britains best.selling electronics magazine discover le space ......britains best.selling electronics...

BritainS Best.Selling Electronics Magazine

No. 82

FULLSOR

Discover leSpace Telescope!How to DesignTransistor Amp

,Circuits Easily

Intelligent CarInterior/ LightControllerProject

Learn AbiMains Safety

Low Cos!VersatileAlarm %asters,*

Psychedelic*Wah Wah PedalProjectIt's Far Out Man!\

Take your senses for a trip to LIVE '94, the UK's

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London El 9AT.

OCTOBER 1994VOL 13. No. 82 IN 71-1,S

FOR YOU TO BUILD!

PSYCHEDELIC WAH WAHPEDALRecapture the classic sound of the '70swith this superb project. Using moderntechniques, this Wah Wah pedal givesunbelievable performance at a veryeconomic price. A ready-made foot pedal

mechanism makes construction easy too!

INTELLIGENT CAR INTERIORCONTROLLERMost car interior light extenders do justwhat their name suggests, they keep the

courtesy light on after the car doors areshut. This project does far more! It works

as a courtesy light extender, plus it will

turn off the light as soon as the ignition is

switched on, plus, if you accidentally leave

a door open, it will turn the light off after

ten minutes preventing a fiat battery.

INDUCTANCE/CAPACITANCEMETER ADAPTORDoes your digital multimeter measureinductance and capacitance? No! Wellbuild this project and it will!

LOUDSPEAKER PROTECTORPrevent your valuable loudspeakersexpiring with a bang with this easy -to -build

and use loudspeaker protector. The unit is

self powering and simply connects in line

between the amplifier and theloudspeaker.

16

33

49

56

FEATURES ESSENTIAL READING!

ALL ABOUT THE HUBBLESPACE TELESCOPESeveral years ago Hubble was hailed as

the biggest astronomical observationproject ever. However. due to a number of

mistakes and failures Hubble failed toperform as expected. Thanks to anamazing space shuttle repair mission,Hubble is now working as intended, giving

astronomers an unprecedented view ofour universe.

MAINS SAFETY IN HOBBYPROJECTSAndrew Chadwick gives life-saving advice

on how to design and construct mainspowered projects so that they are safe.

DESIGNING TRANSISTORAMPLIFIER CIRCUITSThe humble transistor amplifier is afamiliar sight, but designing circuits tobehave as intended can be tricky. Thisarticle explains, in understandable terms,

how modern transistor 'models' andsimple maths can be used to virtuallyeliminate all the guesswork.

4

24

29

HOW TO IMPROVE YOURCIRCUIT DESIGN SKILLS 39The humble scientific calculator can play

an important role when designing circuits.

This article starts with basic ohms lawcalculations and works up to reactivecircuit calculations. Worked examples are

included throughout. This article is theideal starting point for the beginner whowants to learn more.

FILTERS - HOW AND WHY?Filter circuits are commonly encountered

in electronic circuits. John Woodgate,explains how filter circuits work, why they

are needed and how they can bedesigned in this authoritative new series.

GETTING TO KNOW TESTEQUIPMENTKeith Brindley continues his look at testequipment. There's plenty of practicalguidance on choosing and using testequipment, plus easy to understandexplanations on how various testinstruments actually work.

AUDIO POWER AMP ICs63In the final part of Ray Marston's maxi -

feature on Audio Power Amplifier ICs more

tried and tested circuits are presented for

you to build and use.

UNDERSTANDING CIRCUITSWITCHED NETWORKS 69Despite the current trends for packetswitched networking, circuit switchednetworks, such as the dial -up telephonenetwork and ISDN, have a lot to offer the

computer user. Frank Booty takes a look

at modern circuit switched networktechnology and Dean Hodgkinsdemonstrates working examples of thistechnology in action.

HOW TO PROTECT YOURPROPERTYThe contents of your electronicsworkshop, shed or garage are easypickings for today's criminals. What'smore, borrowing all those lovely toolsmakes it easier for the burglar to break into

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October 1994 Maplin Magazine 1

ABOUT THIS ISSUE ..Hello and welcome to this month's issue ofElectronics! Instead of telling you about what's inthis issue (I'm sure you'll enjoy finding out foryourself), there's some other interesting information.

All ChangeIn case you weren't already aware, from 1.00am on16th April 1995 all UK national telephone diallingcodes will be changing (just after everyone hasrecovered from the London dialling code change awhile back)! This national major upheaval has beennamed Phoneday. When dialling a nationalnumber, you will need to add a 1 after the 0 at thebeginning of the code. For instance 0702 becomes01702 and Inner London becomes 0171. Fivecities. Leeds. Leicester. Nottingham, Sheffield andBristol, will get entirely new codes. Premium rate(0891. 0898, etc.), free (0800), low cost (0345) andmobile phone numbers remain unchanged.BT have set up a free number to deal with anyqueries that you may have about Phoneday.Tel: 0800 01 01 01.

Why am I telling you this now, I hear you ask!No, I don't have shares in BT! It is because thePhoneday change is going to affect everyonewho uses the telephone network, and whilstreprogramming the MODEM. fax machine, alarmauto -dialler, is an easy once -only task (if you canfind the manual), trying to re-educate the kids orGreat Uncle George may take a little longer ...On a global front, the international access codein the UK. currently 010 will change to 00. whichbrings us into line with (guess who!) the rest ofEurope. The change also affects people diallingfrom overseas. Currently, overseas callers, afterdialling their international access code and the UKaccess code (44), dial the UK national code lessthe leading 0, followed by the actual number.

The Phoneday change means that afterremembering to miss -off the 0 they will also haveto remember to add a 1 as well! So, if you don'tget a call from Auntie Nellie in Australia to wishyou 'a very happy birthday' next year, it won't bebecause she's forgotten, it'll be because she can'twork out how to get through! So you had betterwrite and tell auntie your 'new' number ...

Phoneday doesn't stop there as some equipment.such as least cost routers (for Mercury), call barringunits and private telephone exchanges, may requirea firmware upgrade (new EPROM, etc.) to workproperly. The supplier/manufacturer should be ableto advise what modification, if any. is necessary.Good suppliers/manufacturers will provide suchafter sales support, the bad ones will tell you tobuy a new model (so don't buy from them again!).Similarly if you are about to reprint your family orbusiness stationery. make sure you rememberto change the dialling code at the same time.

The good news is that because our telephonesystem is now based on digital technology.exchanges are intelligent enough to acceptboth the existing and new dialling codes in thecross -over period, so until Phoneday you willget through whichever code you use! AfterPhoneday (for about a year afterwards) if youforget to add the 1 you will get a recordedmessage telling you what you've done wrongand how to redial correctly.

So until next month, from the rest of the team andme, have fun with the phones and enjoy readingthis issue!

Exclusive Subscribers' Club Special Offer

On offer this month. to subscribers only. is the Helping Handand Magnrf ler. pictured above - it is available for just £4.95,a saving of over 16°C' If you area subscriber. full details ofhow to order this really useful item are included on thespecial otter leaflet in this issue - if the leaflet is missingcontact Customer Services on (017021 552911. If you are nota subscriber and would like take advantage of future specialoffers and other benefits of subscribing. turn to page 13 ofthis issue to find out more.

ABCCONSUMER PRESS

Front Cover Pictures.

Fractal Background Image.

Jim Bowler, Maplin Electronics plc.,

CO Copyright 1994. NI nghts reserved.

Sticernekenlevy9colloon with Jupiter.

Allan Barn. Copyright 1994

All nit reserved. used by permission

Jm Hendra, Hutbon-Deutsch Collecbon &

Maplin Electronics* .© Copyrip 1994.

All rights reserved, used by permission.

EDITORIALEditor Robert Ball AMIPRETechnical Authors Robin Hall FBIS, G4DVJ,Mike HolmesPrint Co-ordinators John Craddock,Len CarpenterNews Editor Stephen Waddington BEng (Hons)Technical Illustrators Ross Nisbet,Lesley Foster. Paul Evans, Nicola HullProject Development Chris Barlow,Dennis Butcher, Alan Williamson, Nigel Skeels

PRODUCTIONArt Director Peter BlackmoreProduction Controller Steve DrakeDesigners Jim Bowler, Matt BuhlerLayout Artists Tracey Walker. Adrian Dulwich,Paul Andrews, David HoltPublished by Maplin Electronics plc..P.O. Box 3, Rayleigh, Essex, SS6 8LR.Tel: (01702) 554155. Fax: (01702) 553935.Lithographic Reproduction byPlanagraphic Studios. 18 Sirdar Road. BrookRoad Ind. Estate, Rayleigh, Essex SS6 7UY.Printed in the United Kingdom bySt Ives (Caerphilly) Ltd.Mid -Glamorgan. CF8 3SU.

MANAGEMENTPublications Manager Roy SmithDevelopment Manager Tony BricknellDrawing Office Manager John DudleyMarketing Manager Vic Sutton

ADVERTISINGJackson -Rudd & Associates Ltd..2 Luke Street, London, EC21 4NT.Tel: (0171) 613 0717. Fax: (0171) 613 1108.Advertisement Managers Eric Richardson,Jim Slater

UK NEWSTRADE DISTRIBUTIONUnited Magazine Distribution Ltd.,16-28 Tabernacle Street, London EC2A 4BN.Tel: (0171) 638 4666. Fax: (0171) 638 4665

I:Copyright 1994 Maplin Electronics plc.

Copyright Al mater -al a suttee to worldwide copyright protection. and reproducoon

mrtattn in whole a part a boxes* fattdden Permission to reproduce pulled awlboard Layouts corkiw.ialy or marketing of lets must be sough from the publisher.

Advertisements: West every reasonable precaLton a undertaken to protect the

nterests of readers by ensuring. as far as possble. Sat aderteerrera metre in the

cunen rived Elea aria are bona fide the atisher and seal d he mejszne Carreldive any undertaicogs m respect of statements a darns made by atartienk Maderon punted page a a loose rise:. Readers who have reasonable giants to before

star they nave been reeled are advwed to canted Ter eel Tradng Standards ORce.

Editorial: The news d individual contnbutors authors are en neceseady hose eterther the peer eckta Where errors occur carecbas lei be publeved assoon as possble attenvards

Publisher's Statement Maar Eleceunw PC take al reasonable are to preyedmury. loss a damage of any knd bang caused by any matter published in Electrones.

Save radar as prohibited by Engeh law lobby of every kid ridu%g negligenne%darned as regards any person on respect tiered.

Project RatingsProtects presented (n Trs de% are rated on a I to 5 for ease a dein" ofconstruction to help you decade %fathom a Ohm your POrletructkpn capabdtes before

you undertake the protect. The ratogs are as follows

Serge to buai and undereand and subtle fa absolute begrrers Baseof 'cols repured (e g solderng iron. We %NM piers. me stapes and

screwdriver). Test gear not rewired and no setting -Lc needed

Easy to turd. but rot sisals for absolute begmers. Sane test gear ie g

milometer) may be required. and may also need setting -Lb a testing

Average Sane sell in construcoon a more extensve setong-up required

Advanced. Fatty hip la el of ski n constructor. speaalised test gear asetting -iv may be required

Candler high level ot sloe in corisaucbor. geoaksen test gear may be

recurred Catetructon may pram complex venng Reconvnended for skilled

conenctirs only.

Ordering InformationKes. carporents and products stocked by mon can be easily Maned n a lumberof ways

Vat your local Magri stare. where you lei find a wide range of electronic products.

If you do not know where your nearest store s. role to the advert n this rive or Tel

101702) 552911 To avoid dsapponment when Penang to purchase products from a

Maple store. PuStomers are advised to check avalabity before raveling any distance

Wnte your order on the form tainted n this issue and send it to 1.4apin Electronics. P 0

Box 3. Rayecfi Essex. SS6 BLR Payment can be made tong Cheque. Pose) Order.a Creit Card.

Telephone your order. cal the Mapir Edema Came Card Hoene on (01702)554161

you have a personal =Cuter equipped with a MOOEM. del up Maoist's 24 -has

wane database and ordering service. CashTel. CashTel supports 300, 1200- and

2400teud MODEMS usnig CCITT tones The format is 8 data brts. 1 stop bit no panty.

ful duplex with XoriXolf handshaking. Ai existing Customers with a Map% customer

number can access the system by sin* dating (017021 552941 If you do not have a

customer n.mber Tel 01704 552911 and we we Nippy issue you with onePayment can be made by aell card

if you have a lone del (DTMF) telephone a a pocket tone dialer you an access our

computer system and place orders dreaty onto the Mean computer 24 hours a day by

simply ciaang 1017021 556751 You lei need a Madan customer number and a personal

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narrow a a PIN nattier Tel: t01702) 552911 and we lei Imp* ssue you with one

Overseas customers can place orders through Mark Export. P 0 Box 3 Rayleigh.Essex. S56 8LR. England: Ter .441702 554155 Eat 326 a 351: Fax. .44 1702 553935

Alternanvery contaa your nearest overseas Mapin Agent see the advert a the issue.

Full %mils of al of the methods of ordenng from Magn can be found in tie currentMackin Catalogue

SubscriptionsFull area of how to subscribe may be found on the Subscripeon Coupon in this issue.

UK Subscroon Rate £21.9612 maths. £10 969 months

PricesPrices d products and services available from Maphn. shown in Vas ssue. include

VAT at 175' (except (terns marked NV whot are rated at 0'.) and are valid Damien

210 Septerrter 1994 and 26th February 1995 errors and amens excluded. Prices

shown in rr. intlydv -ail order postage and handing Merges. Aka are breed at the

Technical Enquiriesif , - . to Maar meas. components and productsfeatured Eectroncs me Customer Technical Services Depaonent may be ableto help. You can obtain hop in several wayss, over the phone. Tel (01702) 556001

behmen 9 and 5.30pn Monday to Fnday. except public hoiclays. by searing a

head. Fax: 101702) 553935: a by writing to Customer Tednical Services. MaplinElectroacs plc . P 0 Box 3. Rayleigh. Essex SS6 8LR. Dont forget to include astamped set -addressed envelope if you want a written reply! Customer Technical

Services are unable to answer enquiries retaeng to tiled -party prodixcts a components

`Get You Working' Servicett you get oornacc and you are unable to gel A working. take

advantaged the Service This service is available fa alMayer kits and craws wrth the excepoon 'Data ries': protects era built on Maierready etched PCBs: protects bat with the meaty of components not suppled by

kitapkn. Cram Maker dies Mn Circus a other similar building blodi and appeaser

orcuts To take advantage d the sew. return he corrplete ktt to ReturnsDepartment Capin Bedroom plc . P 0 Box 3. Rayleigh. Essex. SS6 SLR Enclose

a cheque a Postal Order based on to price of the et as shown n the table below

(rrerenurn £17).1109 fault is due to any after On our part. the project val be repaired

free of charge if the fault a due any error on yaw pad you we be charged the standard

semcng cost pea parts

KR RAVI Price Standsrd Servicing Costicto£2493 £1700U5.00 to £39.99

£40.00 to £59.99

£50 00 to £79.99

NO 00 to £9999

£100.00 to £149 99

Over £15000

Readers Letters

£24.00

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csozoE60.00

£60.00 rninimurn

We very much regret that the edtorier team are unable to answer oschnral queries or

arty kod homier. we are very ceased to mom rat corrnents axis Electrones andsuggestions la prates, bares. sees. etc. Due to the sheer wax* of enters reCerved.we are unlatinaley unable to reply to every letter. I every letter a read - you

one and war a gm* axis:toed. Letters of partetiar Merest and signficance maybe plished at the Editors ciso-seon. Any correspondence not nterced for pleicaommust be dearly marked as such.

Write to. The Eder, Bemuses - The Mapin Msgazne. P.0 Box 3. Rayleigh Esse.SS6

2 Maplin Magazine October 1994

TECHNOLOGY

'Local loop' is a telephone term, relating to theindividual connections between a user's phoneterminal and a public telephone operator'slocal exchange. It is. in fact, most users' singleconnection to the national and internationaltelephone system. In all but a handful of cases(only about 3% of installations), local loopsare wire circuits run under or overground,and are maintained by and rented from BritishTelecom. This is partly historical, in that mosttelephone installations were originally installedin the days when the only telephone operator,of course, maintained all local loops (as wellas all national loops for that matter). But, itis partly technical, in that the means to createnew types of local loops were non-existent.Basically, to date. if you want a telephone,you have little option other than to use BTslandline local loops.

This is all set to change withradiocommunications. Many telephoneoperators (BT included) are developingradio -link methods of providing local loopservices. These are all digitally -based, usingcode division multiple access (CDMA) ortime division multiplex (TDM) methods tojoin large numbers of customers to the localexchange. At least one company (lonica) isnot a telephone operator at all, but is merelyconcentrating on providing a system ofradio -link local loops which customers canuse, merely, to gain connection to thetelephone operator of their choice (whetherit be BT, Mercury, British Waterways,British Rail Telecomms or whoever). Whileits not going to happen overnight (earliestestimates of widespread availability are1996), it is inevitable that it will happen,and BTs effective stranglehold of the wholetelephone system could finally be broken.

Data DayCommunications between humans arecomplex enough (as we have just seen),but communications between computersis a much more vague area of electronics.You only need to go regularly into your localbuilding society to know that. The number oftimes the 'computer is down', whenever you

with Keith Brindleyneed money, is often beyond belief. After all,the building society's branch office is merelylinked by private telephone line to headoffice, and computers are supposedly set upto send and receive data to and from eachother. Most times, of course, the computerscommunicate effectively and well. It's just thetimes when I go into the building society thatthe link happens to be broken!

More seriously, though, somethingrecently (well, last month's columns actually- see later) has cast my doubts on howeffective two-way data communication reallyis. You see, the expected communicationsrevolution, in the shape of the informationsuperhighway, is almost upon us. For this towork well, though, a significant improvementin the way data is handled betweencomputers will be required, and this meansa significant improvement in two main areas.First, the data has to be standardised tothe extent that devices can understand eachother. All computers (PCs, Amigas, BBCs,Apples and so on) must be able to transferinformation between each other so that theycan understand and reuse transmitted data.Either the data has to be standardised to theextent (preferably) that all computers ofwhatever type can use the data directly, orthe computers must have in-built software tobe able to translate incoming data into anunderstandable format.

This is not as easy as you would expect.Take existing standards of data, such asencapsulated PostScript (EPS) or taggedimage file format (TIF, or TIFF depending onyour computer type). These are supposedlystandards, yet try swapping an EPS or a TIFfile between two different computingplatforms without translation software, andyou will nearly always have problems.

However, even if your data is standardisedto the extent that the two devices can easilyswap and understand each other's datafiles, there is the problem of transferringthe information between them, whether bywire, through optic fibre, or over the air. Theproblems of standardisation here are evengreater. Each transmission medium (copper,glass or ether) has its own transmission

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properties and, hence, transmissionproblems.

Yet the problem is not insurmountable.Data, after all, in computer terms is merelya collection of logic 1s and logic Os. It is onlyhow those logic levels are arranged whichdefines whether the data forms an EPS ora TIF file, for a PC or an Amiga for example,or for whatever file on whatever platform.Rearrange the logic levels and the data canmean whatever you want it to, to whatevercomputer you choose. There is no endto this way of thought. Given any twocomputers and proper translation software,any file should be properly transferablebetween them and, after transfer, beproperly usable too.

This has got to be the case, of course,if any vision of the information superhighwayis to work. Can you imagine anybody'sdream of the information superhighwayturning into reality given today's computercommunications? A case in point isTechnology Watch - or, to be more specific,last month's Technology Watch. Last monthwas a bit hectic. I was booked to go on myholidays, but had rather overbooked myworkload. As a consequence, TechnologyWatch was a little late, and fast approachingthe final deadline to get it to the Maplineditorial offices in time for press. To preventpostal delays, the Editor suggested I sendthe copy in by modem. Nonchalantly, I saidyes, and we began the 20 -second processof uploading the file from me to him. Fourand a half hours later, we succeeded, withmy phone bill somewhat the worse for wear,and his frustration only matched by mine.Even with the literally thousands of pounds -worth of ultra -modern computer equipmentat Maplin, and my half -crown's- (12'p)worth at home, we had the greatest ofdifficulty in exchanging the data -bits betweenus in anything like a correct manner, withouta considerable amount of nail-biting,swearing, chiding, goading and hacking.What chance the information superhighway?

The opinions expressed by the author are notnecessarily those of the publisher or the editor.

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by Douglas Clarkson

Photo 1. Launch of the HubbleSpace Telescope on 24 April1990 by the Shuttle Discovery.

Shoemaker -Levy 9 Jupiter Collisioncopyright 1994 Julian Baum.

all rights reserved, used by permission.

The recent successfulshuttle mission, to correctthe flawed optics of theHubble Telescope, has putback on track the world'smost ambitious astronomyproject. Success has,however, come slowly.The funding of the Hubbleproject named after Edwin PHubble, one of the greatestobservational astronomersof all time, was only prisedfrom the US Congress afterEuropean co-operation wassecured. After having takena decade and a half todesign, the launch of theHubble observatory wasdelayed by four years withthe explosion of the shuttleChallenger in1986.

IT was not until 24 April 1990 that theshuttle Discovery finally launched theF lubble Space Telescope (HST) into orbit as

shown in Photo 16The next day the telescopewas released from the payload bay. Photo 2shows the view from the shuttle as the billiondollar satellite is released by the robot arm. Itwas one of the major scientific hiccups, how-,ever, when it was discovered that the optics ofthe main primary mirror were flawed. Thusinstead of clear crisp pictures being availablein 1986. these have onlit begun to be obtainedlate in 1993.

Aperture

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As light from the stars travels through theturbulent air of the atmosphere..wavefronts oflight which have travelled across countlessmiles of space become distorted due to therapid variation of density of air in their path.This tends to 'spread' light so that there is alimit to the size of an object which an earthbased telescope can resolve (no matter thequality of the optics used). This limit is approx-imately I arc second. By observing above theearth's protective atmosphere, this problemwould be removed and a resolution of around0.1 arc second was expected to be achieved.

Figure 1. Ray diagram showing opticalimage error created by the primary mirror.While light from the centre of the image isfocused correctly, light from the peripheryis brought to a focus behind the imageplane - resulting in a blurred image.

Focus forcentre of mirror

Focus foredge of mirror

There has been a development recently ofso called 'adaptive optics where an array ofground based mirrors can be moved rapidlyin real-time to minimise distortion due to suchatmospheric turbulence. It is unlikely, how-ever, that such a mechanism will achieve theresolution available with the HST. Also, mostultraviolet radiation is absorbed by the atmos-phere - so that most UV observations must bemade from space.

The other advantage of Hubble is that it canbe pointed with great selectivity to any part ofthe sky. This is in contrast with earth basedtelescopes which tend to look at the northand south hemisphere star fields only.

A considerable degree of intelligence wasbuilt into the satellite in order to allow it to tar-get specified star locations as part of itsobserving mission. Photo 3 indicates a sectionof Hubble's Star Guide Catalogue which isused to locate specific star fields. The cata-logue contains degrees of brightness andpo'sitions for approximately 19 million objects,15 million of which are used for aiming andstabilising the I lubble Telescope. In the image

Top left: Photo 2. Deployment of the HubbleSpace Telescope as it was released fromthe payload bay on 25 April 1990. The goldcoloured solar panels generate 2kW of power.

Bottom left: Photo 3. Extract from tipGuide Star Catalogue - Hubble's Guide tothe Universe. This Contains data on 19 millionobjects - 15 million of which can be us4dfor aiming and stabilising the HST.


shown. the zones on the periphery containthe guide stars with the target area located atthe centre.

The telescope has to be carefully manipu-lated during each 90 minute earth orbit inorder to achieve its photographic mission.During each half of the orbit the telescopehas to be fixed to guide stars, locked in posi-tion for observation and then calibrated: forabout half of each orbit the space telescopecannot take images.

Data from the Hubble Telescope is firstbeamed to the Tracking and Delay Satellite ingeosynchronous orbit. From there it isbeamed to a ground station at New Mexicoand bounced off a domestic communicationssatellite for routing to space controllers at theGoddard Space Hight Centre in Greenbelt.Maryland. Then the data is relayed to theSpace Telescope Science Institute inBaltimore where it is processed byastronomers.

Fuzzy PicturesFigure 1 shows Hubble's main telescope,comprising of a large primary mirror (dia-meter 2.4m: 94in.) and a smaller secondarymirror. The tolerance of grinding the main

HUBBLE SPACE TELESCOPEFAINT OBJECT CAMERA

COMPARATIVE VIEWS OF A STAR

BEFORE COSTAR AFTER COSTAR

Supernova 1987A In UltravioletHubble Space Telescope

Faint Object Camera

Sep 1992

Dec 1991

May 1993

Jan 1994. with COSTAR

mirror was highly critical. Unfortunately theouter surface of the mirror was ground0.002mm too flat. Thus while light reflectedclose to the centre of the main mirror wasbrought to a focus in the desired focal plane.light reflected from the outer area of the mainmirror was brought to a focus behind the trueone. Thus even a bright star would appear asa bright feature surrounded by rings of lightwhich were converging on an image planebehind the camera focus plane.

The Servicing Mission(STS -61)From the moment that the problems withHubble became apparent. the significance ofthe first planned servicing mission began to

; grow in importance all the time. Initial con-cerns about making the best decisions aboutthe repair mission led to the formation of anInternational Strategy Panel. This eventuallyset the policy for the process of repair of thesatellite. The key element of the mission wasto be the deployment of COSTAR (CorrectiveOptics Space Telescope Axial Replacement) -a package which would correct the light pathsfrom the faulty mirror. In the process the HighSpeed Photometer Experiment would have tobe removed from Hubble.

While the key problem with Hubble was thedefective mirror, the telescope was demon-strating other problems. Late in 1990, one ofa set of gyroscopes failed with another failingin the following May. Such gyroscopes wereused in the positioning of the satellite duringobservational manoeuvres. A fault also devel-oped in a computer memory circuit. A fault

Top right: Photo 4. 'Before' and 'after' picturestaken with the Faint Object Camera (FOC).Prior to the service mission, images of brightobjects were surrounded by a 'skirt' of light.With the COSTAR objects in place. true imagesof resolution 01 second of arc become visible.

Bottom left: Photo 5. The improved Hubblepictures allow the features of the 1987Asupernova to be clearly seen as separate fromthe previous interference from companion stars.

Maplin Magazine October 1994

M100, a Spiral Galaxy in the Virgo ClusterHubble Space Telescope/Wide Field Planetary Camera

WFPC-2: Wide Field Camera

WFPC-1: deconvolved

WFPC-1: Wide Field Camera

Palomar 5m on a good night

had also been observed in the design of thesolar panels on Hubble. Rapid temperaturechanges of the supports of the panelsoccurred as the telescope made transits fromday to night and night to day while in orbitsome 370 miles above the earth. This causedthe telescope to shudder and so preventobservations at such times from being made.

A series of magnetometers to sense theearth's magnetic field were used on Hubble toassist is stabilising the satellite during obser-vational periods. When problems with thesebegan to occur it was decided to replace themall by bolting on additional devices over theoriginal units. The astronauts obtained train-ing in their replacement by visiting the

M100A Spiral Galaxy in the Virgo Cluster

Hubble Space TelescopeWide Field Planetary Camera 2

Upper panel shows the innerregion of M100 imaged withthe Planetary Camera atfull resolution.

Image at left shows a mosaicof the three Wide FieldCameras plus the PlanetaryCamera.

Museum of Air and Space in Washington andsimulating repair activity on a full-scale replicaof the telescope. Other service procedureswere extensively tested in simulated weightlessconditions under water.

During the extensive training of the astro-naut team, it became more and more obvi-ous that a three day mission would be unlikelyto provide enough time to complete all therequired service procedures. At least five dayswould be required. There was even a con-sideration of two separate shuttle missions tocarry out the repair. It was obvious, however,that the Endeavour shuttle would be the onlyone of the available four shuttles capable ofsupporting a five day mission. It was thus atriumph of 'going for it that the Endeavourshuttle blasted off on 2 December 1993 intothe dark night sky.

The first two days were used for the serviceof non -imaging systems such as the gyro-scopes, electronic fuse box and faulty solarpanels. One panel would not wind up and sowas allowed to 'float free from the satellite -another piece of (expensive) space litter. Onthe third day the Wide Field Planetary Camerawas replaced and new magnetometers fitted.On day four the COSTAR module was suc-cessfully deployed by Kathryn Thornton. Thepackage with a mass of over 200kg slid effort-lessly into the heart of the Hubble Telescope.There was considerable anxiety that the deli-cate mechanism would be damaged duringinstallation when its protective covers wereremoved. Fortunately all went well: at thisstage the Hubble s computer system wasupgraded to a 386'. and improvised covers forthe magnetometers added.

On the last day. repairs were made to theSolar Array Drive Electronics. This was whenthe famous 'hunt the screw' manoeuvre tookplace as Storey Musgrave was securing thedelicate mechanism. Work was also under-taken on the electronics of the Goddard HighResolution Spectrograph. The time came atlast when Endeavour released its captive holdon the satellite and Hubble was again alonein orbit with all the visible universe to scan ingreater detail.

The Party Back HomeScientists were apprehensive about the suc-cess of the mission. It rapidly became clear,however, that the corrective optics had beensuccessfully deployed. Slight adjustmentswere made to the sets of mirrors to optimisethe quality of the final images. An embargowas kept on released images until a pressconference on the 13 January, at which time

Top left: Photo 6. This sequence of imagesindicates how Hubble's improved opticsallows 'new. stars to be observed in thisexample in the M100 spiral galaxy in theVirgo Cluster: (top left) the best that Hubblecan now display: (top right) pre -COSTARwithout image enhancement: (lower left)pre -COSTAR with image enhancement:(lower right) Mount Palomar on a goodnight.

Bottom left: Photo 7. Spectacular detail ofstars in the M100 Spiral Galaxy taken withred. green and blue filters to give a true colourpicture. The pinkish regions are huge clouds ofglowing hydrogen gas and identify sites of newstar formation. Individual stars can now easilybe resolved in this galaxy thought to be some40 million light years distant.

October 1994 Maplin Magazine

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Hubble mission will be to store and processthe vast sets of data that will be created by theHubble Project.

The volume of data from previous NASAspace probes including even Magellan will bemore than dwarfed by the sheer extent of theHubble mission.

CD-ROM would be an ideal of distributingimages from the Hubble Telescope, andwould pay considerable dividends in terms ofawakening educational interest in science andtechnology.

The problem with the optics, however, hasgiven considerable insight into the whole prob-lem of 'how far' we can see into the universe.This limit has been dramatically extended.

Astronomers for the present are 'content'to get on with the process of observingand assimilating the data being obtained andnot worrying overmuch about the next leapforward.

Observations are only going to improve ifbigger and better telescopes are built in spacethough with present space technology theseare going to remain expensive. However, it willtake some time to scan the universe with theresolution of the present Hubble Telescope.

The proper work of the space telescope hasreally only just begun. NASA, however, afterhaving been shaken to its core by the prob-lem with the mirror, is perhaps more cautiousin its public relations statements and is exer-

cising great care over the release of newimages from the Hubble project. The publicrelations aspect of the project is managed bythe Space Telescope Science Institute inBaltimore. One of the excellent features ofNASA space projects is the relative ease ofaccess to picture material - they are only aphone call away.

Point of Contact:Space Telescope Science Institute,3700 San Martin Drive,Baltimore,MD 21218, USA.

Tel: +1 410 338 4707.

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6, Maplin Magazine October1994

Acorn Go OnlineThe Acorn Computer Group hasannounced its entry into the interactivemultimedia market with the launch ofOnline Media, a new operation within theAcorn Computer Group.

Online Media. based in Cambridge,aims to exploit the technical expertiseof the Acorn Computer Group to bringaffordable interactive multimedia prod-ucts and services to the huge marketemerging from the information super-highway.

To achieve this aim, Online Media willbe working with major players in thisarena. Examples of current partnersinclude: Olivetti; Advanced RISCMachines: Advanced Telecommuni-cations Modules: Northern Telecom;News International: Anglia Television;Oracle: and Cambridge Cable.

Online Media's revenues will deriveinitially from the supply of set -top boxequipment. Longer term. Online Mediais looking to develop additional revenuestreams in the areas of design licencesales, authorising software sales, arange of on-line services and consul-tancy.

The first of the new company's prod-ucts, the Online Media set -top box, hasalready generated interest from largecommunications providers such asNorthern Telecom and Alcatel. In thefuture. Online Media plans to developproducts relevant to the broader multi-media market. These will include soft-ware for creating services running onset -top devices. services to support thecreation of new applications for inter-active use and personal multimediadevices. Contact: Acorn Computers.Tel: (0223) 254 479.

Low Voltage LogicTwo new 3-3V CMOS logic families fromTexas Instruments (TI) enable designersof portable. battery -operated and otherlow -voltage systems to match logic priceand performance to the needs of lowand mid -range applications such astelecom equipment. notebook personalcomputers and point -of -sale systems.Without sacrificing effective system per-formance, the families give designers amigration path away from traditionalhigh -cost 3.3V logic or 5V logic.

TI's low -voltage (LV) logic familyaddresses the relatively low -perfor-mance requirements of cost -sensitiveapplications such as those in palmtop

and point -of -sale systems. As the mostcost-efficient of all TI's low -voltage fam-ilies. LV-HCMOS can help you createinexpensive portable and desktop PCsand other consumer items.

The low -voltage CMOS (LVC) familybest fits in mid -range performance appli-cations. LVC's meet 3.3V system needscomparable to those the '74 familymeets in 5V systems. The 0.8 micronCMOS LVC family is made up of gates,medium -scale integration (MSI) flip-flops, latches. multiplexers and eight -bitand Widebus devices with a low stand-by power consumption rate of 20pAand a worst -case propagation delay ofseven nanoseconds. Contact: TexasInstruments, Tel: (0234) 223511.

Laptop VisionPCMCIA wise guys Portable Add-onshave recently complemented their rangeof PCMCIA-compatible products with thelaunch of the Plug -n -Scan 256 handheldscanner, which can be interfaced to alaptop computer via the PCMCIA 2.0compliant ScanCard supplied.

The scanner, which derives its meagrepower requirements from the host com-puter via ScanCard, captures photo-graphic quality images with 256 shadesof grey at resolutions of between 100and 400 dots per inch. For versatility,greyscale. halftone and line art/textscanning modes are provided.

A TWAIN device driver is included withPlug -n -Scan to keep obsolescenceat bay. TWAIN (Tool Without AnInteresting Name) is a scanner applica-tion programming interface standard,originally developed by a consortiumincluding HP. Kodak, Logitech andAldus, to ensure that software andperipherals do not suffer from incompat-ibility problems.

The TWAIN driver, part of the ScanKitsoftware included in the package,includes automerge and intelligentmerge so that you can scan any size ofpicture or document with any TWAIN -compliant software.

Applications of Plug -n -Scan include.with the appropriate software, documentinput via OCR, barcode reading for stockcontrol purposes, the production of com-pany newsletters. and even the scan-ning of business cards by executives onthe move.

The Plug -n -Scan package retails for£299 + VAT and includes three sets ofsoftware - ScanKit, which providesTWAIN functionality and basic scannerfunctions; PhotoPlus. for retouching art-work and photographs; and Perceive, anOCR package. Contact: PortableAdd-ons, Tel: (0483) 440777.

Carrera Beats theBottleneckCarrera Technology has been doingclever things to try and speed upthe humble PC. The company hasannounced a new Enhanced IDE con-troller card dramatically improving DOSand Windows performance.

Enhanced IDE increases the speedand efficiency of PC operation throughgreater synchronisation of motherboardand hard drive. This raises data transferrates to around 10M-byte/sec -a 150%improvement on existing IDE drives anda figure that even expensive cachinghard -drive controllers would have diffi-culty matching. 'Fast' IDE controllers areonly capable of transfer rates of4M-byte/sec.

"IDE drives transfer data faster thanexisting IDE interfaces can accept it,leading to inefficient performance andfrustrated end -users". said Jez Deacon,managing director of CarreraTechnology. "The new card eliminates

this problem, matching the performanceof the best SCSI controllers but at aquarter of the price."

Carrera's optimism is echoed byWestern Digital, responsible for much ofthe early development of Enhanced IDE."New IDE controllers allow end -users totake advantage of the hugely increasedbandwidth now available". said PaulCalderwood, marketing communicationsmanager at Western Digital. "Anyoneusing the new generation of localbus systems will notice the differenceimmediately."

Another key benefit of Enhanced IDEis its ability to overcome the 528M -bytelimit imposed by the system BIOS underDOS. Enhanced IDE uses Logical Blockaddressing to give each data block aunique 'address' allowing users to rundrives of up to 8.4G -bytes. EnhancedIDE also has a secondary IDE connec-tor supporting up to four IDE peripheralssuch as back-up or CD ROM devices.

Contact: Carrera Technology,Tel: (071) 830 0486.

Direction LaunchPentium NotebookTo satisfy the needs of power-hungrymobile computer users, Guildford -basedPC manufacturer Direction Technologyhas just launched the 3600 notebookPC. Built around a 60MHz Pentiumprocessor, the machine is capable ofrunning Windows NT with a consider-able speed advantage over existing 486notebooks.

Features include two PCMCIA slots(one Type II card, and one Type III)for network adaptors or fax modems,8M -byte Ram (expandable to 40M -bytes),a TFT colour VGA display screen with32 -bit local bus controller and Windowsaccelerator functions, and enhancedpower management facilities to ensurelonger life from the NiMH battery pack.

The 3600 series case incorporates a for-ward -mounted 25mm trackerball, palmrests for long-term user comfort, and aremovable hard disk (120M -bytes as stan-dard) for security. A speaker and micro-phone are built into the case. for use withthe 8 -bit stereo sound card incorporatedinto the unit as standard.

A £319 docking station is also availableto convert the 3600 notebook into a high-performance desktop workstation withsuperb expansion potential - giving usersthe best of both worlds. Features includeextension speakers for the sound card. afast SCSI interface for hard disks and

CD-ROM, a monitor stand, serial andparallel ports, and a total of four ISA andVESA expansion slots.

During the next quarter. Direction willbe releasing a 90MHz Pentium note-book that makes use of the latest P54Ctechnology. In addition to this. Directionhopes to be launching a new era in note-book technology by combining the bestof both worlds - with a machine thatfeatures both Pentium and RISCprocessors in the same case.

Contact: Direction Technology,Tel: (0483) 454400.


AUDETEL TestTransmissions LaunchedA consortium of broadcasters, consumerelectronics companies and organisa-tions working with visually impaired andolder people have come together tolaunch a trial of AUDETEL, a systemwhich brings a new aural dimension totelevision.

For the next four months, selected pro-grammes on both ITV and the BBC willbe broadcast with a newly -developeddata signal which carries an audio com-mentary describing what happens inthe programme during the gaps in thedialogue. A special set -top receiver isnecessary to hear the commentary,either through headphones or a loud-speaker.

As part of the trial, 140 AUDETELreceivers have been installed throughoutthe country. mostly by the RoyalNational Institute for the Blind (RNIB), aspart of an audience research survey. Itsuse in the domestic environment will beassessed as a prelude to what maybecome a permanent service in thefuture.

Although originally developed for visu-ally impaired and older people to give

them greater access to television, theadditional audio commentary is intendedto appeal to other viewers who may wantto follow a programme while lookingaway from the screen to cook, do theironing or service the car.

The AUDETEL project received asignificant accolade from the RoyalTelevision Society in May winning itsprestigious Communications InnovationTechnology Award.

Audio description of television hasalready been on trial in Japan and theUnited States but the system used thereis unsuitable for Europe because of theincompatibility of transmission stan-dards. The objective of the consortiumhas been to develop a system using thesame signals used by the BBC's Ceefaxand Teletext on Independent Television.

Computerised audio description work-stations, developed within the consor-tium. are being used by both the BBCand the ITV programmes for all types ofdrama, wildlife documentaries and fea-ture films. One of the reasons for theexperiment is to determine the time takento prepare and record descriptions ofprogrammes and films of various typesand different degrees of complexity.

ITV and the BBC will broadcast about

Pp

three hours of described programmesper week during the four -month period tothoroughly test the system and thosepeople with receivers will be surveyed byRNIB about their experience of livingwith the system.

Further development work is under

way by Seleco, the Italian consumerelectronics company, to build threeprototype television sets with built-inAUDETEL receivers and Philips haveproduced a modified VCR which canrecord audio described programmes.

Contact: RNIB. (071) 6361153.

EPROM EmulationSquareWave Electronics has launchedMicroRom, a solid-state device thatemulates a conventional EPROM,removing the need to erase and repro-gramme an EPROM each time code ordata is changed.

The MicroROM EPROM emulator hasbeen designed into an 11mm-high plugmodule and is compatible with standardEPROM sockets. The module ishandled exactly like a conventional de-vice, and plugs into the EPROMsocket. Connection is made to a PC viathe centronics interface.

Once programming is complete andthe connector removed. the module canremain in circuit like a normal EPROMdue to its non-volatile storage capability.Consequently there is no need to blowan EPROM after programme develop-ment.

Consider it, perhaps, as an EPROMwith a built-in programmer and eraserthat has an unlimited number ofwrite cycles. Contact: SquareWaveElectronics, Tel: (081) 880 9889.

Fast Access SRAMIBM has announced a family of one -megabit SRAMs that are claimed to beamong the semiconductor industry'sfastest.

The new SRAMs allow system design-ers to optimise performance by reducingthe delays associated with accessingdata from memory. In second -level (L2)cache applications supporting high-per-formance microprocessors, operatingfrequencies up to 167MHz are achiev-able with a pipeline access time of fournanoseconds or flow -through accesstime of eight nanoseconds.

To optimise performance withPowerPC and Pentium microproces-sors. IBM is offering burst mode versionswhich enable the SRAM to support func-tions specific to these CPUs. Availabilityof one -megabit SRAM with burst modeoperation will alleviate system complex-ity and enhance overall performance ofRISC- and high -end CISC-basedprocessors.

Contact: IBM. Tel: (0705) 561000.

Hewlett Packard JoinHands with IntelHewlett Packard and Intel haveannounced a joint research -and -devel-opment project aimed at providingadvanced technologies for end -of -the -decade workstation, server and en-terprise -computing products. Thecompanies' efforts will encompass 64 -bitmicroprocessor designs, advancedsemiconductor processes and softwareoptimisation.

The companies said that by poolingtheir respective strengths, they expect tocreate powerful new solutions that willdeliver unprecedented performance tomeet the needs of users well into thenext century. The planned architecturewill maintain binary compatibility withboth companies' software bases.

The companies said they will worktowards optimising their fundamentaltechnologies to enhance their futureproduct lines. The work will be con-ducted jointly to take full advantage ofcomplementary capabilities in the twocompanies.

Contact: Intel, Tel: (0793) 696000.

7 Million for SchoolTechnologyA total of £7 million is being spent toenhance technology teaching in schoolsand colleges. The funding is being pro-vided by The Gatsby CharitableFoundation for the TechnologyEnhancement Programme (TEP), man-aged by The Engineering Council.

TEP aims to increase the capability ofstudents, aged 14 to 19. in technology,mathematics and science through amore practical, vocational approach.

Over the last three years schools andcolleges receiving an award through theprogramme have had to work in part-nership with local industry. In additionto the £7 million funding about £500.000sponsorship in cash has already beendonated by industrial partners to theirlocal schools and colleges involved inthe programme. Support from local com-panies in developing curriculum materi-als has also been given as help in kindby their engineers and by the donationof equipment and resources.

Contact: The Engineering Council.Tel: (071) 240 7891.

Scalable ATM ChipAT&T Microelectronics has introducedtwo components which will enablenetwork equipment manufacturers todesign competitively priced asynchro-nous transfer mode (ATM) switches orhub cards. Both the T7650 2 -by -2 switchand the T7652 ATM layer interface (ALI)chip architectures are scalable sothat network hardware can be easilyexpanded to meet growing demands.

When fully deployed. ATM will supportreliable and cost-effective transmissionof large amounts of data. video andvoice traffic over a common fabric oflocal and wide area networks at speedsup to one gigabit -per -second.

Contact: AT&T. Tel: (0344) 865927.

Better Pictures fromCitizenCitizen Europe has launched a 7.8in.Dual Scan Passive Colour LCD featur-ing Citizen's unique 'Chip -on -Glass'(COG) technology.

Citizen claim this innovative technol-ogy delivers significant benefits overconventional manufacturing techniques,which mount the chip -sets onto a boardor film. Instead. the Citizen chip -on -glasstechnology mounts the chips directlyonto the display glass. This allows dualscan addressing which doubles therefresh rate of normal passive matrixdisplays.

Dual scanning leads to clear, vividcolours. significantly faster responsetimes and excellent screen clarity -unsurpassed by any other products intheir class. Additional benefits includethe ability to manufacture thin. light-weight and compact products.

Based on passive matrix technology,the module provides three RGB640x480 VGA resolution. Illumination isprovided by a Cold Cathode FluorescentTube (CCFT) and light guide. The qual-ity and compact size make this productideally suited for the sub -notebook andportable market.

Contact: Citizen, Tel: (0753) 584111.

BT Unveils New DigitalSatellite NewsGathering ServicesBT's Visual and Broadcast Servicesdivision announced in May the develop-ment of a new range of digital, satellitenews gathering (SNG) services. at theannual Inter -Union Satellite OrganisationGroup (ISOG) meeting in London.

A key element of the new range of ser-vices is a prototype SNG communica-tions truck, equipped with an 8M-bitislink, able to act as a KU -band satelliteearth station. BT intends to launch thenew service within the next six months.

To complement the services offeredusing the new SNG services, BT willalso be launching a non-pre-emptible C -band digital TV distribution service onIntelsat 602. satellite 63' East. This willhelp to meet broadcasters' needs foroccasional use services in areas not yetcovered by a KU -band satellite footprint.The new service will enable transmis-sions to the UK from regions currentlygenerating a high volume of news sto-ries such as Rwanda. Yemen, SouthAfrica and Israel.

New digitally compressed SNG ser-vices will enable broadcasters to makegreater use of existing satellite spacecapacity. thereby reducing costs.

Contact: BT Visual & BroadcastServices. Tel: (071) 728 3409.


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Following the successes of LIVE '93.Britain's most exciting consumerelectronics event will be held this year atEarls Court for an extended period ofsix days. LIVE '94, which will take placebetween Tuesday 20 and Sunday 25September 1994, is being run by theexhibition arm of News International,publishers of the Times, the SundayTimes, Today, the Sun and the News ofthe World.

The show will encompass the wholespectrum of entertainment. communi-cations and home office equipment, andexpects over 200,000 people to gain ahands-on preview of the consumerelectronic goodies to be launched duringthe Christmas season Apart frommanufacturers' displays, LIVE '94 willplay host to a number of seminars andfeatures presented by experts in theirfields - ranging from the home office toeffective camcorder use.

Television will be well -represented -by live broadcasts from both terrestrial

and satellite networks. All the majormanufacturers will be demonstratingsatellite receivers and TV sets in allshapes and sizes. Video, photographyand - that latest buzzword - homecinema will also be making a strongpresence, and the public will be able togain hands-on experience of the latestVCRs, camcorders, video editingequipment. Dolby Surround decoders.still cameras and accessories.

If home computers, games machinesand the multimedia experience are yourbag, then LIVE '94 won't let you down.Apple, Commodore. Acorn. Microsoft,Philips CD-i, 3D0 and Nintendo areamong the big names scheduled toappear. Audiophiles. meanwhile, will findtheir spiritual home in 'The Real Hi-FiExperience', which will includemanufacturers of the calibre of LynnProducts, KEF, Pioneer, NAD, Quad.Kenwood and Marantz. Special areaswill be dedicated to in -car audio, and -for those who like to make their own

music - instruments from companies likeKorg, Roland, Yamaha, Impact andFender.

Of interest to many Electronicsreaders will be the communicationsstands, where amateur radioorganisations such as the RSGB, Icom,Yaesu. Lowe Electronics and Maplinaffiliate Waters & Stanton will beshowing their wares. BT and Mercurywill also be at LIVE '94, together with the

latest analogue and digital cellularphones.

And, of course. Maplin will be theretoo. on stand 3360. The 1995 Catalogueand copies of Electronics will beavailable on the stand, together with avaried selection of products from thenew catalogue on show. These willinclude some recent additions to the Foxand Vixen home and security ranges,car audio equipment and a wide rangeof projects for the electronics enthusiast- such as the acclaimed Millenniumvalve power amplifier. We look forwardto meeting you!

LIVE '94 ticket prices have beenreduced since last year, with a singleadult ticket costing £4 on weekdays, or£7 at weekends. Tickets for children are£3, while a £16 family ticket admits upto two adults and three children. Allchildren up to the age of 16 must beaccompanied by an adult.

For booking information, NewsInternational has set up a hot line on(0891) 500103. (39p/minute off-peak:49p/minute at all other times). Ticketswill also be available on the door, and atall stations on the London Undergroundsystem for the four weeks prior to theevent. Earls Court is easily accessible bytube, via the District and Piccadilly Lines.

Ten lucky Electronics readers neednot pay a penny for admission, and canbring a friend along for free! Just fill in thecoupon on this page and send it to 'LIVE'94 Competition'. P.O. Box 3. Rayleigh,Essex SS6 8LR, to arrive between 5 and12 September. The first ten coupons tobe drawn from the Editor's soup bowlon 13 September will receive a weekdaydouble ticket by return of post, but will benotified by phone first. Please note thatall coupons received outside the statedperiod will be disqualified. Good luck!

Name

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Daytime telephone numberIf you do not wish to cut your copy of Electronics photocopies or post -cardsare acceptable.

DIARY DATESEvery possible effort has been made National Hall. London. Tel: (071) 370 26 September. Top Band DF 24 October. ORP by Norman Fieldto ensure that the information 8207. Construction by Geoff Foster G8UKT. G4L0F. Stratford upon Avon andpresented here is correct prior to Stratford-upon-Avon and District District Radio Society.

publication. To avoid disappointmentdue to late changes or amendments

8 September. Hi-Fi Show, Ramada,Heathrow, London. Tel: (081) 6862599.

Radio Society. Tel: (0789) 740073. Tel: (0789) 740073.

please contact event organisations toconfirm details.

3 September. Wight Wireless Rally,National Wireless Museum, Arreton

11 September. The 13th Lincoln HamRadio Festival - Hamfest, LincolnshireShow Ground, Lincoln. Tel: (0522)525760.

26 to 28 September. The SixthInternational Conference on RadioReceivers and Associated Systems,University of Bath. Tel: (071) 836 0190.

26 to 29 September. Business

26 to 27 October. Instrumentation,Sandown Exhibition Centre,Sandown. Tel: (0822) 614671.

26 to 28 October. PEVD '94, PowerManor, near Newport, Isle of Wight.Free admission and free stands to 11 to 24 September. Plasma Light & Computing 94. Grand Hall, Olympia,

London. Tel: (071) 486 1951.

Electronics & Variable Speed DrivesConference, Institution of Electrical

both public and traders. Tel: (0983) Sound Show, Earls Court, London. Engineers, London. Tel: (071) 240567665. Tel (071) 244 6433. 26 to 29 September. Mobile Business 1871.

4September. Applied Optics & Optic- 12 September. Opening Night and Show, Olympia. London. Tel: (071)486 1951.

Electronics Conference, Institute of Dayton '94 by Herb Asmussen 1 November. Talk & Demonstration

Physics, York. Tel: (071) 235 6111. OZ7SM and George Beasley G3LNS.Stratford upon Avon and District Radio

4 October. Talk on Electrical Safetyand Regulations, Sudbury and District

on First Aid by St. Johns Ambulance,Sudbury and District Radio Amateurs.

4 to 6 September. European Society. Tel: (0789) 740073. Radio Amateurs. Tel: (0787) 313212.Tel: (0787) 313212.

Computer Trade Show, BusinessDesign Centre, London. Tel: (081) 742 20 to 22 September. CEMEX '94 - 4 to 6 October. EDI 94. ICC, 1 to 3 November. Windows Expo,2828. Circuit Equipment Exhibition, National Birmingham. Tel: (081) 742 2828. Olympia. London. Tel: (0256) 381 456.

Exhibition Centre, Birmingham. Tel:5 September. EMC - 9th (0705) 665133. 10 October. Inside your PC by Martin 5 to 6 November. Eighth North WalesElectromagnetic CompatibilityConference, Institution of Electrical 20 to 25 September. Live '94. The

Rhodes G3XZO. Stratford upon Avonand District Radio Society.

Radio and Electronics Show,Aberconwy Conference Centre & The

Engineers, Armitage Centre,Manchester. Tel: (071) 240 1871.

6 September. Annual General

Consumer Electronics Show, EarlsCourt, London. Visit the Maplin stand,number 3360, and have a chance tospeak to us in person. Tel: (0891)

Tel: (0789) 740073.

11 to 13 October. Voice 94. Olympia,London. Tel: (0733) 575 020.

New Theatre, Llandudno. Tel: (0745)591704.

Meeting at Wells Hall Old School,Sudbury and District Radio Amateurs.

500103.15 to 16 October. Warley National

Please send details of events forinclusion in 'Diary Dates' to: The

Tel: (0787) 313212. 25 September. SDX Cluster Support Model Railway Exhibition, National News Editor. Electronics - The

6 September. Energy '94, OlympiaGroup Junk Sale, Maryhill Road,Glasgow. Tel: (041) 638 7670.

Exhibition Centre, Birmingham.Tel: (021) 558 8851.

Maplin Magazine. P.O. Box 3,Rayleigh. Essex SS6 8LR.


2PROJECTRATING

ciommisftesm

Jimi lieridrix at the Isle ofWight Festival 1970. 1994The Hilton Deutsch Colechon.

Design by Nigel SkeelsText by Nigel Skeels andRobin Hall

Yes, another Wah Wah pedal that uses the infamousLM 13700 transconductance op amp. Unlike otherdesigns that failed to reach the benchmark of theoriginal Jim Dunlop Wah Wah Pedal, the PsychedelicWah Wah surpasses it with a number of advantages.These include adjustable resonance, which determinesthe subtlety of the effect and adjustable range, usedfor both electric and bass guitars; in fact, on any electric

music instruments such as keyboards and electricviolins. The circuit also features a compander

that reduces noise in the circuit and improvesthe harmonic output content, which

makes for a very warm rich sound.

61teW A 11

VI Pi 11PEDAL

The PsychedelicWah-Wah Pedal

readly for action!

Ideal foruitop ander musical

instruments


FEATURES* Powered from 9V 12123 battery or

regulated power supply* Regulated reference to prevent low

battery voltage drift* compander to minimise noise* Traditional Wah Wah sound (without

the crackle)* Rich warm harmonic content* Minimal adjustment* IC design, no laborious coils to wind* Adjustable resonance and range* Economically priced with

unbelievable performance

R the uninitiated, the sound effectof a Wah Wah pedal Is synonymouswith many recordings of bands of the

seventies. For an example listen to 'VoodooChile' by JIml Hendrix, the track opens witha prime bit of wailing. The Wah Wah actsas a kind of tone boost control, and movingthe pedal adjusts the frequency point atwhich the boost occurs.

Rhythm or lead guitar usually playthrough the device. When playing rhythm,the pedal is moved in time with the 'strum',and when playing lead, extra expressiveabilities become available enablingalmost 'infinite sustain' withoutscreaming feedback.

IN

InputBuffer

VoltageRegulator

SPECIFICATIONPower supply:Current consumption:Maximum boost @Minimum frequency:Maximum frequency:

+9V DC14 7mA20d8901-1z

20khz

Assembled PCB mounted in foot pedal box.

Compressor

Pedal

Om.

LM13700 DualTransductance

OP Amp

OUT

Expander

Figure 1. Block diagram of the Psychedelic Wah Wah Pedal.


O NE

Close-up of assembledPsychedelic Wah WahPedal PCB.

Circuit Description

The block diagram of the Psychedelic WahWah Pedal is given in Figure 1, this as wellas the circuit diagram in Figure 2, will helpillustrate how the circuit operates. The inputto the Psychedelic Wah Wah is fed to theinput stage of IC1 an LF351 op amp. ThisIs a low -noise J -FLT device and is used tobuffer the input signal, thus preventing therest of the circuit loading the input. Thereis no gain in this part of the circuit as seenby the direct link between the inverting

input and the output.The next stage in the circuit is IC2

.e an NE571. This is set up to compressthe signal by the ratio of 2 to 1 before

It is fed Into IC3 the LM13700 dualtransconductance amplifier. This is where

the magic takes place, and is used hereto provide two voltage -controlled tunablepeaks in the audio band. This is achievedby placing both parts of the LM13700 inseries with each other. The voltage changeis obtained by using a 100k linear pot; thewiper of this pot is then connected to bothof the amplifier bias inputs via R19 andR25. These two resistors could be of thesame value thus causing both of the peaksto be on top of each other, but this is notwhat we are after, as the overall peakwould be too narrow and not give us thatcharacteristic Wah Wah sound that is sodesirable. With this in mind, the peaks areslightly separated thus widening the overallpeak and is performed by having differentvalues for R19 and R25.

The diode bias inputs are tied to thesupply rail via resistors R17 and R20 both15k, these help to linearise the input stageof the amplifier. Capacitors C15 and C16also set the frequency at which the gainoccurs.

The gain of the first part of the LM13700Is adjusted by varying RV1, this is knownas the resonance control, by increasing this,the effect is more pronounced. The overallgain of 1C3 is determined by R15.

The final stage in the circuit is to expandthe signal back to its original form, IC2 Isagain used for this operation. The idea of

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compression and expansion is to preventany noise from being amplified in thesystem. R6 sets the output impedanceof the unit and C6 is to ensure hFstability.

PCB ConstructionConstruction of the Psychedelic Wah Wahis fairly straightforward, refer to the PartsList and to Figure 3 for the PCB legendand track. The sequence in which thecomponents are placed is not critical.however, the following instruction will beof use in constructing the project. If you arenew to project building please refer to theConstructors' Guide (XL79L) which isincluded with the kit.

Fit the smaller components first, suchas the resistors and diode, noting correctpolarity for the diode. Next fit the wire linkswhich are made from wire offcuts from theresistors. Fit the capacitors next, makingsure that the electrolytic capacitors andthe tantalum capacitor are fitted the correctway round. Next fit the PCB pins into therelevant holes. Identify the horizontalresistor presets, fit and solder. Next, installthe regulator RG1, making sure that itsoutline conforms to the package outlineon the legend. Fit the IC sockets, correctlyorientating them on the board noting thatthe notch at one end matches that on thelegend.

Next fit the power socket 5K1, and thetwo jack sockets Jttl. and Jt2. Identify and

fit the ICs into the correct DIL sockets,correctly orientating them so that the notchon the ICs matches those on the sockets.

Finally attach the battery clip making surethat the leads are correctly positioned.

Once the PCB construction is complete,check over your work to ensure that allcomponents have been correctly fitted andthat there are no short circuits caused bysolder bridges or splashes. Finally, cleanall the flux off the PCB using a suitable PCBcleaning solution.

Pedal ModificationThere are a number of modifications requiredto be made to the foot pedal box, refer toFigure 4 for box drilling and modifications.

1 2 1

9

Runner Screw

White Plastic Runner

Slot for PCBFixing Screw

Insulating Sticker(Modify to cover screwhead)

121

83.5 7rHOLE DATA

REF SIZE No.A 07

0 1 2 3

All dimensions in mm

Figure 4. Foot pedal drilling and modification details.


- SK I(Ext. DC In)

) I I I Plastic Nut

/////A__

JK2(Output)

JK 1

(Input)

A

(

PCB

PP3 Battery Clip

O

Remove the existing cable clamp andscrew. Cut a hole for the foot switch, andif necessary remove any plastic obstruction(where the original cable clamp waslocated). Drill a hole for the output socket,refer to Figure 4, and also cut a slot forthe input socket, the reason for the slot willbecome apparent when the PCB is fitted.Next drill a hole for the power socket.Remove the variable resistor from the pedal(it may be helpful to loosen the white plasticrunner). Carefully take the split pin fromthe end of the shaft and remove the cogand the washer. Next place the cog andwasher onto the 100k linear potentiometersupplied in the kit. figure 5a showsan exploded assembly view of thepotentiometer. Using the holes on the cog,

RV4 100k Lin. Pot

1114.Rm=19mm

Washer- Nut Cog

SplitPin

00.5mm (See text for Split Pin fitting)

Figure 5a. Exploded potentiometer assembly.

0

SW I Switch

0

=

Velcro Strip(for fixing battery) II II Plastic Nut

M4Sho kep roof

Washer

Figure 5b. Exploded assembly of the Psychedelic Wah Wah Pedal.


9V OC IN --C-.)) ((Battery

(PSYCHEDELIC)_ WAH"OD) era

Baseplate label in position.

Inside the foot pedal box - the foot switch can be seen on the right.

drill through the shaft of the potentiometerand place the split pin through the hole,next bend the split pin in order to securethe cog to the shaft. Now put the finishedpotentiometer back into the pedal andtighten up the lock nut.

Before mounting the PCB in position,cover the white runner screw, with thestrip provided on the main base panel label(KP70M), cut the strip off and stick over thescrew, thus preventing contact with the PCBand possibly shorting out.

Next mount the PCB into the case, referto Figure 5b which shows an explodedassembly view. Fit one end of the PCB withthe jack socket into the hole on the sideand then slide the other end into the slotprovided. Mount the PCB onto the spacer,fit a shakeproof washer and bolt in position.Discard the washer from the footswitch, andmount the switch in position, adjusting it sothat it will only come into contact at thevery end of pedal travel (in order to switchit on or oft).

RV4

Ir

PP3 Battery Clip

Black

4-P-OrangeYellow

- 84- DIR EFF

SK1(Ext. DC In)

=< <

JK(Input)

B Y

Brown

SW1 Switch Solder Tag

LI LILIdy .

Black

Green

CHASSIS

C

.........)jRETURN

JK2(Output)

Rubber Foot/Screw

Earth Assembly(Top Right Base Plate Fixing Post)

Base Plate

Solder Tag

Pedal

Figure 6. Wiring diagram for Psychedelic Wah Wah Pedal.


Low Frequency Adjustment

High Frequency Adjustment

Figure 7. The resonance, low and high frequency presets on the PCB.

Referring to figure 6 the wiring diagram,connect up the potentiometer and theswitch, and solder an earthing wire from thePCB to the baseplate. Lastly, attach one partof a pair of Velcro pads to the battery andthe other part inside the box, this is in orderto secure the battery in position.

Test and Set upWell now that it is built, now comes thenerve racking moment of truth, adjust thepresets as near to those in Figure 7 as

Use this for insulating runner screw

9V DC IN --C- +BatterY PP3x

(PSYCHEDELICWAH

4VAlt OUT

Figure 9. Psychedelic Wah Wah Pedal baseplate label.

possible, plug in a guitar and give it a go(see figure 10). Make sure that the unitis switched to effect and not to bypass.If it does not work check for misplacedcomponents, dry solder joints, and solderbridges between tracks.

To set the correct range of the unit thepedal must be pushed forward to the endof its travel Gust before the switching point).In this position the higher frequencies areamplified, so now (first checking that yourguitar is in tune!) play the highest note.Whilst letting the note sound adjust preset

RV3 (the high and low frequency presetsare shown in Figure 7) it is correct whenyou hear the note at its loudest point Oustlike radio tuning). It may help you to hearthe effect by turning the resonance to full,this is when RV1 is turned fully clockwise.

To set up the lower frequencies, movethe pedal to its lowest point and play thelowest note (usually the open E string),with this note sounding adjust RV2, you willhave to decide if you want it to boom out(ideal for Reggae music, special effects andfeedback) or to be more conventional to

-o

a

20-

15-

10-

Frequency Response Graph

Middle ofPedal travel

1.28kHz 013dB

Bottom ofPedal travel

256Hz 06dBA5-I

I \

0 1 \

- -/- -

5

Top ofPedal travel

2.52kHz 015dB

200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 3000 3200Frequency(Hz)

* Graphs compiled from testing and setup (see text)

Figure 8. Typical frequency response graph.


Amplifier

Psychedelic WallWat, Pedal

Regulated Power Supply

Figure 10. Psychedelic Wah Wah Pedal used in conjunction with guitar and amplifier plus optionalpower supply.

have a more subtle bottom end. Note,when adjusting RV2 the sound maydisappear, the level that you are lookingfor is just before this point.

Before proceeding recheck youradjustments and realign if necessary. Atypical frequency response graph is givenin Figure 8.

The last setting to adjust Is theresonance, you can turn it down nowunless of course you want loads of lovelyfeedback. This setting is again a matter ofpersonal preference, for subtle effects turnRV1 anticlockwise, for feedback turnclockwise.

To complete the project, fit the baseplate

panel label, this Is shown In Figure 9 and issupplied in the kit and available separately(KP70M). Peel the backing paper off thelabel and stick In position on the baseplate.

OperationFigure 10 shows how the Psychedelic WahWah is used in conjunction with a guitarand amplifier (plus optional power supplyif required). To operate the device once ithas been set up, insert a jack plug into JK1from a guitar or some other suitable electricInstrument. The plug connects the OV railto the battery as well as providing a signalpath. Connect a lead from the Wah Wah tothe amplifier.

The main switch is then operated bypushing the foot -pedal as far forward aspossible, this determines whether the signalpasses through without modification (noeffect) or with effect. Once switched on,moving the pedal back and forth providesthe Wah Wah effect.

So all you need now, are the flaredtrousers, psychedelic shirt, afro hair style,and granny specs to complete the image.

PSYCHEDELIC WAN WAN PEDAL PARTS LISTRESISTORS: All 0.6W 1% Metal Film (Unless Specified) 16 -pin DIL IC Socket 2 (BL19V)Ni 10k 1 (M10K) 3A hook-up Wire Black 10m (FA26D)R2,3 220k 2 (M220K) 3A hook-up Wire Blue 10m (FA27E)R4 6805I 1 (M680R) 3A hook-up Wire Yellow 10m (FA36P)R5 100k 1 (M100K) 3A hook-up Wire Orange 10m (FA31J)86,12,14,22,24 1k 5 (M1K) 3A hook-up Wire Green 10m (FA29G)87,9,16,25 22k 4 (M22K) 3A hook-up Wire Brown 10m (FA28F)R8 18k 1 (M18K) Single -ended PCB Pin810,11,18,21 4k7 4 (M4K7) 1mm (0.041n.) 1 Pkt (FL24B)R13,17,20,23 15k 4 (M151t) Press Switch SPDT 1 (F1192A)

R15 5k6 1 (M5K6) M3 Solder Tag 1 Pkt (LR64U)R19 130k 1 (M130K) M4 x 16mm Steel Bolt 1 Pkt (.1Y165)R26 22052 1 (M220R) M4 Shakeproof Washer 1 Pkt (BF43W)RV1,2 220k Enclosed Preset 2 (U1-10711) M4 Steel Hut 1 Pkt (JD60Q)RV3 1k horizontal Enclosed Preset 1 (UMOOA) 4BA x 1/41n. Spacer 1 Pkt (FW31J)RV4 100k Linear Potentiometer 1 (FW05F) PCB 1 (GH88V)

Panel Label 1 (KP70M)CAPACITORS Instruction Leaflet 1 (XU89W)Cl 220nF Monolithic Ceramic 1 (RA50E) Constructors' Guide 1 (>9179L)

C2,5,10,13 2p2F 100V Radial Electrolytic 4 (FFO2C)C3 10pF 50V Radial Electrolytic 1 (FF04E) OPTIONAL (Not in Kt)C4,8,9,11,12 4p7F 63V Radial Electrolytic 5 (FFO3D) Regulated Mains Adapter 1 (1523A)C6 2n2F Ceramic 1 (W)(72P) 9V PP3 Battery 1 (JY49D)C7 47nF 16V Miniature Disc Double Screened Straight

Ceramic 1 (YR74R) Jack Lead 1 (YZ3011)

C14,18 100pF 25V Radial Electrolytic 2 (FF11M) Neon Lead Green 1 (CC36P)C15,16 330pF Ceramic 2 (W10625) Neon Lead Pink 1 (CC375)C17,19 100nr 16V Miniature Disc Neon Lead Orange 1 (CC38R)

Ceramic 2 (YR755) Guitar Lead 1 (CC39M)C20 1pF 35V Tantalum 1 (WW60Q)C21 1pF 100V Radial Electrolytic 1 (FF01B)

SEMICONDUCTORS

The Maplin 'Get -You -Working' Service is available for this project,see Constructors' Guide or current Maplin Catalogue for details.

IC1 LF351N 1 (WQ3011) The above items (excluding Optional) are available as a kit,IC2 NE571N 1 (W87U) which offers a saving over buying the parts separately.IC3 LM13700N 1 (Y1 -164U) Order As LT43W (Psychedelic Wah Wah Pedal) Price f34.99aRG1 LM317LZ 1 (RA87U) Please Note: Where 'package' quantities are stated In the PartsD1 1N4001 1 (QL73Q) List (e.g., packet strip, reel, etc.), the exact quantity required

to build the project will be supplied in the kit.MISCELLANEOUSJK1 Stereo PCB tin. Jack Socket 1 (FJO5F) The following new items (which are Included in the kit)

JK2 Mono PCB 1/4in. Jack Socket 1 (FJO0A) are also available separately.

Foot Pedal Box 1 (XY28F) Psychedelic Wah Wah Pedal PCB Order As GI188V Price f3.29

PP3 Clip 1 (1-1F28F) Psychedelic Wah Wah Pedal Base Panel Label

lin. Velcro Strip 1 Pkt (FE45Y) Order As KP70M Price f2.49

PCB Mtg Power Socket 1 (RK375)8 -pin DIL IC Socket 1 (BL171)


qThis article, part one of

a two part maxi- feature

examinesin detail all

aspects of mains safety 4.1

by Andrew Chadwick B.A., C.Eng., M.I.E.E.

V1\11r\ 1A

;13\

PARTONE

MANY of the projects that appear inElectronics are low voltage circuitsthat can be satisfactorily run on bat-

teries. However, where any significantamount of power is required, a mains -derived power supply is necessary. Usuallythe neatest solution is to incorporate this inthe same enclosure as the rest of the circuit(see Photo 1); this is fine, as long as the poten-tial hazards of working with the mains arerecognised. Poor standards of design andconstruction with battery -powered circuitswill, at worst, lead to component damage ora flat battery; but with mains voltages theresults can be lethal. Despite this dire warn-ing the intention of this article is not to dis-suade you from constructing any circuitsusing the mains, but to describe and explainthe precautions that must be taken to ensuresafety.

If you would rather not deal directly withmains voltages, even after reading this article.then a separate, ready-made battery elimi-nator type power supply might be suitable forsome projects. There are a number of BritishStandards on the subject, which are listed inthe References section (in next month's edi-tion). These are very comprehensive, andalthough they cover the safety of commer-cially manufactured equipment. they are stilla good guide to safe practice. The two mostrelevant to hobby construction are BS 415

24

and BS EN 60950. I have tried to extract all theappropriate material, but if you are interestedin further details, I suggest you have a lookfor yourself. British Standards are available atmost main libraries, and provide a great dealof useful technical information on all sorts ofsubjects.

The standards on safety consist of a seriesof requirements and details of tests to ensurecompliance. Many of the tests are not feas-ible for hobbyist constructors, but I havementioned some of them to give an idea ofwhat is expected.

Although every effort has been made toensure accuracy the author accepts no lia-bility for any accidents arising as a result offollowing the advice contained in this article.

Shock

The most obvious potential hazard of themains is that of electric shock. Anyone whohas experienced this will tell you it is veryunpleasant! The current flowing through thebody upsets the operation of nerves andmuscles. and causes involuntary muscularcontractions. If the muscles of the diaphragmand chest contract, there is a risk of asphyxi-ation. However, the usual cause of death isdue to the current triggering the heart into astate known as fibrillation, in which it beatsvery rapidly and without co-ordination. Painis felt at the points where the current enters

r-Current (rnA)

.5 to 2.0-0 to 10

10 to 25

EffectThreshold of perception.Painful sensation. increasing with current.Cramp and inability to let go. Danger ofasphyxiation from respiratory muscular contraction.

5 to 80 Severe muscular contraction possibly leading tobone damage. Loss of consciousness. Heart orrespiratory failure.

>80 Burns at points of contact. Death fromventricular fibrillation.

urrent passed hand to hand for more than I second.a mpt conduct your experimen

Above: Table I. The effect of various levelsof AC current on the human body.Left: Photo I. A well laid out project:Maplin's Ni-Cd charger.

and leaves the body and, in severe cases,burns may be caused.

The effect of various levels of current andvoltage on the human body has been inves-tigated, quite extensively, in order to deter-mine safe limits. Table I summarises theresults of experiments where varying AC cur-rents were passed between cylindrical elec-trodes held in either hand, for periods of atleast one second. It is evident that sensitivityvaries a lot. but currents of 10 to 25mA aresufficient to give a serious shock. Otherexperiments showed that the body is slightlyless sensitive to DC currents.

In the light of Table I, it may seem surpris-ing that the recommended trip current ofresidual current devices (RCDs), designed toprotect against the worst effects of electricshock, is as high as 30mA. However, the


Right: Photo 2. Electricity pylons.

severity of a shock is very dependent on itsduration. Table 1 is for shocks lasting for asecond, whilst an RCD is required to operatewithin 40ms.

The resistance of the body actually varieswith the applied voltage but, at 240V is typi-cally 1.000 to 2.0000 -by applying Ohm'sLaw, it would appear that any contact withthe mains would produce a lethal level ofcurrent. However, in a practical situation, theresistances at the points of contact are usu-ally far more significant. These vary greatly,depending on such things as the dampnessof the skin, and the type of shoes being worn.There is also a good chance that muscularcontractions will throw the victim clear in amuch shorter time than the one second usedin the experiments.

Mains Supply Basics

Most readers will be familiar with the live. neu-tral and earth conductors in the power circuitsin their homes. The way in which these arederived from the supply authority's circuits isshown in Figure 1. T i Is the secondary of athree-phase transformer whose primary issupplied from the grid .

The consumer's neutral terminal is con-nected to the star -point of the transformer,which is earthed.

The live terminal is connected to one ofthe three-phase conductors denoted by thecolours red, yellow or blue. This is sometimesshown by a coloured disc near the authority'sterminals.

The main earth terminal may be con-nected to the transformer earth by means ofa separate conductor, often the lead sheathof the incoming cable, as shown in Figure1(a). Alternatively, it may be connected to theneutral conductor at the authority's termi-

nals, thereby connecting to the transformer'searth via the neutral conductor of the supplycable, as shown in Figure 1(b). In the lattercase, the neutral conductor is earthed at anumber of other points to reduce the risk ofa fault, in the neutral conductor, resulting inloss of the earth return path to the trans-former. This system is known as protectivemultiple earthing (PME).

Whichever system is used, the function ofthe three conductors within the consumer'spremises is the same; any load is connectedbetween the live and neutral conductors. Thelive conductor should be at roughly 240Vwith respect to earth throughout the installa-tion. The neutral conductor will normally bea few volts above earth potential due to loadcurrent flowing through the finite resistance

of the neutral cable. However, faults in thewiring, or crossed connections. can meanthat the neutral may be at 240V, and so both.conductors are classed as phase conductors,and are treated with equal respect. The earth,or protective conductor, provides an alter-native path to the neutral of the supply, andcan be used to give protection against elec-tric shock, as will be described shortly.

Types of Shock

Simultaneous contact with the live and neu-tral conductors of the supply is likely to resultin a significant shock, due to current flowingthrough the body, as shown in Figure 2(a).

T1

RedYellowBlueNeutralEarth

ConsumersA premises

0 0Live Neutral Earth B

RedYellowBlue

Additionalearths

Consumerspremises Live Neutral Earth

Figure 1. Derivation of live, neutral and earth conductors from the supply authority's circuits: (a) TN -S system: (b) TN -C -S system (protectivemultiple earthing).

Detached liveconductor touchingunearthed metalcase

Figure 2. Types of shock: (a) Direct contact with live and neutral conductors simultaneously: (b) Direct contact with live conductor - currentflows to neutral via the ground: (c) Indirect contact with live conductor - current flows to neutral via the ground.


Basicinsulation

Metalenclosure

Basicinsulation

Failure of basicinsulation

ra

LmmiB C

Metalenclosure

D Supplementaryinsulation

Fuse blowsL

Path offault current

However, this kind of situation is rare: shocksare usually the result of touching a live partwhilst being in contact with the earth (or anyobject which, itself, is in contact with theearth) Current can then flow from the livepart, through the body to earth, and there-by back to the neutral of the supply whichis, of course, connected to earth at somepoint. This is sometimes known as an earth -loop shock and is shown in Figures 2(b)and 2(c).

The live part causing the shock may be alive conductor as shown in Figure 2(b), oralternatively, a conductive part made live bya fault as shown in Figure 2(c). These twoalternatives are described as direct and indi-rect contact respectively. Although the dis-tinction may seem academic (especially ifyou are the victim), the concept is importantwhen considering the design of equipment.

If the mains supply was not earthed, thenthere would be no danger of shocks to earth.Unfortunately, this is not possible in the caseof the National Grid for other reasons.However, the principle is employed on asmall scale in the case of a shaver socket, forexample, where a safety isolating transformerreduces the danger of shock.

Protection Against Shock

It is internationally agreed that. for an accept-able level of safety. there must be two inde-pendent provisions for shock protection. Thisensures that there is no hazard both undernormal conditions, and if a single faultoccurs.

For domestic equipment. the first safetyprovision usually consists of insulating liveparts. Insulation used in this way is known asbasic insulation, and provides protectionagainst direct contact. The second protectiveprovision guards against the risk of shock inthe event of failure of the basic insulation.There are two common ways of implement-ing this, and this gives rise to two types ofequipment, known as Class I and Class II.

Above right: Photo 3. Damaged mainscable. with inner conductors clearly visible.Right: Photo 4. A solder tag earthconnection.

Above: Figure 3. Principles of Class 1 andClass II construction:(a) Basic insulation on live parts is the firstprovision against shock:(b) Failure of basic insulation leads to arisk of indirect contact shock:(c) In Class 1 construction. connection ofthe metal case to earth protects againstshock if basic insulation fails:(d) In Class II construction. a layer ofsupplementary insulation protects againstshock if basic insulation fails;(e) Another implementation of Class 11construction. where the insulatingenclosure forms the supplementaryinsulation.

Class I EquipmentFigure 3(a) shows a piece of equipmentwhere the live parts are enclosed in basicinsulation, and the outer case is wholly orpartly made of metal. A breakdown of thebasic insulation between a live part and theinside of the case could lead to a risk of an

'indirect contact' type of shock, as shown inFigure 3(b).

However, in Class I equipment, all 'access-ible conductive parts', such as the case, areconnected to the protective or earth con-ductor of the supply, as shown in Figure 3(c)(a solder tag earth connection is pictured inPhoto 4). This provides the required secondprotective provision. Now, any fault in thebasic insulation will simply cause a current toflow through the protective conductorwhich, if sufficiently high, will blow thesupply fuse. Although the potential of thecase will rise, due to the impedance ofthe protective conductor, correct sizing ofthe conductor will ensure that the size andduration of the increase are insufficient tocause a hazard.

The requirement that all 'accessible con-ductive parts' must be earthed applies only tothose parts which might become live, on fail-ure of the basic insulation. Small parts, suchas the metal foil in a plastic badge, need notbe earthed as long as they meet the Class IIrequirements (described in the next section).


Function of Insulation Creepage (mm) Clearance (mm)Operational 2.5 1.7

Basic 2.5 (3.0) 2.0 (3.0)

Supplementary 2.5 (3.0) 2.0 (3.0)

Reinforced 5.0 (6.0) 4.0 (6.0)

Insulation in primary (mains) circuits and between primary and secondary circuits.Working voltage 250V AC, pollution degree II, material group IIIb. Main values fromBS EN 60950. Values in brackets from BS 415.

Above: Table 2. Creepage and clearancedistances for insulation.

Right: Table 3. Dielectric strength andresistance tests for insulation.

Class II EquipmentIn Class II equipment, the second protectiveprovision is another independent layer ofinsulation, known as supplementary insu-lation, between the outer accessible surfaceand the basic insulation, as shown in Figure3(d). If there is a breakdown of the basic insu-lation, it is unlikely that the supplementaryinsulation will fail simultaneously. This typeis known as 'double insulated' construction;although the outer case is made of metal, itdoes not have to be earthed to ensure safety.

Note that even if the outer case is made ofinsulating material, it is assumed that damp-ness could effectively form a conductivelayer on the outer surface, and so two inde-pendent layers of insulation are still required.An example is shown in Figure 3(e).

Alternatively, the standards allow a singlelayer of insulation, known as reinforced insu-lation, to be used in place of both basic andsupplementary insulation. Obviously thereinforced insulation must have propertieswhich are at least as good as the propertiesof the basic and supplementary insulationcombined.

Low Voltage Circuits

The preceding descriptions are particularlyrelevant to electrical equipment such as drillsor hair -driers, where all wiring is at poten-tially hazardous voltages. In most electronicequipment, only a small part of the circuit isusually at mains voltage, and it is unneces-sary (and impractical) to apply the same stan-dards to the low voltage circuits. Instead, ifthe maximum voltage between any two partsof a circuit does not exceed 42.4V peak AC or60V DC, there is considered to be no dangerof electric shock, and so any part of this cir-cuit may be accessible to the user. Note thatthe figures quoted are taken from BS EN60950, and there is some minor variation inother standards. Obviously, the low voltagecircuit must also be reliably separated fromthe hazardous circuits, both under normaland faulty conditions. This can be achievedby employing double or reinforced insu-lation, or an earthed conductive screen,between hazardous circuits and the low volt-age circuit. Other methods are also men-tioned in the standards, but note that they allinvolve the use of two independent protec-tive measures or. an equally reliable singlemeasure. A double insulated mains trans-former is probably the most familiar embodi-ment of this principle in hobbyist projects.

Interested readers may also come acrossthe following terms in the various standards:


Using a multimeter to test the effectivenessof the earthing is unsatisfactory, as no signifi-cant current flows. Care is also needed toensure that the contact resistance of the mul-timeter test probes does not swamp the earthpath resistance.

Earth conductors must be reliably con-nected which in practice means using metalscrews (not self -tappers), engaging in threadsformed in metal, not in insulating material.

Function of InsulationOperationalBasicSupplementaryReinforced

Test voltage (RMS VAC)1500150015003000

Resistance (MO)22

2

4

Insulation test voltage for working voltage of 250V AC; resistance measured at 500V DC.

safety extra -low voltage circuit; functionalextra -low voltage circuit; and protectiveextra -low voltage circuit. These are all slightlydifferent interpretations of the fundamentalconcept described above - limiting the volt-age in a circuit to safe levels, and reliably sep-arating the circuit from potential sources ofhazardous voltages.

Application to HobbyistConstruction

Many authors advise the use of an earthedmetal enclosure for mains projects. This,together with the other precautionsdescribed in this article, will provide thesafety of Class I construction. It is probablythe simplest alternative for the hobbyist,although one disadvantage is the cost of ametal case.

Plastic enclosures are, generally, a lotcheaper. However. since the case is non -con-ducting, Class I construction is no longerapplicable. Instead, the requirements forClass II construction must be met. This mayneed a little more thought, but is certainly notbeyond the hobbyist - it is not sufficient tosimply earth the metal of the mains trans-former! The other disadvantage of plasticenclosures is that they are not as robust asmetal cases.

Earthing

The establishment and maintenance of aneffective earth connection is essential for thesafety of Class I equipment. The resistancebetween any accessible metal part and theearth terminal should be less than 0-1C2,when tested at a current of 1:5 times thecapacity of the hazardous voltage circuit, or25A, whichever is less. The open -circuit volt-age of the test current source should not bemore than 12V. At first sight, the test currentmay appear to be unnecessarily high, but thisis, in fact, the sort of current that the earthpath might have to carry in the case of afault. It is, therefore, sensible to use a similarlevel of current during the test, so that anyweak connections will be shown.

To obtain a realistic measurement, it is alsoimportant that any high resistance oxidefilms are not broken down during the test -hence the limitation on the source open -circuit voltage.

If there is a chance of vibration then a lock -nut or shakeproof washer must be used.

Any insulation used on earth conductorsshould be green/yellow. There must also bereliable contact between different parts of theenclosure, such as the lid and the base.

In order to give some protection againstthe earth becoming unknowingly discon-nected, the wire to the earth terminal, in boththe mains plug and the equipment, should belonger than that to the live or neutral. Then,if the flex slips, the live or neutral will be dis-connected before the earth.

Apertures

As already explained, the principle of Class Ior II construction is to ensure that partsaccessible to the user are not at a hazardousvoltage, despite a single fault in the insula-tion. The standards say that this must beproved by using a test finger, which must notbe able to touch live parts (either bare orcovered only with enamel or lacquer), orbasic insulation. During the test, the finger isinserted into any apertures in the enclosure,such as ventilation holes. If the enclosure hascovers that can be removed by hand, thesemust be taken off before the test begins. Capsof fuseholders, or battery compartment lids.must also be taken off, even if they require atool for removal.

Although it should be normal practice toisolate equipment from the mains beforechanging a fuse, a fuseholder must alsoprevent access to live parts should afuse be inserted or removed with the cir-cuit energised. For older type fuseholders.this normally means that the live connec-tion must be made to the rear terminal.Touch -proof types do not require this pre-caution.

A further test is made with a straight metalrod. 15mm long. and roughly 3.5mm indiameter. The rod must not touch bare, liveconductors when inserted into any aperturein the enclosure (with covers in place).Finally, openings above bare hazardous partsshould be designed to prevent objects fromfalling through. Alternatively, they should notexceed 5mm in any dimension, or should beless than imm wide and of any length.

The above requirements are those speci-fied in EN 60950. BS 415 has slightly differentrequirements, but the principle is the same.More stringent requirements apply to holesprovided for adjustment of preset controls.

27

Above: Photo 5. Insulating boots in place inthe Ni-Cd charger.

Right: Figure 4. Defining creepage andclearance distances:(a) Between terminals:(b) Between conductors on a PCB;(c) Across grooves in insulation.

In this case, a 2mm diameter pin I00mm long,simulating a screwdriver, must not touch liveparts when inserted into a hole, at any angle.

Protection of ServicePersonnel

The precautions described above areintended to protect operators during normaluse of the equipment. Operators are con-sidered to be oblivious to any hazards, butwill not deliberately tamper with protectivemeasures.

The same degree of protection cannot begiven to service personnel who may need toremove covers, and work on circuitry whilstit is live (for fault-finding purposes). However,they are assumed to be more aware of thehazards, and are expected to take reasonableprecautions. such as isolating the mainswhenever possible, before making internaladjustments. Protection should be providedagainst unexpected hazards. for exampleshielding exposed parts at mains voltages, orearthing parts that would not otherwiserequire earthing. for operator safety. The con-cept of operators and service personnel isstill applicable in the hobbyist context. Anymember of the family using the projectbecomes an operator, and must be protectedon the assumption that they have no know-ledge of hazards. You are likely to be calledon to fulfil the role of service personnel. andso it makes sense to build in protection foryour own safety when fault-finding!

One simple, but effective, precaution is toinsulate all bare parts at mains voltage thatcould be accidentally touched. For panel -mounted mains connectors and fuseholders.special insulating boots and covers are thebest solutions, as pictured in Photo 5. Forother connections, use sleeving or simply afew layers of insulating tape.

Even if power is isolated, stored charge oncapacitors may present a shock hazard or

A

B

Creepage Clearance

Insulation

Clearance

A groove lessthan 1 mm wideis ignored when

C measuring creepageCreepage

energy hazard (from the spark that couldbe produced if a large value capacitor wasaccidentally shorted). If necessary. a bleederresistor may have to be fitted across a capaci-tor to reduce the charge to a safe level.

InsulationReliability of insulation is of paramountimportance in all forms of construction.

Insulation may be of solid material, or bean air gap of suitable size which is unlikely tobe reduced or bridged. Natural rubber orhygroscopic materials are not permissible.Wood is acceptable. but it must pass a com-plicated humidity test which, therefore,makes it an impractical insulator for the hob-byist constructor to use. However, it can beused for such things as enclosures, where itis not acting as insulation.

If a solid material, such as PVC, isemployed as insulation, it must be at least0-4mm thick. It is permissible to use thin sheetmaterial of any thickness. but in this casemore than one layer must be used, and evenif one layer fails, the remainder must con-form to the requirements.

The concepts of 'creepage' and 'clearance'arise when dealing with insulation. Figure 4shows how creepage and clearance distancesare measured. Creepage distance is the short-est path measured along the surface of theinsulation, and provides protection againstbridging of the insulation by moisture anddirt. Grooves of less than 1 mm width are dis-regarded, as shown in Figure 4(c). Clearancedistance is the shortest distance between theconductors, measured through air. and mustobviously provide insulation at least equiva-lent to the insulating material.

Insulation must comply with the creepageand clearance distances shown in Table 2.These are worst -case values for 220 to 250VRMS supplies. They were extracted frommore detailed tables in the standards whichcover other operating voltages, different lev-els of pollution and variation of the trackingindex of the insulation. The term 'operationalinsulation', mentioned in the table, refers toinsulation used to separate live parts in orderto avoid short circuits. It does not have anyfunction in protecting against shock.

Finally. the insulation must stand the re-sistance and dielectric strength tests, sum-marised in Table 3. The standards actuallyspecify that the equipment must meet theserequirements even after a humidity test.

For Class I equipment, the basic insulationcan be effectively tested by applying the testsbetween the live and earth. and the neutraland earth conductors. For Class II equipment,metal foil might have to be applied to the sur-faces of insulation in order to perform thetests. In some cases, it would still be difficultto test the basic and supplementary insula-tion separately.

The high voltage tests in Table 3 must notbe carried out repeatedly because they candegrade the insulation, and this would leadto failure in service.

Reduction of Clearances

Having established the necessary clearances,it is important to ensure that these are main-tained during operation. The clearances inTable 2 must not be reduced if a force of 30Nis applied externally, or a force of ION intern-ally (30N is roughly the weight of three Ikgbags of sugar). Again, there is some variationbetween the different standards, with BS 415specifying a force of 50N externally, and 2N.internally. Wiring should be fixed effectivelyby suitable cable clamps, or cable ties. Mainswiring and low voltage wiring should not berun in the same loom.

Clearances could be reduced if a wirebecomes detached: in general, a means offixing must be provided which is indepen-dent of that ensuring the electrical connec-tion. For soldered joints, 'hooking -in' the wireis considered sufficient. as long as there is norisk of vibration. It is not considered likelythat two wires will become loose simulta-neously, and so attaching one wire to anotherby twisting or sleeving, for instance. is also anacceptable solution. Precautions may needto be taken against the possibility of creepageand clearance distances being reduced bydust and dirt, or by loose screws or nuts.

Next month, the concluding part dealswith fire hazards. fuses, general precautionsand maintenance. and examples show howthese principles can be applied to hobbyistconstruction.


Designing Linear Circuits

with BIPOLARTRANSISTORS

byJ. M.Woodgate B.Sc(Eng.),C.Eng., M.I.E.E., M.A.E.S., F.Inst.S.C.E.

In the early days of transistors, circuit designappeared a very complex subject, basicallybecause overly -complex models of the tran-sistor were used. To a certain extent this wasnecessary, because of imperfections in theearly transistor types. These models were allright for research scientists, but gave hugeand unwieldy equations even for quite simplethings like voltage gain, which were virtuallyimpossible to remember and not readilyapplied to practical design. They also requiredvalues for certain characteristics of the tran-sistor to be known, which manufacturersusually did not specify. so the designfailed for lack of data. Since then, thependulum has swung perhaps too far theother way, so that we see a lot of 'make itabout enough' so-called design methods.

We shall be looking at powerful designmethods, which can be used in very simpleforms to give approximate but practically use-ful answers, and which can easily be extendedto deal with such things as low -noise designand high -frequency operation, even tunedamplifiers.

DC ConditionsThis is the best place to start, although it begsthe question of how you decide what DC con-ditions you want. Some of that question willbe answered almost at once, but some partwill have to wait a while.

We start with a somewhat idealised case,but one which is still capable of giving usefulresults. In order to set up the DC conditions,expressed normally as collector current andvoltage, we have to provide a bias current, toflow between the base and the emitter.Common -emitter and common -base stagescan be biased in the same way, but common -collector (emitter -follower) stages often use asomewhat different circuit configuration.Generally, we shall be considering silicon NPNcommon -emitter circuits, but not exclusively.PNP transistors use exactly the same circuits.but with the supply polarity reversed.

The bias current can flow only if thereis a voltage between base and emitter.Unfortunately, the current that flows is a very


steep function (exponential, in fact) of the volt-age, and is temperature -dependent. as well asvarying quite a bit from one transistor toanother, even of the same type. Luckily, wecan make our first simplifying assumption,provided that we always remember that, likeall assumptions. it is one that we have to aban-don if it is not justified in any particular case.This assumption is that if there is any base cur-rent flowing at all, the base to emitter voltageis near enough to 0.6V for any small -signal sil-icon transistor, and for any silicon bipolar tran-sistor at all if the collector current is less thanabout 0.5A. (The corresponding value forgermanium transistors is 0.2V.) This, however,applies at room temperature, and it is the tem-perature inside the transistor that matters, ofcourse. If the transistor is hot, we must reducethe assumed value of base to emitter voltageby 2.5mV per degree above 20-C (or 'perkelvin', for the purists).

Figure la. Primitive bias circuit.

Figure lb. Simple bias Circuit.

Figure la shows the very simplest possibil-ity, in which the collector load resistor takes nopart in the actual biasing (provided its value isnot too high). V, is the supply voltage, and weknow that the base -emitter voltage is 0.6V, sothe base current is (V, - 0.6)/Rb. We nowneed just one transistor characteristic, thesmall -signal current gain, (3 (beta), which is vir-tually always specified by the manufacturer. It

is the ratio of collector current to base current,and varies from one transistor to another ofthe same type. The range of values may beonly 2:1. but it may be 4:1 or even more. Ourbias circuits must ensure that any transistor inthe range will work satisfactorily. The collec-tor current I-, then, is (3 (V, - 0.6)/Rb, and thecollector voltage is V, - 41:2,. Now we can seethat the collector resistor value must not betoo high. because if it were, I,R, could belarger than V,. In that case, a current as largeas I could not flow: the collector voltagewould be near zero and the transistor is saidto be 'bottomed or 'saturated'. For powertransistors, the collector -to -emitter voltageunder these conditions is often specified, withthe symbol but for small -signal silicontransistors it is typically about 200mV, verymuch smaller than so we can make asecond simplifying assumption, that =0. Note that the collector voltage is less thanthe base voltage under these conditions, butthe forward bias on the collector -base diodeis only about 0-41V, so no appreciable reversecurrent flows at MOM temperature.

The problem with this circuit is that it cannotcope with a wide range of 13 values, because ifthe collector current is adequate with a low -(3device, a high -f3 device will probably bebottomed, and for this reason, it is very

Figure lc. Improved bias circuit.

Figure I d. Bias circuit for an emitter follower.

Figure le. Emitter follower with bootstrappedbias. Rb2 can be much larger than R,1.

29

seldom used. An improved circuit is shown inFigure lb. The connection of the base resistorto the collector actually provides DC (andsignal -frequency) feedback, which is veryhelpful in coping with both 13 values and temp-erature effects. We can analyse this circuit asfollows. The collector voltage V, is equal to thebase voltage (0.6V, of course) plus the voltagedrop across Rb. It is also equal to the supplyvoltage minus the voltage drop across Rb:

Vc=0.6+l,Rh

V, = V - [3IbR

We have two equations, and two unknowns,V, and Ib, so we can solve them. It is most con-venient to find lb first, and then to calculate I,and V, from it:

ib Vcc - 0.6I Rb PR'

Let's put in some typical values:

V = 10y, (3 = 200, Rb = 1MQ, R, = 5.6k52.

Then lb = 443µA, so that

V. = 0.6 + 4.43 x 10-6 x 1 x 106 = 5.03V

Is this a good value for V,? In fact, at (nearly)half V, it is very good, because it allows usto get the maximum possible symmetrical'undistorted' output voltage swing of 5V peak(10V Pk -to -Pk, or a 3.5V, sine wave) with thecollector going down to OV and up to 10V. Thequote marks around 'undistorted' are therebecause, while the onset of gross distortion isquite sharp as the collector voltageapproaches zero, it is not sharp as itapproaches V, and the distortion increasesgradually. So, in practice, we might tweak thevalue of V down a little, to move away fromthe distortion zone. However, there is no pointin being very precise about this. because ofwhat happens if we change to another tran-sistor of the same type, but with a differentvalue of (3. First, let [3 = 100. Then lb =6.03p.A, and V, = 6.63V. So the collector canswing 3.37V up but 6.63V down. Now try p =400. This gives lb = 2.9,uA and V, = 3.5V,which means a swing of 6.5V up but only 3.5Vdown. It is clear that we cannot expect morethan about 3V peak undistorted (or a 2.12Vimssine wave) from this stage, if we want to beable to use any transistor in the current gainrange from 100 to 400.

This simple circuit is useful for small ampli-tude signals, for lowest possible cost andwhere space is at a premium, such as in mod-els. A much better circuit is shown in Figurel c. Here, the emitter sits at about 1V normally,due to the voltage drop across R,. Ideally, thisvoltage should be large compared with thebase -emitter voltage Vb,, but that is normallyonly possible for germanium transistors,unless the V is unusually high. The base biasvoltage comes from the potential divider Rbl,Rb2, which should pass a current of 5 to 10times the base current, so that the base volt-age is not affected much by the value of (3. Atypical numerical example would be:

Transistor BF173, with V = 12V to run at= 5mA in a high -gain RF amplifier. In this

case, there would be no R, but a tuned cir-cuit as the collector load. This makes no dif-ference to the bias circuit. To get Ve = 1Vapproximately at I, = 5mA, make R, = lsoaThen Ve = 0.9V and thus Vb must be 1.5V.Minimum 13 for a BF173 at 5mA is 40, so the

maximum lb. = 125µA, and we make thedivider current about five times this, thus usingRbi = 151d2 and Rb2 = 2.7kS2 as the nearestpreferred values, giving Vb = 1.83V. Note that,in this simple procedure, we neglect the basecurrent flowing through Rip}, but compensatefor this by choosing resistor values to give aslightly high value of Vb. We also assume thatthe emitter current is equal to the collectorcurrent, i.e. that (3 is very much larger than 1(this is our third general simplifying assump-tion). The maximum p for this device is 100,and it is not too difficult to calculate the truevalue of Vb, taking the base current intoaccount. The results are I, = 5.18mA for p =40 and I, = 6.07mA for (3 = 100, which islikely to be acceptable.

In this circuit, the emitter resistor providescurrent feedback, which substantially reducesthe gain available, so the resistor is oftendecoupled by means of a high -value parallelcapacitor. When used with a collector loadresistor, the optimum DC conditions dependon whether the emitter resistor is decoupledor not. The decoupled case is easier, becausethe emitter voltage is fixed: there is no signalvoltage at the emitter. In this case, thecollector can swing up to V, but down onlyto V. It can be shown (not difficult, but tedious)that the value of V, giving the maximumoutput voltage swing is given when:

v - Vcc (R, + Re)2R, + R,

If R, is not decoupled, as the collector voltagefalls, the emitter rises to meet it, and theoptimum working point is then:

V, - V, (Rb + 2R,)2R, + 2R,

Emitter followers are often biased as shownin Figure 1d. Here. the feedback due to thelarge value R, ensures that variations in p haveonly a small effect. For maximum output volt-age swing, the emitter should sit near V/2.Rb is included as a protection against R,.being accidentally short-circuited, whichwould otherwise cause the transistor to passexcessive current and probably fail. Thevariant shown in Figure le has a very highinput impedance, because Rb2 has almost nosignal voltage across it, so very little signalcurrent can flow through it. This is an exampleof bootstrapping.

AC Coupled LoadThe above analyses are all very well, providedthat the next stage has an input resistancehigh compared with Rb (or Re for the emitterfollower, but that is almost always true).However, if the next stage is capacitivelycoupled, as usual, and does not have a highinput resistance, we have to shift the DCworking point of the first stage to get themaximum output voltage swing. It is easiestto explain how we do this by means of a setof curves of collector current against collectorvoltage with base current as parameter (Figure2). However, you do not need the curves inorder to find the answer: they just help (?) theexplanation.

We begin by drawing the DC load line,representing R. This is a straight line joining Von the horizontal axis to V/R, on thevertical axis. The slope (gradient) of this linerepresents Rb. (For the Figure I c circuit, with Redecoupled, substitute (V - Ve) for V. Theundecoupled case is more difficult.) The DCcollector voltage must be somewhere alongthis line. Now imagine another line, whoseslope represents R, the parallel combinationof R, and R. We don't know where to put thisline yet, but it must cross the Rb line at theoptimum collector voltage Vc, where the voltagecan swing down and up by equal amounts.This means that, since we assume that thecollector can swing down to zero volts kerset)= 0), the R, line must cut the horizontal axis atV, = 2V,, and, since we know its slope, that isall we need to define its position on the graph.We just 'slide' the dotted line down, keeping thesame slope, until it crosses the horizontal axisat twice the voltage at which it crosses the 121d2load line. We can then see that:

- V - V,Rb

and

(V -V )- . -Rt Rt

but

= 2V,

from which we can find:

VccRtV,R, + Rt

3

2

0

Y

0 2 4 6Collector voltage V

x

8

Vco

X - optimum working point with 12kY - optimum working point with 12k

and 12k A.C. coupled load

10 12

Vcc

collector resistorcollector resistor

14

'b= 50pA'b= 4OpA

I b = 30pA

I b = 20pA

I b = 10pA

I b = 5PA

Figure 2. Diagram to clarify the optimum working point with an AC coupled load.


For example, if V, = 12V, Re = 121d2 andR, = I2kS2, V, = 4V, instead of 6V, as it wouldbe without Ri. Note that the maximum outputvoltage swing has been reduced from ±6V to±4V.

Signal -FrequencyConditionsSetting up the DC conditions is basic goodhousekeeping, and the glamorous bit is cal-culating gains and input and output imped-ances. This is where we need a good, simplemodel for our transistor, and we are going touse the dependent generator model, becauseit can be used in basic forms and easilyextended to high frequencies and moredetailed analyses. It also has the advantage ofusing transistor characteristics which varylittle, even from one type number to another.They are, however, dependent on thecollector current. Figure 3 shows our model,and the only odd thing about it is, perhaps, thecurrent generator. This is the opposite of avoltage generator: it produces a fixed currentinto whatever is connected to its terminals andits internal (source) impedance is infinite.

The components in the model are:

rbe, the input resistance, which is 2613/1 whererb 'e is in ohms, and I, in milliamps.

gm, the mutual conductance, which is 391,where gm is in millisiemens (mA/V forold-timers) and I, in milliamps.

Figure 3. Dependent generator model of abipolar transistor.

Figure 4. Circuit for setting voltage gain.

Figure 5. Re controls the voltage gain.


Figure 6. More refined dependent -generator model.

In principle, these values work for bothsilicon and germanium, but imperfections ingermanium transistors usually prevent theresults of calculations being more than arough (optimistic) guide.

It may look odd that there appears to be noconnection between the input and the output,but the connection is there in the vbe com-ponent of the value of the current generator.

We can immediately see that we havethe input and output impedances 'on aplate'. Re is, of course, the external collectorload resistor, and the current generator'simpedance is infinite, so to the outside world,the output source impedance is R-, and theinput impedance is simply rbe. The voltagegain of the stage is the output voltage dividedby the input voltage, which is:

gmvbeR,IRc

Vbe

Note the use of lower-case letters todescribe components inside the model, andsignal voltages as opposed to DC voltages.

If the transistor is operated as an emitterfollower, the source impedance looking backinto the emitter is simply 26/1(1 in milliamps).The input impedance at the base is rb e 3Re,

and this also applies to the common -emittercircuit of Figure I d with the emitter resistor notdecoupled. The shunting effect of the biasingresistors has to be added in, of course, but thisis simply Ohm's law.

Amplifier with DefinedVoltage GainThe use of an undecoupled emitter resistor notonly enables the voltage gain of a stage to beset to a required value, but also raisps the inputimpedance and reduces distortion (by feed-back). While we could use the Figure 3 modelto analyse this circuit (Figure Id), we can usea simpler method. In Figure 4, nearly the samesignal current flows through Re and Re (if is

large), so that the total input voltage is vbe +gmybeRe, where vb, is much smaller than thesecond term. The output voltage is gmybeR,, sothat the voltage gain is simply R,/Re. For exam-ple, if Re = 101dll and Re = 1 k12, the voltagegain is 10. If the transistor is operating at a verylow collector current, we have to add to Re aterm equal to 1/gm. For example. if in the abovecase, the collector current were 100µA, theeffective emitter resistance would be 11d2 +26/0.1 = 1.26 kit. The input impedance is rb e+ IR, and again the second term is normallythe larger. For the example, the value would be12601(12, and for 13 = 200, this is 252k.Q.

With the emitter resistor decoupled, thevoltage gain is larger, being gmRr, but gedepends on the collector current, so the gainvaries with different samples of the same typeof transistor. Even a bit of undecoupled emit-ter resistance, such as 3952 in a stage whereI, = lmA, reduces this effect, and incidentallyallows a much smaller value of capacitor to beused for the same low -frequency response, asshown in Figure 5.

Extension of the ModelTwo extra resistors may be added to themodel. For power transistors, the DC collec-tor current is often not nearly independent ofthe collector voltage (as it is shown in Figure2). This can be represented by a collectorshunt resistor rce across the current generator.For power transistors, and for low -noise cir-cuits fed by low -impedance sources (e.g., amoving -coil vinyl disc pick-up), the basespreading resistance rbb , which appears inseries with rbe is important.

For high -frequency operation, two capaci-tors must be added. These are cbe, in paral-lel with rt, e, and cbc, the feedback capacitance.cbe is approximately proportional to thecolector current, and can be calculatedfrom the transition frequency ft, whichmanufacturers usually specify:

391,Cb e

2it ft

with I, in milliamps and cb e in farads. Figure6 shows all the extra components that may beadded to the model, as necessary.

Provided the operating frequency is not tooclose to ft, we can calculate the gain of atuned RF amplifier as follows. The collectorload at the tuning frequency is given by Rd =(DOM (or Q/o..)0C), where coo is Zit times thetuning frequency, L and C are the compo-nents of the parallel tuned circuit, and Q is thequality factor (bandwidth/tuning frequency).The voltage gain is then simply gmRd. If thereis a load resistance due to the next stage, thisjust appears in parallel with Rd. Note that thegain is proportional to gm, and therefore to theDC collector current. It is thus somewhat sur-prising that IF stages in portable radios tendto operate at about lmA, whereas for just alittle increase in current, a lot more gain isavailable. Most of the transistors used workquite well up to 5mA, but to ensure that thestage remains stable, the tuned circuit valuesneed to be changed at such a high gain.

For the FutureWhile single transistor amplifier stages can bevery useful, and are essential study for begin-ners, better performance can be obtained byvarious types of multi -device 'building block'.I hope to cover that subject in a future article.

Don't Forgetthat the New1995 Maplin

Full Colour Guideto ElectronicComponents

is on sale from2nd September '94

31

Hurry aIpn Sharon itme

mone.

New Batteries for OldWhen 'yours truly' (preparing to write thiscolumn) switched on his PC, it politelyasked him to set time and date, and useroptions (if required), adding a discreetrequest for a new set of batteries for itsnon-volatile section of RAM. It was dulyfed a set of four new 1.5V AA cells, theold ones proving to have terminalvoltages of 1.3V, 1.3V, 0.8V and 0-6V.

Just before throwing them away, PCrecalled that, in the forties and fifties,it had been common to recharge (thethen universal, and still widely used)zinc -carbon (Leclanche) type batteries,a process that could be repeated severalor, with some cells, many times, providinga useful extension of their life, andconsiderable economy in replacementcost. The limiting factor was puncture ofthe zinc can, which, besides forming thenegative electrode, also acted as thecell's outer container -come -strengthmember, covered only by a card andpaper sleeve.

In a detailed article in one of thethen popular electronics magazines,the author described the results of hisresearch. Recharging with DC resultedin puncture of the can after only afew recharges, and it was found, onexamination, that the zinc deposited backon the can during rechargng formed a

ovi Where :aid Brucethe. b 0SS 1111

ten me fo gill

slick the iiil

UDS "41

\`---______--

porous friable layer of vanable thickness.Nevertheless, due to the thick gauge ofthe zinc used, several recharges werepossible before a puncture occurred. PC'deconstructed' (the word 'destruct' onlyseems to exist in American English andweapons terminology and, like destroy,it does not convey quite the samemeaning) one of the exhausted AA cellsbefore throwing it away. Sure enough,it used a zinc pot as the outer container,though in the interests of economisingon scarce resources, the zinc waspaper -thin. Inside was the usual paperseparator containing the depolariser, anddown the centre of that ran the carbonrod forming the positive electrode. I

suspect that these particular cells diedfrom drying out, being housed in acompartment sandwiched between thecomputer's main unit and the monitor,which sits on top of it.

A dried -out cell is never going to beany good, but PC bought an extra setof new cells to experiment with. Theintention was to run them down andrecharge repeatedly, to see if they wouldtake it. Following the advice in the articlementioned, they would be recharged notwith 'clean' DC, or even dirty DC (straightfrom a rectifier with no smoothing), butwith AC having a DC component.According to the article, this is a trick

by Point Contact

used by electroplaters to achieve a goodfinish. In the case of the cells, it resultsin the zinc being redeposited in a moresmooth, dense uniform layer, delayingthe inevitable ultimate puncture of thecan, and the subsequent death of thecell through drying out. Although modernzinc -carbon cells seem to have muchthinner cans than those of earlier times,the covering is now an impermeableplastic layer, so drying out may bedelayed, even if the can does puncture.Watch this space for furtherdevelopments.

Spies (and Telephone Exchanges)in the SkyLast autumn, Motorola announcedthat it had completed the first roundof financing, some $800 million, forits Iridium project to launch sixty-sixtelecommunications satellites to createthe world's first global mobile phoneservice. Of several proposed systems,this is the first one to move beyond thedrawing board stage. Meanwhile, on thesame day, American Intelligence officialssaid that the explosion of a Titan IVrocket over the Pacific Ocean haddestroyed its secret payload, a US spysystem of three solar -powered oceansurveillance satellites, intended for use bythe US Navy. This explosion is said to bethe most costly space accident since the1986 Challenger shuttle disaster. Thecost of the three lost spy satellites? -just $800 million again!

TailpieceHere is a nice little story recounted bythat guru of analogue electronics, BobPease of National Semiconductor, whichI am sure he will not mind my passing on.It concerns a state-of-the-art transistormanufacturing company in the 1960s,who had agreed to supply their largecustomers with devices having an AQL(Acceptable Quality Level) of 2% - prettygood going for those days. Sold in boxesof one hundred pieces, the tester wouldfill the box with ninety-eight good partsand then, as instructed, add a coupleof rejects to make up the number. Withboth the supplier and the customersgetting what was expected, this lookedset to go on for ever: but then, one ofthe customers happened to notice thatthe dud transistors were always in thesame corner of the box!

e°iftThe opinions expressed by the author are not necessarilythose of the publisher or the editor.


Design by Alan WilliamsonText by Alan Williamsonand Dean HodgkinsBEng(Hons)

PROJECTRATING

The projectpresented here is avery useful addition tomany older, and base model cars,most of which do not have the sophisticated courtesy light featureof higher models. Fitting this project to your car will allow the courtesylight to remain illuminated for approximately 30 seconds after thevehicle's doors have been closed, unless the engine is running, inwhich case the light will be extinguished immediately. It also adds anextra feature which many top models do not have - in the event of adoor being inadvertently left open, the courtesy light will automaticallyturn off after approximately 10 minutes, to avoid draining the battery.

11111.1101111111111.1.11111.1111111111.111.11.1111.1111.1....6.

Circuit Description

The block diagram of the Car lntenor LightController is shown in Figure 1. Refer toFigure 2, the circuit diagram; the dooropen/closed sensor comprises a slowcharge/discharge R/C network (R3, R4, C1)to detect a definite change in condition,and capacitors C2 & C3 to help preventspurious operation from noise spikes; thetransistor TR1 and associated componentsprovide an inverted switch input.

Because of the slow change on the inputof the NAND gate (IC1b), a Schmitt triggeredvanety was chosen (which has significanthysteresis) to prevent toggling of the outputwhen the input is in between the twothreshold levels.

The inverters IC1c 8 'Cid and associatedcomponents (C4 & R5 and C5 5' R6) arepositive edge detectors (ie. detecting achange from '0' to '1'); the inverter IC1dproduces a pulse when the door is opened,and IC1c produces a pulse when the dooris closed.

IC2 and IC3 are interior light long durationtimers, specifically designed for automotiveapplications. Each of the timers has 'ON', 'OFF'and 'TOGGLE' inputs - the 'TOGGLE' inputsare not used in this project. The timers alsohave an inbuilt 14V Zener diode across thesupply pins, an oscillator, a frequency divider,input debounce circuits, and a (not quite)open collector transistor with protection (loaddump) diode for switching reactive loads. IC2is used as the 'courtesy light' timer, and IC3 isused as the 'door open' timer

KIT AVAILABLE(LT65V)

PRICE e9.99

The components responsible for the time-out period are R15 and C7 for IC2, and R166' C8 for IC3. The 'OFF' input of IC2 and the'ON' input of IC3 are triggered by the doorbeing opened, while the 'OFF' input of IC2and the 'ON' input of IC3 are triggered bythe door being closed.

The 'extra' components around the inputsof IC2 are the 'inhibit and reset' circuitry; whenthe door is closed and the engine is running,diode D11 feeds a jamming voltage to the'ON' input of IC2, preventing it from beingactivated. However, if the courtesy light is ONand the engine is OFF, starting the engine willcause transistor TR2 to generate a pulse whichwill reset IC2 and turn off the light.

Diode D12 prevents damage to the circuitfrom accidental reverse polarity connections.Resistor R17 limits the current to the internal


ON

TO DOOR { 0-7SWITCHES

1_0

IGN

INVERTER

INHIBIT/RESET

Figure 1. Car Interior Light Controller block diagram.

SpecificationsMinimum supply voltage: 9VMaximum supply voltage: 15.5VQuiescent current: 2.5rnA @ 1 2VOperating current: 30mA @ 1 2V

LI)

Zener diodes of IC2 and IC3. Capacitors C9and C10 decouple the supply at high andlow frequencies. The relay RL1 performs thelight switching function; the coil of the relay isconnected between the supply (after D12)and the open collector outputs of IC2 and Ka

Construction DetailsFirstly, the car 'electrics' need to beexamined. If you have a proper servicemanual (not the pamphlet supplied with thecar), such as the popular Haynes books, thena quick glance at the electrical wiring diagramwill tell you whether the interior lights areswitched, via the door switches, to 'supply',

-

DOOROPENING

DET

DOORCLOSING

DET

OFF

ON

10 MIN.TIMER/DRIVER

30 SEC.TIMER/DRIVER

cog tg7fERiogLiGgi

cow-W.:Lotftl,t.tv4

or to 'ground' (if you cannot find thisinformation, you will have to grab yourmultimeter and investigate!).

If the door switches connect the interiorlight to 'ground' (assuming negative earth),then the following components do not need

ORELAY FROM

INTERIORLIGHT

Photo 1. Assembled PCB.

to be fitted: R1, R2, D1, TR1. The link LK1must be fitted in the 'GND' position.

If the interior light is switched to the'supply', then in that case, the link LK1 mustbe fitted to the (+) position. Diode D2 neednot be fitted.

D2TB1 1N4148

+V

R4 ± C347k _10nFm

R3 -o C110k m1OuF

D1 R2TB1 1N4148 10k

> LJ

R1100k_

TR1BC547

R15100k

C74n7F

IC2U6047B

4 2 3

A D3 7771N4148 Cld D8 \ /05 4093BE 1N4148 v5 1uF

R9 -10k _

C2 ow10nFm

IC1b4093BE

2

051N4148

+V

14

/7-IC10

4093BE

C41uF

A R6100k

D41N4148 A

777

IC1c4093BE

910IT

R5100k

R1682k

C8100nF

R1010k

D9 A1N4148/ \

+V

NC

R111 I

10k

VD61N4148

R1210ki I

R13TR2 100k

BC547D11

1N4148 //./.

C61uFIF-

A D10L ---11N4148

R14I I

10k

2

n R7L110k

R81_110k

A D71N4148

+V4

IC3U6047B

RL112V 5A

N C

R17

N

D12

220R 1N4001

C9 elm o C10100nFT T 220uF

N/0

TB1

IGN

TB2

<SW

TB2+(> LK1

i TB2

ND

Figure 2. Car Interior Light Controller circuit diagram.


MARL INO CRR INTERIOR LIGHT CONTROLLER TR I

KM TR2 I.21. ,NMC6 NM c 10 M

C3 MEMIWIA

7UR IS1 ' LEMlc 1 MEM I C6 -Mall -I- C9

R 10C71 -

R8 L -I re

?'C2 ak MIN 1... "r.wc1r IND

EMI-cf

--1

CHo T 2O C4

I RL I

114

.:10.

1111410

i 11 relli .1

ill10.\

Figure 3. Car Interior Light Controller PCB legend and track.

Figure 5. Car Interior Light Controller box drilling details.

Figure 6. Car Interior Light Controllerexploded assembly drawing.

-0

-a3(/)

-o

_c 13C

0

I'd 00,

LK OLK`-' + GND `-/

0 0 0 0 0 0

+ SW GND IGN S -1_

Figure 4. Car Interior Light Controller fittingLK+ or LK GND.

Construction of the module is fairlystraightforward; the general rule being:begin with the smallest components first,working up in size to the largest. All of thecomponents are mounted on a single -sidedPCB, shown in Figure 3, and the assembledmodule is shown in Photo 1.

The recommended component assemblyorder is as follows: First, fit and solder thediodes D1 to D12. Care must be taken tofit the diodes the right way around; thecathode is indicated by a black band on the

body of the diode, and this must face thethick white band on the PCB legend. Next,use any lead offcuts from the 1N4001 diode(D12) to make link LK1, which should thenbe soldered into place (shown in Figure 4).

Mount capacitors C2 to C9 next, followedby the three IC DIL-sockets. Now, fit thetransistors TR1 and TR2, taking care to ensurethat the flat side of the device matches thestraight edge on the PCB legend. Try not tokeep the soldering iron in contact with thedevice leads for longer than two seconds.

Mount the two polarised capacitors (C1and C10) next, taking care to insert thedevices correctly - the negative lead isidentified by a black band and (-) symbolson the capacitor's body.

Fit the two 3 -way terminal blocks next -remembering to slot the blocks togetherbefore fitting to the PCB.

October 1994 Maelin Magazine 35

Important Safety WarningBefore starting installation work, consult thevehicle's manual regarding any specialprecautions that apply. Take every possibleprecaution to prevent accidental short circuitsoccurring since a lead -acid battery s capableof delrvenng extremely high current. Remove allitems of metal Jewellery, watches, etc., beforestarting work. Disconnect the vehicle's batterybefore connecting the module to the vehicle'selectrical system. Please note that some vehicleswith electronic engine management systems willrequire reprogramming by a main dealer afterdisconnecting the battery.

Assuming a negative earth vehicle, disconnectthe battery by removing the (-) groundconnection first; this will prevent accidentalshorting of the (+) terminal to the bodywork orengine. It is essential to use a suitably rated fusein the supply to this project. For the electncalconnections, use suitably rated wire able tocarry the required current. If in any doubt as tothe correct way to proceed, consult a qualifiedautomotive electrician.

Finally, mount the relay, and build up thefour 'wide' sections of tinned copper trackwith a thick layer of solder.

Thoroughly check your work for errors,such as misplaced components, solderbridges, and dry joints, etc. Clean any fluxoff the PCB using a suitable solvent.

The ICs can now be inserted into theirsockets. Remember to observe the standardantistatic precautions before you handlethe ICs - ensure that you touch an'earthed' conductor (domestic water pipes,for example) to remove any static chargewhich you may have accumulated.

The PCB is now ready for testing.

Testing

Connecting a 12V DC supply between the(+) and GND terminals should cause therelay to operate. The relay should takeapproximately 30 seconds to de -energise(i.e. simulating someone opening a car door,getting in, and closing the door).

Connect a lead between the appropriateinput and supply terminal to bigger themodule; the relay should take approximately10 minutes to de -energise (i.e. someone hasleft a car door permanently open). Next,

disconnect the lead from the input, the relaywill energise once again; quickly reconnectthe lead between (+) and 'IGN'(representing the car being started), and therelay should immediate de -energise (thusturning off the light). Leaving the leadconnected to the IGN input, connect a

7a) +V

Switchedto Ground

GND

8a) +V

Switchedto Supply

GND

InteriorLight

InteriorLight

DoorSwitches

DoorSwitches

Figure 7. Interior light wiring pre -installation.

+V1 OOmAFuse

To Switched+V Supply

when engineis running

SW

GNC

IGN

+V

100mAFuse

To Switched+V Supply

when engineis running

SW

GND

IGN

Figure 8. Interior light wiring after installation.

36

Photo 2. Completed unit in its box.

second lead to the appropriate input totrigger the module (representing a doorbeing opened, with the engine running); therelay should energise (turning on the light).

Now that the module has been fullytested, it is ready for installation into avehicle. If you wish to fit the module into theoptional box, follow the drilling instructions.

Box Preparation

The optional box drilling details are shown inFigure 5. Mark out, drill, cut and file all theholes as required. Fit a seal grommet (notincluded in the kit) into the large hole on theside of the box.

AssemblyPhoto 2 shows the assembled PCB fitted intoa box. Refer to Figure 6 for box assemblydetails. Fit spacers to the corners of the PCB,the module to the box as shown.

Installation

STOP! Before proceeding any further, makesure that you have read the warning at thetop of this page. If you are, in any way,unsure about installation, consult a qualifiedautomotive electrician.

Figures 7 and 8 show typical before andafter wiring diagrams; refer to Figure 7 if yourvehicle has a negative earth, otherwise referto Figure 8. Part (a) of the appropriate Figureshows the existing wring of the vehicle, andpart (b) shows how to modify this toinclude the module. Make sure that you usewiring capable of carrying currents of up to5A, and be sure to fit a 100mA fuse in theappropriate supply line. The 'IGN' terminalshould be connected to something thatprovides a positive supply when the engineis running - for example, it could besomething on the dashboard, such as theignition switch, or radio power lead, etc. Youare advised to consult your car manual forfurther information.

The module may be positioned in anyconvenient location, but it is best to ensurethat the unit is not directly exposed to anysource of high temperature, or to water.


A readers forum for your views and comments.If you want to contribute, write to:

***************Active DistributionDear Editor,May I suggest, as an idea for a possibleproject, an 'Audio Distribution Unit',where a number of unity gain outputs,derived from one input, each has itsown output buffer or line driver. As Iwork in electronics I often come acrossa situation where such a system wouldbe better than a passive system.For example, our local hospital radiohas a mixer to which we have matrixeda passive resistor network to drive tapemachines, etc. Although this works,much better results will be gainedfrom an 'electronic' method, but theprofessional equivalents are very costly.Torn Wilson, Banbury, Oxon.This is a very good suggestion, since itprovides each output with an individuallow impedance buffer with theadvantage that if there is a fault on anyline. for example a short circuit, it doesnot affect any of the other lines. It willalso ensure best HF performance byreducing the amount of screened cablethat each buffer needs to drive. A likelydevice could be the LM837 quad opamp, offering very good audio qualityand fast slew rate. Two of these canbe easily accommodated on a pieceof stnpboard providing 1 into 4 -waydistribution for stereo (four ICs for eightand so on). For non -inverting unity gainoperation link pins 1 & 2, 6 & 7, 8 & 9,13 & 14 together, and connect pins 3,5, 10 & 12 in parallel to a common biasresistor (say 47k) connected to the OVof a ±15V supply. The input should becoupled through a 1 uF polyester layercapacitor to the junction of the resistorand pins 3, 5, 10 & 12. Outputs canbe taken from pins 1, 7, 8 & 14 andare able to drive into a 6000 loadimpedance. Ensure the ±15V supplyis regulated and well decoupled; pin4 is +V and pin 11 is -V. It would be agood idea to have two 0 -1 pF ceramiccapacitors connected across the supplylines, immediately adjacent to each IC.

I Want to be a Weather ManDear Editor,I am a radio ham and I have twocomputers, a BBC B and anArchimedes 3000 2Mb and 40Mbhard disk. The problem is that I want toconnect one of these computers (eitherwill do) to a device that will interface witha wind speed and direction indicator,and one of your posh temperaturesensors, giving me an ongoing, on-screen line graph of readings during theday and a stored version for the night-time. I'm an operator rather than aconstructor, but I once threw an SWRx together in 38 hours.Trevor MacDiarmid, Cambridge.

It's a shame you don't have an IBMcompatible PC because the 'PCWeather Station' in Issue 70 ofElectronics sounds as if it is be rightup your street. Second-hand machinescan now be obtained very cheaply fromauctions, boot sales and radio rallies,even an Amstrad 1512 would do thejob at a pinch! There is a wealth ofsoftware and hardware available for IBMcompatible PCs specifically for amateurradio and weather use. Much of it ispublic domain or share -ware so it is alsovery cheap. You could also operate theIBM compatible PC in tandem with yourArchimedes or BBC via the serial portto transfer data, etc.Newly available is the MAPSAT2weather satellite receiver system,this comprises: crossed dipole aerial(LMOOA); a superb synthesisedscanning receiver (A0490), capable ofreceiving transmissions from the polarorbiting satellites, such as the RussianMeteor and American NOM satellites;and a PC interface (A050E) completewith software for IBM compatible PCs


STAR

The Editor, 'Electronics - The Maplin Magazine'P.O. Box 3, Rayleigh, Essex, SS6 8LR.

In this issue, Mr E J. Scudder, of Cumbria,Gwynedd, wins the Star Letter award of aMaplin £5 Gift Token for his letter aboutinfra -red remote controls.

Infra -red MysteryDear Sir,First of all, a quick whinge - I mightbe forgiven for thinking that Maplin iscurrently going through a small cash -flow crisis, only because it would seemthat the machine which prints thesubscription address labels needs anew ribbon at least. The most recentaddress label that I received, stuck onthe Electronics wrapper, is just oneexample. My first initial is 'E', not 'Fand the village is 'BLINDCRAKE', not'BIINDCRAKE'. Is there any chance ofyour magazine despatch departmentbeing able to afford an efficient newaddressing machine soon?Now for my main query. I spend muchof my spare time listening to CDs, andthe player has a remote control whichI find helpful. The instructions tell me tosit facing the CD and aim the remote ata specific point on the front panel.However, I have found that I was ableto bounce the infra -red beam off objects,such as furniture, while sitting beside theCD out of sight of its front panel. I thenfound that it also worked while theremote was not pointing in any particulardirection, in fact that it would operatewhen pointed anywhere in the room,provided the beam crossed in front ofthe player. I am intrigued because I amnot sure how this is possible. Could youexplain, please?

First of all, don't worry - Maplin isn'tsuffering a cash -flow crisis! On thecontrary, despite the recession, Maplinhas just been listed by leading charteredaccountants, Price Waterhouse, as oneof the fastest growing companies inBritain!

***************

Examination of your enclosed labelshows that the shapes of all the lettersare all there, but the imprint is lighter insome places than others. Thesubscription labels are produced bya very efficient high-speed line printerdriven from our mainframe. The qualityis no better or worse than similaraddress labels produced for othermagazines. This method is chosen forvery good reasons - speed and cost.The total number of labels is over14,000. which then have to beimmediately dispatched by courierto the mailing house (we do not mailElectronics ourselves). Given the totalnumber of labels, it is not surprising thatthe odd letter is not quite as clear as itmight be. The Post Office very rarelyhave problems in deciphering the labels.so there is no need to change! We try tokeep our costs down wherever possibleas it keeps the cost of a subscriptiondown - subscribing is already cheaperthan buying each issue over thenewsagents counter anyway.As far as the remote control goes, youare right in that the infra -red beam canbe bounced all around the room likevisible light. Provided the receiver inthe CD player can 'see' an adequatereflection and be able to decode thepulse train with which the infra -red lightis modulated, the player will respond.Before infra -red became the commonmethod of remote control, ultrasonicswere used instead. Sometimes youcould change channels on your TVby jingling a handful of coins or a bunchof keys. I once heard a story about abudgie that could whistle at ultrasonicfrequencies to change the channelwhen Coronation Street came on!

******************(12MHz 286 minimum). With a downconverter and helical yagi aerial it willalso ret,vive transmissions fromgeostationary weather satellites(Meteosat for Europe). See page 216of the new 1995 Maplin Catalogue fordetails.If you want to take advantage of such

weather monitoring add-on hardwareand software you either need to changeplatform to an IBM compatible PC(build your own easily and cheaply fromMaplin's range of PC parts) or, stayingin the Acom camp, upgrade to the newAcom RISC PC with a 386/486 2ndprocessor fitted.

Getting it TapedDear Editor,Having just looked at the survey resultsI noticed that readers wanted morecomputer projects. I have a couple ofproblems that could be solved by newprojects such as these. For example, Irecently purchased an Amstrad Nc100notepad computer, which stores itsdocuments in an internal memory. Noother storage medium is provided, apartfrom the option of using plug-in PCMCIAcards. As these are rather expensiveI decided to look at the alternativemethod of backing up onto tape, as theserial port provides two way signals.The idea was to have a gadget thatwill convert the RS232 to TTL, thengenerate high and low audio tones fromthe logic levels. Playback would requirethe process in reverse, maybe using aphase -locked loop.Experimentation has shown that whileit sounds simple enough, the pulsesare too fast and need slowing downsomehow. I need a method forrecording an RS232 signal ontotape and then playing it back intothe computer. Any ideas?I also need a small microcomputerset up like the Z80 kit you used tohave. How about running a series oncomputers with projects, as anothermagazine has done recently?Damian Gunner, Southwick, Sussex.

Back in the days when affordable microcomputers first arrived on the scene(NASCOM, UK101, PET, et al) the onlyaffordable form of program storage forhome users, other than EPROM, wascassette tape. Since that time, everycomputer user who has ever usedcassette tape has yearned for the speedand reliability of disk storage. I can stillvividly remember the jubilant feeling ofactually managing to find a file I hadsaved on tape and load it without errors!Memories of frustrated evenings spentunsuccessfully trying to re -load aprogram that took a week to type in stillhaunt me! Even during the 80s, floppydisk drives were considered a luxuryby home computer users. Now we havethe choice of floppy, hard. optical orRAM disc. Tape is only in regular usefor data back-up, even then usingspecial drives and high quality media.You really wouldn't want to use tapewould you?If you really do want to torture yourselfyou could try connecting up a old 300baud modem, obtained from a computerauction, radio -rally, etc. to the RS232port. Just output the data to the port andrecord the modem's audio output! Sincethe data rate is low, it is ideal foracassette tape recorder. All you needto do is match the input and outputlevels properly, and. when it comesto reloading through the serial link, itshould simply be a case of setting upyour computer to receive then pressingplay'. Be sure that you have thenecessary communications softwarethough. Bet you end up back with thePCMCIA cards though!The 280s kits we used to have? We stillsell them! Kits LK67X and LT15R formthe Z80A Development System', seepage 199 of the new 1995 MaplinCatalogue. This pair of projects weretreated to a 'Second Time Around' inIssue 58 of Electronics. For projectsand applications for the Z80 see the'Microcomputer/MicroprocessorInterfacing' books WS85G, WT98G andWT14Q. While these are still popular, itis not really viable to progress furtherwith projects for most of the old 8 -bitmicroprocessors. as they are rapidlyheading towards obsolescence.If you are after designing and buildingyou own processor controlled projectsthe best bet is to follow the current trendtowards microcontrollers, such as theST6, PIC16Cxx, TMS77C82, 80C31,

37

68HC11. These contain CPU. ROM orEPROM, RAM and 10 all on one chip.Low-cost development systems areavailable that enable assembly codeto be written and debugged on an IBMcompatible PC. This is so much easierthan 'hand assembling' code andentering it via a hex keypad. If you addup the cost of all of the bits necessaryto write and debug code. programEPROMs. plus the CPU boarditself. it will cost more than a basicmicrocontroller development system.See pages 693 of the new 1995 MaplinCatalogue. the ST6 Starter kit DC23Acosts under £150.00 and includesassembler, linker and simulator software(runs on an IBM compatible PC), adevice programming unit and fulldocumentation. We hope to publishan article giving full details of the ST6in the future.

Where are all the RailwayProjects?Dear Sr.Some time ago I asked whether somesimple model railway projects couldbe included in the magazine. my maincomplaint being that you only seemto be catering for people that areelectronics engineers, and not hobbyistslike myself. While I can build circuits upand repair things. I cannot design myown circuits, which is why I amsuggesting model projects such asa two-tone train horn, train chuffer.lighting effects, etc. All I get in responsehowever. is promises and nothing else.so how about it?

One other suggestion - could youpublish some simple circuits to help ustest 555 and 741 ICs, just to make surethat it is not the IC that has gone wrong,and time and money is not wastedbuying new ones?K. Hall, Coventry.

A Train Chuffer project was publishedin Issue 80. so how can you say wedon't keep our promises! The followingrailway projects are also imminent: TrainHom & Whistle will be published nextmonth (Issue 83) - this has real soundsamples of a diesel two tone hom and asteam whistle stored in an EPROM andcan be interfaced with the train chuffer.Head & Tail Lamps, Auto Loop Controland Train Detection circuits will bepublished the month after (Issue 84),and Signal Lights will be published themonth after that (Issue 85). You mightthink that we just 'popped them in' thoseissues to keep you happy. but that isn'tthe case, they were already scheduledto appear in those issues before youwrote your original letter! You willappreciate that projects take a little whileto reach publication. The reason beingis that we don't just cook up a circuitand publish it: We check with themanufacturers of all the componentsused to be sure that they are not goingto stop production the following week.We take into account the spread ofcomponent tolerances to prevent 'onlythe prototype worked syndrome'. Thedesign is thoroughly tried and tested sowe know that it will work properly if builtcorrectly - we actually guarantee thatour kits will work as stated. Find another

magazine that goes to those lengths!As for the 555 and 741 ICs. the 555Timer Module kit (Issue 69) and the OpAmp Development Board kit (Issue 68)will make useful testbeds. See alsobooks '10555 Projects' (LY04E) and'How to Use Op Amps' (WA29G).

Putting the Case for Add-onsDear Sir.I have been a subscriber for a little whilenow. and although the magazine is. inthe main. a good read. I believe it wouldbenefit from having more projects tobuild. You provide excellent kits but donot appear to follow up these items. Aparticular example is the 24 -Line I/OCard for the PC.Surely this is an ideal platform for add-ons like an EPROM programmer. an ICtester. robotics control, data sampling.etc. There must be interest in theseotherwise you would not be selling therather expensive stand-alone itemsshown in the catalogue. It seems to methat we are losing the hobbyist elementof the electronics/computing arena as aresult of everything being treated as acommodity which needs to be a 'marketgoer.P. A. Murphy, Carlton,Nottinghamshire.

As far as reasonably possible wealways try to follow up projects withother projects that are compatible. or areadd-ons to the original project. This is tosome extent dictated by what readersactually want. based on readers letters.so please continue to tell us. After all.we don't want to develop projects that

only one or two readers are interestedin!The 24 -Line I/O Card is extremelypopular. but it is questionable whetherit is worth developing add-ons for whatis already an 'add-on'. What is a betteridea is to develop dedicated add-onsthat do what is required from the groundup. We've already gone down this routewith dedicated PC sound card, PCweather station card, digital IC tester,opto isolated card and relay card kits.The major cost tends to be the PCBrather than the electronics so havingeverything on one plug in card is neater,easier to build, requires less wiring andis cheaper! We're developing thesededicated add-ons as we think of them.but any suggestions you. and otherreaders. have are welcome! A -to -D,D -to -A and Teletext are already on thelist. For those wishing to develop theirown add-ons, there is a PC prototypingcard with all the address decodingalready on board, If any readers havecircuits for general add-ons for the24 -line card send them in and we mayconsider publishing the circuit as a'non -kit' project.An EPROM programmer, to beworthwhile. would have to be universaland be upgradeable to include newdevices as they appear. Such a unitwith suitable software would end up ata similar price to a commercial unit. Ifreaders do want such a project pleasewrite in. Developing 'one -offs' for specificcases is quite feasible using the 24line I/O Card. hence it encourages theDIY 'hobbyist element"! The book'Experiments with EPROMs'. StockCode VVT13P. is also recommended.

CAR INTERIOR LIGHT CONTROLLER - Continued from page 36.

Once the module has been installed,check and double-check the Wring beforereconnecting the battery.

Final Testing

Switch on your vehicle's courtesy light (sothat it is activated when a door is opened),and open any one of the vehicle's doors;the light should turn on. Now, close the

door. The light should remain on for about30 seconds, before being automaticallyextinguished.

Open the door again (the light shouldturn on) and wait for approximately10 minutes. After this time, the lightshould be automatically extinguished.

Finally, get into your vehicle and closethe door (the light should remain on).Now, start the engine - the light shouldturn off immediately.

CAR INTERIOR LIGHT CONTROLLER PARTS LISTRESISTORS All 0 6W 1% Metal Film 14 -Pin DIL Socket 1 (BL18U)R1,5,6,13,15 100k 5 (M100K) PCB 1 (GH89W)R2,3,7-12,14 10k 9 (M10K) Instruction Leaflet 1 (XU90X)R4 47k 1 (M47K) Constructors' Guide 1 (XH79L)R16 82k 1 (M82K)R17 2200 1 (M220R) OPTIONAL (Not in Kit)

16.0mm Seal Grommet 1 Pkt (JX77J)CAPACITORS M3 x 10mm Insulated Spacer 1 Pkt (FS36P)Cl 10pF 50V Radial Electrolytic 1 (FF04E) Plastic Box Type 2BA 1 (BZ28F)C2,3 10nF Ceramic 2 (WX77J) Fuseholder 1 (RX51F)C4,5,6 1pF Polyester Layer 3 (VA.V53H) 1.25in. 100mA Fuse 1 (WRO8J)C7 4n7F Polyester Layer 1 (VWV26D) 6A Wire Black As Req. (XR32K)C8 100nF Polyester Layer 1 (WW41U) 6A Wire White As Req. (XR37S)C9 100nF 16V Miniature Disc 6A Wire Red As Req. (XR36P)

Ceramic 1 (yR75S)C10 220pF 16V Radial Electrolytic 1 (FF13P)

The Maplin 'Get-you-Wodong' Service is available for thisSEMICONDUCTORS project, see Constructors' Guide or current MaplinD1-11 1N4148 11 (QL80B) Catalogue for details.D12 1N4001 1 (QL73Q) The above items (excluding Optional) are availableTR1,2 BC547 2 (QQ14Q) as a kit, which offers a saving over buying theIC1 4093BE 1 (QW53H) parts separately.IC2,3 U6047B 2 (AH44X) Order As LT65V (Car Interior Light Controller) Price £9.99

MISCELLANEOUS The following new item (which is included in the kit)RL1 12V/5A Miniature Relay 1 (JM18U) is also available separately.TB1,2 3 -Way 5mm PCB Terminal Blocks 2 (JY94C) Car Interior Light Controller PCB Order As GH89W Price £2.99

8 -Pin DIL Socket 2 (BL17T)


IMPROVE YOURCIRCUIT DESIGNWITH THE AID OF ASCIENTIFICCALCULATORby Clive W. Humphris

How many electronics hobbyistsdesign and build their circuits ona trial and error basis?

Sometimes, the costly consequence willbe one of destroyed components or, morelikely, faults will be generated which areoften difficult to find (as the circuit underconstruction never worked properly in thefirst place).

Electronics is a mathematically basedsubject. Circuit parameters are deter-mined by the values, and characteristics.of the components used to make up thedesign. As a result, circuit voltages andcurrents will, or can be made to. changein a predictable manner by carefullychoosing both the active devices (transis-tors and ICs. etc.) and passive compo-nents (such as resistors, inductors andcapacitors).

The scientific calculator is an inexpen-sive aid to producing circuits that workthe first time. This, in turn, greatlyincreases your confidence and enjoymentof this absorbing pastime, thereby encour-aging you to progress in your developmentof more complex circuit arrangements.

The features and functions of the scien-tific calculator will be introduced withexamples of, firstly, simple DC circuits,before progressing to the more complexand interesting AC analysis.

The key presses used correspond,mainly, to CASIO models, but similarlymarked functions will be available onmost scientific calculators (from a varietyof manufacturers).

DC Circuit DesignAn essential requirement of circuit designand analysis is 'knowing roughly' what toexpect, in terms of voltages and currents,in any particular part of the circuit underconstruction. For example, in a circuitwhich uses a battery as its power source,it would be unreasonable to find largecurrents flowing; we would expect a fewmilliamps at the most. At the same time,the highest voltage measured would bethat of the battery. To meet these con-ditions, resistors would be likely to havevalues measured in Ica Mains operatedequipment could, on the other hand,have a much wider range of circuit volt-ages and currents.


These are general observations, butthey should be borne in mind when mak-ing calculations. If, as a result of yournumber crunching you produce a resis-tor value of only a few ohms for a resis-tor which is to be connected across arelatively high voltage, then a large cur-rent will flow, and this may be undesir-able. Trapping the problem at this stagecould prevent a costly mistake.

For DC circuits, an understanding ofOhm's Law is necess2ry. To demonstratethis, we will use a simple circuit (seeFigure 1) of a resistor connected acrossa battery (which is the source of an elec-tric current). Under ideal conditions, anew battery with negligible internalsource resistance will have a voltage pres-ent, across its positive and negative ter-minals, of 1.5V We can now calculate thecurrent through the resistor and deter-mine the correct power rating of the resis-tor, by using the scientific calculator'sbuilt-in functions, by applying the DCOhm's Law and power formulae.

'lb find the current flowing using Ohm'sLaw

I =V {1}

Input to your calculator 1.5 divided by10,000.

The display will show 1.5-04What does this result tell us? The -04 is

the exponent part of the answer; thismeans that, to find the decimal equiva-lent, the decimal point should be movedfour places to the left. This now becomes0-00015A. This is not a great deal of helpwhen trying to understand 'just howmuch current is likely to flow under theseconditions', especially when very smallcurrents are present (due to large resis-tor values). By applying some simplerules, the results obtained can be mademore meaningful.

10k

1V5

Figure 1. Simple circuit to demonstrateOhm's Law.

If. as in this case, the resistor value ismeasured in ki1 , then the current will becalculated in naillianaps. Similarly, if theresistance is measured in Mil then thecurrent will be in microamps. Where theresistor value is in SI, the current will bein amps. We can now look at the prob-lem again, and obtain a more under-standable result.

Input to your calculator as follows:

1.5 divided by 10 = 0.15mA

The answer is automatically producedin mA, as the 10 means 10ka

Using the same example, if we knowthe current and voltage, we can deter-mine the circuit resistance. The currentis in milliamps, so the resistance must bein

V = 1.5V I = 0.15mA

What is the resistor value?

Fbrmula:

R = -v (Ohm's Law) {2}

Using your calculator, input:1.5 divided by 0.15 = 10 (as the currentis in mA, the resistor must be 10162)

Knowing the current and resistance,we can then determine the source voltage.

Formula:

V=Ix R (Ohm's Law) {3}

The current is in mA and the resis-tance in kfl. Fbr practical purposes, thevoltage will be measured in volts.

Input to your calculator as follows:0.15 x 10 equals 1.5V, which is our orig-inal battery source voltage.

By applying these simple rules, theopportunities for human error, causedby calculating with large numbers ofzeroes, are minimised. The calculator willnot make mistakes. If you have an error,it will either be your interpretation of thecalculator display that is at fault, or youhave pressed the wrong buttons.

Values of resistance and current canrange from, say, millions of ohms, to afew microamperes. Fbr our purposes. wecan consider voltages to be measured as

39

V I R Prove Fbrmulae30V 20mA 1.5ka V i V= I x R60V 10mA 61c0 I I = i25V 0.05mA 5001d2 V V= I x R100V lmA 100kil I

VI = T

250V 0-1mA 2.51v10 R R = vT

volts or possibly tenths or hundredths ofa volt, i.e. 0.1V or 0.05V

Now, practise following the examplesin the table above.

Finding the ResistorPower RatingAs the current flows through 'R, the resis-tance to the current flow will be dissi-pated as heat within the resistor body. Ifthe power rating of the resistor is under-valued, then it will become too hot, andeventually be destroyed. Similarly, usinga physically large resistor will look untidy,be costly (albeit it will last for ever), andpossibly make circuit layout difficult.

The resistor power rating can be foundin one of three ways, depending upon thecircuit parameters available. Each willproduce the same power rating

P=VxIV2

P =

P = 12 x R

{4}

{5}

{6}

This provides us with the opportunityto make use of the calculator's 'square'key, usually marked x2 similar.

Using the previous example of a 10kresistor connected across a 1.5V batterywe proved previously that 0.15mA willflow through the resistor.

From formula {4} , we find the powerrating.

Input by pressing the following keys:

1.5 x 0.15 = 0.225mW

As the current is input in rnilliamps,the answer to the equation will be inmilliwatts. If the current were in amps,then the result would be in watts, and ifcurrent was measured in microamps, thenthe power rating would be in microwatts.Note 'volts' remains as volts.

Using the formula {5}, find the calcu-lator button marked

Input as follows:lx21

1.5 X2, 2.25 will appear on the calcu-lator display. Now divide by 10 (which isthe resistor value in MI), and the samepower value of 0.225mW will be found.

Next, apply Power formula {6}, usingthe E key to find the square of thecurrent, before multiplying by the resistorvalue of 10. Remember it is in Ica Thesame value of 0.225mW will be the result,thus proving that whichever of theformulae is chosen, the power rating willbe the same. The choice of the resistorrating should be higher or equal to thecalculated value, never lower. It is oftenwise to allow some degree of 'headroom

for supply voltage variations, componenttolerance, and reliability. Fbr example, ifthe calculated power dissipation is600mW, it would be better to choose aresistor rated 1W, rather than running a0.6W resistor right on its limit.

Resistors in SeriesConnecting resistors in series (see Figure2) will increase the value of the resistancepresent to:

RTOTAL = R1 + R2 + R3 + ...R. {7}

This is simple enough, until we findthat the resistances to be added includemixed values (measured in 161 and M(1).The larger values, if written using ageneral format, could include manyzeroes. 10MQ, for example, in series with120ka 10,000,000 + 120,000. Plentyof opportunities for mistakes. These maywell be extremes, but it demonstrates thepoint. When we try to multiply thesevalues, the problem becomes even moreerror -prone. Our objective is to make lifesimpler.

The scientific calculator has a built-inExponent function. This is available onthe keyboard, and is usually labelledM or similar. This is simply to allowyou to tell the calculator where to placethe decimal point or, how many zeroes toinclude before or after the decimal point.

For example, add the following values:22kil + 331d1 + 4.71c0 - this could beinput as 22,000 + 33,000 + 4,700. Theequivalent value would be 59,700 or59.71ca A much simpler method is toinput as follows - try it you will get thesame answer:

22 IM 3 + 33 M 3 + 4-7 F3d513H 59,700

R1 R2 R3

Figure 2. Resistors connected in series.

R1

R2

R3

1-Requiv

Figure 3. Resistors connected in parallel.

However, as the values all have thesame x 1000 multiplier, the values couldbe added as 22 + 33 + 4.7 = 59.7 with

added at the end of calculation.When zeroes are included within

numbers, either before or after thedecimal point, they have no actual value'in themselves'. Nought is nothing or zero.Noughts have what is called 'positionalvalue' - placed after the whole numberand, the value is multiplied by 10 for eachnought; placed immediately after thedecimal point and, the decimal or fractionalpart is to divide by 10 each time. The powerdissipated in each resistor can be foundby first calculating the current flowingthrough each, which is equal to thevoltage across all of the resistors (inseries) divided by the sum of the resistorvalues. Then, multiplying the square ofthe current by the resistor value will givethe power dissipated (power formula {6}).

Resistors in ParallelThere are two commonly used formulaewhich can be applied to solve this prob-lem (see Figure 3), and the answer can befound without having to resort to notingdown the intermediate values occurringwithin the calculations.

Fbr two resistors:

RAL

1R1) x (R2) {8}(121) + (R2)

This can be written as: (121 x R2) / (R1+ R2)For any number of resistors:

RTOTAL1 4_ 1 4_

1

1 4_ 1{ 9 }

RI R2 R3 R.We can use the example of having to

find the equivalent value of just two resis-tors to introduce the scientific calculatorfeature of 'brackets', also known asparenthesis.

Supposing the values of RI and R2 were47052 and 8200 (the previous point aboutthe MI key still applies for larger values):

Input formula {8} to your calculator:

El 470 E 820 EEE 470 E 820LI press E, and the answer is 298.76a

Practise yourself, but do not forget toclose the brackets, or your values willbecome what is called 'nested' -a featurewe shall use later.

The next function I would like to intro-duce is the 'reciprocal key', markedor similar.

Taking parallel resistor formula {9} forany number of values, and you will see itis necessary to find the reciprocal (dividethe value into one) for each resistor value;add the reciprocals, and find the recipro-cal of the total. Sounds complicated, butthe scientific calculator performs this featwith ease, without you having to writedown intermediate calculations.

Suppose we use our previous values of22k52, 33k0 and 4.7ka Input to yourcalculator as follows:

22 0 E 33 El 4-7 0 HUI H 3-47

We know the value will be in Ica andso we can write it as 3.47ka The re-sultant value will always be lower thanthe smallest value.


Voltage and CurrentDividersFurther examples in using the scientificcalculator to assist with your circuitdesign are: determining the circuit para-meters within voltage and currentdividers.

Voltage DividerThis circuit consists of a number of resis-tors connected in series. The source volt-age is applied across the extreme ends.Each resistor junction forms a voltagetapping point. A familiar example of thiscan be seen in Figure 4 -two resistorsconnected across the supply voltage ofa common -emitter transistor circuit;the junction of the two resistors supplysthe correct voltage to set the transistorbias conditions. The formula to find thejunction voltage of just two resistors is:

R2VJUNCTION = VCC X {10}

R1 + R2

Suppose Vcc is 9V, and the resistorvalues are: R1 = 101d1; R2 = 3.31(11.Calculate the junction voltage.

Input to your calculator as follows:

9E1 E3.3 MIO 1113.3E0E2.23V

This example is chosen to demonstratethe ease of using nested brackets. In mostcases, circuit evaluation using the scien-tific calculator requires nothing morethan first taking care to write down theformula onto one line. Then, it is input as

there are some excep-tions where the value is required beforethe function is selected - finding thereciprocal of nought will, as you mightexpect, generate an error; this is one suchexample.

ACC

R1

R2

OV

Figure 4. A 2 -resistor voltage divider usedto bias a transistor.

Current DividerWhen resistors are connected in parallel,or a number of resistors meet at a junc-tion, the net result of the current flowinginto that joining point will equal thecurrent flowing out. This is known asKirchhoffs current law - all the branchcurrents, when added, will equal thesource or supply current (see Figure 5).

If the resistor values are known alongwith the source current, each branchcurrent can be found. I1, 12 and 13 arecalculated as a proportion of the totalcurrent, depending upon the resistorvalue relationships. The lower the value,the more current will flow in that branch.Branch currents are said to be inverselyproportional to the resistor values.


Example: If the resistors have thefollowing values, determine the branchcurrents when a source current of 20mAis made to flow. RI = lkf2, R2 = 21(.(2and R3 = 3kf2.

The total current equals the sum of thebranch currents and so, in practice, weonly need to find two.

= I X RT°TALR1

= I X RMTALR2

Is=Ix RTurAL

3

For this calculation it is first necessaryto establish `12Tcrim: which is then used ineach subsequent calculation. Here, thecalculator memory keys come in useful:

or similar to write to memory, andmemory recall.

MinMR

The resistors are in parallel and so thefollowing formulae establish Thum:

1Ea E 2 El 1;13 E Min

0.55 (note kill will be permanently storedin your calculator memory until youdecide to change it, and is now availableas 'IlircrrAC.

Clear the display, and press MR ; yourcalculator should show 0.55.

Calculating II:Input as follows (note the relationship

between mA and IQ means we can inputthe numbers directly):

20 E 111 MR 11111111 10.91mA

Calculating 12:20 H n MR 1112 05-45mA

Calculating I320 Ell ( IlmR El 3 EIE 3.64mA

The memory keys can be extremelyuseful for storing fixed values (which arethen available for repeated use), or forwhen calculations become more complex(break each formula into a number ofsub formulae, and use the memory tostore intermediate results). The memorycan be also be added to, or subtractedfrom, using the M+ and M - keys.Some scientific calculators will have morethan one set of memory buttons.

Before we leave DC circuit calculations,the importance of ensuring you avoidproblems by inputting the wrong numberof zeroes, must be stressed. A 471c0resistor value can be input as 47 EXP 3.5mA, for example, would be input as 5

E 3 = 0.005A. The E key willchange the exponent to a negative value,which in this case is the same as shiftingthe decimal point 3 places to the left.

EXP

I

-I I-R2 /2

41-1 1-*R3 13

Figure 5. Current flow in parallel circuits.

However, you must ensure you haveselected the correct mode on your calcu-lator to display sufficient significant fig-ures, or this function might appear not towork.

AC Circuit AnalysisThe formulae used for AC analysis are alittle more complex than those used tofind the relationship between voltage, cur-rent and resistance, expressed as Ohm'sLaw. However, once the relationshipbetween the values is understood, thenthe scientific calculator can make lifemuch simpler, and most importantly, youwill have confidence that the answers toyour mathematical problems are correct.

AC circuits comprise capacitance,inductance, resistance and the appliedfrequency of voltages and currents. Wecan now look at various aspects of ACtheory to demonstrate how the scientificcalculator can be used for mathematicalanalysis.

Capacitors in ParallelThe formulae for capacitors connectedin series and parallel are the same asfor resistors, except that the series andparallel formulae are reversed. Connectingcapacitors in parallel effectively increasesthe surface of the plate area (see Figure6), and so the values are added directly,which is the opposite to that for resistors.

GrarAl, = Cl + C2 + C3 + ...Cn {11}

One essential difference between ACand DC calculations are the very smallnumbers used, when expressing somecapacitance values (as µE nF and pF) or,inductance values (as p.H, mH and H). Aslong as the conversion table is referred to(or remembered), calculation should notbe a problem.

Capacitance is measured in farads:= 10-6, nF = 10-9 and pF = 10-12.

Inductance in henries: mH = 10-3 andAH = 10-6.

Here the IM calculator key comes inuseful.

Example: Three capacitors have beenconnected in parallel (with values of0.25pF, 2pI and 10µF). As the values areall expressed in microfarads, we can adddirectly:

0.25 + 2 + 10 = 1225µF

However, suppose the values to beadded are expressed as follows: 0.1µF,4,700pF and 220nE

The equation can be written as (0.1 x10-6) + (4,700 x 10-12) + (220 x 10-9)

Now, input to your calculator as fol-lows (note the E key will provide thenegative exponent):

0.1220

ExPiEXF1

E 6E+ 4700 1M1 1] 12 ElEl 9

The calculator will now display3.247-°7 , expressed in farads. To convertto ME reduce the exponent by -6 . Thenumber now becomes 3.247-01. Shift thedecimal point one place to the left(to remove the exponent), and write as adecimal: 0.3247µF Many calculators

41

Cl11C2

11C3

Ctotal

Figure 6. Capacitors in parallel.

have buttons marked 1ENG1 and [EU,or similar, which automatically adjuststhe decimal point and exponent. Whenpressed, the 'engineering buttons convertthe number into the form where theexponent is a multiple or sub -multiple of3, i.e. 10-3. 10-6, 103, 106, etc.

As you become more familiar withusing exponent values, you will find thatthere are further short cuts that you canmake. The previous example used expo-nents of 10-6, 10-12 and 10-9. We couldsimplify the values by reducing the expo-nents by -6. The equation now becomes:

0.1 + 4700 x 10-6 + 220 x 10-9.

Input as follows:0.1 11 4700 E 6 E 220 wo3 E0.32µF

No further number conversion is nec-eqsary as the number is expressed inmicrofarads.

I stressed earlier that it is necessary tohave some idea of the magnitude of theanswer to an equation. My experienceproves that whenever I am using a cal-culator or computer for my calculations,if I get an error, it is usually a big one, andproviding I know roughly what to expect,I can detect it early, before the problembecomes compounded by furthercalculations. For example, using theprevious values, if the answer producedan exponent of say -12, this would indicatean answer expressed as pF - clearlyincorrect.

Capacitors in SeriesConnecting capacitors in series (seeFigure 7) reduces their total value. Thisis because the plates are now effectivelyfurther apart'.The formula being:

Gram', - 1 {12}1 4. 1 1 1

CI C2 C3 CnUsing the previous values of 0.1µF,

4,700pF and 220nF, first write theexponent values in microfarads:

0.1, 4700 x 10-6, 220 x 10-3

Input as follows, finding the recipro-cals of each individual value:

0.13

47001M 1116 ,-51- 220

1./.1 1=1

The calculator display will show4.399-03 which is expressed in µF. lb con -

42

vert to the decimal equivalent, move thedecimal point 3 places to the left. Thisnow becomes 0.004 µF or 4,399nF

The formulae for series and parallelinductance can be applied in the sameway, remembering of course to includethe MI 3 for mH and WO H 6for µH.

Inductive andCapacitive ReactanceReactance is the AC resistance to currentflow' of a capacitor, or inductor, and ismeasured in ohms. The reactance of acapacitor decreases with increasingfrequency, whilst for an inductor, thereactance increases.

The formulae ,pKovide the opportunityto introduce the or El calculator key.Pressing this will display a constant valueof 3.141592654, frequently used inelectronic AC calculations. lb access thisfunction, we may need to use the

(also known as INV1) key, butthis depends upon your calculator type.Usually situated at the top left-hand cor-ner of the calculator extended keys, itprovides access to additional functions,including the 'inverse trigonometric' func-tion (to be used later).

First, taking the formula for inductivereactance:

SHIFT

= 27tiL {13}

Suppose the applied frequency is100Hz, and the inductor has a value of0.2H. What is the inductive reactance?

Input to your calculator:

2 El 1001x10'2[]2

which equals 125.660.Example: Frequency of 2kHz and an

inductance of 5mH.

Input as 21:1 1112 W1 31115E 3 E

you should now have 62.83 on your dis-play which is 6351.

Capacitive ReactanceThe formula for capacitive reactance is asfollows:

1Xc 27tfC

Example: Find the reactance of a 0.1µFcapacitor to an applied frequency oflkHz.

Input to your scientific calculator asfollows:

26

X x 1 EXP 3 E 01 MI El

Your calculator display will show1,591.55 expressed in ohms. Convertedto k.Q. = 1.5911d1

Resonant FrequencyWhen a capacitor and an inductor areconnected in a series or parallel configur-ation, they will have a 'natural frequencyof resonance' to an applied alternating

signal. The resonate frequency is deter-mined by the formula:

F0 1 {14}2n NI LC

This example allows the use of thesquare root calculator function.

Example: Find the resonant frequencyof a 0.1µF capacitor and a 250mH induc-tor connected in either series or parallel.

To avoid the use of brackets, and theneed to store subtotals, input the formulato your calculator as follows, using theequals sign to perform subtotals:

250 EM 11 3 Eo.imE6EEIE2EEE gg

This returns an answer of 1,006.58(1or, 11c.Q.

Note the 1 =limy must be pressed afterthe values for L and C have been multi-plied to ensure that the square root iscalculated as (L x C), and not just thesquare root of C. Also, use the H keybefore finding the reciprocal of thecalculation.

Cl C2 C3-I I-I I-I I-Ctoto I

Figure 7. Capacitors in series.

R C

Figure 8. Series RC circuit.

R L

Figure 9. Series RL circuit.

Circuit Impedance 'Z'Impedance calculations are base on thePythagorean theorem, where the squareof the hypotenuse of a right-angle triangleequals the sum of the squares of the othertwo sides.

RC and RL Series CircuitsAs there is a 90° phase relationshipbetween the applied voltage and the cur-rent flowing in a capacitor or inductor, wecannot, simply, add the individual volt-ages appearing across R and, C or L,when calculating the impedance. The for-mula for the RC series circuit (see Figure8) is:

Z 1{ 15 }

\/R2 +X2

and for the RL series circuit(See Figure 9):

z -4R2 xt2

Using the series RC circuit as anexample. it is first necessary to find the


{16}

reactance of C at the applied frequency.Using our previous example of a 0.1µ,Fcapacitor, with an applied frequency oflkHz, the capacitive reactance was foundto be 1,5910. Making the series resistor1k; what is the circuits impedance?


1 [WI 3 w E 1591 x2 111

The resultant impedance, 'Z', is1,879.1752.

Again, to find the series impedance ofthe series RL circuit, using a previousexample where a 0.2H inductor has areactance of 12551 at 100Hz, and con-necting the inductor in series with a 5000resistor, we find the resultant seriesimpedance from the following keypresses:

500 x2 E 125

This returns a value of 515.390

Impedance of ParallelRC and RL CircuitsThe formulae to find the resultantimpedances are shown below.

Parallel RC (see Figure 10):

R x X,Z - {17}JR2+

Parallel RL (see Figure 11):

Z R x XL {18}+

Using the RC parallel circuit as ourexample: Suppose R = 1MQ and Xc =500161


( 1

1 MI6 H500 EXP1] 1 EXPI 6 I] 500

WH

3 WEIM3EXP x2

The value obtained should be4472136052 (4471d2) which, as we knowfor a parallel circuit, will always be lowerthan either R or

The parallel RL circuit can be calculatedin the same way.

Determining theResultant Phase Angle ofthe Series RCL CircuitThe scientific calculator has built intrigonometric SIN, COS and TAN functions.To demonstrate their use, consider aseries RCL circuit(see Figure 12) whereR = 10k51, and the reactance's of C andL at the applied frequency are 181(12 and25k52 respectively. Calculate the resultantimpedance and phase angle.

The formula for RCL series impedanceis as follows:

z = R2 + ocL - xd2 {19}

Note, if Xc is larger than XL, reversetheir positions. All values are in Ica


10 E 25-18 M x2

The circuit impedance is 12.211Q.


CIOFigure 10. Parallel RC circuit.

Figure 11. Parallel RL circuit.

R C LHA u_rmeme-vm_

Figure 12. Series RCL circuit.

Phase AngleTo find the phase angle, first ensure yourcalculator is in degrees mode, and notradians. Degrees mode is normally selectedby a key marked DEG , or similar.

The formula is:

PHASE 4) = tan-' Xc {20}


251- 181=11+110 1=1 SHIFT tan

The display will show 34.99The resultant phase angle is therefore

34.99°.

A More ComplexFbrmula for ParallelRCL ImpedanceMaking R = 5001Q; XL and Xc 251d2 and181d2 respectively (note, if Xc is largerthan XL, reverse their positions):

Z - R x XL x Xc {21}11XL2 x Xc2 + R2 (XL - Xc)2

Note the use of nested brackets. Allvalues are in kit.


©500 x25H 1818 1]

lx2

180 [la] 25111I1500P01( 25M E E

x1

The circuit impedance is 63.76162.

Finding Values of R andXc or XLIf we know the series RC or RL circuitimpedance and its phase angle, we candetermine the values of R and, Xc or XL,using the SIN and COS trigonometricfunctions.

As an example, make the series circuitresultant impedance 'Z' = 5k.C2 and thephase angle 34°. What is the value of Rand, Xc (for a capacitive circuit) or X,.(if inductive)?

If Xc and XL are both present, then thereactance is the difference between thevalues.

Fbrmulae:To find R:

R = COS 4) x Z

lb find Xc or XL:

Xc or XL = SIN 4) x Z


34 cos E 5 EXP 3

You will find that R = 4.145ka

34

{22}

{23}

You will find that Xc or XL= 2.796ka

Ratios of Current,Voltage and PowerGain/LossDetermining the gain of an amplifier(measured in dB), as either voltage, currentor power gain, gives us the opportunityto explore the log function key. Wherethe input and output circuit impedancesare equal, we can use the simple formulae:

Voltage gain (dB) = 20 log V0 {24}

Current gain (dB) = 20 log lout {25}

Power gain (dB) = 10 log Pout {26}

Example of Voltage GainVoltage in = 0.2V; Voltage Out = 5VCalculate the voltage gain.

Calculator input:

5+0.2=111 E 20

The voltage gain is found to be27.96dB.

Example of Current GainCurrent in = 5mA; Current Out = 50mA.Calculate the Current gain.

Calculator input:

50 E 5 E log 1x120

The current gain is found to be 20dB.Note, as the decibel measurement is a

ratio, we do not need to concern our-selves, in this instance, with the fact thatcurrent is measured in mA, or performany conversion.

Example of Power GainPower in = 20mW; Power Out = 5mW.Calculate the power loss.

Calculator input:

5 1] 20 E z 10The power loss is found to be -6.02dB.

Continued on page 48.

43

NV

FILTERPart 1: Why and How?

J. M. Woodgate B.Sc.(Eng.), C.Eng., M.I.E.E., M.A.E.S., F.Inst.S.C.E.

Regular readers will know that many of my articles mentionthe use of filters, so it seems appropriate to discuss them,generally, in a devoted series.

What are Filters, andWhat are They For?Originally, a filter circuit was one with adefined, non -flat frequency response. Theconcept has been extended to include cir-cuits with flat frequency response but, witha defined, non -flat phase response, includ-ing 'phase proportional to frequency'(which is a pure time -delay). Even furtherextensions of the concept have been madein software engineering, as opposed to cir-cuit design, but we will not be dealing withthose.

Filters seem to have originated in fairlyearly developments of long-distance car-rier telephone circuits, enabling one pair ofwires to carry more than one conversation.Although the tuned circuits used in earlyradio equipment were, in fact, filters, theywere generally not recognised as such.

Figure 1. A filter considered as a 'black box',with a source and load.Figure 2. Argand diagram showing sigmaand }omega axes, and two conjugatecomplex poles.Figure 3. Plot of Tis) as a surface on theArgand diagram.

(-0.71,+j0.71)

al .....-3 -2 -1

(-0.71 ,-j0.71)

+ 6.)

-2

Because the first filters were used inmatched transmission lines (the traditional'6000 circuits'), filter theory was con-strained for many years to this one appli-cation. Most filters, however, are not usedin transmission lines, and new designmethods have greatly eased the achieve-ment of near -theoretical performance,even of complex filters.

In principle, filters are used to discrimi-nate between signals by their frequencies.One of the most obvious cases is the passivecrossover network, used in multiple -driverloudspeaker systems, which is a set of filterswith their inputs connected in parallel tothe amplifier, and their outputs connectedto the appropriate drivers. However, thereare many other applications of filters incommon use; for example, treble and basstone -controls, and graphic equalisers arefilters, as are the fixed- and variable -

- 1

-2

- w

I +a2 3

tuned circuits in receivers. More clearlyidentifiable as filters are the circuits whichstrongly attenuate sub -carriers, such asthe 19kHz and 38kHz sub -carriers of thepilot -tone stereo radio system, and the4.43MHz colour sub -carrier of the PALtelevision system.

Filters are used widely in measuringinstruments; one basic purpose is to definethe bandwidth within which measure-ments are made. This is very important,since out -of -band signals may produceserious errors. I recall that soon after Ientered industry I was unable to obtainsensible answers in trying to measure thenoise level of an audio amplifier, whichseemed to measure far more noisy than itsounded. It was then pointed out to me thatthe meter I was using had a bandwidth of5Hz to 10MHz, and I was measuring thenoise in the whole of this wide band,instead of the 20Hz to 20kHz audio band!Since the noise voltage is, in principle, pro-portional to the square root of the band-width, the out -of -band noise could havebeen up to 22 times larger than the in -bandcomponent.

Modern Network TheoryThe old filter design methods referred to'constant -k' and 'm -derived' configurations.If you remember these, please forget allabout them. If filters of these types need tobe studied, that study should come afterthe new methods have been understood.Figure 1 shows a filter with source and loadimpedances (pure resistors), representedas a 'black box', i.e. we do not know (orcare) what is inside it, only how it behavesas seen from the input and output termi-nals. Generally, we are interested in theratio of output voltage Uo to source voltageEs, but current -mode filters exist, where weare interested in the ratio of output to inputcurrent The same design methods workfor both, but the circuits are, naturally,different.

Here, we go into a little mathematics, butit is nothing to be frightened of. The ratio T= Lb/Es depends on the impedances of thecomponents within the box, some of whichmust be either capacitors or inductors(both may be present); a filter cannot bemade exclusively from resistors! Thismeans that the ratio is, in general, a com-plex number, containing that mysterious

Figure 2. Figure 3.


1

0.8

g 0.6a.

cae) 0.4

0.2

00

Figure 4a.

2 3 4 5 6 7 8

C..) (angular frequency) rad/s

9 10

quantity j, the square -root of minus 1, alwaysassociated with 21C times the frequency f,to which the symbol w (omega) is given. Inother words, Tdepends on, or is a functionof, jw. The writing of equations is easier ifwe use s instead of jw. Furthermore, byallowing s itself to be a complex number, a+ jco, we can not only find out some veryimportant properties of the filter (as itaffects repetitive signals (sine waves orcombinations thereof)), but also its effectson impulsive signals (isolated pulses orsteps). The variable a is associated withthe energy loss in the circuit, while w isassociated with the energy storage (incapacitors and/or inductors).

In general, T(s), the transfer function ofthe filter, is the ratio of two functions of s,which we call NO (for Numerator) and D(S)(for Denominator):

D(s)

Now, interesting and important thingshappen when N(s) = 0, and when D(s) = 0.The first case is easier to understand;if N(S) = 0, T(s) = 0, so Uo = 0, i.e. theoutput voltage is zero: the filter has infiniteattenuation (in theory). Not surprisingly, thevalue(s) of Sat which this occurs is called azero. However, if D(s) = 0, the value ofT(s) is infinite (almost anything divided byzero is infinite). It does not seem very likelythat we can produce an infinite voltage inthis way! We have to remember that S isnot an ordinary number but a complexnumber. When we convert T(s) back into afunction of jw, and then derive the modulusby a mathematical process called rational-isation, we find that I T641 (read: 'mod Tof j -omega') is not infinite at any physicallyrealisable frequency. Even so, the values ofs for which T(s) is infinite are important,and they are called 'poles'. An example mayhelp; suppose:

N(s) = 1(simple), and:

D(s) = s' + as + 1(apparently not so simple, but we shall see!).

This combination of N(s) and D(S) repre-sents a second -order filter, so-calledbecause the highest power of s in T(s) is thesecond (52).

Then:

NW) is always 1, so there are no zeroes, while

D(S) = 0 if s = ÷_

i.e. there are poles at these two values ofs. Now, while we are concerned only with


0

-10

- 20

- 30

- 40

- 50

- 60

Slope zero. .

Semi-infinite..../approximotion

44.4444.4%,"%Bode fillet/ .,,o7

cove

0 1 1

a) (angular frequency) rad/s

Figure 4b.

10

Figure 4a. Response of the normalizedsecond -order filter plotted on linear axes.Figure 4b. Response of the normalizedsecond -order filter plotted on logarithmicfrequency and logarithmic (decibel)response axes.Figure 5. Second -order low-pass filter for zerosource impedance and 1l l load impedance.

sinusoidal signals, s = jw, so we cansubstitute for Sin T(S) to get T(joi:

Tqui) - -1co2 - iZjco - 1

We have to get rid of the j terms in thedenominator and we do this (rationalise)by using the fact that:

Ix +jyllx- jy) = +y2)

We also need the ideas of the modulus ofa complex number, written I (x + jy) I ,

which is an ordinary ('real') number'1)0 +y2, and the argument, which is anangle cp, equal to arctan(y/x).

Thus:

- (co' - 1) + 4jco(co - 1)2 + 2co2

and the modulus of this, which representsthe amplitude/frequency response of thefilter, is:

''cL'2-1)2+2w2(o)2- 1)2 + 2CO2

1

(CO2 - 112 + 2c02

1

Nia)4 A- 1

Now,whenS= -1/4 -±,a, a= -1/ aand co = ± 1/4. wean be either positive ornegative, but for practical purposes thenegative sign is of no significance: negativefrequencies are exactly the same as positivefrequencies. In fact, complex poles can onlyoccur in pairs, with j -terms of opposite sign(conjugate pairs). Furthermore, while thej -term may be zero (resulting in a 'realpole' the a term must be negative. It canbe shown that a positive a ('real') term

implies that the network possessing such apole is a net emitter of energy, i.e. thecircuit is oscillating. This is, of course,impossible for a network of only resistors,capacitors and inductors, and if we useactive devices as well, oscillation is not at alldesirable if we intended to make a filter.

The argument angle 0 is the phase -shiftintroduced by the filter, and thus:

4-2c0tamp =co2 - 1

There is one special case to consider: If,for some value of s, both N(s) and D($) arezero, T(s) is of the form 0/0, which can takeany value. In this case, we have a coincidentpole and zero, and we have to use deepermathematical analysis to find out whatI TOO I actually does. Luckily, this problemdoes not arise very often.

Graphical PresentationWe can present any complex number, a +/co, by a point (colon a graph; this is calledan Argand diagram (Figure 2). It is just likean ordinary graph, but it allows us to findgeometrical representations of complexnumbers, and operations on them.Consider the positive part of the a -axis.Movement to the right represents anincrease in the value of a. Movement to theleft of the origin (a = 0) brings us into thearea of negative numbers. If we draw anarrow (vector) from the origin to a point /on the positive a -axis, multiplication of thatvalue E by -1 brings us to the point -Land the arrow has rotated by half a turn (icradians or 180°) anticlockwise. So, multi-plying by -1 is equivalent to rotating thearrow by half a turn, and this is thereforeequal to multiplying by j twice. Thus, mul-tiplying by j is equivalent to turning througha quarter turn (it/2 radians or 90°), whichis the same as measuring along the co -axis.

This means that we can plot the pole loca-tions (- ji,d-2) of our example, andthey are conventionally indicated bycrosses, as shown in Figure 2. We do nothave any zeroes, but if we did - they wouldconventionally be indicated by small circles.

With the aid of the trusty computer, wecan actually plot a perspective view of T(s)as a surface above an Argand diagram(Figure 3), and we can indeed see the twopoles or, at least, the surrounding 'mountains'.

NormalizationIf we plot a graph of I Tqco)1 against co, wefind that it is nearly constant, at a value of1, until w approaches 1. The shape of this

45

graph is much easier to understand if weplot the value of 20log I T(ja. I instead, anduse a logarithmic frequency scale as well.This is, in fact, the 'level' of I T(ja)) I indecibels, relative to the response at zerofrequency, and the fact that decibels givesimpler plots of filter response shapes is amajor reason why electronic engineers usethem. Figure 4 shows the result, and, infact, the use of decibels makes the actualpoint plotting much easier because:

20log 1 - -10 log (w4+ 1)w4+ 1

We can note some important facts fromthis graph:

We have a low-pass filter, i.e. theresponse is nearly 0dB (or x 1) up to acertain freqUency, with no peaking, andabove that frequency it falls off uniformly.

The response is -3dB (i.e. the outputpower is half the input power) at thefrequency w = 1.

The slope of the response at highfrequencies tends rapidly to a constantvalue, which may be expressed as 12dB/octave (an octave is a frequency ratio of 2:1)or 40dB/decade (a decade is a frequencyratio of 10:1).

We can, therefore, draw two straightlines which give a close approximation tothe frequency response, except in theregion around the -3dB point.

These straight lines are called the 'semi -infinite approximation' (because the linesstart at a finite point and extend indefinitelyin one direction only), and the spacebetween them and the actual response isknown as the 'Bode fillet', after H. W Bode,chiefly noted now for a seminal book onfeedback amplifier design. The approxi-mation is very useful because, for example,we can tell immediately that, for all filters ofthis type, the response at five times the-3dB frequency is -28.5dB.

We seem to have made a low-pass filterthat begins to attenuate significantly at andabove the frequency co = 1, i.e. f = 112n Hz,which is 0.16Hz - rather a low frequency.This does not matter a bit, because todesign a filter to have its -3dB point atanother frequency, we can just carry out ascaling operation.

One circuit which gives the transfer func-tion 7-(5) of our example is shown in Figure5. This is a filter that works between a zeroimpedance source, and a 111 load. Inmodem circuit design, it is often convenientto provide a very low impedance source(e.g., an emitter follower or an op ampfollower) to drive a filter which, in turn,feeds a high impedance load (emitter fol-lower or non -inverting op amp). A filter de-signed the other way round, i.e. to be fedfrom a high impedance source (a transistorcollector, for example) and to feed a lowimpedance load (a common -base stage, for

0

10

20

30

40

50

601k

Figure 8.

10k 100kFrequency Hz

1M

Output 60dBgain at d.c.

OutputUnity gainat d.c.

example) would be an example of a current -mode filter.

There may be some surprise that thisexample contains an inductor. For manyyears, the use of inductors was very difficultfor the home -constructor because, neitherready-made components, or parts to makeinductors, were readily available. As aresult, inductors became unpopularthrough the 'sour grapes' reaction - 'wecannot get them, so we don't like them'.The situation is different now; both ready-made inductors and parts are available. Itis, however, necessary to bear in mind thatinductors inevitably also have resistance,and the inductor will not work well unlessits reactance (2)TfL) is much larger thanits resistance R at all the frequencies f ofinterest, i.e. its Q is reasonably high, saygreater than 10.

Figure 6. Frequency response of the 501(11z(- 3dB) second -order low-pass filter withloss -free components.Figure 7. Sallen and Key second -ordernormalized low-pass filter.Figure 8. First -order normalized low-passfilters with identical corner frequency w = 1.Figure 9. A first -order active low-pass filterwith a corner frequency of 160µHz (just forfun!).Figure 10. Third -order passive low-passfilters: (a) R /R' between 0 and 1; (b)between 0 and 1.Figure 11. Third -order active low-pass filter.


First-order

Figure 14.

Second-order Second-order To furthersections

Figure 12. General passive low-pass filterwith low source impedance and high loadimpedance.Figure 13. Basic first- and second -orderactive low-pass filter building blocks, or'sections'.Figure 14. Build-up of odd -order activefilters.

To design a filter to feed a load valuedifferent from 10, we can just carry outanother scaling operation. This means thatwe need only find out how to design filterswhose -3dB angular frequency w (notf)is 1,and whose source impedance is zero (ornearly so) and load impedance is 112. Thisconcept of reducing everything possible toa value of 1 is called normalization, and thescaling processes are thus denormalization.We could also find out how to designfilters with a finite ratio of load to sourceimpedance, but these are less often requiredin home -constructor designs.

A Usable Low -Pass FilterLet us use the above ideas to design a pass-ive LC filter with the following specification:

Source impedance: 1000Load impedance: 10152-3 dB frequency: 50kHzAttenuation at 200kHz: >20dB

The semi -infinite approximation inFigure 4 tells us that, for the only type offilter that we know about at present, theattenuation at four times the -3dB fre-quency (often called the 'cut-off frequency;which is misleading, or the 'corner fre-quency', which is much better) is 23dB,which comfortably meets the specification.

Because the load impedance is at least 10times the source impedance, we can use


the 'zero source/152 load' model, but theload is 10452, not 152. We do the scalingoperations as follows:

Frequency scale factor 2PIEF = 271 X 50 = 103

We divide all the inductance and capaci-tance values by2PIEF:

L = -It X 105

F= 1 -Cx Tr x

.2.25µ

Impedance scale factor Z =104

We multiply the inductance values by thisfactor, and divide the capacitance values(this is logical, because these operationsincrease the impedance of both types ofcomponent). We get

L = 4.5 X 10-6 X 104 = 45mH

C = 2.25 X 10-6 X 10-4 = 225pF

Effects of LossesThe theoretical response of a filter withthese values is shown in Figure 6. However,we have assumed that the components areideal, i.e. that the inductor has no resistanceand the capacitor no losses. The effectsof losses are quite difficult to deal withtheoretically, and the best course of actionis to aim for low losses. This means ferrite -cored, rather than air -cored, inductors atlow and medium frequencies, and low -lossceramic, silver mica, polystyrene or poly-propylene capacitors, rather than polyester.However, there is another solution.

Active filtersThe LC filter, taken as an example above, isonly one way of arriving at the chosen T(s).Consider the circuit shown in Figure 7,

which is an example of a unity -gain single -feedback or Sallen and Key active filter(named after the two authors who fullydescribed this type of filter). Since itsoperation seems mysterious, a detailedanalysis is called for, but to make itreasonably brief, we will use two somewhatsophisticated ideas in circuit analysis. Thefirst is simply to use s instead of jco, whichis not a new idea now, and the second is torecall that, using s, the current / through acapacitor C is simply SCU, where U is thevoltage across it: SC is the susceptance of C,the reciprocal of the reactance.

Since the op amp follower has a gain(from the non -inverting input to the output)very close indeed to 1, we have:uo = U2 =

SC2so that

= h(1 +1 (because Rz = 1.(2)Also

/2 = (U1 - U9)SCI

and

Es = U, + + i2) x 1 (because R1 = 10)

Thus

= U0SC2

so that

= Uoll + saland

/2 = U0S2CIC2

Therefore

E5= Uoll + 2SC2 + S2C,C2)

and 1

T(s) = 1 + 2s1.4 + S'U/ U2

Now, the denominator D(s) is a quadraticin s, so that, if the roots of D(s) = 0 (i.e. thepoles) are S = -a ± A we have:IS + a + PIS + a - J13) = 0,so that

s' + 2aS + 132) = 0

or1 s2 2a ) s +1 = 0

k ce+/3z k a2+/32

Hence

acz-a2+fp,also

2C2 - 2a

Thus

a= oe+/32'also

G= 4f -Now, we know that the poles are atS= -11,1T± soa= 1/a,and/3=1/aas well. Therefore:

Cz = 1/>(Z and C, =

These values are in farads (!) because thisfilter, too, is normalized. We have to scale forfrequency and impedance, as we did for theLC filter. However, there is one difference:our new filter has a highlish) input imped-ance and a low output impedance, whichdoes not depend on the values of the filtercomponents. So, we do not use the imped-ance scaling to suit the load impedance but,to get practical values for the components.Instead of 10 resistors, the popular 10kg isquite suitable. So our Z factor is 104, and the

47

Second-order

Figure 15.

Second-order Second-orderTo further

sections

capacitor values change to 71µF and 142gF.These are very big because co is still equal to1. Suppose we want the corner frequency tobe 15kHz; the 2PIEF factor is 27t x 1.5 x 104and the capacitance values change to 753pFand 1.51nF, which are very practical values.Note that the resistor values do not changein the frequency de -normalization process.

Filters of Other OrdersWe started with a second -order filterbecause the pole -zero analysis of first -orderfilters is so trivial that it is difficult to extendto second- and higher -order filters. Figure8 shows two identical passive first -orderlow-pass filters which have a pole at S = 1,and a -3dB response at co = 1. The finalslope of the response, above the cornerfrequency, is 6dB per octave, or 20dB perdecade. In fact, for the types of low-passfilter that we shall study, the final slope isalways 6ndB per octave, where n is theorder of the filter. It is possible to make first -order active filters, as shown in Figure 9,but they are normally only used where thecorner frequency is to be very low, suchthat a passive filter would require very highresistor and capacitor values. The gain ofthe op amp, controlled by the externalfeedback resistors, is used to multiply theeffective capacitance value; for example,with a FET-input op amp set for a closed -loop gain A of 1,000, a 1MS/ resistor and ali.LF capacitor, a low-pass filter with acorner frequency of 1/27 ACR = 1.6 x10-4Hz can be made. Such filters are notnormally required for home -constructorprojects, but you never know....

First order filters always have a single realpole at a = 1. Third -order filters can bemade with two capacitors and an inductor(or vice versa), as shown in Figure 10, or bycombining a passive first -order filter withan active second -order filter, as shown inFigure 11. The procedures for findingcomponent values for filters of higher thansecond -order are not simple.

TABLE 1Order L1 Cl L2 C2

2 1414 0.7073 1.500 1.333 0.5004 F531 1.577 1.082 0.383

TABLE 2Order Cl C2 C3

2 1414 0.70713 3.547 1.3920 0.20244 1.082

2.6130.92400.3825

Figure 15. Build-up of even -order filters.

Table 1. Component values for Butterworthpassive low-pass filters.

Table 2. Component values for Butterworthactive low-pass filters.

Butterworth orMaximally -Flat ResponseWe noted that the response of our second -order filter was 0dB at very low frequencies,and did not peak or rise above this value atany frequency. This characteristic is called'maximal flatness', and filters with thisproperty are called 'Butterworth filters',after S. Butterworth who first describedthem (in 1930). He showed, mathematically,that the sharpest possible rate of increaseof attenuation, coupled with the absenceof peaking, is achieved if D(s) is what hasbecome known as a 'Butterworth poly-nomial'. 'Polynomial' simply means a sumof powers of s. There is a procedure forgenerating Butterworth polynomials ofany order (highest power of S), but for thepresent purpose a list will suffice:

Butterworth ButterworthOrder polynomial in S polynomial in jco

1 S '1+W2r2 S' + N2s + 1 N 1 + Ctri3 s' + 2s2 + 2s + 1 i 1 + of

Beyond the 3rd order, the coefficients arenot simple and can only be expressed, inpractice, as numerical approximations.Notice that the forms of the polynomialsare quite different in terms of s and fru: it isimportant not to use the wrong form!

Component Values forButterworth PassiveLow -Pass FiltersExtensive tables of component values arepublished in handbooks on filter design.For the home -constructor, filters of higherthan 4th order are rarely required, and wecan concentrate on the case with very lowsource impedance and high (but finite) loadimpedance. Figure 12 shows the generalcircuit form, and the normalized designdata is given in Table 1.

Component Valuesfor Butterworth ActiveLow -Pass FiltersThe general forms of active low-pass filtersof the Sallen and Key type are shownin Figures 13 to 15. The third -order filterconsists of a second order section and afirst -order section. The fourth -order filterconsists of two second -order sections incascade. Once again, tables of componentvalues for filters of many orders are foundin published handbooks. A few values aregiven in Table 2, which will cover themajority of fairly simple applications.

Active or Passive?Both types of filter have their advantages:For low -frequency, small -signal use, activefilters are less costly and more accurate,since capacitors are much less lossy thaninductors. It is a pretty poor capacitor thathas a Q of 100, whereas that is a high valuefor an inductor. However, they need apower supply, and accuracy falls off abovethe audio frequency band unless wide -band op amps are used. There are alsosome types of filter (not low-pass) whichrequire many op amps, but rather fewcomponents in the passive version.

Why Low -Pass Filters?We have only looked at low-pass filters thistime because it is possible to deduce thecharacteristics of most other types of filterfrom them: high-pacs; band-pass; and band -stop. That will be the subject next month.

IMPROVE YOUR CIRCUIT DESIGN - Continued from page 43.

SummaryI have attempted to show how easy it isto improve your circuit design with theuse of the scientific calculator. As you willsee from examining the array of keyspresent, there are many that we havenot used - nor do we need to.

The secret of good circuit analysis isknowing what to expect from the circuitthat you are designing. To ensure thatyou get accurate results from your analy-sis, it is helpful to be able to break yourcircuits into manageable parts. A complexcircuit will have many current paths inparallel with the ones you are trying tomeasure, and they are not always easy to

48

spot. To avoid this problem, design yourcircuit in modules, making sure that youcheck each stage before proceedingfurther. Ensure that the DC conditions arecorrect before determining the values ofcoupling or decoupling capacitors.

Where a capacitor appears in parallelwith a resistor, the combination willpresent an impedance to the AC signalwhich, as we have seen, can be calculated.You probably will not be able to measure'Z' directly, but it could represent apossible design error by effectivelyshorting -out your AC signal (at oneparticular/all frequencies). I have shownthat you can easily determine the AC gain

of your amplifier by using a few key-strokes (in the right order of course!).

Any good textbook, of which there aremany, will provide a comprehensive listof commonly used formulae. Practisewith your scientific calculator, and checkyour answers with those (usually)supplied. As you practise, you willbecome familiar with certain componentcombinations, which can be easilyremembered. For example, the reactanceof a 0.1µ.F capacitor will be approximatelythe same as that for a 250mH inductorat an applied frequency of lkHz, or whenthe impedance phase angle is 45°, theresistive and reactive values will be equal.


Text by Robin Hall

PROJECTRATING

How often have you thought,that you would like to checkthose spare unmarkedcapacitors and inductorslying about in the bits box'?They are normally kept withthe thought that one daythey might be useful, butwhen the heat is on tocomplete a project, they areleft behind. Well, this neednot be the case!

THis project will enable the hobbyistto measure those unmarkedcapacitors and inductors, and

find out if they are in satisfactoryworking order, or finally retire them tothe rubbish bin. Also it will be usefulwhen having to make up preciseinductor values for other projects,especially if they are not readilyavailable as off the shelf values.

The operation of the IJC MeterAdaptor is straightforward oncecalibrated. A digital multimeter(DMM) or high impedance analoguemultimeter is required to display thevalue of the component and is usedin conjunction with the LJC MeterAdaptor.

Circuit DescriptionReferring to Figure 1, which showsthe block diagram for the L,/C MeterAdaptor, and Figure 2, the circuitOctober 1994 Maplin Magazine

SpecificationDC power supply

voltage: +9V DCSupply

current: 7 5mAInductance

ranges: 10pH to 10mHCapacitance

ranges: 1pF to 2pFPCB size: 120 x 102mm

Above right: Theassembled PCB inthe optional case.Right: Close-up ofthe Assembled L/CMeter Adaptor PCB.

diagram, circuit operation is asfollows:

The power for the circuit is obtainedfrom a +9V PP3 battery which is thenregulated to +5V DC by a 7805regulator.

There are effectively two circuits,each working separately, on theboard. One is the capacitance sectionand the other the inductance section.Both operate on similar principles,that of comparison with a 'reference

* Easy to Build

* On -board Regulator

* Calibrationcomponents included

Please note the caseshown is not includedin the kit and must bepurchased. separately.

L.C-1

component. The difference betweenthe two comparators is thenconverted to a voltage and this ismeasured by the meter. Using verysimple multiplication factors thesemeasurements can be equated witheither inductance or capacitancevalues, depending on the form ofmeasurements being taken. Boththe capacitance and inductancemeasuring circuits have two ranges,for high and low values.

49

The CapacitanceSectionThe capacitance section usestwo separate comparators. Thecomparators form the basis of twosections, one which has a referencecapacitor C5, located on the PCB,and the other section which has thecapacitor under test. An oscillatorrunning at about 400Hz is basedaround IC Ul:F; R2 adjusts thefrequency of operation, and sets thelow capacitance calibration point.

R7 and R8 are switched out of circuitin the low range, and the oscillator'ssignal is coupled to the unknowncapacitor through R6 and Dl. Twovoltage dividers are used, one formedbetween R6 and the capacitor undertest, and the other by R8, R10 and 010.When the voltage across C5 reachesthe trigger point of the Schmitt triggerUI :D, it switches. As the value of C5 isconstant, IC Ul:D will always switchat the same point - it is known asthe 'reference comparator'.

The unknown capacitor usesUl:E, another Schmitt trigger, andin this case when the trigger pointis reached, it also switches - at adifferent point because of thedifference in capacitance betweenC5 and the unknown capacitor. Whenthe outputs of the reference and thetest comparator are compared, anaverage DC voltage is generatedand this is taken to sockets J1 and J2to be read by an analogue or digitalmultimeter. The larger the capacitorvalue the longer it will take for the testcomparator to switch. This presents aneven lower average DC voltage on

comparator, as in the capacitancesection.

A simple oscillator is formed by ICUl:A, C7, C9, R12, R13, R16 and R17.Two separate frequencies areavailable in this section, a highfrequency for small values ofinductances and a low frequencyfor larger values of inductances. Theoutput of this oscillator runs into abuffer stage using IC Ul:B, and is fedthrough a current -limiting resistor R18.

the test comparator's output resultingin a larger difference between thetwo comparators. The DC voltage isaveraged by C2 and provides astable reading for the meter.

The InductanceSectionThe inductance section operates ina similar fashion to the capacitancesection, but it does not require a

Inside the assembledL/C Meter Adaptor.

Capacitancelow adjust

S3A

Tc/7 B1

9V

U1:F

Oscillator400Hz

Regulator+5V

Voltagedivider comparator

Test

jack Capacitanceunder test

D1R6

U1:E

CapacitancetestHigh

S1A

Capacitancehigh adjust

0Low°

HighCapacitancezero adjust +5V

R11B Standard

comparatorDCOff

Low° SetUl:D

High

Voltage

1_

dividerR13

S1 C Si DR17

0 0 BufferstageLow

Oscillator R18U1:B

U1 :A Currentlimiter

L

JInductanceundertest

Out

-Vout

InMultimeter

Out

+Vout-0

In

Comparator

J

U 1 : C

Figure 1. Block diagram of the L/C Meter Adaptor.


Capacitance

-1=}R1 low adjusiti

47k

13

u01750nF

S3:A

B17 9V

U1:F

74HC14

cw

12

VR1

7805In Out

R2250k

+5V

C3 C4710uF 10uFT

HighSi

0 -Low

R121=122k

-C9710nF

R16-6-1=1 R17

cw 25k

01

1N4148R6-1=1-1M

High

R3 11

10k

U1:E

74HC14

S1:A R7

-0 2700 -.- R82k2cwLow

10

Capacitancehigh adjust

R4 R5

10k

R9-1=1

22kD2

R10

Capacitancezero adjust

cw 25k

1N4148

High41:B

-0 LowInd.

adjust

R13cw 25k

22kou C771nF

Ind.adjust

+5V

U1:A2

High

row

74HC14

U1:0

74HC14

C5390pF

U1:B

74HC14

100k

C2luF

C101Out10nF

Function.Inductance = INCapacitance = OUT

Out

0

-V Out

(0)In J1

Multimeter

+V Out

(0)In J2

Test jack6

R18CI

J3

n R20U1000

2700

Cap.

+5V

Ind.

C6m 77 D3 (-)t Ind.zero10nFT -Y-1N41483 R14

33k

25k

U1 :0

74HC14

C8T1OnF

R19I=110k

R15100k

Figure 2. Circuit diagram of the L/C Meter Adaptor.

This resistor and the coil under testpresent a voltage divider to the inputof the next comparator. When theinductance is small, it presents verylittle AC voltage drop across the coilbeing tested, and a very short voltagepulse is read by the IC UI :C in thenext stage. The averaged DC levelof these pulses is fed to the socketsJ1 and J2 and then to the multimeter.A larger value of inductance willpresent a longer duration pulse on ICUl:C, resulting in a higher averagedDC output.

Hints and TipsThere are a number of suggestionsthat could enhance the use of theL/C Meter Adaptor preferably beforecompletion. Using an IC socket isgenerally good practice, but notessential. One is not provided in thekit, and if one is to be used a 16 -pinDIL IC socket is required.

Afresh' 9V PP3 battery isrecommended for use in the unitwhen built. This way the circuits willperform at their optimum levels withless likelihood of errors being made.To finish off there is a case availablefor the kit (RU39N), and this includesparts not supplied with the main kit.There are a number of boxes from The kit as supplied.


the Maplin Catalogue that could bemade equally suitable as a casewith some drilling.

A suitable DMM such as one ofthe White Gold, Precision Gold orAcademy models can be used, andare available from Maplin; or a highimpedance analogue type with atleast 1MQ input impedance.

ConstructionThe 1./C Meter Adaptor kit includes thePCB, the legend and track are shownin Figure 3, and all the componentsrequired to build the project,excluding the case which is availableseparately. Sort out and identify thecomponents, there are some verygood instructions supplied with thekit, and they show a logical path tofollow. If you are new to projectbuilding, refer to the Constructors'Guide (order separately as XH79L)for helpful practical advice on howto solder, component identificationand the like.

Start first by soldering in the testconnector. This must be mountedso that it hooks onto the board formechanical stability, Now identify theresistors and fit and solder to the PCB.Install the trimming potentiometersand solder in position.

Next identify the diodes makingsure that the are correctly orientatedon the board. Fit and solder thecapacitors, making sure that theelectrolytic capacitors which arepolarised are fitted correctly. Thepositive lead is slightly longer than thenegative, which is normally identifiedby a series of (-) symbols markeddown one side of the capacitor.

With wire off -cuts from either theresistors or capacitors, preform theoff -cuts into jumper leads, then fitand solder to the relevant positionsin the board.

Install the voltage regulator, withthe metal heatsink facing C4. Nextfit IC Ul, making sure that the IC iscorrectly orientated on the board.A 16 -pin DIL IC socket is not suppliedwith the kit but if required should befitted beforehand also noting thecorrect orientation.

Locate and fit the switches, makingsure that each switch is flush againstthe PCB. Now locate the wires fromthe PP3 battery -clip through theappropriate holes in the PCB, andsolder in position.

Set-up and CalibrationMake sure that the power switch S3is off before connecting up the 9VPP3 battery. Connect your analoguemultimeter or DMM to the outputsockets of the kit (J1 and J2) and setit to a low voltage range 2 to 2 5V DC.

Use a small screwdriver or trimmingtool and rotate all the potentiometersto the centre of travel. Next switch onthe power and select theCapacitance Function (S2 in the 'OUT'position). Note the reading on themeter should be fairly close to zero,

tr) .

0.1

0oc

v-- LJJ_J

04 >"" 201140 S1TO3J3 Y32MAS1.-0002Z) WqM r. V3S1 r-0JSe\e\er

a)_J 04

Meill 1:1. A321 1-01 Z6/6/01.SOINO2A10313 A3SINV2:1

Figure 3. PCB legend and track.


if this does not read zero then adjustR10 until it does so. Some autorangingdigital volt meters will change tomillivolts, on other types you mayhave to switch to a more sensitivevoltage range to make sure that itis zero. If you do not get a readingof zero volts, switch off and checkthe components and the PCB forany assembly mistakes.

If all is well, select the InductanceFunction (S2 in the IN position),the reading should now readapproximately 2 5V but thisis not critical.

Now place a wire jumper acrossthe pins of J3 and adjust R11 until areading of zero volts is obtained onthe meter. Remember that the pinsin socket J3 are doubled up, and itis possible to accidentally put thewire between commoned pins. Thiswire simulates an extremely smallvalue inductance. Switch back to theCapacitance Function (S2 OUT), andselect the low range (S1 IN). Insert a

Figure 4. Typical interconnection between the unit and multimeter.

1 nF capacitor (marked 1 nF or 102)and adjust R2 until a reading of IVis obtained. Next, select the highrange (S1 OUT), insert a 100nFcapacitor (marked 0.1 or 104) andadjust R8 until a reading of 100mVis obtained.

The L/C Meter AdaptorRange Multiplication Factor

Inductance High Range 1 my - 10pHInductance Low Range ImV = IpH

Capacitance High Range I mV = 1 nFCapacitance Low Range ImV = 1 pF

Table 1. Multiplication factors for the conversions.

Available RangesLow High

Inductance 10pH to 800pH 100pH to 10mHCapacitance 1pF to 2000pF 1000pF to 2,uF

Table 2. Available ranges.

Switching to the inductance range,leave it on the high range, insert the1800pH inductor (this is a large roundgreen component marked 182) andadjust R13 until a reading of 180mVis obtained.

Now switch to the low range, andinsert the 100pH coil (this is a smallround green component marked101K), and adjust R17 until a readingof 100mV is obtained.

The kit is now fully tested andcalibrated and ready to go.

OperationNote the multiplication factor ofthe readings, as shown in Table 1,when using your multimeter, also therange you are using on the L/C MeterAdaptor, Figure 4. Remember, thatthe high and low ranges of thecapacitance and inductance sectionsare quite different. Table 2 shows theavailable ranges obtainable with thisproject.

L/C METER ADAPTOR PARTS LISTRESISTORSR20 1001/ 1

R7,18 27052 2R3,4,19 10k 3R9,12,16 22k 3R14 33k 1

RI 47k 1

R5,15 100k 2R6 1M 1

R8 2k2 Potentiometer 1

R10,11,13,17 25k Potentiometer 4R2 250k Potentiometer 1

CAPACITORSC5C7C6,8,9,10ClC2C3,4

390pF Disc Ceramic1nP Disc CeramiclOnF Disc Ceramic50nF Disc Ceramic1pF 50V Radial Electrolytic10pF 50V Radial Electrolytic

SEMICONDUCTORSUl 74HC14 Hex InverterVR1 7805 5V RegulatorD1-3 1N914 (1N4148)

MISCELLANEOUS1nF Capacitor100nF Capacitor

1

1

41

1

2

1

1

3

1

1

100pH Inductor1800pH InductorPCB

S1-3 Switch ArrayBattery HolderPP3 Battery Clip

J1-2 Banana JacksJ3 6 -pin Test Connector

121n. Hook-up WireInstruction Leaflet

1

1

1

1

1

1

21

1 PM1

Note that these components are used for calibration only.

OPTIONAL (Not in Kit)CLC Plastic Case

(Includes Knob Set)9V PP3 BatteryConstructors' Guide

1 (RU39N)1 (JY49D)1 (XF179L)

The Maplin 'Get -You -Working Service is availablefor this project, see Constructors' Guide or current

Maplin Catalogue for details.The above items (excluding Optional) are

available in kit form only.Order As RU38R (L/C Meter Adaptor) Price £39.95

Order As RU39N (CLC Case) Price £14.95

Please Note: Some parts, which are specific to this project(e.g., PCB), are not available separately.


CORRIGENDA

Issue 79/July 1994.Model Train Chuffer, page 8, PartsList, RV1 & RV2 were incorrectly listed asUH15R the correct code is UHO2C.

Issue 81/July 1994.In This Issue, page 1, page numbersincorrect, Getting To Know TestEquipment is on page 39: Secrets ofSurround Sound Revealed is on page52.

PWM Drill Speed Controller, page 21,Figure 8b, the budge rectifier is shownincorrectly. The correct onentation of thesymbols are as follows: top left negative(-), top right ac (-). lower left ac (H, andlower right positive (+). On page 22,Figure 9a, the rectifier is again shownincorrectly, the correct symbols shouldshow top left negative (-). top right ac(-), lower left ac (-), and lower rightpositive (+). The red and blue wires fromthe transformer must be connected tothe top right ac (-) connections on thebridge rectifier, and the yellow and greywires must be connected to the lower left(-) on the bridge rectifier. The red wire

IN (Re) OUT (12x)

C1

.,

, LiI._. 00-

ALLCH1

CH2

R1

C1

L.Z i-I ---0. Li.

CS+

4 GNO

[1

CE I_=

113

r'i 11,)013JT 1-1IC1

2 GND

A

from the positive (+) on the rectifier mustbe connected to the positive (+) locatedtop middle of the PCB and the black wirefrom the negative (- -) on the rectifier mustbe connected to the negative (-) top lefton the PCB.

Parts List, page 23, optional footcontroller XY28F missing from Optional List.

VHFUHF Preamplifier, page 33, circuitand block diagrams are incorrect, seebelow. Photos la and lb, are transposed.Please note that on issue 1 PCBs there isno positive (--) symbol shown on the legend

A Figure 7. PCB legend of theB Figure 3. Circuit diagram ofC Figure 2. Block diagram of tD Figure 9a. Low -voltage and

PWM Drill Speed Controller.

VHF/UHF Preamplifier.the VHF/UHF Preamplifier.he VHF/UHF Preamplifier.transformer primary wiring of the

Red

Blue

.. ..a

'11111111,01.......k.s.....1,........,....... Transformer

Yellow114111111Grey Olt

Black-d. Red

Orange

11L

4/_Jr"---)r"---)A e,

n.Ar

I

Black

Red

(11( )...4(1.'...4

1= 'rr,7---IT,:1 1: 11 ;1 To Main

llll Switch

Black RedII

I iliiiill''."'"'"')1 1 connectors

0 01 & 4

..1. v.

Push -OnRecep Cover

Switch 1111 111shown in\ OFF position

D

IIRF Choke? IIRF Choke1 i

RF

RF Input MAR -6 Output

[><1[>1 ti I

DC Output DC 419dB DC DCLPF HPFBlocker Blocker Input

C

for C5. C5 should be orientated with itspositive lead nearest Ri.

Getting to know Test Equipment,page 40, incorrect text, see correct textbelow; caption lower left should read"Right: A typical moving coil panel meter- 100pA."

When in series, the converter (knownas a multiplier), effectively raises the totalresistance, allowing the combination ofmovement and multiplier to measure ahigher voltage. When in parallel, theconverter (known now as a shunt)effectively lowers the total resistance,allowing the combination to measurehigher currents. Note that in both cases,no more current passes through themovement's coil than before - i.e. onlythat sufficient to produce full-scaledeflection of the pointer.

With an external power source such asa cell, the movement can also be usedto indicate values of resistance. Byconnecting the movement in series witha cell and the resistance to be measured,the pointer indicates the current flowingthrough the resistor. As this current, fromOhm's Law, is inversely proportional tothe resistance, the indication on thescale of the movement is also inverselyproportional to the resistance. In effect,the scale graduated for resistance mustbe marked in the opposite direction tocurrent and voltage scales - that is, thefurther up the scale the pointer isdeflected, the lower the resistance beingmeasured.

Using mechanical switches to switchmultipliers, shunts and cells in and outof circuit with a moving -coil movement,an analogue multimeter - one capableof measurement of many ranges ofvoltage, current and resistance - can beconstructed. Such an instrument is alsoknown (particularly across the Atlantic) asa volt-ohm-milliam-meter (VOM). Thebasic multimeter is shown, in blockdiagram form, in Figure 2.

DC Voltage MeasurementsA typical configuration of multipliers andswitch, for a 50pA meter with a coilresistance of 200052, is shown in Figure3. Six ranges are available, five of which(2.5V, 10V, 50V, 250V, 1000V) areswitched into the circuit via themechanical rotary switch, while the sixthrange; 5000V, is selected by the user by

connecting the positive probe into aseparate connection socket that hasa 80MS2 resistance in series. A sep-arate socket is needed for tworeasons. Firstly, rotary switches cannotalways withstand voltages over 1000V.Secondly, plugging a probe into aseparate socket for such high -voltagemeasurements tends to give the user a'psychological push' into taking extremecare with the measurement.

With the switch in the 2.5V rangeposition, it is easy to calculate that thetotal series resistance is 50k12 (that is,48k11 + 2k.(2), so the input resistance(see later) of the meter is 50.000/25,or 20,000S2N. This is a good sort ofinput resistance to aim for - any lower.and the circuit you're testing with themultimeter may be loaded by the meter'sresistance.

DC Current MeasurementFigure 4 shows a moving -coil movement,shunts and switch, connected into a DCcurrent measuring circuit. This particularmeter circuit is known as a ring -shuntconfiguration. The ring -shunt's mainadvantage is that the movement isalways shunted, even when the rangeswitch is turned from one position toanother. This protects it from accidentalburnout even if the test leads areconnected to a 'live' circuit.

AC Voltage MeasurementsThe circuit for a typical AC voltagemeasurement is shown in Figure 5. It issimilar to the DC voltage measurementcircuit of Figure 2. but the inputresistance is lower (around one third ofthat of the DC circuit). This is mainlybecause the applied AC voltage must berectified by a diode rectifier before beingapplied to the meter. The metermovement responds to the average valueof the rectified AC voltage - 0.318 of thepeak value for a half -wave rectifier, as inthis example. In turn, therefore, the inputresistance...

Intelligent Split Charge Unit, page 49,first paragraph right column should read:Remove the +12V connection...

20 Metre 20W Linear Amplifier, page62, second paragraph, stock code forthe 40 Metre 20W Linear Amplifiershould read (RU33L).


1994 Catalogue 44'

4part from being an excellent referencesource, listing and describing in detailover 200 components, our newcatalogue answers many of yourmost asked questions.

Bang up to date with sections on fixed andmotonsed systems, distributing UHF andIF signals and much more. Probably the best-.£1.9.6 you'll ever lend..Inc. post and packing. Refunded on next purchase

TRAC Commerc o waySkippers LanaMiddlesbrough

Cleveland TS6-6UR

Last few Ferguson SRBI D2MACconverted BSB receivers at theexceptional price of £29.95 !Brand New boxed Ferguson .5RB1 receivers withD2MAC software for reception of CLEAR D2MACsmissions : Currently 4 German channels fromTVSAT, plus ARTE and occasionally MCM fromthe French sat TDF1, all with digital sound.On screen graphics, 16:9, Audio Mix, andother usual MAC refinements.P&P 16.90 Matching sqaurials .E20.00

0642 468145 IE210642 452555

FAX 0642 440927 visa

TRACtran-

There are more terrific projects and fea-tures heading your way in next month'ssuper issue of Electronics - The MaplinMagazine, including:

PROJECTSTELEPHONE BELLREPEATERQuite often, the telephone cannot beheard in other parts of the house, or in

the garden, shed or garage. This simplePhone Repeater is very useful, to thosepeople with only a single phone socket(or even a hard -wired phone). TheTelephone Bell Repeater 'sits' besidethe telephone, and a piezo buzzer isremotely wired from the TelephoneBell Repeater to the required 'repeat'position.

MODEL TRAIN HORNAND WHISTLESound -effects can be used to add anextra dimension of realism to anymodel train layout. With the introduc-tion of digital recording techniques, it isnow possible to obtain good qualitysound reproduction. Since the soundsamples for the train horn/whistle are ofa relatively short duration, it is practi-cal to store them in a non-volatile dig-ital memory IC. This is an excellentcomplementary project to go with theTrain Chuffer in Electronics Issue 79.

41 8MHZ ENCODEDTRANSMITTER ANDRECEIVERThis project together with its compan-ion, the 41SMHz Encoded Receiverare a boon for security and other appli-cations. Using these projects means thatstand-alone or wire -interconnectedequipment can now be linked to goodeffect by totally wireless means.

400W MONO/STEREOAMPLIFIERThis superb amplifier is small and com-pact, and it certainly packs a punch! Itis very versatile, being ideal for domes-tic use, car or caravan, and PA systems.

FIATUOISSpecial features include an informativelook at 'All About Intcrnee; 'DesigningOp Amp Stages' explains about theoperational amplifier. Two new series,'Digital Signal Processing' an introduc-tion to this vast subject with exampleprograms written in QBASIC;'Repairing Radios' takes the mysteryout of radio repairs. Other featuresinclude 'Quo Vadis? Computers andInformation' about the direction ofInformation Technology and FutureElectronics, plus the concluding part of'Mains Safety'.

PLUS ALL THE REGULARITEMS FOR YOU TO ENJOY!

ELECTRONICS -THE MAPLIN MAGAZIN' EBRITALN'S BEST SELLLNG ELECTRONICS NL-1GAZLNE

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maiF+85

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4071

RB 3.33 W.iMy

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use 3.3 RI

IC2

Runs onPC/XT/AT/286/386 withHercules CGA, EGA orVGA and manyemulations.

Design Single sided,Double sided andMultilayer boards.

Use Surface Mountcomponents.

Standard output includesDot Matrix / Laser / Inkjetprinters, Pen Plotters,Photo -plotters and NCDrill.

Award winning EASY -PCis in use in over 18,000installations in 80Countries World -Wide.

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For full info' please phone, fax or write to:

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DESIGN


2PROJECTRATING

Text byAlan Williamson andDean Hodgkins BEng(Hons)This project has been designed to protectHi-Fi loudspeakers from damage due to faultsoccurring within the output stage of a poweramplifier. A fault of this nature nearly alwayscauses a large DC current to flow, and it isthis current which 'burns out' the loudspeaker.This module will 'disconnect' the speakersas soon as a DC voltage is detected.

SpecificationMaximum input voltage:Maximum contact current:Minimum safety voltage:No external supply neededPCB:

90V DC10A10V DC

40 x 67mm

Photo 1. Close up ofassembled PCB.


HE module can be mounted inthe loudspeaker cabinet, andis also suitable for protecting

car stereo systems (one moduleper channel, of course). Note thatthis project must not be used inconjunction with traditional valveamplifiers; they do not require thistype of protection, and the use ofthis module could cause extensivedamage to the amplifier. This moduleis also unsuitable for amplifierswith less than 10Vp, present atthe output.

Circuit DescriptionThe circuit (shown in Figure 1)is very simple; the resistors R1 toR4 provide current limiting throughthe Zener diode chain. Note thatthe number of 5W resistors fittedwill depend upon the peak outputvoltage of the amplifier (see Table 1).

The 5W resistors have a dualfunction: (a) they form a potentialdivider with the relay, and (b) theydetermine a time constant (in

+LS IN0

Frompower

amplifier

-LS IN0

R1

68R

R268R

ZD14.3VZD24.3VZD34.3V

ZD44.3VZD54.3VZD64.3V

o C2I2200uF

C31=2200uF

RY)12V

R368R Loudspeaker

R468R

ELC1-1uF

+LS OUT

0 LS OUT

Above left: Figure 1. Loudspeaker DC Protectorcircuit diagram.Above right: Photo 2. Assembled PCB installed in anamplifier.Below left: Figure 2. Loudspeaker DC Protectorwiring diagram.Below right: Photo 3. Assembled PCB installed in thespeaker cabinet.

- I

V)

+0+LS IN+ -LS

PowerK4701 -/

amplifier V) I--LS IN1IC

Amplifierpeak output

voltage10 to 30 30 to 50 40 to 70 50 to 90

Power outputinto 8Q load

(Wr7115)

6 3 to 56 3 563 to 156 3 100 to 3063 1563 to 506 3

Power outputinto 412 load

(Wrms)125to1125 1125to312.5 200 to 6125 312 5 to 1012 5

Number of 5Wresistors fitted 1 2 3 4

Table 1. Resistor build table.

conjunction with the electrolyticcapacitors C2 & C3) that providesa short time delay, which preventspremature 'drop out' duringtransient clipping.

The Zener diodes prevent'over voltage' of the electrolyticcapacitors in the event of therelay coil becoming open circuit, aswell as clamping the induced emfvoltage across the relay coil duringthe collapse of the magnetic field.Note that half of the Zener diodesare orientated in the opposite

direction to the others; this is toallow an equal clamping voltage ofeither polarity to be provided.

In the event of the protectioncircuit prematurely 'dropping out'due to excessive clipping (especiallyat low frequencies), the DC voltagepresent at the output of theamplifier will operate the relayafter a short time - determinedby time constant; the capacitorC1 will 'snub' the relay contacts,and prevent arcing when the load(the speaker) is reconnected tothe amplifier's output.


tw-Getting toknow

PART 3Oscilloscopes

PMENTNo electronics lab, amateur or professional, hobbyist orindustrial, school, college or university, is complete with-out the oscilloscope. Keith Brindley takes the lid off it inhis series about test equipment.

Strictly speaking, an oscilloscope is any device which candisplay waveforms. The term 'oscilloscope' is derivedfrom the Latin word oscillore, meaning to swing back-wards and forwards, and the Greek skopeein, meaning toobserve, aim at, examine. A number of electronic testinstruments have this capability: XY plotters and penrecorders are two examples. However, the term has tobe used in a general way to refer to the particular type oftest instrument covered here.

While the oscilloscope is not the most common lab-oratory test instrument (multimeters hold that crown) itis by far the most useful one, allowing the innards of a cir-cuit to be analysed in real-time, i.e., the oscilloscope dis-plays what is actually occurring in the circuit at thatmoment. Compared with a multimeter, say, an analoguemoving -coil multimeter, which can only display directvoltages and currents (or, at most, very slowly varyingones), the oscilloscope can display rapidly alternatingvoltages and currents. Also, a meter can only display ameasurement in one dimension, i.e., the measurand'samplitude at any given time. The oscilloscope, on theother hand, displays two dimensions, the measurand'samplitude against the second dimension of time. In thisway rapidly varying amplitudes are displayed.

This ability of the oscilloscope to display two-dimen-sional measurements arises from the type of display deviceit uses. A more technical description of what is normallycalled an oscilloscope is a cathode ray oscilloscope (CRO),which gives a clue to the display used: a cathode ray tube

(CRT). Operation and composition of cathode ray tubesare covered in the last section.

All we need to know here is that the cathode ray tubemay be used to display the two dimensions of amplitudeand time. This is done by moving the electron beam in thecathode ray tube across the screen in a controlled way,so that a representation of the measurand is displayed, thevertical amplitude of which corresponds to the measur-and's amplitude and the horizontal amplitude of whichcorresponds to units of time over which the measurandis observed.

What the Oscilloscope DisplaysThe real-time or general-purpose oscilloscope is essen-tially used for displaying a representation of periodicallyvarying (i.e., repetitive) measurands. Such a measurand isshown in Figure 12a, and is seen to be a fairly simple per-iodic voltage waveform something you might get froma microphone picking up a steady note from a musicalinstrument.

To display the waveform the oscilloscope firstly has tobreak it down into manageable, screen -sized portions, asin Figure 12b. These portions of the waveform are thendisplayed in turn on the screen, as in Figure 12c, just likeleafing through a child's flick -book the only differencebeing that the pictures in a flick -book are minutely differ-ent so that flicking through them creates the illusion ofmovement, whereas the oscilloscope's pictures are allidentical, creating the illusion of a constant picture. So, witha continuous periodic waveform, the apparent displayedrepresentation would be just one of these portions, as inFigure I 2d - trick photography at its best!


The final display is, in fact, a graph of voltage against timefor this apparently single portion. This representation ofa measurand which an oscilloscope makes on its screenis commonly called a trace - referring to the way theelectron beam seems to trace out on the screen the dis-played waveform.

The Basic OscilloscopeFigure 13 shows the action of a basic oscilloscope in blockdiagram form. Waveforms at various points around thediagram illustrate circuit action.

To enable the image of the measurand to be displayed,the electron beam must be moved across the cathode raytube screen in a controlled manner. Firstly, it has to movehorizontally from left to right (as viewed). When the beamreaches the right-hand side of the screen it must returnto the left to begin a new trace. This whole proceduremust occur at regular intervals. Secondly, the amplitudeof the displayed waveform must correspond to the ampli-tude of the measurand, so the electron beam must bevaried in a vertical manner, too. Horizontal movement ofthe beam creates an axis corresponding to time, verticalmovement creates an axis corresponding to amplitude.

Horizontal beam movement is the effect of voltages,called sweep or timebase voltages, applied to the hori-zontal deflection plates of the cathode ray tube. A time -base voltage waveform which might be applied to one ofthe horizontal deflection plates is shown in Figure 14 - it sbasically a ramp voltage. A second timebase voltage, theexact inverse of this, would be applied to the other plate.When the voltage is low, say, at point I, the beam isdeflected to the left-hand side of the screen. When thevoltage is at point 2 the beam is not deflected so is in themiddle of the screen. Finally, when the voltage is at point3 the beam is deflected to the right-hand side of thescreen, and soon. The shape of the timebase voltage tellsus how the beam traces across the screen. When it movesfrom left to right it does so in a steady manner, at a con-stant rate determined by the steepness of the increasingvoltage. However, on its return trip from right to left(known as the flyback) it instantly jumps back, ready tostart the slower left to right movement again. An instantjump back from right to left (i.e., in zero time) ensures thata user does not see a trace on the screen as the electronbeam flies back. In reality, timebase generator circuits can-not produce such an ideal timebase voltage - typicallytheir ramps do not increase at a constant rate but varyslightly, and their flybacks take a finite but very small time.

Ramp length, i.e., the time the beam takes to sweepfrom left to right on the screen, is user -adjustable to allowdifferent measurands with different timeslots to be dis-played.

In order that the same part of the waveform is displayedby each sweep of the beam across the screen, a triggercircuit is used to detect when a particular point on therepeating waveform occurs. Consequent triggering causesa pulse to occur which literally kick-starts the timebasegenerator. This circuit is user -adjustable so that the pointin the timeslot at which the sweep starts can be selected.

Circuits which change the incoming voltage of the mea-

LPortion 1

_J LPortion 2

(c) Portion 4Portion 3

Portion 2Portion 1

I

I

I

II

II

_J LPortion 3

(d)

Time

Portion 4 Time

surand to the voltages required by the vertical deflectionplates are typically amplifiers, known as vertical amplifiers.Two stages of amplification are used. The first stage ampli-fier, known as the vertical preamplifier, converts the incom-ing signal waveform to a standard -sized waveform. Tocater for different amplitudes of input signal, gain of thevertical preamplifier must therefore be user -controlled,although gain of the vertical amplifier itself is fixed.

There are a number of additions to this basic circuitwhich make the oscilloscope far more useful. Additionsare shown in a more complex block diagram in Figure 15.At present, it's only capable of displaying DC waveforms,so the first addition is of a capacitor and switch at the Yinput allowing the user to select between AC or DCwaveforms.

If large DC voltages, or AC voltages with a large DCbias, are to be displayed, a user -adjustable control to adjustvertical deflection voltages must be included. This is theY shift control, which allows zero Y voltage to correspondwith, say, the centre horizontal line. A similar controladjusts the horizontal deflection voltages, known as the Xshift. The X shift allows the complete timeslot of theobserved waveform to be moved left or right over thescreen.

In many instances a user may require the oscilloscopeto be triggered not from the viewed waveform, but froman external source. Thus the viewed waveform can beobserved in direct comparison with a separately occur-ring trigger source. The trigger selector provides this func-

Figure 12. Showing how anoscilloscope displays ameasurand as a steady image:(a) the measurand as a signalof amplitude against time;(b) breaking the signal downinto screen -sized portions(c); displayed as successiveimages;(d) as viewed.

Below left: A basicoscilloscope.

Below right: A storageoscilloscope.


COInput

Verticalpreamplifier

Verticalamplifier

Triggercircuit

Verticaldeflection plates

Timebosegenerator

As.

Horizontalamplifier

1 3

Horizontaldeflection

plates

O

Figure 13. Block diagram ofan oscilloscope.

Right: Figure 14. A possibletimebase voltage, as couldbe applied to one of thehorizontal deflection platesof an oscilloscope.

Figure 15. More complexblock diagram of anoscilloscope.

tion. The point at which triggering of the timebase gen-erator occurs is adjusted by the trigger level control. Theaddition of an auto -triggering circuit allows automatic trig-gering of the timebase generator, without the need toadjust trigger level. Triggering may take place on a posi-tive or negative going edge, depending on the triggerpolarity switch.

One final external input is the X input, which effectivelyallows the timebase generator to be disabled and anexternal voltage to control horizontal deflection. What isdescribed here, although it is a real-time oscilloscope, isnot particularly common.

Advanced OscilloscopesMost practical oscilloscopes have a dual -trace display, thatis, two traces are available so that two different waveformscan be displayed. In dual -trace oscilloscopes the electronbeam is switched rapidly from one waveform to the otherso that the eye perceives a constant display of two wave-forms. Four -trace and even greater oscilloscopes are alsoavailable. Of course. sharing the single electron beambetween two, four, or more traces means that less andless time is spent by the beam displaying any one wave-form and the traces will be correspondingly dimmer.Some oscilloscopes have separate electron guns to de-feat this problem, and are known as dual -beamoscilloscopes.

Advanced, but still real-time, oscilloscopes exist whichhave other features, too. Separate timebase generators(one for each trace), timebase generator delay facilities (toallow displayed traces to be offset), and input waveformdelay facilities (to counteract the effect of circuit delayswhich would otherwise render impossible the observa-

Y input

Externaltrigger inputO

Verticalpre-

amplifier--

Auto0-Int -0.*-0-

0ExtTrigger

selector

Verticalomplifier

Delaylines

Y shift

Auto-triggering

circuit

Triggercircuit

-77-Trigger Polarity

X input level0

On/Off

Timebosegenerator

H CRT

Horizontalamplifier

Speed X shift

60

tion of rapidly occurring signal changes), are some of themany features.

However complex real-time oscilloscopes are, they canonly be used to observe rapidly repetitive waveforms.Also available are non -real-time oscilloscopes which allowthe observation of non -repetitive, i.e.. singly occurringwaveforms which real-time oscilloscopes could not dis-play. An example of such a singly occurring waveform is,say, the interrupt request signal to a microprocessor. Thismay only take place once every so often and is of only afew microseconds' duration. To display this kind of wave-form a storage oscilloscope can be used, in which the dis-played trace represents a single timeslot of the completewaveform which is stored within the oscilloscope.

There are two main types of storage oscilloscope: onewhich depends on a special cathode ray tube which main-tains its display long after the electron beam has sweptacross it; while the other samples and stores the wave-form digitally, ready for later recall and display. As youwould expect, the cathode ray tube storage oscilloscopeis cheaper then the digital storage oscilloscope (DSO), butis correspondingly less versatile. The trace stored withina storage cathode ray tube cannot be repositioned oraltered, whereas the digitally stored trace can be moved,magnified, contracted, erased, and displayed yet again atthe user's whim, long after the recorded event. Most

storage oscilloscopes have the facility to be switchedbetween real-time (that is, non -storage) and non -real-time (that is, storage) operation. so two separate oscillo-scopes are not required to give all functions.

SpecificationsWe now turn our attention to those aspects of oscillo-scopes which define one device as being better or worsethan another. Put another way, if you are going to buyyourself a 'scope what should you be looking for?

Most important criterion of importance - in fact theonly one, really - is bandwidth. Like any electroniccircuit, the oscilloscope only passes a limited range of fre-quency components. Components outside the bandwidthrange are drastically reduced in amplitude. Obviously, thewider the bandwidth, the better the oscilloscope.Bandwidths of I OMHz or 20MHz are common in general-purpose oscilloscopes, but oscilloscopes with bandwidthsup to around 250MHz (very expensive) are available forhigher -frequency work. More specialist oscilloscopes(very, very expensive!) have bandwidths up to I GHz.

Also of consequence and related to the bandwidth isthe range of amplification factors of the vertical amplifier.The greater the amplification the smaller the waveformdisplayable on screen. Oscilloscope screen displays aremarked in a grid of I 0 -by -8 centimetre square divisions,known as the graticule. Amplification factors are denotedin volts/division or volts/centimetre, so that a factor suchas I OV/cliv corresponds to a total screen height of 80VTypical amplification factor ranges are from aboutI OmV/cliv to 5V/div. The better the oscilloscope thegreater this range, so higher quality oscilloscopes haveranges which extend down to I mV/div and up to 50V/div.

Bandwidth considerations discussed here apply to theY, i.e., vertical, circuitry within the oscilloscope. However,there's little point in having a high Y bandwidth if the Xcircuitry does not have a similar high -frequency ability.The X 'bandwidth' corresponds to the range of settingsavailable in the oscilloscope's timebase generatorcontrols. Timebases in the range of 2 seconds to about I 0microseconds (times given indicate the time taken for thebeam to sweep from the left-hand to right-hand side of


the CRT screen - and are generally denoted per division,e g , 0 2 second/div to I microsecond/div) are common,but with oscilloscopes of higher Y bandwidths total time -base times must decrease to about 10 nanoseconds (i.e.,

I nanosecond/div) for Y bandwidths of around 250MHz,or less for specialist oscilloscopes.

Oscilloscope ProbesThe standard input resistance of oscilloscope verticalamplifiers is I Mi.2. Such a high resistance would, you mightthink, allow measurements of most measurands tobe taken without the oscilloscope loading the measurandcircuit in any appreciable way. This should be reflected inthe accuracy of the measurement. However, such a highinput resistance necessitates use of screened input lead toconnect the oscilloscope to the measurand circuit, toprevent excessive hum pick-up. Typical screened leadcapacitances are around 50 to I 00pF/m (one metre ofinput lead is a convenient length) and this, coupled withthe vertical amplifiers' own input capacitances of around15 to 50pF (depending on make and model), makes a totalinput capacitance of about 100 to 150pF. With inputcapaci-tances of this order, giving a reactance of around14012 at 10MHz, you can see that considerable loading ofhigh -frequency measurands could easily occur, thus affectingmeasurement accuracy.

The usual solution to this problem is to use a standardaccessory: the passive divider probe. Such probes have anattenuator within, which has the effect of increasing theresistance presented to the measurand circuit, whiledecreasing the capacitance. A typical probe attenuator is10:1, which presents a resistance of ten times that of thevertical input of the oscilloscope, i.e., 10MQ, and onetenth of the input capacitance, i.e., about 10 to I 5pF,

effectively reducing loading. The attenuation which apassive 10 x divider probe causes, can be counteractedmerely by increasing the vertical amplifiers' gain by thesame amount.

Active probes are an alternative, employing a field-effecttransistor (FET) amplifier which presents a high resistanceand low capacitance to the measurand circuit, with unitygain (i.e., no attenuation) or even amplification. Activeprobes are more expensive than their passive counter-parts and require a power supply, generally a small cell,for the internal FET amplifier.

Oscilloscope AccuracyThere are three main sources of error when using an oscil-loscope to take measurements. It is worth bearing thesein mind simply because they can generally be compen-sated for if you want.

First, although probes reduce loading of the measurandcircuit to a defined amount, loading still occurs, so volt-age measurements are not necessarily as accurate as theuser thinks. When using a passive probe its internal com-pensation capacitor must be adjusted to keep waveformdistortion and amplitude error to a minimum. As ageneral rule this adjustment should be done every timethe oscilloscope is used. The check is quite simple; themost convenient way is to observe a I kHz square waveand adjust the capacitor for clean and square transitioncorners. Figure I 6a shows the observed square wave

when using a correctly compensated probe. Figure I 6b

shows the observed waveform when the probe is under -compensated and Figure I 6c shows it when the probe isovercompensated. Many oscilloscopes have a squarewave output available from the front panel, specifically forthe purpose of compensating passive probes.

The second main source of errors comes from theinternal oscilloscope circuits. Errors can exist due toincorrect gain in the vertical amplifiers, or to incorrecttiming in the timebase generator circuits. With the verticalamplifier controls set in the calibration position, an inputof, say, I V, must produce a display corresponding to I V

vertical displacement on the screen. Similarly, with thetimebase generator controls set in their calibration posi-tion, a pulse input of, say, I second duration must producea waveform display corresponding to that horizontal dis-placement. The same square wave output used to com-pensate probes can often be used to check and adjust forvertical amplifier and timebase generator errors.

Finally, user errors make up the third main source.There are a number of possible problems. Parallax is themost prevalent. In most oscilloscopes the graticule is infront of, so is in a different plane from, the layer of phos-phor on which the trace is inscribed by the electron beam.So, if the user observes the screen at anything other thanperpendicular, parallax errors will occur in the readings.Some oscilloscopes have graticules which are on the insideof the screen, virtually in the same plane as the phosphorlayer, thus reducing parallax errors to a minimum.

Non -linearity can occur on the edges of cathode raytube screens and so important measurements should betaken in the central, say 6 by 4 division, portion of the 10by 8 division graticule, whenever possible.

As a general rule the beam intensity should be keptas low as possible, because a dim beam produces anarrower trace than a bright beam, and so it is easier tocorrectly assess the beam's true centre. In a similarvein, beam focus should be checked regularly throughoutmeasurements.

The Cathode Ray Tube (CRT)Shown in Figure 17 is a simplified cathode ray tube whichillustrates the basis of all CRTs. It's made up from an evacu-ated glass tube, widened and flattened at one end to forma viewing screen. The inside surface of the screen is coatedwith phosphor, a material which is fluorescent and glowswhen electrically excited, say, when hit by electrons. Anumber of electrodes are positioned inside the tube toperform various functions. The main electrodes and theirfunctions are:

a heater - literally a coil of wire which is heated toglowing point by the application of an electric current atlow voltage. The area around the heater becomes rich inelectrons

an acceleration anode electrode - held at a high posi-tive potential (with respect to the heater) so that it attracts

Figure 16. Viewedwaveforms for an appliedsquare wave:(a) when a passive probeis correctly adjusted;(b) when the probe isundercompensated;(c) when the probe isovercompensated.


CathodeHeater

Control Deflectiongrid

Acceleration platesanode electrodes

Electrongun

Phosphor

Screen

Figure I7. The cathode raytube (CRT).

electrons from the heater. As the electrons reach thiselectrode their kinetic energy is such that they pass rightthrough the electrode and towards the front of the tube,in a beam (the 'cathode ray')

a control grid - which is held at a variable negative poten-tial to the heater, thus repelling or not repelling the negativeelectrons and allowing a controlled amount through

focusing electrode - the potential on this electrode isvariable to create a beam with as small a cross-sectionalarea as possible when it strikes the screen. The heater,accelerating electrode, control grid and focusing electrodeare often collectively known as an electron gun

deflection plates - the potential across each pair ofplates (vertical and horizontal), and the polarity of thepotential (i.e., positive or negative) defines how much theelectron beam is deflected away from the centre of thescreen. If, say, the upper plate is positive and the lowerplate is negative, the beam will be deflected up (towardsthe positive plate and away from the negative plate).Similarly, by varying the potentials applied to the horizontaldeflection plates the beam is deflected in a left or rightdirection.

So, the beam direction (and thus the position where it hitsthe screen), strength (i.e., the brightness of the trace), andwidth (the thickness of the trace) are all controllableelectronically, merely by changing the voltages applied tothe various electrodes. Many refinements may be addedto this basic cathode ray tube to improve performance,sensitivity, or beam brightness, but the operation remainsmore or less as discussed.

K4701 LOUDSPEAKER DC PROTECTION

Construction DetailsTable 1 shows the number of 5Wresistors which must be fittedto suit the maximum peak outputvoltage of the amplifier. Rememberthat amplifiers with less than10Vpk output are not suitablefor use with this module.

There are two methods whichcan be employed to find theapproximate peak output voltageof the amplifier: the first, simplymeasure the voltage across theLl PSU supply rails; the secondis to determine the voltage bycalculation, as shown below:

W =Vrrne

Vpk - Vrms X 2

Where W = Power of amplifier;R = load impedance

VPk = peak output voltageof amplifier.

With a little transposition, we have:

Vpk = MxR X

= JWx R x 1 -414

Example: 50W into an 8S2 load

Vpk = J50x8 x 1.414 = 28-3V

Construction of the module isfairly straightforward; all of thecomponents are mounted on asingle -sided PCB. The assembledmodule is shown in Photo 1.

The recommended componentassembly order is as follows: First,fit and solder the Zener diodes.Care must be taken to fit thediodes the right way around; thecathode is indicated by a blackband on the body of the diode,and this must face the thickwhite band on the PCB legend.

Mount the capacitors next,taking care that the two polarised

UNIT - Continued from page 57.devices (C2 and C3) are inserted

correctly - the negative lead isidentified by a white band and (-)symbols on the capacitor's body.

Fit the four mounting tabs next,followed by the 5W resistors.The number of resistors requiredcan be determined from Table 1.Remember to fit wire links on thePCB, in place of the resistors, ifless than the full number areneeded.

Finally, mount the relay, andsolder the three supplied lengthsof wire along the solder resistfree (tinned copper track) areas,building up the tracks with a thicklayer of solder.

Thoroughly check your workfor errors, such as misplacedcomponents, solder bridges, anddry joints, etc. Clean any flux offthe PCB using a suitable solvent.

Testing / InstallationConnect a 12V DC supply(minimum) between the +LS IN& -LS IN terminals; the relayshould operate. Disconnect thepower supply and reverse theconnections. When the powersupply is reconnected, the relayshould operate again.

The Loudspeaker DC Protectoris now ready for connection. Fit aflat push -on receptacle to each ofthe amplifier and speaker leads,and link the module between theloudspeaker and the amplifier'soutput, as shown in Figure 2.

The project is now ready for use;it can be installed in an amplifier(shown in Photo 2), or mountedinside the speaker cabinet (shownin Photo 3).

K7401 LOUDSPEAKER DC PROTECTORPARTS LISTRESISTORS: All 5WR1 to R4

CAPACITORSC1C2, C3

SEMICONDUCTORSZD1 to ZD6

MISCELLANEOUSRY1+LS IN, -LS IN,+LS OUT, -LS OUT

680

1pF100V Minimum2200pF 16V Electrolytic

4.3V 1.3W Zener Diodes

12V 10A Relay

Flat Push -on Receptacle& Mounting Tabs

PCBTinned Copper Wire

4

1

2

6

1

41

As Req.

The Maplin 'Get -You -Working' Service is available for this project,see Constructors' Guide or current Maplin Catalogue for details.

The above items are available in kit form only.Order As VF44X (Loudspeaker DC Protector) Price £9.49

Please Note: Some parts, which are specific to this project(e.g., PCB), are not available separately


Ray Marston presents a selec-tion of high -power (6 to 40W)circuits in this concluding partof his mini-series.

The opening two parts outlined some ofthe basic principles of modern audiopower amplifier design (including that

of 'bridge mode' amplification and powerboosting). They presented a variety of prac-tical designs, based on ICs with maximumoutput power ratings in the approximaterange 325mW to 5-5W. This month continuesthe 'audio power amplifier' theme by look-ing at a further selection of power amplifierICs and their application circuits - in thiscase, devices with maximum output powerratings in the range 6W to 40W.

Table 1 gives basic details of the IC types;note that two of these ICs (the LM2879 andTDA2004) are 'dual' types which each housea pair of independently accessible amplifiers.The TDA2005M is a 'bridge' type whichhouses a pair of amplifiers that are perma-nently wired in the bridge or 'power boost-ing' configuration. Throughout this article,all ICs are dealt with in the order in whichthey are listed in Table 1. Practical applicationcircuits are given for each IC type, but insome cases, only very brief descriptions ofindividual IC circuit theory are given.

Many of the ICs mentioned in this articlehave featured in Data Files in Electronics,whilst others have been used in previousMaplin projects. These are a source of furtherinformation and application ideas, and issuenumbers have been given, where appro-priate.

TBA8 10S/1)The TBA8I0 is a very popular medium -powerIC that features internal protection againstaccidental supply polarity inversion, andhigh supply -line transients. etc. It is widelyused in automobile applications, in which itcan deliver several watts of audio power intoa 40 or 20 load when operated from a 14-4V(12V nominal) supply. Early versions of this ICcarry an 'S' subscript, and have a typical sig-nal power bandwidth of only 15kHz; modernversions carry a 'P' subscript and feature anumber of performance improvements,including lower noise and a 20kHz band-width.

Figure 41 shows the outline and a practi-cal application circuit for the TB/18105/P Thiscircuit can deliver 7W into a 20 speaker load;its voltage gain is determined by R2; R 1 is an

DEVICE

NUMBERSTOCKCODE

AMPLIFIERTYPE

MAXIMUM

OUTPUT POWERSUPPLYVOLTAGE RANGE

DISTORTIONINTO 8S1

INPUTIMP.

VOLTAGE

GAINBAND-

WIDTHQUIESCENTCURRENT

TBA810STBA810P

QL13P MONO 6W Into 4.0 4 to 20V 0.3% @ 2.5W 5MS2 37dB 40Hz to15kHz40Hz to20kHz

12mA

LM383TTDA2003V

WQ33LAH52G

MONO 7W into 4Q 5 to 25V 0.2% cii,, 4W 150k 40dB 30kHz 45mA

LM2879 NOTAVAHABLE

DUAL 9W/CHANNEL 10 to 35V 0.04% @4W/CHANNEL

3M0 34dB 50kHz 12rnA

TDA2004 NOTAVAILABLE

DUAL 9W/CHANNEL 8 to 18V 0.2% @4W/CHANNEL

200k 50dB 35kHz 65mA

TDA2006V WQ66W MONO 12W into 40 -±6 to ±-15V 0-1%@ 4W 5M.Q 30dB 150kHz 40mA

TDA2030AV WQ67X MONO 18W into 40 ±6 to ± I8V 01% ki. 8W 5MQ 30dB 140kHz 4OmA

TDA2005M YY7OM BRIDGE 20W into 40 6 to 18V 0.25% @ 12W 100k 50dB 20kHz 75mA

TDA I 520 UN79L HIS MONO 22V7 into 412 15 to 40V 0.01% g 16W 20k 30dB 20kHz 54mA

LM1875N UH78K MONO 25W into 412 20 to 60V 0-01% @ 20W IM0 26dB 70kHz 70mA

TDA2050V CP88V MONO 32W into 40 9 to 50V 0.05% g 15W 500k 30.5dB 20kHz 55mA

TDA1514A-N7 UK75S MONO 40W into 4Q -±8 to ±30V 0.003% @ 32W 1Mi2 30dB 25kHz 60mA

Table 1. Basic details of the ICs described in this concluding part. DIS after Stock Code means discontinued.


Figure 41. TBA810S/P outline and application circuit foruse as a 7W amplifier in cars.

Figure 43. LM383 (TDA2003) 5.5W amplifier for use in cars.

output biasing resistor that is bootstrappedvia C8, and R3 -C7 is a Zobel network. A PCBfor this IC is available from Maplin (StockCode BRO2C); see the Semiconductorssection in the current Maplin Catalogue forfurther details.

LM383 (TDA2003) CircuitsThe LM383 and the TDA2003 are identicaldevices, and are described in the manufac-turer's literature as 8W audio power amplifierICs. Figure 42 shows the internal circuit andpin notations. The device is specificallydesigned for use in automobile applications,in which the 'running' supply voltage has anominal value of I 4.4V; at this voltage, the ICcan typically deliver 5.5W into a 4i1 load. or8.6W into a 20 load. The IC can, in fact, oper-ate with any supply voltage in the range 5Vto 20V and can supply peak output currentsof 3.5A. It also has a current -limited and ther-mally protected output stage.

The LM383 (TDA2003) is housed in a 5 -pinpackage, as shown in Figure 42. and is a veryeasy device to use. Figure 43 shows how towire it as a 5.5W amplifier for use in cars.Here, the closed -loop voltage gain is set atx 100, via the R1 -R2 -C3 feedback network.

64

Figure 42. Internal circuit and pin notations of the LM383 (TDA2003)8W audio power amplifier IC.

Figure 44. LM383 (TDA2003) 16W amplifier for use in cars.

The IC is operated in the non -inverting modeby simply feeding the input signal to pin 1 viaCI . Capacitors C2 and C4 are used to ensurethe high -frequency stability of the IC; it is vitalthat C4 is wired as close as possible betweenpins 3 and 4.

Figure 44 shows how a pair of LM383 ICscan be connected as a I 6W bridge amplifier(for use in cars). Preset potentiometer RVI isused to balance the quiescent output volt-ages of the two ICs and, to thus minimise thequiescent operating current of the circuit.

The LM383 is featured in a 8W PowerAmplifier project, and a kit is available fromMaplin (Stock Code LW36P); see the Projectsand Modules section of the current MaplinCatalogue for more details.

LM2879The LM2879 (Figure 45) is a dual 9W audiopower amplifier IC that is housed in an I 1 -pin

package, incorporating a large ground -con-nected metal heat tab that can be bolteddirectly to an external heatsink without needof an insulating washer. The IC can operatefrom single -ended power supply voltages inthe range 10V to 35V It can deliver 9W perchannel into an 8S1 load. and incorporates

internal current limiting and thermal shut-down circuitry, etc. It typically generates only0.04% distortion at 4W per channel output,and gives a 50kHz bandwidth.

The LM2879 is a very easy device to use;Figure 46 shows how it can be used as a 7W+ 7W stereo ceramic cartridge amplifier, withintegral bass controls that enable theresponse to be cut or boosted by up to 13dBat 100Hz. Figure 47 shows it connected as abridge amplifier that can feed 12W into a 16S1load when using a 28V supply. Note, in bothof these circuits, that pin I provides a voltagethat is used to bias the amplifier's non -invert-ing input terminals.

TDA2004The TDA2004 (Figure 48) is a dual 9W ampli-fier that is housed in an 11 -pin package, sim-ilar to that of the LM2879 but with differentpin notations. The IC can operate fromsingle -ended supplies in the range 8V to 18V,and can provide peak output currents of3-5A. It can deliver 9W into a 40 load fromeach channel when using a I 7V supply

Figure 49 shows how to wire the IC as asimple stereo amplifier that will deliver 4Wper channel into 4S1 loads while powered


TOP VIEW

11

10

I 2 V+) Output 2

9 I GND

B ) Input 27 I 2 Feedback 2

CO 6 I GND2 5 I 2 Feedback 1

4 I 2 Input 1

I ) GND

I 2 Output 1

I Bias

Figure 45. Outline and pin notations of the LM2879 dual 9W audioamplifier IC.

Figure 46. 7W 7W stereo ceramic cartridge amplifier.

Figure 47. 12W bridge amplifier.

TOP VIEW

Oj_N4Lif.

11

10

2 Bootstrap 1

Output 1

00

0

9

8

6

5

4

2 +V

2 Output 2Bootstrap 2GND

Input +22

Input -2SVR

2 Input -1Input +1

Figure 48. Outline and pin notations of the TDA2004 dual 9W audioamplifier IC.

Input

10uF 120k

a+

1=1

2u2F OV

+15V

I100nF

II

2u2F ov

TDA2004

220uF 100uF1k

11-1=1OV

012R

OV

1R

7100nF

7

100uF

3R(Minimum)

1R11

220uF2k

100nF 1 2 R

Figure 49. 4W + 4W stereo amplifier.

Input +15V Input10uF 120k I 100nF 2

1=1

= 2u2F OV OV 2u2F

TDA2004

11I 10

.220uF1k2

-1=1

03R3

1R

100nF

2200uF

C( 4R

CV

7.

1 OOuF 220uF1k21=1

2200uF 1R

4R 10OnF 3R3

Figure 50. 20W bridge amplifier.

TO -220 Plastic Package

1 ) 5 V+1 ) 4 Out

Insulated 3 GNDHeat Tab 1

I ) 2 -InputI ) 1 +Input

Figure 51. Outline and pin connections of the TDA2006 and TDA2030


Figure 52. TDA2006 8W amplifier with single -ended supply.

65

Figure 53. TDA2006 8W amplifier with split power supply.

Figure 55. TDA2030 24W bridge amplifier with split supply.

from a I5V supply, generating total harmonicdistortion of less than 0.2%. Note that eachchannel has its voltage gain set at about x 364by the ratio of the 1k2 and 3.352 resistors, andeach channel thus needs an input of only12mV rms to give full output.

Figure 50 shows how to wire the TDA2004as a bridge amplifier that can deliver a max-imum of about 20W into a 352 load (the min-imum allowable value) when using a 15Vsupply. This circuit needs an input of about50mV rms for full output.

Figure 54. TDA2030 15W amplifier with single -ended supply.

TDA2006This is a high -quality amplifier that can beused with either split or single -ended powersupplies, and can deliver up to 12W into a 452load, which typically generates less than 0.1%distortion when feeding 8W into a 40speaker. The IC is housed in a 5 -pin T0220package (see Figure 51) that has an electri-cally insulated heat tab which can be bolteddirectly to an external heatsink without theneed of an insulating washer.

Figure 52 shows how to use the TDA2006with a single -ended supply. The non -invert-ing input pin is biased at half -supply volts viaR3 and the R i-R2 potential divider. The volt-age gain is set at x22 via R5/R4. DI and D2protect the output of the IC against damagefrom back EMF voltages from the speaker.and R6 -C6 form a Zobel network.

Figure 53 shows how to modify the abovecircuit for use with split power supplies. Inthis case. the non -inverting input is tied toground via R 1. This circuit also shows howhigh -frequency roll -off can be applied to theamplifier via C5 -R4.

TDA2030This very popular IC can be regarded as anuprated version of the TDA2006, and ishoused in the same 5 -pin TO220 packagewith insulated heat tab. It can operate withsingle -ended supplies of up to 36V or withbalanced split supplies of up to I 8V. Whenused with a +28V single -ended supply, itgives a guaranteed output of 12W into 452 or8W into 852. Typical THD is 0.05% at I kHz with7W output, and is still less than 0.1% at 8W.

Figure 54 shows how to connect theTDA2030 as a 15W amplifier using a single -ended +30V supply, a 452 speaker load, anda voltage gain of 30dB. Alternatively, Figure 55shows how to wire a pair of these ICs as asplit -supply bridge amplifier that can deliver

10uF

EFL

2u2F OV

120k17

+14.4V100nF

100uF

OV 2u2F i 1OV

TDA2005

51°

111 10..220uF

1k 100uF

-l=1OV

8 71---U .220uF

100uF

SPKR2R

2k

1R 12R

100nF 100nF 12R

Note:- All capacitors

TOP VIEW

rate 025Vwk, minimum

11

(I) 10

9

8

70O 6

5

4

I Bootstrap 1

1 _) Output 1

1 ) +V1 ) Output 2I ) Bootstrap 2

GND

I Input +2I ) Input -2I SVRR

I ) Input -1

OV\\_GrN4_:if I ) Input +1

Figure 56. TDA2005M 20W power booster for use in cars.


24W into a 4I speaker load while generatingtypical total harmonic distortion of less than0.5%.

The TDA2030 was featured in a 15W PowerAmplifier project, and a kit is available fromMaplin (Stock Code LT23A); see the Projectsand Modules section of the current MaplinCatalogue, or issue 64 of Electronics for moredetails.

TDA2005MThis is a 20W audio power booster IC specif-ically designed for use in cars, and is fullyprotected against output short circuits, etc.The IC actually houses two power amplifiers,which are internally connected in the bridge

Figure 57. TDA1520 22W power amplifier.

configuration to provide the high power out-put (into a 20 load) from the 14.4V (nominal)power supply of a car. The IC is housed in an11 -pin package, as shown in Figure 56 (whichalso shows a practical applications circuit forthe TDA2005M). Note that all capacitors mustbe rated at 25V minimum.

TDA1520This is a very high performance device thatcan deliver up to 22W into a 40 load, or 11Winto an 8Q load, when powered from a 33Vsupply, and which typically generates a mere0.01% distortion at 16W output into a 4i2load. The IC is housed in a 9 -pin package thatcan be bolted directly to an external heatsink

Figure 59. LM1875 20W amplifier using single -ended power supply.

(without the need for insulating washers) insingle -ended supply applications. Figure 57shows the IC outline and a practical applica-tion circuit for this device.

LM1875The LM1875 is a very high quality audioamplifier that can (when using a 60V supply)deliver a maximum of 25W into a 4Q load; itwill deliver 20W into a 40 load when using a50V supply, generating a mere 0015% distor-tion. The IC is housed in a 5 -pin T0220 pack-age that does not require the use of aninsulating washer between its metal tab andan external heatsink in single -ended supplyapplications; note, however, that an insulat-

TO-220 Package

OJ 5 V+

) 4 Out3 GND2 -Input1 +Input

Figure 58. LM1875 outline and pin notations.

VIN

C14u7F

C24u7F

R1100k

R2100k

+25V C5

+11000uF

C61

100nF

2

5 OV

LM1875

14R32k2

C34147uF

C447uF

100k

C7+1000uF

C8100nF

R52R7

Spkr4R

C91 00nF

OV -25V

Figure 60. LM1875 20W amplifier using dual (split) power supply.

Figure 61. TDA2050 outline and basic application circuit, giving 25W into an 852 speaker, or32W into 452.

ing washer must be used if the device is pow-ered from dual (split) supplies.

Figure 58 shows the outline and pin nota-tions of the LM1875, and Figures 59 and 60show practical 20W application circuits, usingsingle and dual power supplies respectively.In Figure 59, the IC's output is biased to half -supply volts via R3 and the R1 -R2 divider,and the voltage gain is set at x 45 via R4 -R5;R6 -C7 form a Zobel network across the loud-speaker and help enhance circuit stability. InFigure 60, the IC's output is zero -referencedvia R2, and the gain is set at x45 via R4 -R3;note that capacitors C1 -C2 and C3 -C4 arewired back-to-back, to enable them to accepteither polarity of input signal.

The LM1875 was featured in a 20W AudioPower Amplifier Data File in issue 30 ofElectronics. A PCB for this IC is available from


Mounting Base

\V1.7-71 -Input2 Soar3 Mute4 V-5 Output6 V+7 Bootstrap8 OV (Common)9 +Input

Figuie 62. Outline and pin notations of the TDA1514A 40W amplifier.

Maplin (Stock Code GE 13P); see the Projectsand Modules section of the current MaplinCatalogue, or issue 30 of Electronics for moredetails.

TDA2050The TDA2050 is a high quality audio ampli-fier that can (when using a 45V supply)deliver a maximum of 25W into an 85I load,or 32W into a 411 load. The IC is quite versa-tile, and can use any supply in the 9 to 50Vrange. but performs best with a supply in the38 to 45V range. It typically generates a mere0.05% distortion when feeding 15W into an811 load from a 38V supply. The IC is housedin a 5 -pin 10220 package that does notrequire the use of an insulating washerbetween its metal tab and an externalheatsink in single -ended supply applications;

Figure 63. Bask TDA1514A application circuit, as a 40W amplifier.

note, however, that an insulating washermust be used if the device is powered fromdual (split) supplies.

Figure 61 shows the outline and pin nota-tions of the TDA2050, together with a basicapplication circuit that tic's a single -ended45V power supply, and can be used with a 40or 8S2 loudspeaker.

TDA1514AThe TDA15 1 4A is a very high quality devicethat can deliver 40W into an 80 load whenusing a split 55V supply, or 40W into 40 froma split 42V supply. It typically generates amere 0.003296 distortion at 32W output, andincorporates auto -mute circuitry that elimi-nates switch -on and switch -off surges (clicks)in the speakers. The device is housed in a 9 -pin flat package that incorporates a metal

plate (internally connected to pin 4) that canbe bolted directly to a 4-3 °C/W heatsink(which must be insulated from ground).Figure 62 shows the IC's outline and pin des-ignations.

Finally, to complete this look at modernaudio power amplifier ICs, Figure 63 showsa practical 'split power supply' applicationcircuit for the TDA1514A. The IC's non -invert-ing input is ground -referenced via RI. Its volt-age gain is set at x30 via R3 -R2. R7 -C8 forma Zobel network, and R6 -C7 provide auto -muting at power switch -on and switch -off.

For further information, see Electronics(issue 40). where this IC was featured in a 50WIC Power Amplifier Data File application cir-cuit. A complete kit for this module is avail-able from Maplin (Stock Code LP43W); seeissue 40 or Projects and Modules section ofthe current Maplin Catalogue for details.

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CIRCUIT MODE SWITCHINGb 11. I _JD ,1111

LATER-NEIWORK communication is accom-plished through the three primary switch-ing technologies of circuit, message, and

packet switching. In circuit mode networks,each circuit is dedicated to a predeterminednumber of users during the image period. Thetelephone system is the most frequently citedexample of a circuit mode switching network.In message mode networks, circuits areshared on an ad hoc basis, and entire mess-ages are stored at intermediate nodes enroute to their destinations - telex networksuse message mode switching. Packet modenetworks use the same circuit -sharing prin-ciples as message mode networks, but largemessages are broken into smaller packets formore efficient and flexible transfer. Most ofthe popular local area networks (LANs) in usetoday are based on packet mode switching.The three types of inter -network communi-cation outlined here are shown in Figure 1.

All three network switching techniqueshave advantages and disadvantages. Forexample, circuit mode networks offer usersdedicated bandwidth, but, on the other hand,they can also waste bandwidth (under certaincircumstances). Because packet mode net-works have traditionally offered more flexibil-ity and utilised network bandwidth moreefficiently than circuit mode networks, someconsider packet mode networks to be higheron the evolutionary scale than circuit modenetworks. Recent advances in circuit modenetworks however have again brought thistechnology to the forefront of networkresearch and application. Hence, this articleseeks to explore the state of circuit modeswitching networks.

As a starting point, it is worthwhile to con-sider some basic concepts. One of the mostcommon interfaces in networking is the inter-face between data terminal equipment (DTE)and data circuit -terminating - sometimescalled data communications - equipment(DCE). DTE, typically embodied in computers,terminals or routers, requires a circuit forinformation transfer. DCE, typically embodiedin modems or similar devices, terminates andprovides docking for a circuit. DTE is usuallyprovided by private computing -equipmentvendors. DCE is commonly offered by com-munications product vendors. DCE providesconnection from DTE into a communicationsnetwork and back again. The DTE/DCE inter-face, and its relationship to a communication'snetwork, is shown in Figure 2.

Physical -LayerSpecificationsThe DTE/DCE interface is described throughvarious physical -layer communication specifi-cations; they provide information about fourdifferent aspects of physical -layer interfaces:


mechanical, electrical, functional and proce-dural. Mechanical aspects of physical -layerinterfaces indude connector specifics, circuitto pin assignments, and connector latchingarrangements. Electrical aspects of physical -layer interfaces indude voltage levels that rep-resent binary values and resistanceuripedance.Functional aspects of physical -layer interfacespecifications assign functions (control, data,ground) to particular circuits. Fmally, proce-dural aspects of physical -layer interfacesdefine those procedures associated withvarious data exchange operations. One exam-ple is the various electrical actions over spe-cific circuits that together specify the loopbackoperation.

The best known physical -layer interface isthe recommended standard 'RS -232' (whichis often referred to as Electronic IndustriesAssociation - EIA - 232). For international cir-cuit connections the Consultative Committeeon International Telephone and Telegraph(CCITT) created a physical specification verysimilar to that of RS -232. This specification isreferred to as 'V24'. A third popular physicalinterface, 'RS -449', is the successor to RS -232.RS -449 was created to ease the physical

limitations of RS -232, specifically the cabledistance limitation. Yet another physicalinterface is defined by the X.21' specification.X.21, also created by CCITT, includes a gen-eral-purpose DTE/DCE interface specificationfor synchronous transmission.

Another standard is the High Speed SerialInterface (HSSI), which is a de facto industrystandard developed by Cisco Systems (a keymanufacturer in the circuit mode switchingarena) and 13 plus Networking (a manufac-turer of DS3/E3 digital service units and band-width managers). HSSI operates over ashielded cable at speeds of up to 52M-bits/sand, distances of up to 50ft. Functionally, itserves the same purpose as lower -speedserial interfaces (such as V35 and RS -232) inthat it provides the interface to DCE for widearea network (WAN) communications. HSSIhas been standardised by the ANSI EIA/TIA(TIA - Telecommunications IndustriesAssociation) TR-30.2 committee as EIASP -2796 for the physical interface specifica-tions, and as EIA SP -2795 for the electricalspecifications. A table comparing variousaspects of several of the most popularphysical -layer specifications is shown in Table 1.

Circuitmode

c \-... ...7p 4)\.... ...,,

d

--X---,e \--'`.

---b

--X-- ););,e. \e\-- -...

"11)cr

Switch A

Message

Messagemode

Message 1

Message 2

Switch B

Message

Switch A

r

LP1 P2 P3

Message

Packetmode

P7 P9

Message 3

Message 4

Switch B

E

LP4 P5 P6

Message

I

J

Switch A

P12 P14

Switch B

Figure 1. Three Types of switched network.

DTE(Router)

DCE(Modem

Datacommunication

network

DCE(Modem

DTE(Router)

Figure 2. The DTE/DCE interface.

69

Numberof pins

Maximum cable length(Twisted pair)

Maximumsignalling rate

RS232 25 50 feet 20K-bit/sRS449 37 4,000 feet 10M-bit/sX21 15 not specified 1.92M-bit/s

V35 35 50 feet 4M-bit/sHSSI 50 50 feet . 52M-bit/s

Table 1. Comparison of some popular physical -layer interfaces.

Router

Remote office(Hong Kong)

Modem

Head office(London)

Modem Router

Modem

Remote office(New York)

Figure 3. Connecting corporate and remote offices.

Dedicated and SwitchedCircuitsCircuit mode networks can be used in differ-ent ways. For example, private circuit modenetworks are often composed of dedicatednetwork links. Although these networks fol-low the circuit switched model, no actualswitching occurs. Because these links andtheir associated telecommunications equip-ment are dedicated to use by a particularorganisation, they are often called 'dedicatedcircuits' or 'leased lines'. In contrast. publiccircuit mode networks typically offer manycustomers shared access to a large pool ofswitched circuits. Links of this type are called'switched circuits' or 'dial -up lines'. Dedicatednetwork links offer (potentially) a higher callvolume and no blockage from other organ-isations. Switched circuits, on the other hand,offer route flexibility, and cost advantages forlow call volumes.

The choice of a dedicated or a switchedline is most dependent upon how often datawill be flowing over the line. Dedicated linesare of fixed cost. Switched line costs are basedon usage. In general, with all other factorsbeing equal, dedicated lines become a moreeffective solution the more a line is used.Various studies have been run to determinethe crossover point of the two functions.Naturally, these studies yield different conclu-sions depending upon: type of line; line costsin the area of interest; when the study wascompleted; and other parameters. In the USA,for example, it becomes more economical touse a dedicated, rather than a switched,70

56K-bit/s line when the usage exceeds fourhours (according to AT&T figures in 1992). InJapan, the economies come in after eighthours of usage (according to NTT Japan,INS -Net 64 versus Super Digital Service).

Analogue SwitchedServicesThe most commonly used example of ananalogue switched service is provided by theworld's telephone system. Other popular ana-logue switched applications include: businessservices, LAN to LAN connections, and disas-ter recovery. Each of these applications hascontinuing utility for users, and a relativelyundiminished popularity over recent years.However, significant growth in LAN to LANconnectivity and disaster recovery makesthese applications particularly interesting.

During the 1980s, many companies estab-lished office -based LANs as a means to sharedata between users at a particular site. As inter -networking and distributed system technolo-gies matured, during the late 1980s, many ofthese companies saw the benefit of extendingtheir LAN applications and data retrieval capa-bilities over wider areas. Companies beganconnecting their remote offices to oneanother and to corporate offices (see Figure3). LAN -to -LAN connection offers access fromany desktop computer to all data within anenterprise, hence the name enterprise net-working.

As companies continue to move their'mission -critical' applications from centralcomputers to networks of smaller computers,

network `up -time' becomes essential.Network failures can cost companies hun-dreds of thousands of pounds in irretrievablelost revenue. To help prevent network failuresfrom having such an impact (on the bottomlines of their accounting balance -sheets),many companies are purchasing disasterrecovery equipment and applications. Often,these applications utilise switched analoguelinks to open automatically new routes overwhich mission -critical data can flow. They canalso provide backup channels for communi-cation of network failure information to cen-tral network management sites. When themain network lines are down, this may be theonly way in which notification of a networklink failure can be communicated to a centrallocation; the troubleshooting procedures canthen begin.

Generally, analogue transmission technol-ogy has several serious constraints that limitits usefulness. First, analogue transmission isnot as immune to transmission impairments,including problems resulting from the mod-ulation process, as digital transmission.Second - and most importantly - analoguetransmission in the telephone network is lim-ited to speeds of under 20K-bits/s, whereasdigital transmission can achieve far greaterspeeds. Many newer applications, such asimaging, video, and certain factory automa-tion applications, require substantially higherbandwidth and/or lower latency than ana-logue services can provide. As a result of theseand other benefits, digital transmission israpidly replacing analogue transmission.

Digital SwitchedNetworksSwitched digital services are offered by car-riers, including Ameritech (USA based), BellAtlantic, British Telecom, Telecom Finland,France Telecom, Pacific Telesis, and others.These services are also offered by USA -based'inter -exchange' carriers (IECs) such as Sprintand AT&T. Services offered include 56K-bits/s,n x 64K-bits/s, and others. These services sup-port not only the applications listed previously(voice, business services, LAN to LAN con-nectivity and disaster recovery), but alsoexpand the range of pncsible applications toinclude those that require a very high datarate, and low latency. Applications falling intothese categories include: imaging, video andfactory automation.

Over in North America, the switched56K-bits/s service is usually regarded as themost popular switched digital service.Switched 56K-bits/s service allows users todial a 56K-bits/s data circuit, pay for usageby the minute, and then disconnect the callin much the same way as a voice call isdisconnected. Switched 56K-bits/s service hasseveral important strengths, not the leastof which is price/performance. It is oftenused for such applications as video conferenc-ing, Group IV fax, LAN to LAN connectionsand backup. A switched 'n x 56/64K-bits/s'service is an extension of this technology,providing multiple channels of either56K-bits/s or 64K-bits/s service.

Switched T1 is a relatively new offering thatprovides high bandwidth capability for appli-cations such as mainframe connections, chan-nelising data, and video conferencing.Switched T1 is based on T1 technology, whichwas introduced by AT&T for the purpose ofinter -office trunking in metropolitan areas.


In Europe, similar switched services aredelivered as switched X.21 at 64K-bits/s,n x 64K-bits/s or as ISDN (Integrated ServicesDigital Network).

Integrated ServicesDigital Network (ISDN)The term used to refer to a new set of digitalservices becoming available to end -users is'Integrated Services Digital Network' (ISDN).ISDN involves the digitisation of the tele-phone network so that voice, data, text,graphics, music, video, and other sourcematerial can be made available to end -usersfrom a single end -user terminal, over existingtelephone wiring. Proponents of ISDN imag-ine a world-wide network much like the exist-ing telephone network, except that digitaltransmission is used, and a variety of new ser-vices are available.

ISDN is an attempt to standardise sub-scriber services, user/network interfaces, andnetwork and inter -network capabilities.Standardising subscriber services are aimedat ensuring a level international compatibility.Standardising the user/network interface stim-ulates development and marketing of theseinterfaces by third party manufacturers.Standardising network and inter -networkcapabilities is necessary to achieve the goal ofworld-wide connectivity by helping to ensurethat ISDN networks easily communicate withone another.

The ISDN reference model is shown inFigure 4. ISDN components include: termi-nals, terminal adaptors (TAs), network termi-nation devices, line termination equipment,and exchange termination equipment. ISDNterminals come in two types - specialisedISDN terminals are referred to as 'terminalequipment type 1' (TE1), and non-ISDN ter-minals, such as DTE equipment that pre -datesthe ISDN standards, are referred to as 'termi-nal equipment type 2' (TE2). This connect tothe ISDN network through a four -wiretwisted -pair digital link. TE2s connect to theISDN network through a terminal adaptor.The ISDN terminal adaptor can either be astand-alone device or a board inside the TE2.If implemented as a stand-alone device, theTE2 connects to the terminal adaptor via astandard physical -layer interface (for example,EIA 232, V24 or V35).

The next connection point in the ISDN net-work is the NT1 or the NT2. These are net-work termination devices that connect thefour -wire subscriber wiring to the conven-tional two -wire local loop. In North America,the NT1 is a customer premises device thatallows up to eight terminal devices to beaddressed. In most other parts of the world,the NT1 is part of the network provided by thecarrier. The NT2 is a more complicated device,typically found in digital private branchexchanges (PBXs), which performs layer -2and layer -3 protocol functions, and concen-tration services. An NT1/2 device also exists;it is a single device that combines the func-tions of an NT1 and an NT2. Basic rate access(BRA) is an example of an NT1/2 interface.

A number of reference points are specifiedin ISDN. These reference points define logi-cal interfaces between functional groupingssuch as TM and NT1s. These reference pointsindude 'R' (the reference point between non-ISDN equipment and a TA), 'S' (the referencepoint between user terminals and the NT2),'T' (the reference point between NT1 and


TE2R TAJS/T

TE2

U U

NT1

NT2+NT1

TA+NT2+NT1 r

Centraloffice

u

Figure 4. The ISDN reference model.

NT2 devices) and 'U' (the reference pointbetween NT1 devices and line terminationequipment in the carrier network). The U ref-erence point is relevant only in NorthAmerica, where the NT1 function is not pro-vided by the carrier network.

ISDN's Basic Rate Interface (BRI) serviceoffers two B -channels and one D -channel(2B +D). BRI B -channel service operates at64K-bits/s, and is meant to carry user data;BRI D -channel service operates at 16K-bits/s,and is meant to carry control and signallinginformation - although it can support userdata transmission under certain circum-stances. BRI also provides for framing controland other overheads, bringing its total bit rate

to 192K-bits/s. The BRI physical -layer specifi-cation is CCITT 1.430.

ISDN Primary Rate Interface service (PRI)offers 23 B -channels, and one D -channel, inNorth America and Japan, yielding a total bitrate of 1.544M-bits/s (the PRI D -channel runsat 64K-bits/s). ISDN PRI in Europe, Australia,and other parts of the world provides 30B -channels plus one 64K-bits/s D -channel,and a total interface rate of 2.048M-bits/s. ThePRI physical -layer specification is CCITT L431.

To signal between end -user equipment andthe ISDN network, a signalling protocol foruse over the D -channel is defined. This pro-tocol comprises layers 1 to 3 of the OSI refer-ence model. Layer -2 of the ISDN signalling

COMMONLY USED NETWORKING ACRONYMSANSI

ATM

BISDN

BRI

CCITT

DCE

DTE

EIA

HDLC

HSSI

IEC

ISDN

IAN

IAPB

IAPD

NT1/2

OSI

PBX

PRI

SDH

SONET

TA

TCP/IP

TE1/2

TIA

WAN

American National Standards Institute.

Asynchronousc Transfer Mode.

Broadband ISDN.

ISDN's Basic Rate Interface.

Consultative Committee on International Telephone and Telegraph.

Data Communications Equipment.

Data Terminal Equipment.

Electronic Industries Association.

High-level Data Link Control.

High-speed Serial Interface.

Inter -Exchange Carriers.

Integrated Services Digital Network.

Local Area Network.

Link Access Procedure for a B -channel.

Link Access Procedure for a D -channel.

Network Termination device 1/2.

Open System Interconnection.

Private Branch eXchange.

Primary Rate Interface service.

Synchronous Digital Hierarchy.

Synchronous Optical Network.

Terminal Adaptor.

Transmission Control Protocol/Internet Protocol.

Terminal Equipment type la

Telecommunications Industries Association.

Wide Area Network.

71

protocol is 'Link Access Procedure' for a D -channel, also known as LAPD. This is similarto HDLC and LAPB, and as its acronym expan-sion indicates, is used across the D -channel toensure that control and signalling informationflows and is received properly. LAPD's frameformat is very similar to that of HDLC; LAPDuses supervisory, information, and unnum-bered frames. The LAPD protocol is formallyspecified in CCITT Q.920 and CCITT Q.921.

Two Layer -3 specifications are used forISDN signalling: CCITT 1.450 (also known asCCITT Q.930) and CCITT 1.451 (also knownas CCITT Q.931). Together, these protocolssupport user -to -user circuit switched andpacket switched connections. A variety of callestablishment, call termination, informationand miscellaneous messages are specified;they include set up, connect, release, userinformation, cancel, status, and disconnect.These messages are functionally similar tothose provided by the X.25 protocol.

ISDN applications include high-speedimage applications (such as Group N fax),second telephone -lines in homes to serve the'telecommuting' industry, high-speed filetransfer, and video conferencing. Voice, ofcourse, will also be a popular application forISDN. Many carriers are beginning to offerISDN under tariff.

It is an acknowledged fact that transmis-sion costs are the most expensive part of long-term Wide Area Network (WAN) use. Indeed,independent research condudes that trans-mission costs make up nearly 90 per cent oftotal WAN inter -networking operating costsover a five year period; With this level ofinvestment, it is comprehensible that signifi-cant added value in WAN communicationssolutions equates, at least in part, to reducedtransmission costs. The main way an inter -net-working vendor can offer users reduced trans-mission costs is to offer features that decreaserequired use of expensive WAN communica-tions links. There are many ways of doing this,but arguably two of the most prominent con-cern data compression, and on -demand net-working. Data compression features caneffectively reduce the actual number of bitsthat must traverse a line for a given mesageor session. For example, compression algor-ithms for both TCP/IP and LAN traffic can beimplemented. On -demand networking is aconcept that turns routers into intelligentdialling -devices, capable of determining whenremote networking must occur, and then sup-porting it efficiently.

The Future for SwitchedCircuit TechnologiesBroadband ISDN (BISDN) is being hailed asthe WAN service of the future. This technol-ogy offers high bandwidth, and a singleswitching technology for everything from lowspeed data, to voice, to full motion video.Synchronous optical network (SONET) inNorth America, and Synchronous DigitalHierarchy (SDH) in Europe, representsBISDN's transmission technology.Asynchronous Transfer Mode (ATM) isBISDN's switch technology. ATM utilisesregularly spaced micro -packets (cells), each53 bytes in length, to transfer information.A body - the ATM Forum - has been set upto oversee and accelerate ATM standards, witha further mission of ensuring that users will beprovided with the best technologies for solv-ing large-scale inter -networking problems.

DIGITAL SWITCHED NETWORKSAND THE COMSUMERby Dean P Hodgkins B.Eng(Hons)

so, how exactly does ISDN benefit theaverage user? Well, ISDN is a CCITTmodel for the integration of voice and

data, and that is exactly what it provides. Theuser is able to access a network via an ISDNline, and this single line is able to provideaccess for a number of channels (voice com-munications, data transmission, etc.).

British Telecom state that ISDN is currentlyavailable on 98% of UK exchanges - thereason for the 'missing' 2% being due tosignal loss, which would be too great overthe distance between the exchange and theend -user.

BT offers what is known as ISDN-2 andISDN-30 lines. The former provides two sep-

arate data channels on one line. Each channelhas a capacity of 64K-bits/s, and can be usedfor either voice or data transmission. The lat-ter, ISDN-30, provides a minimum of 15 chan-nels (it is still called ISDN-30 for historicalreasons), and it is primarily used for voicecalls, switched through digital exchanges. Dueto its high capacity, ISDN-30 is usually imple-mented by using fibre -optic cable, although itcan be implemented with a microwave link.

Here, at Maplin Electronics, ISDN-2 is usedby our DTP (Desk Top Publishing) depart-ment. The DTP department is responsible for,amongst other things, production of the mag-azine pages (on computer). Once the pagesare completed, they are downloaded, via

Photo 1. A Quadra 700, used to run 4Sight's ISDN manager.

Photo 2. Inside the Quadra 700, the ISDN card is visible at the rear.


14

TELECOM ISDN 2

ET RT

ISDN, to a 'repro bureau' (to transfer the'pages' onto high resolution (2540 dpi) photo-graphic film) a few miles down the road.

The DTP DepartmentThe DTP department at Maplin makes use ofApple Macintosh computers (see Photos 1and 2). The computers are all inter -linked bymeans of a coaxial, Ethernet based LAN; theyalso have access to an ISDN-2 line (see Photo3). The Macintosh computers 'access' theISDN line via an 'interface', and a special ISDNcommunications package; the one commonlyused is `4Sight's ISDN Manager'.

The 4Sight ISDN Manager is easy to use,and it supports two concurrent data channels,

Above left: Photo 3. An ISDN-2 line terminating unit (BT).Above right: Photo 4. A handset enables voice communication on the ISDN-2 line.Below left: Photo 5. An SDX 420N digital exchange unit.Bottom left: Photo 6. Inside the SDX 420N; the ISDN-30 interface card is easily identifiedby the two white coax cables.Below right: Photo 7. A BT ISDN-30 interface unit; the ISDN-30 fibre optic lines terminateat this unit, with the link to the digital exchange being made by coax cable.

a-

...... ....- -

........

ge,.....

_ A

-7"."-'41"n"i ei fla......... - ip

I,*osuutm." . tre,..%-f\'

both of which have the same 'phone number'(in other words, a normal I3T ISDN-2 line).This means that, if both channels are used forsimultaneous, synchronous data -transfer, atransfer rate of up to 800K/minute is achieved.

ISDN enables users to have voice e-mail(electronic mail), whereby, instead of theusual 'textual' messages, the user will be ableto hear actual spoken messages. 4Sight'sManager does not provide voice e-mail, but itdoes allow the user to make a 'voice call' (seePhoto 4). This is similar to making a call witha conventional telephone, but all dialling andconnection are made by the computers. Thesoftware displays details of the call and of thecaller - in other words, you can see who is call-ing, and decide whether or not to answer!

Telesales and Customer SupportWhenever you call Maplin, your call is routedinto a modern digital exchange (see Photos 5and 6) via one of two ISDN-30 lines; the con-nection from the local BT exchange to theMaplin exchange being achieved by a fibre -optic link and a BT ISDN-30 terminating unit,shown in Photo 7. At present, Maplin haveallocated 14 channels (i.e. the 14 channelsshare the same telephone number) for CreditCard Sales, and 6 channels for CustomerServices. As with ISDN-2, ISDN-30 does nothave to be used exclusively for voice; variousother channels are used for the Cashtel ser-vice and key call, etc. The system is fully digi-tal, and is totally controlled by computer, withchannel usage being determined from a 'net-working option' in the software.

Further InformationBritish Telecom ISDN Helpline, Tel: 0800 1815144 -Sight Support Line, Tel: 0202 764401

73

By InstallingThe VersatileMulti -Purpose -INTRUDER _4..

ALARMText by Robin Hall

Ideal forpfr Garage* * Caravan

Summerhouse * Garden -shedTool -shed Workshop Lock -Up

* Barn * Stable Outhouse* Stock room Boat

* Loop alarm

FEATURES* Internal battery or external +12VDC operation * Long battery life* Key operated Integral loud siren* Compact * Visually attractive* Easy to use * Easy to install* Integral Ni-Cd charger circuit* External siren option * Normallyclosed, normally open and tamper(24 -hour) security loops * Integralanti -tamper switch Siren timer

h thi

ob beenthere overburglariyear!Rememberare one of thecleterrants

ALL too often these daysone hears or reads about

homes that have beenbroken into, and that all thethief apparently had to do wasgo to the unsecured tool -shedselect an appropriate tool andthen break into the home, andthat valuable tools kept in thegarden shed were also stolen.Not in the dead of night, butduring the afternoon in broaddaylight.

'Up -and -over garage requires a Garage DoorSensor to operate the alarm


NO MAINS?NO PROBLEM!Not all structures are attached tomain buildings, and are not so easilyprotected. This is especially true ofgarages, summerhouses, sheds and forthe more rural areas, barns, stables andouthouses. In many cases they may besituated some distance away from themains electricity supply. In the case ofboats and caravans, mains electricitymay not even be available, and thiswill cause a problem, as most alarmsystems are mains powered, withbattery back-up. The alarm can alsobe used as a 'loop alarm' for protectingvaluable goods on display in a shop, oras a secondary -alarm for protecting a

Priceonly

VERSATILEMULTI -PURPOSEinTRuckert ALARM

shop stock room against unauthorisedentry during opening hours (i.e. whenthe main alarm is disarmed).

There is, of course, absolutely noreason why the alarm cannot be utilisedas a low-cost home or flat alarm!

ESCALATING CRIMEWith security ever in our minds, withthe escalation of crime according to thelatest surveys, it is gratifying to find thatthere is a product that fits the bill.

The AnswerThe Versatile Multi -purpose IntruderAlarm has been designed for low currentoperation, and can be powered froma multitude of different power sources,

SpedficationInternal powersupply:

Standbycurrent (armed):External powersupply:

Internal sirensound level:

Arrn/disarrntime period:Alarm timeoutperiod:

Operating current(alarm sounding):

+9V DC (PP3)

2504 @ 9V (typically)

+12V (nominal)

80dB at 1 metre

20 seconds (nominal)

10 to 15 minutes

70mA typical

Outer: The Versatile Mufti -Purpose intruderAlarm Is suitable to protect a diverse rangeof property.Centre: The alarm unit as supplied.Bottom: The alarm unit Installed and readyfor use.

ranging from an internal battery to anexternal mains derived supply, even solarpower is possible! Built into the alarm isa charging circuit, so that a Ni-Cd batterycan act as a back-up in the event of themain supply failing. Operation of the unitis controlled by an unambiguous twoposition key -switch. There is a piercingalarm included within the case of theunit, but there is also provision for anexternal siren to be connected as well.So even on its own, the alarm canprovide a very capable base for an alarmsystem without the complications thatusually surround intruder alarms. Thebest news of all is that the unit is veryeasy to install and use, and is a fractionof the cost of a commercially installedalarm system. Anyone with basic DIYskills and tools can quickly and easilyfit the alarm.

WHAT YOU GETThe Versatile Mufti -purpose IntruderAlarm is supplied with approximately fivemetres of cable, cable clips, a surfacereed switch, a set of keys, screws andwall plugs, and an inst., uction manual.With the exception of the battery whichis not included, these are enough toget a basic system up and running


The three terminal blocks at the rear of unit.

numerous accessories are also availableto expand the system.

The stylish alarm control unit itself ismade of high impact plastic, and housesthe electronics at the heart of thesystem. A key controlled arm/disarmswitch is fitted on the front panel.The internal siren is housed behind thefront panel. The alarm is secured to aconvenient surface by means of amounting plate - if any attempt is madeto remove the unit from the mountingplate the internal siren and external sirenif fitted, will sound.

Three terminal blocks are located onthe rear of the unit these enable externalconnections to be made. The (optional)internal battery is housed in a recessedcompartment behind the rear panel.

POWER SOURCEAttention has been given to the powerrequirements of the unit. With negligiblecurrent drain when the unit is in thestandby condition, and modestconsumption when activated, the unitcan be powered by an Alkaline 9V PP3battery for up to six months. This isespecially important, where there is noother supply available, for example aboat or caravan or remote building.However, provision has also been madeto power the alarm from other externalpower sources. An external supply canbe connected to the terminals marked' +12V' and 'OV'. If an external powersupply is used, and battery back-upis required, a rechargeable 8.4V Ni-CdPP3 battery must be fitted instead of anormal 9V PP3 Alkaline battery. This isbecause a DC charging voltage is presenton the alarm's battery connectionswhen an external supply is used -applying such a voltage to an ordinarybattery could cause it to leak or evenexplode!

The ability to connect an externalpower supply opens up other possibilitiesfor alternative power sources such assolar panels or wind generators. In therural or maritime environment suchpower sources are already frequentlyused. In such instances it would benecessary to include a Ni-Cd batteryotherwise the alarm would only workwhen the sun shines or the wind blows!

Please note that under no

circumstances must this alarm controlunit be directly connected to the ACmains supply it is designed for extra lowvoltage use only. The recommended ACmains to 12V DC adaptor, or a suitableequivalent, must be used if mainsoperation is required.

SECURITY LOOPSThe three terminal blocks on the back ofthe unit facilitate connection to externalsecurity loops. Three distinct securityloops are provided on the unit, and eachone has a different part to play - this isjust one of the factors that make thisalarm so versatile.

The loops are as follows:

Normally Closed LoopThe Normally Closed Loop is for use withsensors that have contacts that open('break') when the sensor is operated(triggered). The loop may comprise justone sensor or several sensors connectedin series, such that operating any of thesensors will break the Normally ClosedLoop. The Normally Closed Loopconnects to the terminals marked 'COM'and 'NC'. These terminals are suppliedfitted with a 'shorting link'. This linkmust be removed before connectingthe Normally Closed Loop. The majoradvantage with a Normally Closed Loopis that cutting the wires to a sensor willtrigger the alarm. The Normally ClosedLoop is only monitored when the alarmcontrol unit is in the Armed state.Triggering the Normally Closed Loopstarts the Entry Delay Timer, if the alarmis not disarmed within the timeout periodthe siren will sound.

24 -Hour (Tamper) LoopThe 24 -Hour Loop is a Normally ClosedLoop that is continuously monitoredwhen the alarm has power applied toit (i.e. the battery is fitted). Operationof this loop is exactly the same as theNormally Closed Loop described above,except that it does not matter whetherthe alarm is in the Armed or Disarmedstate. The Normally Closed Loopconnects to the pair of terminals marked'24HR'. These terminals are suppliedfitted with a 'shorting link'. This linkmust be removed before connecting the24 -Hour Loop. This loop will trigger thealarm if the loop is 'broken' by wiresto a sensor being cut or if the cover toa sensor, fitted with a tamper switch,is removed. The loop may also beconnected to a Panic/Personal Attackbutton; operation of the button willtrigger the alarm and cause the siren tosound immediately. This loop is normallyused in tandem with the Normally Closedand Normally Open Loops so doublyensuring security. Note that the 24 -HourLoop does not latch when triggered.

Normally Open LoopThe Normally Open Loop is for use withsensors that have contacts that close('make') when the sensor is operated(triggered). The loop may comprise justone sensor or several sensors connectedin parallel, such that operating any of the

sensors will make the Normally OpenLoop. The Normally Open Loop connectsto the terminals marked 'COM' and'NO/EXT+'. The Normally Open Loop isonly monitored when the alarm controlunit is in the Armed state. Triggering theNormally Open Loop starts the EntryDelay Timer, if the alarm is not disarmedwithin the time-out period of the EntryDelay Timer, the siren will sound.

Please note that the Normally Openand Normally Closed Loops are notmutually exclusive, both types can beused simultaneously in an installation.

INSTALLATIONPlanningInstalling the unit is very straightforward,but it is advisable to carefully plan exactlywhere the alarm control unit is to belocated and what doors, windows, etc.,are to be protected. From this you will beable to decide which accessories, if any,are required.

It is advisable, if possible, that thealarm control unit is not installed in fullview, i.e. opposite a window, and yet atthe same time must be easily accessibleby 'authorised' persons in order todisarm the alarm within the permitted 20second entry period. The position of thealarm control unit must be such that it isnot directly exposed to water. Cables willalso need to be run to the alarm controlunit so consider suitable routes to runthem. If the alarm control unit is to bepowered from an external power source,locate it accordingly, e.g., adjacent to anAC mains socket.

Fitting a Door SensorApproximately five metres of 4 -wirecable is included with the unit, and thisshould be sufficient to wire the suppliedmagnetic door sensor. Choose a suitablelocation for the sensor, see Figure 1. Thesensor is in two halves; one half containsa magnet and the other a reed switch.The reed switch may be readily identifiedby the presence of the wiring terminals.The magnet should be mounted on thedoor itself and the reed switch should bemounted on the frame. The two halvesshould be positioned as close togetheras possible, without fouling each other or

Door Sensor (magnetic reed switch) Installed.


Magnetic reed switch.

the door (maximum separation with doorclosed 10mm).

The wiring diagram is given in Figure 2,and shows how to wire up one pairof wires to the Normally Closed (NC)contacts on the sensor. The other pairof wires, which are for the 24 -Hour Loopare joined together on a spare contact.When wiring up the sensor, make a noteof the colour coding used on the cable.Tack the cable in position, out of view ifpossible, and route it to the alarm controlunit. Before cutting the cable, ensurethat there is sufficient slack cable atthe alarm control unit end to allow theconnections to be made.

If no other sensors or accessoriesare to be fitted (i.e. basic installation)continue onto the next section otherwiserefer to the section on Expanding theSystem.

Fitting the Alarm Control Unit

plate is to be fixed is subject tocondensation, it is recommended that awaterproof sealant be used to seal themounting plate to the wall. If sealant isused remove the bottom cable entryknockout on the mounting plate to allowany water droplets that may accumulateto run out. Cable entry knockouts willneed to be removed as necessary toallow entry of the cables. Fix themounting plate to the desired surfaceusing the screws and wall plugs supplied.

At the alarm cot rtr 01 unit end the wiresare connected as shown in Figure 2, withthe Normally Closed (NC) Loop wiresconnected to 'NC' and 'COM' terminals -make sure that the shorting link hasbeen removed. The tamper '24 -HR'Loop wires which have been connectedtogether on the sensor, are wired to the'24 -HR' terminals on the unit - againmake sure that the link has beenremoved first.

If a multimeter or continuity tester isavailable then check that the 24 -HourLoop is continuous, and that theNormally Closed Loop is continuouswhen the door is closed and brokenwhen the door is open.

These tests are to minimise possibleproblems in the installation.

Fitting the BatteryOnce all wiring is complete, remove theback panel of the alarm control unit andinstall either an Alkaline or Ni-Cd batteryas previously discussed. Note: the sirenwill sound as soon as the battery isconnected. Since the tamper circuit

Reed half of sensor

To alarmcontrol panel

Magnetic half of sensor

Cable entryknock-out

Figure 1. Fitting the magnetic reed switch to door.

0

0 0

10mmMaximum operating gap 10mm(ensure contacts are parallel

at top, as shown)

Normally closedloop

Optional

Magnetic door/window switch(normally closed sensor)

2 wires joinedunder terminal

Alarm back panel

Optional

Figure 2. Wiring diagram showing alarm back panel and wiring up of rnagnetk door/window sensor.


operates regardless of key -switchposition, hold in the anti -tamper micro -switch in order to inhibit the siren. Makesure that the battery lead is locatedthrough the cut-out located on the edgeof the PCB, and that it is not proud. Nextrefit the rear panel, ensure that therubber battery 'pad' is located above thebattery. The siren will sound once themicro -switch is released. The siren willcontinue to sound until the alarm controlunit is correctly mated with themounting plate.

USING THE ALARMUsing the key provided, rotate the key -switch anticlockwise from 'Disarm' toArrn' and remove the key. The alarmallows approximately 20 seconds in orderto exit the area, a loud warning bleep isthen heard which announces that thealarm is now armed. Upon triggering asensor, by opening a protected door,etc., the alarm allows 20 seconds forthe area to be entered and the alarmdisarmed. To disarm the alarm, returnthe key -switch to the 'Disarm' position.Failure to disarm the unit within theentry time, will cause the siren to sound.If the alarm is powered by a 9V alkalinebattery, it is advisable to replace thebattery every six months. Whatevermeans of power is used, it is advisableto carry out periodic tests to confirmthat all aspects of the alarm are fullyfunctional.

If the alarm is set off, eitherinadvertently or by a deliberateunauthorised entry, the siren may besilenced by returning the key -switch tothe 'Disarm' position. Note that if the 24 -Hour Loop is broken, the key -switch willnot silence the siren. In such instances,the siren will continue to sound until the24 -Hour Loop is restored or the sirentimer switches off the siren.

EXPANDING THE SYSTEMThere are a number of optionalaccessories available that fullycomplement and expand the system.These are easily wired to the main unitusing 4 -wire cable.

Magnetic Reed SwitchesExtra magnetic reed switches areavailable for the system and are wiredin series with the Normally Closed Loop.There are different styles of magneticreed switches available and are usedaccordingly. All these devices triggerwhen a door or window is opened.

External SirenThe External Siren is a particularly goodidea if the garage, outhouse or shed issome distance from the main building.It is wired to the main unit using cablesupplied with the siren, and wired upbetween the 'NO/EXT+ ' and 'EXT-'.Note that the 'NO/EXT+ ' is shared withthe Normally Open Loop. The design is

Optional External Siren

such that the siren does not draw morethan 200mA in use so it still enables theunit to obtain power from a 9V AlkalinePP3 battery. The battery has sufficientcapacity to operate the siren for anumber of 15 minute cycles.

It is recommended that the outdoormini -siren is mounted on an outside wallas high as possible, such as under theeaves, to prevent tampering. It is fixedby using one screw at the top of theunit. The cable must sit in a groove atthe back of the case, as this allows it tomount flush against the wall. The cableto the siren should pass through the walldirectly behind the siren, or enclosed inconduit to prevent cable damage.

External Alarm BoxAlso available is an External Alarm Box,which when installed in a prominentposition acts as a highly effective visualdeterrent to would be thieves. if desired,the External Siren can be fitted into thisenclosure.

Garage Door SensorAnother optional extra is the GarageDoor Sensor, and this comes completewith tilt -switch, prefilled junction box,4 -wire cable and cable ties. The sensorhas a mounting plate attached whichenables it to be fitted to up -and -overtype garage doors. The sensitivity isadjustable so that it can either activateon just a slight movement of the garagedoor, or when the door is in its fully openposition. Figure 3 shows typicalinstallation (Normally Open operation).

The sensor can be set to eitherNormally Open, or Normally Closedoperation see Figures 3 and 4, whichmode is selected depends on how it ispositioned with respect to the verticalplane, and the way it is rotated. Junctionbox wiring is shown in Figure 5. ft is

Junction Box(Mounted on GarageWall or Door Frameclose to Tilt Switch)

0.5m of Cable

Tilt Switch(to be Mountedat 90' to insideface of garage

door)

Sticky Pad(see text) Pivot Point

Fixed to Frame

Tie Wraps (see text)View in direction 'A'

Inside face ofgoroge door

Pivot PointFixed to Garage Door

View from inside of garoge.

Optional garage door sensor fixed In position.

78

Figure 3. Fitting the garage door sensor (Normally Open operation).


Optional breaking glass sensor.

recommended that the sensor isconfigured for normally open operationand connected in parallel with theNormally Open Loop.

Vibration SensorAnother useful device for incorporationinto the system is the vibration sensor.This can protect areas not covered bythe other sensors or switches. Insidethe device is a small weight on a spring,and this is adjusted for sensitivity byan acijusli T tent screw, the contacts canbe set for either Normally Closed orNormally Open operation. NormallyClosed operation is preferable, and ismore secure, see Figure 6.

Window ProtectionExtra protection is given by usingBreaking Glass Detectors and WindowFoil. Both are connected to the NormallyClosed Loop. The Breaking GlassDetector is simply stuck in position onthe window to be monitored. tt is thenconnected, through a junction box on thewindow frame, to the Normally ClosedLoop.

Window Foil, which is supplied on a33m reel, should be carefully applied toa window and folded at the turns. It istaken to Window Foil Terminals whichare then connected to the main unit by4 -wire cable.

Extra 4 -wire cable and junction boxesare available if needed, and these enableinterconnection of different sensors.

O

Figure 4. Garage door sensor orientated forNormally Closed operation

Fitting window foil and termination blocks.

To NormallyOpen (n/o)or NormallyClosed (n/c)connectors ofcontrol panel

Wire in serieswith tamper(24hr) loop

Spare

Figure 5. Junction box wiring for the garagedoor sensor.

Figure 6. Vibration sensor wiring.

Panic ButtonThe Top Operated Panic Button isa useful accessory in case there is anemergency, such as in the home or in aworkshop, when outside attention isrequired. It can also trigger the alarm toscare off intruders, especially useful forsingle person occupancy.

The panic button is wired in serieswith the 24 -Hour Loop, breaking this loopimmediately sounds the alarm. There isalso a tamper -proof connection available,and this can also be wired in serieswith the 24 -Hour Loop if desired, seeFigure 7. Another element of security,is that once the top operated button has

Figure 7. Panic button wiring.

been activated, resetting the buttonis only possible by using the keyprovided.

Pressure MatTo protect other areas near doors, apressure mat can be used. Placed undercarpet or mats near the door. The matsare Normally Open and should be wiredto the Normally Open Loop via a junctionbox. When stepped on the pressure matwill activate the alarm. It provides acompletely hidden detection method.There is a smaller sized pressure matavailable that can be it to be placedalong the top of a step or stair.


Optionalbreaking

glasssensor

(NC)

(NC)

Optionalvibrationsensor

Smoke alarmwith relay

(NC)

OV +12V0 0

Optionalwindow foil

(NC)

Optionaldoor mat

(NO)

Optionalgarage tilt

sensor

(NO)

Optionalmagnetic sensor

(NC)

Magneticsensor

(NC)

(NC)

Optionaloutside siren

enclosure

Optionaltop operatedpanic button

NO/Ext

24Hr

Corn

NC

Ext

+12V

Optionalexternal

mini-siren

OV

Alarm back panel

Versatile multi-purposeintruder alarm withoptional accessories

Optional 12Vmains adapter

A wide range of accessories may be connected to the alarm control unit.

External +12V DC Power SupplyThe Versatile Multi -purpose IntruderAlarm has been designed to operatefrom an Alkaline 9V PP3 battery, or froma rechargable Ni-Cd battery. In somecases it may be preferable to chargethe 9V Ni-Cd PP3 battery (used inplace of the Alkaline PP3 battery) on aseparate charger for its initial charge, asthe unit contains a trickle charge circuit,

and will take 24 to 36 hours to chargethe battery.

Running the Versatile Multi -purposeIntruder Alarm from an external supplyopens up a number of opportunities toexpand the system by utilising otheraccessories which require power, thusgiving further protection. Figure 8shows how the accessories are wiredto main unit.

Ordering Information for the Alarm and Accessories

ProductVersatile Multi -purpose Intruder AlarmExternal sirenExternal Alarm BoxAC Mains Adaptor 12V DC 500mARechargeable Battery (Ni-Cd PP3)In -line Fuseholder500mA FuseDoor LoopWindow Foil Terminations5 -way Junction Box8 -way Junction Box12 -way Junction BoxSurface Reed SwitchRecessed Reed SwitchFlush Reed SwitchGarage Door Sensor (for up -and -over -doors)Glass Break DetectorWindow FoilTop Operated Panic ButtonMetal Panic ButtonPressure MatStair Pressure MatVibration SensorSmoke Alarm with Relay

Stock Code Price50491 £19.99CVV50E £14.99YP5OE £7.25BZ83E £8.99HW31J £6.25RX51F 25pWR1OL 10pYW48C £2.75YW51F 52pYVV49D 60pFK76H 90pRC59P £1.75YW47B £1.70YVV46A £1.75FK77J £1.45CW51F £9.99FP11M £2.65YVV50E £1.99KROOA £2.99YZ67Z £5.99YB91Y £3.99FK79L £2.99FK78K £3.49KC39N £19.99

Smoke Alarm with RelayWith safety in mind, a Smoke AlarmSensor with a built-in 85dB siren is alsoavailable for wiring into the system (seePhoto 11). If used, the smoke alarm'srelay output connects to the NormallyClosed Loop. The supply for the smokealarm is obtained from the '+12V' and'OV' terminals on the alarm control unit(external power supply required). Whenthe smoke alarm is activated, it alsotriggers the main unit's siren and theoutdoor mini -siren, if fitted.

Junction BoxesA range of junction boxes are available,these enable interconnecting cables tobe used with the extra security devices.The three types of junction boxes are5 -way , 8 -way and 12 -way.

Further SuggestionsMention was made of alternative powersources, such as solar panels and windgenerators, and these can be used torecharge the Ni-Cd battery in the mainunit, instead of a conventional mainspowered supply.

Levels of sunlight vary from day today, from summer to winter, but evenon a cloudy day though, it is amazinghow much light falls on the solar panelsto produce electricity.

On farms and on some boats theremay be a wind generator available. If thevoltage is compatible with the unit thenthis can be utilised to charge the battery.

When using a supply with a largercurrent capability, it is recommendedthat a 500mA fuse is inserted into thepositive line.

80Maplin Magazine October 1994

Computers in Schoolsbr Steve Greenwood

The author has targeted this book towardsanyone, but particularly parents and

teachers, who want to learn aboutthe computers that are becoming anindispensable part of school life. This isa topic about which parents show great

concern, but are maybe too frightened

to ask for more information for fear ofnot understanding the jargon. For parentswho do not work with computers, thejargon barrier can cause many problemsbetween parents, children, and theirteachers. Where computers in schoolshave become a significant part of theclassroom learning process, it can resultin a lack of understanding or worse a totaldistrust of the school system. This booksets out to answer the many questionsand queries that parents and teachersmay have. An additional consideration isthat it is the intention of the governmentto integrate computers or informationtechnology (FT) in all areas of the NationalCumculum.

The text explains concepts and ideasin plain English and does not assume

any computer knowledge. Explanationsof computing terms are explained in aglossary at the back of the book. Thisis an accurate, unbiased and informativebook, written by a concerned andknowledgeable parent. It answers manyof the questions being constantly askedby parents and teachers about all aspectsof computers in education.1993. 144 pages. 210 x 146mm,illustrated.

Order As A483E

(Computers In Schools) £5.95 NV

Electronics for EngineersSecond Edition

P r -raddock E D ^A CalcuttThis book provides a comprehensivecourse in electronics for engineeringstudents to higher technician and firstdegree level. Emphasis on new technology

and analytical methods mean that thisbook will be of most use to those wishingto bridge the gap between old andpresent-day integrated circuit technology.

Comprehensive coverage incorporatestopics like Laplace Transform State

Analysis, High -Frequency Amplifiers and

VLSI. The book has summaries at theends of chapters and features learningobjectives like worked examples and

problem soMng exercises. Sections

explain about the operation ofcomponents and the way they interactwith circuits to produce both wanted andunwanted effects. Information includesuse of operational amplifiers, digitaldevices and controlled rectifier devices.

Sections on techniques of analysis explainhow circuits can behave differently at ACand DC, together with transient responseand digital analysis methods. Sequentialanalysis of synchronous and asynchronous

circuits provides a way of analysing digitalcircuits, including useful analysis toolslike Camaugh mapping for realisation

of logic circuits from truth -tables, in both

positive and negative logic types. Topics

in the digital area also include countertechnologies like synchronous types,multiplexers, timing circuits, driver logic,

arithmetic circuits, and various types ofROM and RAM, including EPROMs. etc.

The analogue section explains amplifieroperation. with analysis techniques

and various models to simplify activecomponents and their effects on circuitoperation. In-depth discussions of circuittheory and operation help to impartexactly how a particular circuit will

operate. The book covers most of themodem digtal and analogue syllabusmaterial covered in courses like ONCand HNC electronics, together with agreat deal aimed at first degree level.

A good source of information forstudents, with numerous diagramsand illustrations.

1994. 719 pages. 240 x 190mm.illustrated.

Order As ANO4E

(Elecs for Engineers) £19.99 NV

Practical AntennaHandbookSecond Edition

Jost,nil 1 Gin.For those interested in building anykind of radio antenna or antenna basedproject. this book is for you. All the basic

types of radio antenna are covered by thisbook including: high -frequency dipoles:

multiband and tunable wires; hidden andlimited space; directional phased -verticaland directional beam: VHF/UHF

transmitting and receiving antennas;shortwave antennas; microwave: mobileand marine. They are all covered, in thisin-depth book, with all the relevant theorynecessary for the successful building ofsuch antennas, including examples andconstruction diagrams. Radio wavepropagation, transmission lines, Smithcharts and matching theory are includedand will prove useful when matchingantennas, and working out delay andphasing problems. Sections on wavepropagation and electromagnetic fieldsexplain why an antenna works in the wayit does. One of the main problems withexplaining antenna behaviour is that you

cannot see by looking at it if its workingefficiently. With this book you are showndiagrammatically how the wave -patternand radiation would appear from differentantennas. and there are tips on improvingreception and transmission, regardless of

the arrangement you are using. By theend of chapter five. all the groundworkhas been put down to start looking atantennas in detail. Sections on microwaveand waveguide techniques will beparticularly useful to those interestedin frequencies between 900MHz and300GHz. Theory on waveguide sections,impedance matching and coupling willhelp to show how losses can be reduced,leading to a more efficient overall system.Microwave matching and mixing with

waveguides is also discussed, along

with other factors to do with transverseelectromagnetic waves. Dubbed the'Antenna Builders' Bible', and containingmany additions, this book will show youhow the best results can be obtained fromany antenna, whether you plan to makeit, buy it or repair it.1994. 560 pages. 230 x 190mm.illustrated. American book.

Order As ANO3D

(Pratt Antenna H/book) £23.95 NV

Mastering RadioFrequency Circuitsby Joseph J. Can.This book will be useful to thoseinterested in radio theory, and wishing

to build radio receiver or antenna basedprojects. It gives details of many receiversand radio based projects with explanationsof the concepts behind the design theory,and reasons for approaches in design.

Likely to appeal to students, electronicshobbyists, ham radio operators andshortwave listeners. The book easesthe reader into the subject by explainingfrequency allocations and classifications

of radio signals. before going on toexplain about RF components and theirconstruction. An overview of RF tools andinstruments explains how equipment isto be used for a vanety of tasks and howto make measurements correctly andaccurately. All the important theoryabout electronic and RF components isexplained in the first few chapters of thebook before the details of radio andamplifier designs are presented. Eachchapter is filled with great ideas andcircuit designs, to show by example howcircuits and equipment work. Among the

many projects in this book you can buildvarious RF amplifiers and receiverpreselectors, design and wind inductor

coils for radio circuits, and there is asection on building and using RF bndges.

Dedicated parts of the book are used to

explain frequently used, or popular itemsof equipment like direct -conversionreceivers. signal generators and filters:

equipment that will be of great interestto ham radio operators. At the end of thebook are a couple of chapters on morerecent technology, including pin diodesand microwave integrated circuitamplifiers, and finally a chapter on the

various projects and their construction.Throughout the book there is much radio,transmission line and instrument theory,which will also prove invaluable to thehome enthusiast and constructor forgeneral interest and in the buildingprocess of the projects in the book.1994. 411 pages. 230 x 190mm,illustrated. American book.

Order As ANO2C

(Mastering RF Circts) £17.95 NV

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Part 2 1987Level B:

Quality AssuranceRS12750

Over 800 colour packed pageswith hundreds of Brand New

Products at Super Low Prices.Available from all branches of WH SMITH, John Menzies in Scotland ONLY,

Eason &' Son in N Ireland ONLY, and Maplin stores nationwide. The MaplinElectronics 1995 Catalogue - OUT OF THIS WORLD! Southern Africacustomers please contact Maplin (South Africa) Telephone (024) 51-5124

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