mounting E12 Series 
1000p to 8200p - 3p. 0.01 to .068 - 4p. 0.1- 5p. 0.15, 0.22 - 6p 


Subm. Ceramic Plate 100V Wkg. E12 Series Vertical Mounting 

2%. IP8 to 47P - 3p. 56P to 330P - 4p. 10% 390P to 4700P - 4p 

Plate/disc ceramic 50V. Wkg. Vertical Mount. £12 series 1P0 to 1000P & 
E6 series 1500P to 47000P - 2p. 0.1 — 3p 


Polystyrene Capacitors 63V Wkg. E12 Series Axial Mounting 
10P to 820P - 4p. 1000P to 10,000 - 5p. 12,000P - 6p. 


Diodes 

Zener diodes E24 series 3V3 to 33V 400mW - 8p. 1 watt - 12p. 1N4148 — 2p. 1N4001 — 
3p. 1N4002 — 3.5p. 1N4007 — 5p.1N5404 — 14p. OA91 — 8p. AA143 — 10p. W005 — 20p. 
W01 — 25p. BYX10 — 6p. 

L.E.D’s Red, Green & Yellow 3mm & 5mm - 10p 8mm - 35p 

20mm fuses 0.15A to 10A quick blow - 5p. 0.1A to 5A Anti Surge - 8p 

High Speed drills 0.8mm, 1.0mm, 1.3mm, 1.5mm, 2mm - 30p 

“Expo” “Reliant” drilling machines 12V d.c. with improved 3-jaw chuck — £7.00 

Nicads AA - 90p. HP11 - £2. HP2 — £2.20. PP3 - £5 Universal chargers for all above 
batteries - £6.50 

Glass reed switches with single pole make contacts - 8p. Magnets - 12p. 0.1" stripboard 
2.5" X 1"9 rows 27 holes — 25p. 3.75" X 2.5" 24 rows 37 holes — 70p. lonisers list price 
£16.95 7 year guarantee — £12.00. Jack plugs 2.5mm 4 3.5mm — 10p. Sockets panel 
mtg. — 10p 

All prices VAT inclusive. Postage 30P (free over £5). 


THE C.R. SUPPLY CO. 
127 Chesterfield Road, 
Sheffield S8 ORN. 
Tel: 0742 557771 


UURAD! 
GT SQUEEZE 


STRETCH 
OUT! 


IN THE 
DISPLAY 
PAGES OF PE! 


PHONE DAVID BONNER FOR 
DETAILS ON 081-743 8888 











For full details send 
an A4 S.A.E. to 


PHONE: 0268 793381 





Achieve That Professional Sound And Look 


Full range of pre-amps, Virtual Earth Mixer, Comp Limiter and 
Driver Electronic Boards 
* As Used in Our Commercial Mixing Desks * 
* Kits or Fully Assembled and Tested * 


Metal Work For Use With Above Boards 
Module Frames, Printed Front Panels, Enclosures, Console Frames, Mounting 
Brackets, 19" Frames etc. 
Full Range Of Electronic Components For Above Systems 
Including Sliding Faders, Rotary Pots, Switches, Indicators, Meters etc. 


ARTRIDGE 
ELECTRONICS 


A.C. PARTRIDGE LTD. 
FAX: 0268 565759 _ESSex SS7 SHB 


Classifieds 


Components 


N.R. Bardwell Ltd. (PE) 


Over one million capacitors for sale in small quantities at 
prices which are much less than manufacturers 10,000 off 
prices. Also many, many other types of components at 
silly prices. Stamp for lists. 


288 Abbeydale Road, Sheffield S7 1FL 
Phone 0742 552886 Fax 0742 500689 


Large selection of interesting components at very 
competitive prices. Large S.A.E. for lists to AGS 
Electronics, Unit 2, Haxter Close, Bellver Ind. 
Estate, Plymouth, Devon PL6 7DD. Tel: 0752- 
767738. 


EPROMS £1 each. Type 27256 150ns. 
Programming voltage 12.5V. Used once 
but erased and with no bent pins. 

Tel: (081) 842 3862 









Modernising your workshop? 
Sell your old equipment through PE 
Classifieds pages. 










Manufacturers Original Spares 


For Amstrad, Atari, Commodore and 
Sinclair computers. 


Many TV, VCR & Audio Parts also available —- Send S.A.E. or Phone 
0452 26883 for a ‘Price and Availability’ on your requirements 
Atari YM2149F ‘Sound’ Chip (ST) 

Commodore 906114 ‘PLA’ Chip (C64/C) 

Sinclair ZX8302 'ULA' Chip (QL) 

Membranes: QL-£7.95 Spec. 48K-£4.50 +/128K-£7.90 
Amstrad Original Service Manual (CPC464 + Monitors) 
ditto Original Service Manual (PCW8526/851 2) 
Chips: PEGA1A-£32.72 TEA2000-£4.49 AY38912-£5.74 
Sanyo/Fisher VCR Belt Kit VTC6500/VBS3500 

Hitachi Replacement Video Head VT11/14/33/34 

Toshiba Power Transformer (22224100) ST-U2 £9.99 
Ferguson Tuner Control Door £v35/36/38/39 £1.95 
The above is just a very small sample of our stock. For a Catalogue 
please send 50p CHQ./Stamps/3xIRC's etc. Please add 95p (UK) P & 
P to above orders. All items subject to availability. 


MARAPET (PEG) 1 Hornbeam Mews, Gloucester GL2 0UE 
MAIL ORDER ONLY TEL: 0452 26883 







£10.02 
£6.73 
£10.75 









£8.49 
£13.89 







£2.49 
£26.10 











19" Rack Mount Project Cases 
Also Consoles, Cabinets, Blanking 
Panels (XR1, Jacks, etc.). All sizes (lu, 
2u, etc.). Installation Racks and Flight 
Cases up to 30u. Call (0253) 300690 












Unit D 
318 High Road 
Benfleet 











July 1991 Practical Electronics 55 


Classifieds 


Surveillance 


Miscellaneous 


Miscellaneous 





Surveillance devices, lasers, Tesla coils, 
scramblers, ultrasonic and many more, 
over 150 designs. Send SAE to: 
Plancentre, Old Wharf, Dynock Road, 
Ledbury HR8 2HS for free list. 


NEw VHE MICROTRANSMITTER KIT, 
tuneable 80-135 MHz, 500 metre range, 
sensitive electret microphone, high 
quality PCB. SPECIAL OFFER complete 
kit ONLY £5.95, assembled and ready to 
use £9.95 post free. Access orders 
telephone 021 411 1821. Cheques/ P.O.'s 
to: Quantek Electronics Ltd, (Dept P.E.), 


45a Station Road, Northfield, 
Birmingham, B31 3TE 
Kits, Plans, Assembled Units, 


Surveillance Microtransmitters, Phone 
Recording Switches, Trackers, Defence 
/Protection Circuits, Plus much more. 
Send 2 x 22p stamps for lists, or tel. 
05436 76477 24hrs. Everything for the 
budding 007. ACE(PE). 53 Woodland 
Way, Burntwood, Staffs. WS7 8UP. 


PROFESSIONAL INTRUDER ALARM EQUIPMENT 


One x 1 Coded Keypad Microprocessor Control Panel, 
One Rechargeable Battery, Two x 12mts Passive Infra 
Red Detectors, One self Actuating Siren Bell Box and 
flashing Strobe Light, Four Magnetic Door Contacts, two 
Panic Buttons, 100 mts Cable and one box of Cable Clips. 


Only £189.00 + VAT inclusive of delivary. Allow 28 days 
for delivery. 


Please make cheques payable to: 


Laser Alarms Limited, 
Laser House, 
Newton Avenue, 
Gloucester GL4 7LT 





Oscilloscope double beam 10MHz. 
Portable (telequipment) £120, AVO 8 
Mk5 cased £100, both excellent, geiger 
counter £50. Wanted: Transistorized 20- 
60 MHz oscilloscope not 
working /” cheap!” 0803-882572. 


Pro Alarm Panels, value £44, £65, 
unused, sale £5,£10. 999 Dialler £10, Bells, 
Ultrasonics, Strobes, Sensors, CR 
Components, Tx, New Bench Drill £150, 
Lucky Bags £5. All must go. Sorry no 
mail order, callers only. Please phone 
first. Hockley, 61 Disraeli Terrace, Leeds 
LS11 6NT. Phone Leeds 778444 (Private). 


For Sale, 80C39 Microprocessors, £2.75p, 
TLO94 opamps 75p, TL066 30p, Reusable 
cable clips stick or scxrew down 5p, send 
orders and cheq/PO to: P.I. Ridge, 5 
Conway Groce, Great Barr, B'Ham B43 
5HD 


Oscilloscope TekTronix 455, Delay Time 
Base, 0.05us time base, VGC, £350 ono. 
Romsey (Hants) 517101 

Enthusiast wishes to swap/sell 
electronics related P.D. software. PC, ST 
and Amiga. Non profit making. LUI. 56 
Leamington Terrac, Edinburgh EH10 
4JL. 


Missing that vital PE 
back issue? 
Don't despair! 


Reprints of articles and software 
listings are available for £2 from the 
editorial address. We cannot accept 
credit card orders below £5 value. 





FOR SALE 


Practical Electronics 
in Binders volumes one (1965) to ten 
(1974) and later loose copies. Offers 
for lot (0905) 54318. 





SERVICE MANUALS 


Available for most equipment. 
TVs, Videos, Test, Amateur Radio etc. 
Write or phone for quote. 
Mauritron (PE), 8 Cherry Tree Road, 
Chinnor, Oxon OX9 4QY 
Tel: (0844) 51694. Fax: (0844) 52554 





Writing Classified Advertisements 
That Work 


® Start by mentioning the product or 
service you are selling. By doing so, you 
make it easier for the reader to see what you 
are offering. 

® Always include the price of whatever it is 
you are selling. Research has shown that 
52% of people who read classified ads will 
not respond to advertisements that fail to 
include the price. 

® Keep abbreviations to a minimum. It 
makes whatever you have to say more easily 
understood. 

| @ Pretend to be your prospective customer. 
Ask yourself what it is you would like to 
know about the product or service that you 
are selling that would make you want to buy 
it. For example, you could include 
information such as brand names, colours 
and specifications. 























a — — — — — — — _ _ _ 


: CLASSIFIED COUPON 








Rates are 20p per word plus 4p VAT (lineage, for semi-display advertisements contact our Ad. Dept.). All classified advertisements must be 
pre-paid. Please send your copy with the remittance (payable to Intra Press or payments by Visa or Access accepted) to: Practical 
Electronics Classified Dept., Intra House, 193 Uxbridge Road, London W12 9RA. Tel: 081-743 8888, Fax: 081-743 3062 


en ee eee ee ee eee ee 


56 Practical Electronics July 1991 





ee 





LOTS MORE 


CATALOGUE! 
K539 LED Pack. Not only round but 
many shaped LEDs in this pack in 
red, yellow, green, orange and Clear. 
Fantastic mix of new full spec devices. 

Siupevesosbonadioess 100/£5.95; 250/£11.75 
K575 Plastic Power Pack. Mainly T0126 
and T0220 transistors, SCRs, Triacs etc. 
All new full spec marked devices 
i ebthicd fantastic value. Lots of TIP and 


IN OUR 
















INDICATOR PACK 
K700 Big variety of neons in this pack! 
Round, square and oblong, clip and 
screw fix. Red, Green, Amber and Clear. 
Tag & wire-ended. All are 110V, but 
suitable resistors for use on mains are 
included. Really great value for money! 
K805 M2 screws. Good mix, this. 
Cheesehead, C/s, pan, mostly pozi, few 
slot. Lengths to12mm. All steel with 
various plating 


































K806 M2.5 screws. Various heads - 
mostly pan and c/s pozi. All plated steel. 
Lengths to10mm. 










K807 M3 screws. Good selection of sizes 
including a few brass. Most heads. 
Lengths to 35mm. 

Pack of 100 £1.50 
K808 M4 screws. Huge variety! Pan, C/s, 
cheese, set, slot, pozi. From 4-50mm 
long. Allsteel, plated, black/hi-tensile. 













MOTOR + GEAR PACK 
K579. This pack contains 10 assorted 
battery powered motors (mostly 3V) + 
90 gears etc; 16-60mm dia + worms and 
shafts. Amazing value. 
Pe ieacetiachceunsateimianiancieuetacuceins Only £7.95 
K708 Voitage Regulators. This is an 
excellent pack, made up from a huge 
variety of the +ve, -ve, fixed and 
variable regulators from 1.2V to 37V, 
100maA to 5A, plastic and metal. 

K709 Bridge Rectifiers. Another 
superb value pack - could include 
anything from % amp to 35A, 25V to 
1000V, plastic and metal. 
Price 







































20 for £5.95 
K710 SCR's & TRIACS. Big mixture could 
include all types from TO92 plastic up to 
DOS stud mounting with a chance of 
everything in between! 25V to 1000V, 
100mA to tens Of amps. Marvellous 









mseiphegidisbebeseboneséadbanaseaxeets 25 for £4.95 
K711 74 Logic Pack. Aji brand new 
full spec devices from basic gates to 
complex logic. May include 54 & 64 
types as well as 74 in L, LS, S, ALS, H, HC, 
HCT, etc. 

Price per pack of 100 £6.00 
K712 Crystals. Mostly HC60 and 
HC18U in a wide variety of frequencies 
from a few hundred kilohertz to many 
megahertz and the odd crystal oscillator 
module or two. 

ccscciahscsnsonessensscosoosossessosee 20 for £4.95 
K713 Fusehoiders. Panel and chassis 
mounting from a basic clip to high 
current enclosed types for 15, 20 and 
32mm fuses. 

Price for pack of 50 
K714 Power Supply Capacitors. 
cans, mostly computer grade including 
popular values like 10,000, 40V etc. Big 
mix of values and voltages up to 100V or 
more and 50,000, F. 

Price for box of 25 £12.50 
K536 Bonanza Pack of 74 series chips 
on panels. 200+ chips, may inlude L, 
LS, H, HC, HCT etc. (These are actually 
the 28900 computer panels with all the 
memory missing.) 


GLUE GUNS 


87-0400 Hot melt giue gun. 
Electronically controlled heating 
element which melts the long stick of 
glue when inserted. Trigger feed. Mains 
operated. Normally sells for £8.60. 







































Pee eeeesercceseseeeeseces 














































OUP PTICO ooo. eeeeeeeeeeeeees £4.95 
Glue sticks - pack Of 10... £1.00 
Multiturn Trimpots from 14p 
- details in B/L 68. 

Grey ribbon cable 100ft reels. 
230176 14 way £6.00 
230197 16 way £8.00 
Bulk LED's from 2D .-. sce 
B/L 68. 


Auto answer. 
interface. 













Our special price 


ASCil KEYBOARD 


5V supply, switch on board 
Changes output to emulate AT 


28921 ‘Apricot’ PSU. Beautiful 
unit 160x110x55mm_ with 
switched 
Made by Astec, 
120 240V input. 
+5V@2.5; +12Via 2A. 


t+ ttt + HH 


Mercury Switches 


Metal 
dia x 9mm long. 40mm flange one end. 


4 Digit Displays 
LCD 4 digit 12.5mm high with low batt 
and clock symbol. 


Driver Chips 


For above display. Type ICM 7211 AMIPL, 


PACKS PACKS|VISTEL II VISUAL TELEPHONE|switchH MODE PSU's 


Total communication for deaf people - this brilliant piece of equipment has a full 
QWERTY keyboard and 40 character screen. Text editor. 9,500 character memory. 
Printer interface. 

These are new and boxed but because 
the makers are bankrupt, there's no guarantee. Originally sold for over £500. A 
comprehensive 143 page instruction manual is provided. (Manual only - send £12, 
£10 refunded on return). 


Auto dial. Calculator. 
Modem support V21/23/25. 


Pee meee eee eee eee eee eee eee eee sees eee eee esses eeeeeseses 


or XT keyboards. 94 keys, but ONLY................... £1 100+ 0.5046A output. Supplied with connection 
18 do not have caps. 20 keys MIODEL RAILWAY PSU | 2°.2"'S..we can offer these at fraction of 
have removeable tops. SiZ€ 28897 panel 185 x 105mm with | Prices......... £5.95; 10+ 4.95; 100+ 3.95 
350 x 145mm. mains |/ P & 2x 5V 1A outputs | 28888As above but SV 10A output. 

Order Code 28922 | fully variable 0-12V 1.5A | Prices----- £8.95; 10+ 6.50; 100+ 5.20 
Price......................... £12.00 Y -9‘\1 28890 DC-DC Converter board Panel 


C/O 
IEC mains inlet. 
BM43024. 

Outputs 





132 pages of value-packed 
48 pages in 2 supplements 


Easy to use order form 


Our famous Bargain List 


Price to include Catalogue, current bargain list and next 6 lists. 
reply paid envelope £2.50 (UK & BFPO) £5.00 Overseas. 


Stepper Motors 
Superb 
35mm dia x 21mm deep with a 16 tooth 
9.5mm dia gear wheel mounted on the 
2mm dia spindle. Fixing centres 42mm. 
7’%,°,48 step. 100+ priceis £9.04. 
abe Code 









PCB 9 
2 VTL 


little 12V motor by Airpax. 


25045 
£4.00 100+ 2.50 


enclosed, in case 7.5mm 






Of Co 


Peer cree erceeseeeseseeseseeeseseeeeeseees 


100+ 0.25; aK + 0.18 
to (a) 






Complete with edge 





connector. Can you believe the price?! = chanel Code auugaee eet 25+ 0.75100 a, 60 
aweswcuwceeeewsaccascscetuceces is . Sealed Lead Acid Batter es 
srl bswdpuideastanaanteiecbeas Opto Slotted Switch Vike waeasds 45 Cin <esiediie ciauia 






25+ 0.60; 100+ 0.45; 1k+ 0.35 

















this is a 4 digit LCD decoder driver (uP extremely cheap version of TIL100/ Order Code 28918 
interface). List £3.69. ! PRICES ooo ccecccccecceee. £14.95 10+ 11.20 
— Code Se reer as YUASA NP10-6. 6V 10Ah, size 
qLuwssvusebwsvessivaberssaeadosrtsucesssevawenresvas TICES .0oooo oe . Pae ; : 150x95x50mm. List £18.00. 
100+ 0.10; 1K + 0.07 Order Code 278920 
PRICES occ ccceceeeeeene £10.00 10+ 6.00 
¥,Meg Memory Board 


Job lot of returns - famous manufacturer. All are complete and look OK, only minor 
faults. 
25028 110mm Manual models include 110LF and 110TF, many have built in flash. 


COO eee OOOO OOOO REE EEE ETOH EEE SEES EEE EEE EEE EEE EEE EEE EEE EEE SEES OSES 


25029 110mm Motor driven. Models include 110IF. Prices...€4.00ea 5 for £16.00 
25030 35mm Manual. 
flash. Prices 
25031 35mm Motor driven. Models include DL15, 35EM, 35EFM, 35ESM, most with 
builtin flash. Prices 
25032 Broken cameras. These have parts missing. A parcel of 6 assorted, all 35mm 
including manual, motor driven, autofocus, twin lens types. Price 
Also some brand new cameras and lenses - SAE for detalis. 


To avoid confusion over different VAT rates 
all our 1 off prices will include VAT «15% 
until further notice. 
Quantity prices do not include VAT. 


Models include 35HL, 806, 


CORPO reer eee reer eee eee EEE EEE E EEE EEE EE EEE EEE SEESSESESEEES 


Pee e meee meee e renee eee eee eee sees eres esses eeeeeees 





GREENWELD 


3G ten gfe), ile 
COMPONENTS 


notes, 





27C PARK ROAD, SOUTHAMPTON, ’s01 STB 


LCD DISPLAY 
22123 8 digit 10mm high. No 
edge connector, so 


output (needs 3 components 
(supplied) 
5x12V 


transistor circuits. 
terminals on 
Supplied with circuit and 
wiring diagram. 


1991 CATALOGUE + SPRING SUPPLEMENT 

YOU GET A GREAT DEAL MORE WHEN YOU DEAL 
WITH GREENWELD!! 

The 1991 Greenweld Catalogue is out NOW! 

Many substantial reductions 

Quantity pricing for bulk buyers 


Next 6 updates plus lists included in price 


1st class reply paid envelope 


Motor Panels 


of 11 core cable with socket and stepper 
motor as described above. 


Controiier Boards 
PCB 175x 122mm containing a wealth 


TLO94, CMOS and 74 series chips, 8 x 
T0126 transistors, 13 TO92 transistors 
and lots of R's and C's etc - also a 3V 
lithium battery. 3 connectors on it go 


display panel which is identical to our 
2027 (P111 Of Catalogue). 


vactel Type VTL 10DI - IR emitter and 
detector can be removed from the 
plastic housing 


Corer eereseeseserseseeses 


welcome; min invoice charge £10.00. 
Payment is accepted b 
order, cash, includin 
book tokens, 

Our stores have enormous’ stocks of 
components and our trade counter is open 
om 9-5.30 from Mon-Sat. 


Tel: (0703) seetae 





Totally enclosed Astec Unit 
175x136x65mm with switched and 
fused IEC inlet. 

Model Number: BM41012 

Input: 115/230V, 50/60HZ 











RS232 (V24/28) serial 








































Outputs: +5V 3.75A 

+12V 1.5A 

sidsdinsananstssesisaiedvdees e —12V 0.4A 
Total Wattage: 65W 





cbecnindaiacbesebneonite £14.95; 100+ 11.21 
28887 Made by STC, this 160 x 100mm 
PCB is attached to an aluminium chassis 
165x102x65mm and has a single 5V 
























220x195 requre SOV I/P for 5V 19.5A 






to be fitted). 
relays with 10A 
contacts controlled by 
Screw 
I/P and O/P. 





£7.95; 25+ 5.20; 100+ 3.89 





another Astec model. This one is 
partially cased, the overall size being 
160x104x45mm. The PCB measures 
160x100mm. Input and Outputs are on 
flying leads, all colour coded. There is 
also an additional IEC socket to extend 
mains to another unit. 

Specification: 

Model Number AA12531 

Input 115/230V, 50/60Hz 























































Outputs +5V 5A 
+12V 0.15A 
Total Wattage SOW 
i ne £6.95; 100+ 5.21 





Have you purchased the above 
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We now supply a conversion kit to 
change the outputs to +5V 2.5A; +12V 
2A; -12V0.1A and -5V 0.55A. 

K625 Complete kit and instructions. 








goods + afurther 







NI-CAD BATTERIES 


Regular stocks: AAA £1.20; AA 99p; 
C £2.20; D £2.30; PP3 £3.95 

24150 Ex mobile radio battery. 
58x63x33mm case (sometimes 
damaged) contains 8xAA_ size 
rechargeable Nicads. These can be 
removed by breaking the case open. 
Each cell rated 1.2V 600MA. 
24149 As above but 84~ 66 x 33mm. 
There are again 8 cells but they are 
longer than AA size, being 73mm long. 
Each cell rated 1.2V 900MA. 






All supplied with 







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10D2 opto siotted switches, length 















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mponents - 80C39 CPU, 4x TLO66, 





















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card reader (b) motor panel & (Cc) 






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trickle charged whilst in store. 
150x95x65mm. List price £28.00 
















if required. An 





























z8900 Massive panel 460x 400mm 
smothered in chips. Could be a 
complete computer judging by the IC's 
on the board. Made by Whitechapel 
Computer Works. Contains at least the 
following (some panels have extra chips): 
64 x 4164-15 RAM’s; over 200 74LS, F and 
other logic chips; 3 x 4016-3, 2 x 8253-5, 
8251, 2x 5516, 6 xtals, 3x ‘D' plugs and 
sockets. 3 x DIN 64 way socket, + R’S, C’s 
etc. Price equivalent to 4164's « 30p 
each and rest of chips « 3p each! 







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€4.50ea 5 for £18.00 






























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24397 RS232 data cable - 25 ‘D’ plug 
to 25'D' socket 6 feet long. All pins 
connected. These are identical to our 
Cat. NO. P2875 « £4.95 except for some 
very minor corrosion on socket casing. 





Add 2.17% to total. 
P&P 


No 














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25048 Panel 275x178mm containing 
some excellent components: 2 x D8243 
1/0 expander, 8035 CPU, 8253 timer, 2651 
USART all in sockets, 2 x 2111A-4 RAM, 25 
mostly CMOS chips, 8 x T0126 transistors, 
5 x TO92 transistors, R's, C's etc; 26W IDC 
plug, 2 x 34W IDC plugs, 2 xtals. 

















Come and see 







(0703) 2363 






July 1991 Practical Electronics 57 


Q&A 


Techniques 


From solar heating temperature sensors to guitar string pickups, Andrew Armstrong has 
electronic solutions for everything. 


wo readers have a similar 

request this month. Mr S D 

Craft of Deal in Kent writes “I 
wish to construct a digital 
temperature indicator to show the 
temperature in six different 
positions in a solar heating 
installation”. Mr P Anslow of 
Wakefield in Yorkshire writes “I 
need a circuit to measure the 
temperature of my solar panels at 
several different points. Preferably 
it should use a 9V battery supply, 
and have thermistor sensors wired 
back to a rotary switch”. 

These requirements’ are 
sufficiently similar that one design 
can satisfy both of them. The only 
alteration I have made in designing 
this unit is that I have used 
semiconductor temperature-sensors 
instead of thermistors. This avoids 
the use of complicated and difficult 
to calibrate thermistor linearisation 
circuitry. 

The circuit shown in Fig. 1 
illustrates the basis of a digital 
thermometer operating with a 
number of switched remote sensors. 
The ICL7116 DPM (digital panel 
meter) is chosen because it has 
sufficient accuracy for the job, is 
easily available, has a low current 
consumption, and drives a liquid 
crystal display. If it were not 
necessary to run the unit on battery 
power, then the ICL7117 (a similar 
chip but using an LED display) 
would be equally suitable. Display 
connection details are not shown 
because they are simply a matter of 
following the pin connections in the 
data sheet. 

At first sight, the circuit may not 
appear to make sense, so an 
explanation is in order: the A to D 
(analogue to digital) conversion 
used in the chip cannot measure 
relative to the negative power- 
supply rail. Both input and 
reference voltages are measured 
internally relative to the common 
terminal, which maintains a 
constant voltage below the positive 
supply rail. It is for this reason that 


58 Practical Electronics July 1991 


the reference voltage is adjusted 
using a potentiometer chain 
between positive and common. 


Temperature Sensors 


The AD590 temperature sensor 
passes a current of 1A per Kelvin. 
Thus, at normal room 
temperatures, a current of between 
290 and 300UA will flow. This gives 
approximately 600mV across R1 at 
0°C. To make the meter read zero at 
freezing point, an offset of 600mvV is 
fed to the low input terminal. 
Temperatures of above freezing 
feed a positive voltage to the high 
input relative to the low input, and 
give a positive output reading. 
AD590 temperature sensors 
have different tolerances, so the 
meter can only give a completely 
accurate reading from one of them. 
To calibrate the unit, first adjust 
RV1 to give 600mV between the 
low input and the common 
terminals. Then, set the reading 
switch to the temperature sensor 
whose accuracy is most important, 
and adjust RV2 until the correct 
temperature is displayed, as 
verified by some dependable 
temperature reading instrument. 





~ Liquid 
crystal 
display 





“ Ay 
Fig. 1. Digital thermometer. 





Calibration accuracy on the 
other sensors should be accurate 
enough for most requirements. If 
absolute accuracy is required, a 
second gang could be added to the 
selector switch to switch preset 
potentiometers in series with R1 
(whose value should be lowered to 
1k8), to trim each sensor to work 
accurately. 


On The Pickup 


Mr J M Alsop writes to ask about 
guitar electrics. He wants 
information about pickups and 
control circuits, the difference 
between solid bar and individual 
string pickups and the effect of 
using more than one pickup. 

Though superficially simple, this 
is a big subject. Among guitar buffs, 
many aspects of electric guitar 
design are considered important, 
even the colour of the knobs! But 
seriously, it is true that some points 
which would normally be 
considered electrically insignificant 
can affect the sound enough to 
matter, while other things which 
would be considered important in 
other fields do not concern many 
electric guitar players. 








In principle, a guitar pickup 
consists of a coil of wire wound 
round a magnet, and positioned so 
that the vibration of the string 
disturbs the magnetic field and so 
generates a voltage in the coil. To 
disturb the magnetic field, the 
string must of course be made of 
steel so that it will interact with the 
field. Fig 2 illustrates this effect. 

A typical single-coil guitar 
pickup is illustrated in Fig. 3. In this 
type of pickup, the coil runs round 
a series of pole-pieces, one per 
string. The magnet is behind the 
entire assembly, so that one magnet 
can provide the magnetic field for 


Sona s 
eee eer 





all the strings. The advantage of this 
is that it is simple, reliable and 
economical, and works well. Some 
pickups of this general type use 
adjustable pole-pieces so that the 
output from each string can be set 
individually. 

A major drawback of this simple 
design is that it will also pick up 
any hum field present around the 
guitar. Modern guitars often use 
very light strings, which give a 
lower output from the pickup. To 
achieve the required volume, the 
amplifier gain is turned up further, 














- 


\ / 
4 aie ‘\ 
PO AN Wie 





which amplifies the hum more. 

“Humbucking” pickups can 
almost eliminate hum from the 
guitar pickup output. Here, two 
identical coils pick up the hum and 
their outputs are added in 
antiphase so that the hum is 
cancelled. The antiphase connection 
can be either series or parallel: 
series connection tends to 
accentuate bass at the expense of 
treble, while parallel connection 
does the opposite. The second coil 
on the pickup only picks up hum; it 
doesn’t get any signal from the 
guitar strings, because there is no 
magnet behind the coil. 

To understand why the tone is 
affected by series or parallel 
connection, it is necessary to 
consider the equivalent circuit of a 
pickup, containing inductance, 
resistance and a source of voltage 
(hum or hum-plus-string signal). 
Figs. 4 and 5 show the effects of 
series and _ parallel pickup 
connection using these equivalent 
circuits. Working from the fact that 
inductors have a series impedance 
proportional to frequency, it is clear 
that in Fig. 5 the extra inductance in 
series with the pickup reduces the 
treble more than the bass, while the 
parallel inductor shown in Fig. 4 
tends to short out the bass more 
than the treble. 


Positioning 

The position of the pickup has a 
substantial effect upon the sound. 
This is because the guitar string 
does not vibrate purely 
sinusoidally and _ the 
amplitude of harmonic 
_| vibration varies relative to 
| the fundamental along the 
length of the string. A 
pickup exactly in the 
middle of the string would 
receive maximum 
fundamental and 


Q&A 


minimum harmonic signal, but the 
harmonic output tends to increase 
as the pickup is moved nearer to the 
end of the string. This effect 
accounts in part for the reason that 
the same note played by fretting a 
different string can sound different: 
if a bass string is fretted to produce 
a high note, then the pickup is 
likely to be more or less in the 
middle of the vibrating area of 
string. If the same note is played on 
a high string, most of the length of 
the string will be in use, so the 
pickup will be towards one end of 
the vibrating portion, and so will 
receive a different tonal balance. 
Controls on most electric guitars 
are passive in nature: a selector 
switch will select one, the other or 
both pickups, the volume control 
connected across the pickups will 
adjust the output level, and a 
simple treble cut control, illustrated 
in Fig. 6 will adjust the tone. Passive 





treble and bass tone controls 
generally attenuate the signal to be 
of great value, but some designs 
have used inductors as well as 
capacitors in the tone control 
circuit, to achieve passive control 
without too much loss. 

Some guitars, and especially 
optional extras for guitars, use 
active circuitry which can include 
anything up to a full graphic 
equaliser, and goes beyond the 
scope of this article. = 








July 1991 Practical Electronics 59 


25 Years 


(PRACTICAL 








ELECTRONICS 


SCIENCE AND TECHNOLOGY 


The more things change, the more they stay 
the same. Especially, it seems, in PE. 


July 1966 


ack in the early days of PE the lack of PCBs must 

have been quite a limitation to what circuits could 

be built. Components were usually fixed to 
wooden panels with the leads poking through specially 
drilled holes. These were then connected together by 
soldering wires to them. The projects this month 25 
years ago have a familiar ring to them; fuzz box, 
stabilised power supply, bug locater and microphone 
mixer — they have appeared again and again over the 
years, always in improved forms of course. For all that 
electronics hardware has improved beyond recognition 
in 25 years, the basic ideas and projects don’t seem to 
have changed at all. 


1876 


The cover story this month was the PE digiscope. This 
was an amazingly simple idea that was, perhaps, before 
its time. It comprised a portable hand-held box with a 
probe on one end and a matrix of LEDs on the other. 
The signal fed into the probe could be viewed on the 
LEDs and although not an entirely accurate 
representation, gave some idea of what was going on. 
The main problem was the resolution of the image but 
with modern LCD displays, chips and general 
improvements in technology, it seems likely that the 
time is right for a more modern version. 


1981 


Mike Kenward’s editorial 10 years ago started off by 
saying that there had been “one or two comments 
recently that the light at the end of the depression is just 
beginning to show”. A familiar sentiment 10 years on 
when nothing appears to be any different. Perhaps 
talking the economy back into shape does work - it’s 
nice to think that if you wish hard enough, its bound to 
come true — only time will tell. 


1986 


An interesting article appeared in this issue. It covered 
the recharging of dry cell batteries. According to the 
author “propaganda” had been put out in the UK 
saying that this was a dangerous process. He actually 
tried overcharging some batteries, just to see what 
would happen. The result seemed to be nothing-at-all. 
With the possibility of getting 20 recharges per cell, it’s 
a wonder that the idea didn’t catch on. On the other 
hand, NiCd batteries and chargers are now so easily 
available that no one would want to buy normal dry 
cells and recharge them would they? 





60 Practical Electronics July 1991 








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q 


—~—_ —— mm. — wd we 


PP1 6V batteries or two battery 
holders each with 4 type AA cells. 
The circuit for the prototype is on 
two boards (Fig.4) which slot into 
grooves moulded in the sides of the 
case. if the case you are using has 
different dimensions, cut your 
boards to fit. It is possible also to 
combine the layouts of Fig.4 on to a 
single board and fit this at the 
bottom of the case, using plastic 
self-adhesive pcb mounting strip or 
double-sides self-adhesive pads. 

The power switch, range selector 
switch, input terminals and meter 
are mounted on the lid or front 
panel of the case. 

For testing the circuit, you will 
need a oscillator capable of 
generating signals in the range 0 to 
20Hz. The best is a calibrated signal 
generate but it is possible to make 
do with an astable multivibrator 
based on a 7555 time ic. Fig.5 shows 
a suitable circuit, which can be 
breadboarded for the occasion. 
Using 1% tolerance capacitor and 
resistors gives frequencies close 
enough for calibration. 

Build Board A first and, if you 
have an oscilloscope, check that the 
output from pin 6 of IC2 is a square 
wave. Otherwise connect an LED 
and 180 _ resistor in series between 
pin 6 and the OV rail and see that 
the LED flashes strongly at the 
correct rate (as near as you can tell). 

Next, assemble the sub-circuits 
associated with IC3 and IC4 on 
Board B and make the wired 
connections to board A. Monitor the 
signal at pin 1 of IC4. With a 1.2 Hz 
input, this has a frequency of 
120Hz; with a 12Hz input, the 
frequency is 1.2kHz. 

Finally, assemble the tachometer 
sub-circuit (IC5). Before placing the 





ic in its socket, use a multimeter to 
adjust the combined resistances of 
R16/VR1 and R17/VR2 to 410k 
and 41k respectively. 

Also calculate the series resistor 
required for the meter and adjust 
VR3 accordingly. 


‘Integrated circuits 2 
ICt, IC2 1CL7611 CMOS 
an . . 








Low Frequencies 


When finally tested with a 12Hz 
signal, and with S2 switched to the 
20Hz range, the meter takes a few 
seconds to come up to its final 
reading. This is while the charge is 
accumulating on C4. Its progress is 
unsteady as the VCO gradually 
locks on to the correct frequency. 
The needle finishes a little to the 
right of the centre of the scale. For 
example, if the full-scale reading is 
100A, the needle reads 60u.A. After 
settling, the needle readily follows 
slow changes of frequency, but 
needs a few seconds to catch up 
with a rapidly-changing frequency. 
The same meter reading is obtained 
with a 1.2Hz signal on the 2Hz 
range. There is more tendency for 
the needle to hover around the true 
reading on this range as there is 
more likelihood of the PLL 
occasionally losing its lock at such 


low frequencies. a 


18BE CMOS dual decade 


chometer (8-pin or 





July 1991 Practical Electronics 61 


Barry Fox 


All We Need 





This month Barry tunes into Britain's crowded radio waves to see where all of the programs 
fit into the great scheme of things. 


there were only a few radio 

stations in Britain and no 
frequencies for any more. Then, 
suddenly, there were enough 
frequencies for dozens of new 
commercial stations and the BBC 
started Radio 5. 

In 1979 a World Administrative 
Radio Conference of the 
International Telecommunications 
Union decreed that the whole of the 
VHF radio band (Band II) had to be 
released for entertainment 
broadcasting. In 1984 a Regional 
Administrative Conference in 
Geneva set a timescale - the band 
must be cleared by 1995. 

Pirate radio put the British 
government under pressure to act 
on the WARC ruling and make 
room for more legitimate stations. It 
also set about ending the wasteful 
practice of simulcasting, whereby 
one station broadcasts the same 
programme on two different 
frequencies at the same time. 


On The Right Path 


Frequency allocations in the UK are 
now like footpaths. Those with 
rights risk losing them if they do 
not use them. This has led to 
disputes between the Radio 
Authority and BBC. The BBC says it 
needs all its allocated frequencies, 
to build relay stations. The Radio 
Authority sees any unused 
frequency as a new local station. 
The Home Office asked the BBC 
to stop simulcasting its national 
stations. Radio 2 moved off the MW 
wave last August and its MW 
frequencies were taken over by 
Radio 5. Radio 3 will stop MW 
broadcasting in 1992 and Radio 1 a 
year or so later. This will release 


| seems like only yesterday that 


62 Practical Electronics July 1991 





for 


frequencies 
national radio. 

The main changes are in the 
VHF-FM band which, in Britain, 
stretches from 87.5MHz to 108MHz. 
Inside this band there is a complex 
jigsaw of allocations which will 
baffle all but the most dedicated 
listener. The BBC national, BBC 
local, commercial local and future 
commercial national stations stack 
in a multi-layer sandwich. 


independent 


No More Police 

Until the end of 1989 the police 
used a large chunk of frequencies 
(97.6 - 102 MHz) smack dab in the 
middle of the VHF band - a 
ludicrous situation when you bear 
in mind how touchy the police are 
about crime reporters 
eavesdropping on their radio 
conversations. This chunk is now 
used by Radio One _ and 
Independent Radio. Although they 
do not seem to use them much, 
utility services (gas and rail for 
example) will still hold a large 


chunk (105 - 108 MHz) of 
frequencies at the top of the band 
until 1996. After that there will be 
no more VHF frequencies to 
allocate. 


Who's Got What 


The current situation is that the 
BBC uses the low end of the band 
(88 - 94.6 MHz) for national and 
regional radio, Radios 2, 3 and 4. 
The next chunk (94.6 - 96.1 MHz) is 
used for BBC local radio and Radio 
4 in some areas. The next up (96.1 - 
97.6 MHz) is used by Independent 
Local Radio. BBC Radio 1 has 97.6 - 
99.8 MHz and a mix of new 
independent national and local 
radio stations, mainly yet to come, 
has 99.8 - 102 MHz. Further up a 
slice (102 - 103.5) goes to ILR and 
the highest slice (103.5 - 105) goes to 
BBC local radio. 

But there are even odd 
exceptions to the general rule 
where, for instance, ILR station 
Melody uses a frequency once 
allocated to the BBC and ILR Kiss 
uses a national allocation. Melody 
got its frequency because the BBC 
did not go through with its plan to 
build a new local station in 
Berkshire. 

Using a continuous tuning dial 
or frequency display is now a turn 
off for listeners. The short term 
answer to the worsening muddle is 
a radio with pre-set tuning buttons. 
The long term answer is RDS, the 
BBC’s Radio Data System. Digital 
code in a sub-carrier identifies each 
station. So an RDS receiver can 
have pre-set buttons marked only 
by station name. The listener need 
know nothing about frequencies. 
The pity of it is that so far only car 
radios have RDS. a 





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