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The Maplin ser ies
This book is part of an exc i t ing s e r i e s deve loped by
Bu t t e rwor th -He inemann and Maplin E l e c t r o n i c s P i c .
Books in the se r ies are prac t ica l guides which offer e lec-
t ronic cons t ruc to r s and s tudents c lear in t roduct ions to
key top ics . Each book is written and compiled by a lead-
ing e l ec t ron ics author.
Other books published in the Maplin se r ies include:
Computer Interfacing
Logic Design
Music Projects
Starting Electronics
Audio IC Projects
Video and TV Projects
Test Gear & Measurement
Integrated Circuit Projects
Home Security Projects
The Maplin Approach
to Professional Audio
Graham Dixey 0 7506 2123 0
Mike Wharton 0 7506 2122 2
R A Penfold 0 7506 2119 2
Keith Brindley 0 7506 2053 6
Maplin 0 7506 2121 4
Maplin 0 7506 2297 0
Danny Stewart 0 7506 2601 1
Maplin 0 7506 2578 3
Maplin 0 7506 2603 8
T.A.Wilkinson 0 7506 2120 6
Auto
Electronics
Projects
U N E W N E S
Newnes
An imprint of Butterworth-Heinemann Ltd Linacre House, Jordan Hill, Oxford 0X2 8DP
- ^ J j A member of the Reed Elsevier group
OXFORD LONDON BOSTON MUNICH NEW DELHI SINGAPORE SYDNEY TOKYO TORONTO WELLINGTON
© 1995 Maplin Electronics Pic.
All rights reserved. No part of this publication may be reproduced in any material form (including photocopying or storing in any medium by electronic means and whether or not transiently or incidentally to some other use of this publication) without the written permission of the copyright holder except in accordance with the provisions of the Copyright, Designs and Patents Act 1988 or under the terms of a licence issued by the Copyright Licensing Agency Ltd, 90 Tottenham Court Road, London, England W1P 9HE. Applica-tions for the copyright holder's written permission to reproduce any part of this publication should be addressed to the publishers.
The publisher, copyright holder and author have taken all reasonable care to prevent injury, loss or damage of any kind being caused by any matter published in this book. Save insofar as prohibited by English law, liability of every kind including negligence is disclaimed as regards any person in respect thereof.
British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library ISBN 0 7506 2296 2
Library of Congress Cataloguing in Publication Data A catalogue record for this book is available from the Library of Congress
Edited by Co-publications, Loughborough
£ ^ Typeset and produced by Sylvester North, Sunderland
all part of The Sylvester Press ^
Printed in Great Britain by Clays L t d , St Ives pic
Preface
This book is a col lect ion of art ic les and projec t s previously
published in Electronics — The Maplin Magazine.
Each project is se lected for publication because of its special
features , b e c a u s e it is unusual, b e c a u s e it is e lectronical ly
c lever, or simply because we think readers will be interested
in it. Some of the devices used are fairly specific in function —
in other words , the circuit is designed and built for one pur-
pose alone. Others , on the other hand, are not specific at all,
and can be used in a number of applicat ions.
This is just one of the Maplin series of books published by
Newnes books covering all a s p e c t s of computing and e lectron-
ics. Others in the series are available from all good bookshops.
Maplin Elec tronics Pic supplies a wide range of e lec tronics
c o m p o n e n t s and o ther p r o d u c t s to private individuals and
trade cus tomers . Telephone: ( 0 1 7 0 2 ) 552911 or write to Maplin
E lec tron ics , PO Box 3, Rayleigh, Essex SS6 8LR, for further
details of product cata logue and locat ions of regional s tores .
VÜ
1 Car electrical systems
The modern motor veh ic le is a precision-buil t highly-
t u n e d m a c h i n e . High s p e e d p e r f o r m a n c e , low fuel
consumpt ion and quiet smooth-running engine all rely
on efficient ignition, ba t te ry charging and general e lec-
t r ical sys tems throughout the car .
The e lec t r ica l sys tem is very complex . One only has to
look behind a dashboard to s ee the hundreds of wires of
all s izes and co lours , in te rconnec t ing the ins t ruments ,
high vol tage and high current c i rcu i t s . Also, the e lect r i -
cal sys tem is very prone to breakdown, whether this is a
blown lamp bulb, a faulty dynamo or badly adjusted con-
tac t breaker points .
1
Auto electronics projects
No two models of cars have identical e lec t r ica l c i rcui t s .
The e lec t r ica l c i rcui ts are, however, similar and fall into
ca tegor i e s such as convent iona l ignition or e lec t r i ca l
ignition, dynamo or alternator, positive or negative earth.
This chap te r desc r ibes the bas ic sys tems: it is left to the
individual car owner to interpret the descr ip t ions and
diagrams to suit their par t icular vehic le .
One word of warning. Car e l e c t r i c c i rcu i t s can cause
damage to e i ther the car or to the user if tampered with.
For ins tance a shor t c i rcui t a c r o s s the ba t te ry can gen-
era te hundreds of amperes and a lot of heat, even a fire:
the ignition circui t genera tes very high vol tages indeed:
tampering with the instrument c i rcui t s , can cause mis-
leading readings and a poss ib l e safe ty hazard to the
dr iver . Befo re embark ing on any c h a n g e s to the ca r
e l ec t r i c s , make every effort to understand how the cir-
cuit works. In this way fault finding should be greatly
simplified.
The ignition circuit
The purpose of the ignition circui t (Figure 1.1) is to sup-
ply the high vol tage required to opera te the spark plugs
in the co r r ec t s equence and so ignite the air /petrol mix-
ture in each cylinder. The explos ions generated push the
pis tons and so turn the engine, causing motion. The cir-
cui t c o m p r i s e s the ca r ba t t e ry , an ignit ion co i l , the
dis t r ibutor and four (or s ix) spark plugs. The principle
of operat ion is descr ibed later .
2
Car electrical systems
Figure 1.1 The ignit ion c i rcu i t
Battery charging
All e lec t r ica l sys t ems draw their power from the 12 volt
ba t te ry (Figure 1.2). If the ba t te ry was not cont inual ly
charged it would b e c o m e exhaus ted very quickly, par-
t icular ly if the lights, wipers and s ta r te r motor were in
cons tan t use. The turning of the engine charges the bat-
tery by connec t ing it to a dynamo, via the fan bel t . A
3
Auto electronics projects
pulley network at the front of the engine cons tant ly turns
the dynamo which genera tes enough power to charge up
the bat tery . A control box cont ro l s the charging rate and
informs the driver via the ignition light if the ba t te ry is
not charging. Some cars use an a l te rnator in preference
to a dynamo. T h e s e are more efficient but genera te a.c.
ra ther than d.c. and so require rect i f icat ion of the a.c.
output. Ba t te ry charging is desc r ibed later .
Figure 1.2 The battery charging c i rcu i t
4
Car electrical systems
Lighting
The lighting c i rcui t s are the s imples t of all these , com-
prising a simple connec t ion of the 12 volt lamp to the
ba t te ry via the instrument panel swi tches (Figure 1.3).
T h e s e c i rcu i t s are comple te ly independent of the igni-
tion and charging c i rcu i t s , the one connec t ion to each
lamp being taken via a single wire and respec t ive switch
to the bat tery; the o ther connec t ion uses the car chas -
s is . The lighting c i rcu i t s are desc r ibed in more detail
later .
Figure 1.3 The l ighting c i rcu i t
5
Auto electronics projects
Figure 1 .4 The indicator and accessories c i rcu i t
6
Indicators and accessories
Contained within this circui t is the s ta r te r motor which
draws hundreds of amperes from the ba t te ry to turn the
engine until it fires (Figure 1.4). Heavy duty cab le and a
heavy duty solenoid car ry out this operat ion, which is
prone to t rouble for various reasons . Also there is the
fuel pump which is a small solenoid opera ted device to
Car electrical systems
pump petrol from the tank to the ca rbure t to r , the indi-
ca tor light c i rcui t ry with hazard warning lights, the radio
and c a s s e t t e player c i rcu i t s , the hea te r and wiper mo-
tors , horns , instrument gauges, and heated rear sc reen .
T h e s e c i rcui t s are relat ively s imple and are desc r ibed
toge ther with fault-finding t echn iques later .
Wiring diagram
Car wiring diagrams are often very difficult to read and
interpret . The reason for this is that , in a modern car
with a large number of ins t ruments , l ights, a c c e s s o r i e s
and motors , all are to be in te rconnec ted on one compre-
hensive diagram. Fuses and switches must also be shown,
toge ther with the co lours of the wires and cab le s ; many
manufacturers use an international colour code for easier
identification of the r e spec t ive c i rcui t c ab l e s .
Some of the more popular symbols used in car wiring
diagrams are i l lustrated in Figure 1.5. The cab les are of-
t en c o d e d and c o l o u r e d for i d e n t i f i c a t i o n and a
s h o r t h a n d m e t h o d of s implifying t h e d iagram often
groups all in one bundle (cal led a cable-form) as a single
line. To t r ace the s tar t and finish of one cab le involves
almost m ic ro scop i c analysis of all connec t ions , search-
ing for the required code and colour .
E l e c t r o n i c dev ices such as e l e c t r o n i c ignition or the
dashboard m i c r o p r o c e s s o r are shown as simple b locks .
Fault finding within t h e s e dev ices must be left to the
specia l i s t dealer .
7
Auto electronics projects ~L
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Car electrical systems
The engine
The most common small to medium car engine is the 4-
c y l i n d e r p e t r o l i n t e r n a l c o m b u s t i o n e n g i n e . M o r e
powerful engines have six cyl inders , some have eight;
motor cyc le s and mopeds have one or two. The arrange-
ment of cylinders varies, some being overhead cam shaft,
some pushrod and rocker , and o the r s with cy l inders
aligned in the shape of a V.
This brief descr ipt ion of the 4-cylinder engine, highlights
the impor tance of a c c u r a t e timing so as to maximise
power and performance. Figure 1.6 shows the arrange-
men t of c y l i n d e r s and t h e four s t r o k e s , i l l u s t r a t e d
separa te ly in Figure 1.7:
induction compression
Crank position {degrees)
Cylinder no. 1
Cylinder no. 2
Cylinder no. 3
Cylinder no. 4
0 - 1 8 0 1 8 0 - 3 6 0 3 6 0 - 5 4 0 5 4 0 - 7 2 0
Power Exhaust Induction Compression
Exhaust Induction Compression Power
Compression Power Exhaust Induction
Induction Compression Power Exhaust
Figure 1.6 4-cy l inder and 6-cyl inder engines
9
Auto electronics projects
Figure 1.7 The four stages of combustion
10
Car electrical systems
• induction — the petrol /a i r mixture is sucked into
the cylinder,
• compress ion — the piston compresses the mixture,
• power — the spark plug ignites the mixture caus-
ing an explosion which pushes the piston down,
• exhaust — the piston pushes the burnt gases out
of the cylinder.
The four cyl inders opera te in se r ies so that, at any one
t ime, one is being powered. The crank shaft posi t ions
the pis tons in the c o r r e c t s equence , two comple te revo-
lutions (720°) comprising the comple te four-stroke cyc le .
The e lec t r ica l c i rcu i t s have the j ob of supplying each
spark plug with a high voltage pulse to power the piston
in the co r r ec t s equence , and at the t ime when the piston
is at the top of its s t roke ( top dead c e n t r e ) . The distribu-
tor ensures that the pulses travel in s equence to the four
spark plugs and, at the same time, t ime the pulse to top
dead cen t re .
Basic ignition
The main componen t s of the ignition circui t are the igni-
t i on c o i l — a c y l i n d r i c a l t r a n s f o r m e r wi th two
connec t ions SW and CB and a high tension cab le going
to the dis t r ibutor ( s e e Figure 1.8) — and the dis t r ibutor
— a mechanica l device coupled to the engine via skew
gears . This ac t s as a four-way switch to route the high
tension to the spark plugs, and as a means of generat ing
the high tension vol tage.
11
Auto electronics projects liiilllllillfj
Figure 1.8 Basic high voltage generating c i rcu i t
Figure 1.8 shows the bas i c high voltage generat ing cir-
cuit . The operat ion is as follows, assuming the con tac t
breaker points are initially c losed ( see Figure 1.10):
• the piston in one cyl inder ( say number 1) r i ses to
top dead cen t re , compress ing the petrol /a i r mixture,
• the ro tor arm in the dis t r ibutor cap points to the
a p p r o p r i a t e high t e n s i o n c o n n e c t i o n to s p a r k plug
number 1 and,
• the con tac t breaker points open,
• the magnetic field in the primary of the ignition coil
(Figure 1.9) quickly c o l l a p s e s . The turns ra t io of the
t ransformer of about 10,000 to 1 transforms this col lapse
into a vol tage of about 20 ,000 volts a c ro s s the second-
ary,
12
Car electrical systems
'To distributor • • • •
^ J \ High tension
^ Ι ^ » · ^ ^ Secondary
Figure 1.9 The ignit ion coi l
Θ I Sparking plugs
winding ^ J g n M o n «wteh | I Γ Ι Π Ι Π Ι
winding^ "Jl ^ „. g ^ o ^ ^ ^ ^ a m ^ ^ ^ ^
Figure 1.10 Sparking plugs f i r ing c i rcu i t
13
Auto electronics projects
• the high tension pulse ignites the petrol /a i r mix-
ture in cyl inder 1 causing the engine to ro ta te ,
• the dis tr ibutor shaft ro ta tes to again c lo se the con-
t ac t b reake r poin ts . The c a p a c i t o r a c r o s s the points
suppresses the high voltage pulse genera ted by this c lo-
sure,
• the distr ibutor shaft turns the rotor arm to the next
cyl inder and the procedure repea t s .
The timing of the opening of the points is cr i t ica l . The
dis t r ibutor shaft cam opens the gap as in Figure 1.12,
the posit ioning of the con tac t breaker points a ssembly
is cr i t ica l toge ther with the gap width. The points , after
a period of wear, tend to co r rode and pitting occur s ; a
deposi t which builds up and reduces the effective gap.
The gap is usually about 25 thousands of an inch wide,
opens and c l o s e s s o m e ten mill ion t imes every 1000
miles. One o ther adjustment to opt imise the timing is
the dwell angle. This is the number of degrees that the
points remain c losed; refer to the maker ' s manual for
the recommended value.
Ignition timing is carr ied out in the following sequence :
• c h o o s e cyl inder number 1 — consul t the manual,
Φ loca te the timing marks on the fan belt pulley ( see
Figure 1.13),
• turn the engine crank shaft until the marks align at
top dead cen t re ( t . d . c ) . The engine can be turned by
placing the car on level ground, take out all the spark
plugs, p lace in top gear, r e lease the brakes and move
the car to and fro,
14
Car electrical systems
• ensure that the dis t r ibutor ro tor arm points to the
high tension lead to cyl inder number 1. If not, turn the
engine through a further 360° ,
• connec t a 12 V lamp between the con tac t breaker
spring ( see point X in Figure 1.12) and a good earth point,
• ro ta te the engine by about 20°, then inch it slowly
backwards until the lamp just l ights,
• if the t .d.c . reading is i nco r rec t , align the t .d.c .
mark, then loosen the dis t r ibutor clamping nut (point Y
in Figure 1.11) and turn the ent i re dis t r ibutor ant ic lock-
wise until the light just goes out. Then turn c lockwise
until it jus t l ights. Clamp the nut,
• c h e c k the t .d.c. set t ing once again,
• rep lace the plugs, put on the brakes and take out
of gear! A faster method uses a s t r o b o s c o p e with the
engine running, a Xenon tube flashing as the points open
and c lose .
Electronic timing
The sys tem so far desc r ibed somet imes fails because of
pitting of the points and wear and tear of the moving
parts of the dis t r ibutor . Two types of e l ec t ron ic sys tem
are found:
• t rans i s to r i sed ignition or capac i to r d ischarge igni-
tion — see Figure 1.14 and,
• con t ac t l e s s (opt ica l or magne t ic ) ignition.
15
Auto electronics projects
Figure 1.11 The distr ibutor
16
Car electrical systems
Figure 1 .12 Contact breaker assembly
Figure 1.13 Timing marks on fan belt pulley
Trans i s to r ignition uses a power d .c . -d .c . conver te r , a
two t rans i s to r push-pull osc i l la tor , to genera te 400 V or
so , to feed to the ignition coil and produce a higher volt-
age and heal th ier spark. At the same time, the con tac t
b reakers no longer switch the full 12 volt ba t te ry cur-
rent: they merely switch a 12 volt low current signal to
the d .c . -d .c . c o n n e c t o r . T h e points the re fore last far
longer and the sys tem is virtually maintenance-free.
17
Auto electronics projects
Contac t less ignition uses a moving magnet or infra-red
ray to rep lace the cumber some con tac t b reakers , a tran-
sis tor ised d.c.-d.c. conver ter circuit being used as before
to deliver the high tension pulses to the plugs. Both sys-
tems can be installed into an exist ing c i rcui t in a very
small t ime, a number of modern ca rs having such sys-
tems built in when new.
•12 V O-
Figure 1.14 Transistorised and capacitor-discharge ignit ion
circui ts
18
Car electrical systems
The battery
A car ba t te ry is a real powerhouse and should always be
maintained in prime condi t ion. It is compr ised of a se-
r ies of s ix lead-acid 2 volt ce l l s (Figure 1.15) which,
together , cons t i tu te 12 vol ts at capac i t i e s varying from
about 30 to 100 ampere-hours . A 70 ampere-hour ba t te ry
delivers a cons tan t 70 amps for one hour, or one amp for
70 hours , or on a very cold day, 400 amps for a few s e c -
onds to s tar t the engine.
The negative plates are cons t ruc t ed from spongy lead
plates and the posi t ive plates from lead dioxide. Dilute
sulphuric acid with a specif ic gravity of about 1.2 s ta r t s
the chemis t ry into act ion, current from the ba t te ry turn-
ing the plates into lead sulphate . A ba t te ry charger , by
Figure 1.15 The battery
19
Auto electronics projects
way of the dynamo or a l ternator , r everses this p roces s
by restor ing the ba t te ry plates to their original compo-
sit ion.
Modern ba t te r ies are self maintaining and the e lec t ro-
lyte (ac id ) levels remain cons tan t . Older ba t te r ies are
prone to deter iorat ion and last only 3 or 4 years . The
performance of a ba t te ry falls at low tempera tures , giv-
ing problems on a cold morning and sulphation of the
terminals which causes leakage currents to chass i s ; this
is avoided by smearing petroleum jel ly onto the termi-
nals. A more common cause of bat tery trouble, other than
an old and tired ba t te ry itself, is damp and dirty wiring,
part icular ly around the s ta r te r motor which drains most
of the ba t te ry power.
Ba t te ry charging is carr ied out in one of two ways:
• the dynamo — a d.c. generator , like a motor in re-
verse , which delivers current to the ba t te ry as long as
the engine is running fast,
• the a l ternator — an a.c. genera tor which, although
requiring an a .c . /d .c . rect if ier circui t , has greater effi-
c i ency and charges the ba t te ry even when idling.
Figure 1.16 shows a cut away picture of the dynamo and
the circui t which con t ro l s the charging of the bat tery,
cal led the cut-out or cont ro l box. This unit s enses the
dynamo output vol tage and, if low, cuts the dynamo out
of c i rculat ion. As the voltage r i ses the cut-out connec t s
the dynamo to charge the ba t te ry and if it r i ses beyond
a prese t value, the regulator winding reduces the effec-
tive dynamo output by adjusting the current in the field
winding, excess ive current going direct ly to the car e lec-
tr ical c i rcu i t s .
2 0
Car electrical systems
Figure 1.16 Dynamo and control box
The a l te rnator is shown in Figure 1.17 toge ther with its
cont ro l c i rcui t ry and rect if ier d iodes . The th ree s ta tor
windings are connec t ed internally to the diodes and a
d.c. output is obta ined. A t rans i s to r i sed cont ro l c i rcui t
maintains a cons tan t ba t te ry charging current by adjust-
ing the current in the ro tor winding.
21
Auto electronics projects
Stator windings in which current is generated
Diodes convert alternating current to d.c.
Rotar turns inside stator assembly
Figure 1.17 Alternator and control c i rcu i t ry
Both sys tems have a built-in ignition warning light with
one side connec t ed to the ba t te ry +12 V terminal , the
o ther to the dynamo or a l ternator output. If the genera-
tor is not working, when the engine is swi tched off for
22
Car electrical systems
ins tance , or when the fan-belt is slipping or broken, the
12 V bulb has 12 vol ts a c r o s s it and it l ights. Normally
the lamp has 12 vol ts on e i ther side and it goes out.
Lighting
Little needs to be said about the normal lighting c i rcui t s
excep t to say that the headlamp bulbs can consume sev-
eral amperes each and so cab le of the c o r r e c t size must
be used to prevent heating (or melt ing) of the wiring.
Many bulbs , as in Figure 1.18, have two fi laments for
compac tness . Quartz halogen bulbs, with a gas surround-
ing the tungsten fi laments, give off greater br ightness .
Figure 1.18 Dual fi lament bulbs
23
Auto electronics projects
As the headlamps between them consume several am-
peres , the headlamp (or f lasher) switch has to be heavy
duty and high current wires must be sent to the dash-
board. Consequent ly a relay is often posi t ioned near the
headlamps, as in Figure 1.19, this being act ivated via a
(preferred) low current switch and wiring. Operating the
switch act ivates the relay which connec t s the headlamps
direct ly to the ba t te ry terminal .
One final lighting device in common use is the spring
s tee l flasher unit ( s ee Figure 1.20) which turns the indi-
ca to r lamps on and off.
Figure 1.19 Headlamp relay
2 4
Car electrical systems
While cold, the c o n t a c t s are held toge ther by the dia-
phragm. When current passes through the con t ac t s , by
indicating to turn left or right, the res i s t ance metal heats
up, expands and pushes the con t ac t s apart . They then
cool again, c l o s e and the s e q u e n c e repea t s 60 to 120
t imes a minute. Emergency light units are similar excep t
that heavy duty con t ac t s are used.
Current from A I Current to indicator switch | φ indicator lamps
Indicator lamps on' Indicator lamps off
Figure 1.20 Flasher unit
Starter motor and other accessories
In a similar way to the headl ights being opera ted via a
remote control relay, a s ta r te r solenoid is used as in Fig-
ure 1.21 to switch the 400 amps to the s ta r te r motor .
This wiring is the th ickes t to be seen under the bonnet
and every s tep is taken to minimise any heat generated
25
Auto electronics projects
despi te the c o s t s of the thick copper wire. The s ta r te r
motor engages with the engine via the flywheel to s tar t
the engine, as seen in Figure 1.22. If the ignition circui t
is working well, a few turns of the engine should cause
the engine to fire and cont inue under its own s team. The
s ta r te r motor is then d i sconnec ted from the engine.
Figure 1.21 Starter solenoid
Two methods are used, a pre-engaged motor whose pin-
ion is always linked to the flywheel, a solenoid operat ing
a plunger to engage the s tar ter motor with its pinion (like
a small c lu t ch ) , and the inert ia type whose pinion sl ides
along the shaft to engage with the flywheel as soon as
the s ta r te r motor opera tes . T h e s e are shown in Figure
1.23. Figures 1.24 to 1.28 il lustrate a number of other e lec-
tr ical a c c e s s o r i e s which are essent ia l , and some legally
required, in the modern motor car .
26
Car electrical systems
Figure 1.22 Flywheel
Petrol pumps ope ra t e e i ther via a mechan ica l rocke r
assembly coupled to the engine forming a small mechani-
cal pump (Figure 1.24), or an e lec t r ica l diaphragm pump,
ra ther like a vibrator , which pumps the petrol from the
tank to the engine, as in Figure 1.25. The petrol gauge
opera tes using a small float coupled to a var iable resis t -
ance unit. As the petrol level r i ses or falls, the current
to the gauge r ises or falls accordingly . This unit, similar
to a WC bal l-cock, is sea led for fire r easons , see Figure
1.26.
27
Auto electronics projects
28
c* «— Ο
Ο
ε
<σ
CO
<ν>
α> »_
β>
Car electrical systems
29
Fig
ure
1.
23
Co
nti
nu
ed
Auto electronics projects
Figure 1.24 Mechanical fuel pump
Horns come in all shapes and s izes . Figure 1.27 shows a
simple type, working like a v ibra tor whose diaphragm
output is mechanica l ly amplified to warn pedes t r ians to
get out of the way.
Ammeters can be fitted in any car : a s imple means of
installat ion necess i ta t ing a minor change to the wiring
30
Car electrical systems
Figure 1.25 Electr ic fuel pump
31
Auto electronics projects
Figure 1 . 2 6 Fuel gauge and float
as shown in Figure 1.28. By this means the ammeter does
not record the s ta r te r motor current , but all o ther cur-
rents taken by the car c i rcui t ry .
Figure 1 .27 Horn diaphragm
32
Car electrical systems
Figure 1.28 Ammeter wiring
Finally, a look into the compute r i sed dashboard now
found in a number of high performance ca r s . Transduc-
ers cons tan t ly read r.p.m., p ressures , t empera tures and
so on; t hese are moni tored and the computer checks and
warns the driver of impending t rouble ( see Figure 1.29).
The day of the J a m e s Bond superca r or the Night Rider 's
Kit looms nearer everyday.
33
Auto electronics projects
Figure 1.29 Computerised dashboard
34
2 Electronic ignition
The e l ec t ro -mechan ica l ignition sys tem that has been
used to fire the fuel/air mixture in an internal combus-
tion engine for severa l decades , and which is familiar to
home mechan ic s everywhere , has prac t ica l ly been re-
placed by e lec t ron ic methods in recen t t imes . Some of
the reasons for this are not quite as obvious as you might
suppose , but cer ta inly, as with everything e lse , a mod-
ern e l e c t r o n i c a l t e r n a t i v e is s u p e r i o r to i t s
e lec t ro-mechanica l ances to r . To be fair though, the lat-
ter has had a lot going for it, it originally rep laced a
method so a rcha ic as to be unbel ievable .
Automotive ignition — a brief history
Earl iest motor ca r s , or in fact anything using the new-
fangled gas engine (many of which were a lso used for
35
Auto electronics projects
powering agricultural machinery) , of slightly over a cen-
tury ago had to make do with a device compr is ing a
thin-walled copper tube with c losed ends, supported in
the middle with a porcelain insulator or some-such simi-
lar item. The insulator sc rewed into the cyl inder head,
like a modern plug — indeed the word plug p robably
or iginates from this t ime.
To s tar t the engine, the outs ide end of the tube is heated
with the flame of a spirit burner until glowing. Then at-
t empts can be made to get the engine going, using a
start ing-handle. When the fuel/air mixture arr ives at the
o ther end of the tube, on the inside, in the right quanti-
t ies (a bit of a juggling a c t ) , it should (hopefully!) burn.
Once the engine is warmed up and running, the spirit
burner can be put out and thereaf ter the tempera ture of
the tube will be maintained by the heat of internal com-
bust ion, in the same way tha t the engine of a model
aeroplane keeps its glow-plug hot.
Not surprisingly, while the gas engine was still only a
few years young, engineers thought hard about improv-
ing this less than ideal s i tuat ion. It was only a quest ion
of t ime before the e lec t r ica l ly powered hot wire type of
ignition, a glow-plug then, was pressed into se rv ice for
the petrol engine. The t rouble with glow-plugs however,
is that the wire burns away quite quickly and a s tock of
spares must be carr ied around at all t imes .
Then, just prior to the turn of the century, a method was
devised which, though it s eems obvious now, must have
taken a good deal of working out at the t ime. It was reli-
able in opera t ion like nothing e l se previously, it was
sophis t ica ted , it was state-of-the-art. It was spark igni-
tion.
3 6
Electronic ignition
37
The advantages included much eas ie r s tar t ing — simply
energise the sys tem and crank the handle. Also, b e c a u s e
the plug was no more than a spark gap at the business
end, and the e l ec t rodes were far more robus t than thin
wire or copper tube, it had a working life h i ther to un-
seen.
From the engine des igners ' point of view it ra ised two
important poss ib i l i t ies :
• the moment of ignition of the fuel/air mixture could
be p r e c i s e l y c o n t r o l l e d . P rev ious ly , the c o m b u s t i o n
chamber had to be designed to prevent the charge ignit-
ing prematurely during compress ion , a shape which did
nothing for efficiency (or performance, if you l ike) ,
• engines with multiple cyl inders could be ca te red
for jus t as easi ly as s ingles . Prior to this engines were
most ly a single cyl inder type — the ignition parapherna-
lia for jus t one was usually quite enough to cope with.
There are basical ly two types of e lectro-mechanical spark
ignition sys tems: the magneto, and what ' s cal led coil ig-
ni t ion. T h e only d i f ference is tha t the magne to a l so
genera tes its own e lec t r i c power to opera te . With coil
ignition the power supply is external . In the beginning,
there was only the magneto. In the 1920s , the Americans
p ioneered coil ignition, which used power h i ther to gen-
erated exclus ively for ancillaries — lights and so forth.
The power supply compr ised a d.c. genera tor in the form
of a dynamo, with a back-up for the per iods when the
dynamo couldn ' t provide the n e c e s s a r y current — an
accumulator (a ba t t e ry ) . In Europe there was great re-
s i s t ance to coil ignition, espec ia l ly among the Bri t ish,
who thought it too gimmicky. Customers wouldn't buy a
Auto electronics projects
car if it had coil ignition — manufacturers had to revert
to the magneto in order to be able to maintain sa les .
Would you bel ieve that such a r e spec ted manufacturer
as Rolls Royce couldn ' t shift their la tes t spor t s tourer
until they had put a magneto back into every car? Such
was the r e s i s t ance to change. Perhaps there is a modern
parallel here , about cus tomers (and m e c h a n i c s ) being
frightened of the complexi ty of fuel in ject ion. . .
Spark ignition — the principles
An e lec t r i c arc is an e lec t r i c current flowing through a
gas, which for the purposes of this d iscuss ion, is air. Air,
as with most insulators r es i s t s the flow of e lec t r i c cur-
rent . If forced, it ionises as e l e c t r o n s begin to move
between molecules . As with any o ther res is tor , this mo-
lecular friction genera tes heat — from the amount of
energy required to cause air to succumb, quite a lot of
heat . The arc is a whi te /blue colour , and hot enough to
s tar t a fire.
It is worth descr ibing how the e lec t ro-mechanica l igni-
tion sys tem opera tes first, s ince there is no substant ia l
difference between it and any e lec t ron ic equivalent —
they all have to do the same thing, make a spark. We
shall s tar t here and work backwards .
Air needs a little persuading in order to ca r ry an e l ec t r i c
current and produce an arc . At normal a tmospher ic pres-
sure it is not all that difficult, but still requires a high
voltage to break down the air be tween a pair of e lec-
t rodes . The narrower the gap, the eas ie r it is . However,
38
Electronic ignition
whilst it is quite easy to bridge a gap of 0.02 inches (a
typical spark plug gap) in open air, it is much more diffi-
cult inside the combus t ion chamber . This is b e c a u s e air
ionises more easi ly the thinner it is ( the typical demon-
st ra t ion is an e lec t r i c a rc in a glass vesse l with a vacuum
pump a t t ached ) , it cor respondingly b e c o m e s more re-
s is t ive the more dense it is, like inside the combus t ion
chamber of an engine. Universally, the fuel/air mixture
is compressed before ignition, the main reason being that
this r e leases more energy on combus t ion (but a lso be-
cause the piston, being a rec iproca t ing part linked to a
revolving part, can ' t help i tself) . The upshot of all this is
that it is more difficult to bridge the gap to produce a
spark in c o n s e q u e n c e , requiring a very high vol tage to
do so , which accoun t s for the 25 to 35 kV HT vol tage
range typical at the plug's live end. I labour on this point
b e c a u s e it causes problems for the design of e l ec t ron ic
ignition amplifiers, as will be seen later .
Obviously it is impract ica l for this sor t of potent ial to
be produced and cont ro l led di rect ly from some engine
driven genera tor , so ins tead a step-up t ransformer is
used, which is where the coil c o m e s in. All the genera-
tion and t imed-switching is done at a more manageable
low voltage, and is conver ted by the coil to the neces -
sary high vol tage.
Actually the sys tem is c levere r than that . The s e q u e n c e
shown in Figure 2 .1 (a ) to 2 .1(d) reveals the sys tem to be
a form of flyback converter. Figure 2 .1 (a ) shows the com-
ponents of a mechanica l sys tem at rest. With switch SI
open, nothing is happening. When S I c l o s e s in Figure
2 . 1 ( b ) , current flows in the primary winding LI of T l ,
39
Auto electronics projects
the ignition coil . T l has a laminated s tee l co re and a fi-
n i t e t ime is t aken for t h i s c o r e to r e a c h m a g n e t i c
saturat ion, by which time the primary current will a lso
be at a maximum. This maximum is set by choos ing a
d.c. impedance for LI by using res is t ive wire, or e l se it
will a t tempt to short-c i rcui t the supply after the co re
sa tura tes! For 12 V sys tems the impedance is chosen for
a maximum current of around 3.5 to 4 A, as a typical
value.
(c) GO
Figure 2.1 Sequence of act iv i ty in contact breaker ignit ion
system
40
Electronic ignition
In Figure 2 . 1 ( c ) , SI opens and unwanted effects take place
in its vicinity, but we'll ignore them for the moment . Suf-
fice to say that as the magnetic field col lapses , it a t tempts
to maintain the current flow in LI in the same direct ion,
and at the same t ime induces a current in L2. B e c a u s e L2
has many more turns than L I , its output vol tage is much
higher. In the cha rac t e r i s t i c manner of flyback conver t -
ers , the coil will a t tempt to output the same amount of
power that went into it. If a path on the primary side is
denied it, then the only r ecou r se is to find an outlet on
the secondary side.
The load is the plug air gap, which basical ly doesn ' t want
to know at first, but the coil will keep pushing the volt-
age up until the gap is bridged. If the total power input
was 50 W and the output r e ached 30 kV then the gap
cu r r en t is ini t ia l ly 1.6 mA. However , o n c e the a r c is
s tar ted , the vol tage level required to maintain it can re-
duce substant ia l ly allowing a grea ter current flow and a
nice heal thy spark. This is indicated in Figure 2 .1 (d ) .
The snag is that a smal ler representa t ion of this act ivi ty
also appears a c r o s s the primary, L I . The effect is an ini-
tial pulse of up to severa l hundred vol ts . At the point of
breaking the circui t , the mechanica l switch SI has a very
narrow gap between its c o n t a c t s which might be meas-
ured in m i c r o n s . Such a gap is e a s y for a coup le of
hundred volts to bridge; the coil expends all its energy
in producing an arc between the switch c o n t a c t s , and
there is none left for the plug. If you want to prove the
effect for yourself t ry it with the coil of a relay, a pair of
tes t leads and a ba t tery .
So this is where the o ther c lever bit c o m e s in, the third
componen t in the set-up, C I . To this day it is still cal led
41
Auto electronics projects
a condenser, a very old-fashioned name for a capac i to r .
Its function is to momentar i ly take over from the switch.
As S I opens , current flow is diverted into CI , charging
it. T h e idea is tha t by the t ime the pr imary vo l t age
reaches a high level, the contac t gap is unattainably wide,
forcing the coil to go for the plug gap instead. This has
two main disadvantages:
• it consumes some power which might o therwise
con t r ibu te to the spark, and,
• it s lows down the ra te at which the HT level can
increase , the output of which takes on more of a milder
ramped pulse shape ra ther than a true pulse. The value
of CI is cr i t ica l : if too small , it will encourage switch arc-
ing; if too large, it will absorb too much power and defeat
the whole ob jec t . A value of 220 nF is usually about right.
Switch arcing and power loss still occur , but at accep t -
able levels .
A third anomaly is that , after the main pulse has oc -
curred , what you are left with is LI and CI , with the
supply as a common terminal , forming a tuned circui t
which rings or r e sona tes slightly. Figure 2.2 shows the
vol tage waveforms a s soc ia t ed with this se r ies of events .
It was ment ioned that the ignition coil has an inbuilt d.c.
impedance to limit current flow while the con tac t break-
ers are c losed . During this t ime the coil is drawing its
maximum power of 45 to 50 watts , to no effect o ther than
that this manifests i tself as heat . Consequent ly an igni-
tion coil has been safeguarded against this , and hence is
a lmost universal ly cons t ruc ted as shown in Figure 2.3. It
is supported in the cen t re of an aluminium can, which is
filled with oil . An ignit ion coi l is , the re fo re , a liquid
cooled component .
42
Electronic ignition
Figure 2 . 2 Voltage waveform from Figure 2 . 1 at coi l primary
Brass HT socket
Terminal Terminal
Moulded insulator
Aluminium can
Oil filled cavity
Synthetic rubber support
Twisted pair of wires.
Primary and HT Common
is · + · terminal
Coil windings
Laminated core as a bundle of steel strips.
HT Ίΐνβ* connects to this, and is
passed via the coil spring at the top
to HT socket.
Figure 2 . 3 Internal construction of a typical ignit ion coi l
43
Auto electronics projects
Advantages of electronic ignition
The first two problems are prac t ica l ly solved by e lec-
t ronic switching, the third by using the coil in a different
way. The re are o ther p rob lems that can be solved at a
s t roke , like mechanica l wear.
The heel of the moving half of a con tac t b reaker wears
on the dis t r ibutor cam. The con t ac t sur faces b e c o m e
damaged, developing a hole or pit in the posi t ive side
and a raised pip on the negat ive surface, as the inevita-
ble arcing causes metal to migrate from one surface to
the other . The lumpy result causes irregular timing and
bad separat ion, but it may be poss ib le to rescue them
with the skilful appl icat ion of a fine s tone .
Then there is the ( somet imes be t te r than dreadful) me-
chanica l auto-advance mechanism, with its centrifugal
bobweights , springs, cam con tours and vacuum ass is t
device. To be fair, in p rac t ice a mechanica l sys tem which
is both well designed and 100% fit is difficult to beat ,
even by an e lect ronic equivalent, but sooner or later wear
takes its toll , affecting engine efficiency, and so it needs
per iodic examinat ion and co r rec t ion or even replace-
ment.
But owners put off having the car serv iced until it des-
perate ly needs it b e c a u s e of exorbitant garage bil ls . In
the meant ime the vehic le is wasting valuable fossil fuel
and polluting the a tmosphere in a way that it wouldn't if
properly tuned. Also of conce rn to car manufacturers ,
under pressure to reduce pollution and fuel consump-
t ion, is the D.I.Y, home m e c h a n i c t inker ing with his
engine. If he knows what he is doing then fine. If he
doesn ' t . . .
44
Electronic ignition
Consequen t ly fac tory se t and m a i n t e n a n c e free e l ec -
t ronic ignition, and ca rbure t to r s with secur i ty blanking
plugs seal ing off the vital b i t s , prevent unauthor i sed
hands fiddling with these and getting it wrong. And you
thought it was all done for your benefit . It a lso explains
the lack of really meaningful information in the modern
owner ' s handbook. Refer servicing to your dealer, or
warranty is void, and that sort of thing. Basical ly it means
s lapped wrist to the potent ial D.I.Y'er.
Electronic ignition — how it works
The good news is that e l ec t ron ic ignition for the average
modern car has boiled down to a recognisab le s tandard
formula, with a long t rack record of rel iabil i ty. The bad
news is that if it does go wrong, you can ' t fix it yourself .
Having a c i rcui t diagram is no help (which you won't be
able to get hold of anyway); both the s enso r and the
amplifier are sealed in resin and you can ' t get inside with-
out destroying them. And assuming you could get into
the amplifier you will most p robably find thick film re-
s i s to rs bonded straight onto a ce ramic base which they
share with o ther micro-mount componen t s and a very
spec ia l i sed cus tom chip, with which you will be able to
do nothing.
The h is tory of t rans i s tor i sed ignition goes back as far as
the 1960s . Unfortunately s e m i c o n d u c t o r s of the t ime,
being made of germanium instead of s i l icon, were some-
what fragile, requiring that spec ia l beefed-up ones be
manufactured to cope . Consequent ly e l ec t ron ic ignition
was expens ive and usually only found a t t ached to simi-
larly unaffordable spor t s ca r s .
45
Auto electronics projects
Timing sensors
In the 1970s , solid s ta te ignition with th ree vers ions of
timing sensor proliferated. The simplest was the so called
transistor assisted ignition, which still required a me-
chanica l switch. The second type had an opto-e lec t r ic
timing sensor , which might use e i ther vis ible light or an
infra-red coupler . Here the beam is interrupted by a ro-
tating shut ter with blades like a fan. The third type uses
a magnet ic sensor .
Many of t hese were available as after-market bolt-on kits
for both ca rs and mo to rcyc l e s . After some twenty years
only one type has c om e out on top as the s implest and
most re l iable — the magnet ic sensor .
The senso r genera tes an e lec t r i c pulse which tr iggers
the amplifier, which in turn drives the coil primary. Fig-
ures 2 .4 (a ) and ( b ) show the now archetypal , s tandard
design in operat ion. Here a permanent magnet couples
to a ferromagnet ic e lement which is mounted on the dis-
tr ibutor shaft and rota tes with it. As this element ro ta tes ,
the s t rength of the field var ies , being largest when the
air gap is smal les t . The t ime varying magnet ic field in-
duces a current in the coil which is proport ional to the
rate of change of the magnet ic field, and which outputs
a vol tage waveform as i l lustrated in Figure 2 . 4 ( c ) . Each
t ime one of the teeth , or r idges, on the ro tor passes un-
der the co i l ' s axis , one of the sawtooth shaped pulses is
generated. The rotor has one tooth for each cylinder and
the voltage pulses cor respond to the spark t ime of the
relevant cylinder. Figure 2 .4(d) shows an advanced ex-
ample of this idea following exac t ly the same principle,
46
Electronic ignition
except that the rotor is a star shaped wheel and the s ta t ic
magnet ic sys tem has a cor responding number of poles ,
in this c a s e six of each , for a s ix cyl inder engine.
Auto advance
One reason why this triggering method has come out on
top over rival designs is simply due to one staggering
implicat ion. B e c a u s e the sys tem is magnet ic ; it is, in ef-
fect, a very simple a.c. genera tor on a small sca le , and
its output is, therefore, proport ional to the driven speed.
What this means is that at slow rotor speeds the output
vol tage is low, while for higher speeds the output is a lso
higher by a proport ional amount. If the tr igger thresh-
old of the amplifier 's input is vol tage dependent , then
triggering can be made to o c c u r at the required point
anywhere on the leading s lope of the output waveform.
Figure 2.5 shows how, from different output levels as
produced by cor respond ing ro tor speeds , the t r igger
level is near the peak of the s lope if the output is low,
and near the beginning if it is high. At a s t roke, what we
have here is, by way of an added bonus , an automat ic
ignition advance mechanism, and this with just one mov-
ing part — the rotor!
The need for ignition advance
While the fuel/air mixture in the combus t ion chamber
burns at a cons tan t rate , the engine as a whole however
47
Auto electronics projects
48
Distributor Rotary shaft ferromagnetic
\ element
W il low reluctance \ J U / /
P e r m a n e nt
and r e s u l t s in • §£ m a g n et
s t r o n g m a g n e t i c I Π . j j
f i e l d f o r c o i l / /
P i c k u p co l l • • ^ 1 ^ / /
( ) ^ 1 N a r r o w Gap
V o l t a g e d u e t o m a g n e t i c M a x i m u m n a r r o w f i e l d c h a n g i n g a s g a p v o l t a g e
r o t o r m o v e s t o w a r d s e n s o r ^ /
V o l t a g e d u e t o m a g n e t i c M a x i m u m w i d e f i e l d c h a n g i n g a s g a p v o l t a g e
r o t o r m o v e s a w a y f r o m s e n s o r
(c)
Figure 2.4 Magnetic timing sensor
Electronic ignition
49
Wide air gap offers ^^^S I/ I I high reluctance / / and results in **^e§> —7/
weak magnetic I Π « I I field for coil Η ^ - Ν / /
( b ) + I W ide G a p
Rotor a r m key
/^^^^^^^^^^^^^^^^^^^^^^^^^^\^^\ R e ' U C t 0 r
I / o V ^ ^ ^ T - | (ts. Li^) / ^ ^ V ^ \ | | Coi l a n t
^ m a g n e t 1 | w f r - - ~ " ^ \ Χ ^ Τ Γ ^ Λ / / « || —j—j-i— s y s t e m u n d e r
rT Λ 1 d f r * ^ \ v ^ X / X / / J / Ii d u s t c o v e r
( j | ^ ^t a
* ' ° Po l es
" I — L D is t r ibu to r
l ι b o d y
V J (D)
Figure 2.4 Continued
Auto electronics projects
Figure 2.5 Auto-advance plot using waveform of Figure 2 .4(c )
is required to opera te over a range of crankshaft speeds .
For this reason the moment of ignition must occu r ear-
lier at higher r.p.m. Full combus t ion of the fuel gas must
occu r during the period where the piston has full lever-
age on the crankshaft , and at high revs the burn actual ly
needs to begin well in advance of this point; at lower
speeds , not so much, at idle, hardly at all. The magnet ic
re luc tance type of ignition timing senso r ach ieves this
auto advance act ion in a much more linear manner than
do compromised mechanica l or e lec t ron ic methods , and
barring the odd rare mishap such as a s c rew coming
loose , once se t it does not need readjustment — for any-
one who has persona l ly endured the long drawn out
p roces s of ignition retiming, the subt le t ies of the opera-
tion do not need rei terat ion!
50
Electronic ignition
Furthermore , s ince this requi rement has already been
taken ca re of by the sensor , it makes the amplifier much
s impler . Otherwise e l ec t ron ic advance might take the
form of f requency sens i t ive switches se lec t ing from a
range of t ime delays, the minimum number of which is
two in the crudes t example of such a sys tem. More than
this requires ra ther more logic gates , or a mic roproces -
sor . Instead the magnet ic re luc tor allows the use of a
compara t ive ly very few t rans i s to rs to produce an ampli-
fier.
The electronic ignition switch
Obviously the hear t of an e l ec t ron ic sys tem which simu-
lates the act ion of a mechan ica l switch to opera te the
coil primary in the traditional way is a t rans is tor , and
you might suppose that any power t r ans i s to r ab le to
ca r ry the maximum on-time current of the primary will
suffice. But oh dear me no. Remember that the primary
potent ial is sufficient to produce an arc a c ro s s the me-
chanica l switch, and that the ignition coil as a whole,
primary included, must be allowed to genera te however
high a vol tage is n e c e s s a r y to bridge the plug gap? We
are therefore obliged to use a high vol tage power tran-
sistor , with a V rating of several hundred volts , and such ' ce ° '
devices are notor iously inefficient, which means to say
that the current gain (H f e) is very small, measured in tens
or less ra ther than hundreds.
The usual biasing method is to use a base bias res i s to r
which typical ly c o n n e c t s di rect ly between the t ransis-
to r ' s base and the supply rail, and this r es i s to r can be
51
Auto electronics projects
formidably beefy to provide the n e c e s s a r y bias current
for the t rans is tor to do its job properly, with the attend-
ant power consumpt ion and heat dissipat ion problems.
I have actual ly seen one design where the base bias re-
s is tor is no more than 9.2 Ω!
No, that wasn' t a printing error . It 's an illustration of how
ex t reme base biasing may have to be to ensure that the
switching t rans is tor ach ieves a sa tura ted on s ta te , es-
sential to get the maximum available vol tage ac ro s s the
primary of the coil and therefore the maximum primary
current . Suppose, in a worst c a s e example, that our tran-
s i s t o r has an Hfe of 3 at 1 A ( y e s , j u s t 3 — al though
fortunately later devices are be t t e r than that now), but
then in order to conduct 4 A this value reduces to say
<2. To ensure adequate biasing we assume a current gain
of 1.5, and c h o o s e a base bias res i s to r with a value of
4 Ω, taking into account a base /emi t t e r forward drop of
1 V. This res i s to r is then sinking 2.6 A and dissipating 28
watts; has to be removed from the res t of the amplifier
to avoid cooking it to death, and be provided with its
own heatsink!
Even in the c a s e of the aforement ioned design using the
9.2 Ω component , the res i s tor is of the high power, metal
encapsula ted type ( see the res i s to r s sec t ion of Maplin's
ca ta logue for examples ) and is sc rewed to the outs ide
surface of the amplifier 's die-cast c a se .
In comparison the power dissipation of the actual switch-
ing t ransis tor is not very much at all, which seems almost
perverse . This is because it performs a switching act ion;
it is e i ther on or off. Which leads us to the next cr i te-
rion, namely ensuring that the t rans i s to r commutâ tes
52
Electronic ignition
( swi tches off) as fast as poss ib le . This is n e c e s s a r y s ince
the coil needs to be swi tched off quickly in order to de-
velop its high tens ion output (a slowly swi tched ignition
coil fails to make a spark) .
High speed switching
Figure 2.6 shows the essen t ia l s of a typical ignition am-
plifier as used with a magnet ic r e luc tance type of timing
sensor . To summarise so far, TR5 is the inefficient, high
vol tage power t rans i s to r switch for the coil , and R9 is
the base bias res i s tor . In this c a s e the bias current origi-
nates from TR4, which is cont ro l led by a Schmit t tr igger
comprising TR2, TR3, and res i s tors R3 to R6. The Schmit t
tr igger is essent ia l to produce the fast edged switching
waveform from the s lower changing input, provided by
T R I .
T R I is the bas is of the input s tage which incorpora tes
the input level th reshold as indicated in Figure 2.5. This
cons i s t s of diode Dl and the base /emi t te r junct ion of TRI
itself, which toge ther will not begin to conduc t until the
applied level is >1.2 V. This signal is of cou r se the ramp
shaped output from the s enso r coil and you can see now
that while the amplitude of the ramp is var iable , the in-
put th reshold is cons tan t . Dl a lso b locks the negative
going part of the input waveform, which is superfluous,
while R l is a current l imiter to p ro tec t Dl and T R I in the
event that for example the input is acc iden ta l ly con-
nec ted to the supply while the power is on.
53
Auto electronics projects
Fig
ure
2.
6 E
ss
en
tia
l ig
nit
ion
a
mp
lifi
er
for
a m
ag
ne
tic
relu
cto
r b
ase
d sy
ste
m
Electronic ignition
Protec t ion for the engine 's mechanica l bi ts can be pro-
vided by including CI , which ac t s as a rev limiter. While
it is charged quickly v i a D l , this charge leaks away slowly
via the base emit ter of T R I due to this dev ice ' s current
gain offering a relat ively high impedance , and in conse -
quence the waveform at T R l ' s emi t ter takes on a more
triangular shape . As engine speed inc reases the mean
average d.c. vol tage drop a c r o s s R2 also inc reases until
a point is r eached where even the lowest level of the
waveform exceeds the low threshold of the Schmit t trig-
ger; the amplifier c e a s e s to opera te and no sparks are
generated.
CI a lso affords some RF filtering, but it might be surpris-
ing to learn that the input leads are rarely s c reened . The
senso r coil is of such low impedance that this is unnec-
essa ry and in any c a s e s ince both these wires are run
toge ther as a pair, any external ly induced current will
be equally present in both, cancel l ing each o ther out.
A real working amplifier
Figure 2.7 shows a c ircui t which is the culmination of s ix
months development including test ing in the field on-
board a real motor vehic le which, for ear l ier vers ions ,
proved to be des t ruc t ive ( to the c i rcui t , not the vehi-
c l e ) . Such is the way of r e sea rch and development , and
t h e s e even t s made defini te ind ica t ions tha t the unit
should be:
• e lec t r ica l ly robust ,
• mechanica l ly robust ; and,
• ut terly weatherproof .
55
Auto electronics projects
56
^
-H2V
te
st
^j4V
LLK
-7
S220n
F
22R
S
IOO
uF
y
w/
T20X
M
lOW
T
35V
ο
1
1—
u—
L
J—
3—
<fj
/Q\
BU
208A
I
1
1 d
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e
/ 0
0\
[Ξ N
E55
5 ^
(
be
)
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Fig
ure
2
.7
Re
al
am
pli
fie
r c
irc
uit
d
iag
ram
Electronic ignition
Referring to Figure 2.7, the input s tage is as desc r ibed
for the hypothet ica l amplifier earl ier , with the combined
diode junc t ions of both Dl and TRI forming the input
threshold level, and having Rl as a pro tec t ive current
l imiter. Cl is merely an HF filter in conjunct ion with R l
and does not provide any rev limiting.
To reduce component count , the fast switching act ion
needed to sharpen the pulse produced by T R I is pro-
vided by ICI , a 555 t imer IC used in an unusual way.
Instead of being employed in a convent ional (for the IC)
manner as a monos tab le e t c . both trigger and threshold
inputs (pins 2 and 6) are t ied toge ther to exploit the be-
haviour of the internal b is table , forcing a Schmit t tr igger
act ion. The 555 was chosen b e c a u s e the output s t ruc-
ture can sou rce the driver s tage, TR2, direct ly without
the need for any more t rans i s to r amplifiers.
While there is no input and T R I is off pins 2 and 6 of ICI
are high and the output pin 5 is low, so that TR2 is a lso
off, allowing the bias res is tor , R4, to sa tura te the main
t rans i s tor switch for the ignition coil , TR3, and the coil
is on.
Upon an input ramp vol tage from the timing senso r ex-
ceeding the combined th reshhold levels of Dl and T R I ,
TRI conduc t s and quickly pulls the tr igger input down
to <V3 of the supply level, causing ICI to change s ta te
and switch on TR2, which c lamps R4 to ground and de-
prives TR3 of base drive current . The coil is swi tched
off, ICI is rese t when the ramp is comple ted as TRI col-
l e c t o r g o e s high again , and t h e s y s t e m is r e ady to
genera te another spark.
57
Auto electronics projects
Note that all s tages use the 0 V rail as the so le re ference
and are thus immune to supply rail f luctuations, which
will occu r often in the range of 12 -13 .8 V espec ia l ly if an
e lec t ro -mechanica l regulator is employed, and can be
less than 9 V while the s ta r te r motor is giving the bat-
tery a hard t ime.
Electrical safeguards
The o ther a rea of e lec t r ica l weakness is concen t r a t ed
on TR3. This is b e c a u s e of some horr ib le punishments
that the ignition coil will try to inflict on this device. From
the range of high voltage power t rans i s to rs readily avail-
able the only one to prove itself e lectr ical ly tough enough
to be truly rel iable is the long standing, T 0 3 packaged
B U 2 0 8 d e v i c e d e s i g n e d for u s e in c o l o u r TV l ine
t i m e b a s e s and s w i t c h e d - m o d e power s u p p l i e s . T h e
BU208A vers ion is preferred for its lowest sa tura ted V c e,
essent ial to ensure maximum voltage drop ac ross the coil
and reduce power dissipat ion in the t rans i s tor i tself to a
minimum — it is the more expens ive version, but that
can ' t be helped. The device has a V c e rating of 700 V and
a reasonab le Hf , which reduces bias res i s to r heat dissi-fe'
pation and power loss , as this componen t (R4) has a
conserva t ive value of 22 Ω( ! ) . However TR3 still needs
two essent ia l p ro tec t ion s c h e m e s .
One of t hese must cope with ignition coil back e.m.f.,
which, without a power sapping condense r ( s ee ear l ie r )
is exces s ive . But surely this can only occu r without a
spark plug as a load, e lse how can this happen where
there is an air gap which must s t r ike and conduct and
5 8
Electronic ignition
t h u s l imi t b o t h t h e c o i l ' s p r i m a r y and s e c o n d a r y
vol tages? The truth is that, compara t ive ly speaking, the
air gap takes a long t ime to respond. Until this happens
it is as if there were no load at all and the coil shoves up
the potential enormously . A very simple calcula t ion can
be made to get some idea of the theore t i ca l magnitude
of back e.m.f. from a coil by:
voltage drop across coil
commutation time
where commutat ion time is the t ime taken for the switch-
ing device to swi tch off, which is of c o u r s e not truly
in s t an taneous . Assuming for example a commuta t ion
t ime of 100 ns which even for a BU288 is very much on
the slow side, we get (in theo ry ) :
12V = 120,000 V! 100 ns
This is what we get on the primary side. In p rac t i ce how-
ever it will be prec ise ly 1,400 V. Why so? B e c a u s e this is
the designed c o l l e c t o r to b a s e ( V c b) limit of a BU208,
never mind that this value is double the maximum V ! ce
The b a s e / c o l l e c t o r junct ion is breaking down in the re-
verse direct ion like a Zener diode, and it is not supposed
to be used in this way. Damage is cumulat ive and the
device may fail after even some tens of hours of appar-
ently fault free operat ion.
The voltage limiting protect ion scheme in Figure 2.7 com-
pr i ses ident ica l c o m p o n e n t s SRI and SR2, which are
nothing more e labora te than two mains t rans ient sup-
p r e s s e s in s e r i e s . This c o m p o n e n t is a Metal Oxide
Var is tor (MOV), the r e s i s t ance of which is vol tage de-
pendent . It has a knee vol tage of 340 V ( that is, 1.414 χ
240 V) , which is the peak value of the mains supply. Up
59
Auto electronics projects
to this point its r e s i s t ance is high, but reduces consid-
erably as soon as its knee vol tage is exceeded , and is
normally used to prevent vol tage spikes which would
o therwise exceed the peak mains value from enter ing
mains powered equipment .
Originally it was assumed that two of t h e s e in se r i e s
would be sufficient to limit coil e.m.f. to 680 V (within
the maximum V c e of T R 3 ) on their own, but in real i ty they
are unable to cope . Consequent ly they have to ach ieve
the desired ob jec t ive by the a l ternat ive means of pro-
viding feedback to TR3 base and letting TR3 do the actual
limiting instead. In o ther words, TR3 is made to switch
off up to the 680 V point and then holds this until the
e.m.f. value falls below this level before switching off
properly. Reverse blocking diode D2 detours the current
from SR2 to TR3 base so that it doesn ' t go straight to
ground via TR2.
The o ther pro tec t ion s c h e m e is a provision to prevent
the vol tage ac ro s s TR3 being reversed , i.e. <0 V, which
is inevi table s ince the ignition coil still r e sona tes after
the spark ext inguishes , for while there is no condense r
there is still interwiring capac i t ance , toge ther with that
between TR3 ' s c a s e and its heats ink. The ringing is now
high frequency and very shor t in duration, but still very
much alive and kicking. This is the duty of D3.
Insulation problems
Exper ience has indicated that a g rease less T 0 3 insula-
tor is more rel iable than the tradit ional mica var ie ty for
heats ink mounting. If the mica is not 100% perfect then
60
Electronic ignition
any c racks are weaknesses which can be perforated by
the high vol tage pulses . In the final design the unit was
housed in an extruded modular alloy case ( see Photo 2 .1) ,
with which a slide-in T 0 3 compat ib le heats ink was used.
Although this item comes complete with screws, nuts and
insulator bushes , insulator s leeves were cut from sepa-
rately avai lable T 0 3 bushes and pressed into the holes
before mounting the ent i re a s sembly in the co r r ec t posi-
tion on the s t r ipboard ready to sl ide into the ca se , as
can be seen in Photo 2.2.
Photo 2.1 A complete home-made ignit ion amplif ier in its case
61
Auto electronics projects
Photo 2 . 2 The s t r ipboard assembly of the c i r c u i t of F igure 2 . 7
wi th heats ink in pos i t ion and remote R4 on separate board
Mechanical considerations
Components which are at risk from vibrat ion, e.g. up-
right PCB mounted e lec t ro ly t i cs , should be suppor ted
at their base with b lobs of flexible rubber sealant . ICI
was soldered direct ly without a socke t , or e lse in serv-
ice oxidat ion may cause cont inui ty p rob lems . R4 is a
c e r a m i c b l o c k e n c a p s u l a t e d 10 watt c o m p o n e n t and
should be fitted on a separa te board such that its top
surface is in con tac t with the c a s e and so ldered in this
posi t ion during a tes t fitting. At final fitting this top face
can be smeared with heats ink compound to fill-in the
rough surface. R4 then uses the c a s e as a heats ink.
6 2
Electronic ignition
The reason for the enormous number of external cab le s ,
evident in the example shown in Photo 2 .1 , is that this
unit conta ins an identical pair of t he se amplifiers for a
spec ia l i sed moto rcyc le applicat ion, so there is plenty of
room for one in the case !
Transistor assisted ignition
Trans i s to r ass i s ted ignition simply means that a conven-tional m e c h a n i c a l t iming swi t ch , s u c h as a c o n t a c t
breaker , is not actual ly used to switch the coil d i rect ly
but con t ro l s a solid s ta te switch instead. The ci rcui t of
Figure 2.7 could be used in this role, by merely adding
an ext ra 22 Ω 10 W res i s to r between the input and sup-
ply, as a load for the con tac t breaker . This will greatly
inc rease the life of a pair of normal con tac t b reakers ,
which will consequen t ly require much less frequent tim-
ing readjustment , after which the vehic le will opera te
efficiently for longer per iods with less damage to the
environment . In addition, switching speed is faster mak-
ing more energy avai lable to the spark, although actual
improvement is difficult to measure .
It is worth a mention however that the ignition coil must
be a normal spec type with a r e s i s t ance of 3 - 4 Ω, and
not a high current , high energy type, t hese types will
des t roy the amplifier!
Testing
To be prudent you can c h e c k the operat ion of the ampli-
fier before fitting into the vehic le . A simple tes t requires
a 12 V power supply of up to 4 A output (or a car bat-
63
Auto electronics projects
64
te ry) , and a spare ignition coi l . The amplifier on its own
draws approximately 500 to 600 mA. By wrapping some
tinned copper wire around the + terminal of the coil and
looping the o ther end into the HT socke t , a s imple spark
gap should be formed. This type of sys tem must not op-
era te without a spark gap for a load, or e lse it is likely to
fail.
With the coil wired in, the repeated applicat ion of a 1.5 V
cell to the input should produce crackingly heal thy blue
sparks . For the t rans is tor ass i s ted vers ion, earthing the
input lead for on and re lease for o/ /wi l l have the same
effect. While on, the output ( - terminal on co i l ) will be
0.5 to 1 V.
A more e labora te tes t rig is i l lustrated in Figure 2.8. The
bat tery charger simulates an act ive charging system. The
primary coil voltage can be monitored by an osc i l loscope
using a xlO probe for an effective sensi t ivi ty of 100 V/cm
on the 10 V / c m range . It is very important t ha t the
p robe ' s t r immer be prec i se ly ca l ibra ted for an exac t ly
flat f requency r e sponse using a high quali ty squarewave
signal! The co i l ' s primary winding provides a good rep-
resenta t ion of what ' s going on at the secondary output
end, which can be seen on an 8 cm high grat icule with
the base l ine se t on the bot tom or s econd line.
You may need to turn the br ightness up and shade the
sc reen well, as the whole event is over in less than 3
mi l l i seconds . The t r ace should look like that shown in
Figure 2.9.
Note that the primary 's representa t ion of the gap con-
duction vol tage level is quite low at 80 or 90 V, but this
is because the air gap is at normal a tmospher ic pres-
Electronic ignition
BATTERY CHARGER
12V BATTERY
IGNITION Test COIL gap OSCILLOSCOPE Trigger
' C o Q O O
Ο Ο ο
loi • ο
πι III og
ο
}
X10 Probe
1 αν/cm Y 500uS/Cm Χ
AF SIGNAL GENERATOR
+VS OVE OUT IGNITION
AMPLIRER OVE IN
— c
• ( Ε » f=
sine 20Hz 1V r.m.e.
Figure 2.8 Test rig for monitoring amplif ier output at scope
sure . While providing sparks for a real engine this level
actual ly wanders about all over the p lace in direct pro-
port ion to the gas densi ty in the combus t ion chamber ,
being at its grea tes t while this is high during acce le ra -
tion, and lowest during the over-run while the thro t t le is
c losed . It is for this reason that the upper limit is de-
signed at 680 V and the BU208 chosen in order to provide
plenty of headroom: a different output s tage with a lower
vol tage t rans i s to r will not work properly (as it s tands ,
the design has been found to handle compress ion rat ios
of > 10 :1) . This behaviour a lso explains why any insula-
tion weakness always breaks down during acce le ra t ion .
Such a breakdown is usually total , as I found out the hard
way, leaving me s t randed. So take note!
65
Auto electronics projects
7 0 0 - ,
600
5 0 0 -
ω 4 0 0 -
§ 3 0 0 -
2 0 0 -
1 0 0 -
10_ 0
Limited by protection
scheme #1
© ®
t Limited by protection
scheme §2
5 milliseconds —
+12V
0V
Figure 2.9 Osci l lograph produced by test r ig : (a) in i t ia l e.m.f.
pulse: (b) spark gap ionisation t ime; (c) gap conduction time:
(d) gap extinguishing moment: (e) ringing period
C.D. I .
Who remembers D.I.Y, clip-on ignition b o o s t e r s . At one
point during the late 70s , the popular motor a c c e s s o r y
shops were crawling alive with these things. The selling
point was the third principle ment ioned ear l ier — ca-
paci t ive d ischarge ignition.
CDI employs the ignition coil in a total ly different man-
ne r , n a m e l y as a form of p u l s e t r a n s f o r m e r . T h e
advantage is that the coil is no longer an apprec iab le
part of the e l ec t r i ca l load as in a more conven t iona l
swi tched system; it does not have a heavy current flow-
ing in it for a large part of the t ime and consequen t ly has
6 6
Electronic ignition
an eas ie r life promoting reliabil i ty. In addition, overall
power consumpt ion for the ignition sys tem as a whole is
much lower and is in fact proport ional to engine speed.
As well as by the much reduced power requirement , cold
winter s tar t ing is aided by the very high energy spark
that CDI can genera te , which, if the designer is careful,
is still avai lable even if the ba t te ry vol tage is very low
during start ing.
CDI is e lec t r ica l ly efficient like no o ther a l ternat ive sys-
t em, p r o d u c i n g e n o r m o u s s p a r k s for a m i s e r l y few
hundred milliamps of supply current . Past exper iments
by this author with home grown CDI designs have pro-
duced sparks of V/2 inches! Figure 2.10 shows a typical
sys tem in b lock form, and individual designs do not de-
viate much from this .
The hear t of the sys tem is a d .c . -d .c . conver te r , which
produces a high voltage first (as opposed to the switched
method which derives it at spark t ime by switching the
coil off) d i rect ly from the low tension supply. It is s tored
by capac i to r CI which is in se r ies with the coil primary
winding.
The input stage receives a signal from a magnetic or other
form of timing senso r or a con t ac t breaker , and tr ips a
pulse genera tor , usually a monos tab le . The output pulse
tr iggers on CSR1, which c lamps C l ' s live end to ground.
The coil primary suddenly finds something in the region
of 500 V a c r o s s it, and c o m m e n c e s discharging CI . In the
p roces s , the d ischarge current induces a current in the
secondary winding, where the primary vol tage is multi-
plied by the turns rat io, producing a spark at the HT
output. The counter-e.m.f . from the coil pr imary that
follows turns CSR1 off again. While all this is going on,
67
Auto electronics projects
+ 12V DC IN Ο
σ
DC/DC CONVERTER
+500V DC OUT C1
"II"
INPUT PULSE ) STAGE GENERATOR
Ignition coil
Figure 2.10 Capacitive discharge ignit ion block schematic of
essential parts
the conver te r ' s output is effectively shor t -c i rcui ted to
earth, and it must be designed in such a way that it is
not damaged by this .
The sys tem is that simple, and easy to design, but lat-
ter ly is by and large not taken seriously by most motor
manufacturers . Why should this be? B e c a u s e of two in-
herent , unavoidable flaws in the principle.
One of these is to do with spark conduct ion t ime. The
truth is that this depends on capac i to r d ischarge time,
and as a result can be apprec iably shor te r than that of a
convent ional ly swi tched coi l . This means less gap con-
duction time in the combust ion chamber and, to be blunt,
less than ideal ignition of the fuel gas. In real i ty a be t te r
burn (and less waste and pol lu tants) resul ts from a me-
dium energy long spark than a high energy shor t one —
although this also depends on how the combust ion cham-
ber design can make the bes t use of it; with some older
shapes , which are so inefficient in the first place, it won't
make much difference.
68
Electronic ignition
69
The obvious answer is to inc rease the value of CI to in-
c r e a s e conduct ion t ime, but this aggravates the s econd
problem — which is that the capac i to r should be com-
p l e t e l y r e c h a r g e d p r io r to t h e nex t s p a r k m o m e n t .
Suppose that CI were inc reased to 1 μΡ to provide a 4-
cyl inder engine with r easonab le sparks up to its peak
output speed of 6,000 r.p.m. This requires 200 sparks per
second , further requiring CI to be recharged in the space
of <5 ms. This needs a charging current of 100 mA, which
can be proved by:
100,000 μΑ - < - Λ Π Λ/
- χ 5 ms = 500 V ΙμΡ
and the average power consumpt ion of the conve r t e r
inc reases , by co inc idence , to 50 watts — I say by coinci -
d e n c e b e c a u s e t h i s is a l s o t h e a v e r a g e for a
convent ional ly used ignition coi l . In p rac t i ce the spark
s t rength of CDI always drops off along a s teadi ly wors-
ening curve at h igher r.p.m., aggravat ing i ncomple t e
combust ion , already compromised by gas flow problems
and such. This is not to say that switched ignition doesn ' t
have a similar behaviour , but the roll-off of a swi tched
coil is less acute , and in any c a s e it is eas ie r to se l ec t or
manufacture the coil for the j o b required.
To be fair though, CDI is not a to ta l ly duff idea, but,
should you be toying with the idea of investigating the
principle yourself , be advised that , in order to be able
to deliver the required goods with any semblance of real
usefulness, the conver te r should follow a high frequency
type of swi tched mode power supply principle, using a
ferri te co red t ransformer , and not use a mains t rans-
former in reverse] Mains t ransformers are designed to
tap power from the mains at mains frequency, and are
Auto electronics projects
not very good at doing anything e l se . Given the short-
circuited output p rob lem, the c o n v e r t e r cou ld be a
single-ended flyback conver te r design.
The future
One p o s s i b l e f o r t h c o m i n g i n n o v a t i o n for c a r s is
d is t r ibutor less ignition. Instead of a mechanica l ro tor
delivering the HT current to the required plug as neces -
sary, one i terat ion of the principle is to use high voltage
rect i f iers in a floating secondary circui t to s t ee r HT to
the desired pair of cyl inders in a 4-cylinder engine, the
o ther cylinder, which does not need a spark, is on its
exhaust s t roke and so a spark here is known as a wasted spark. The ignition coi l pr imary is double-ended and
opera ted in push-pull mode by a pair of switching tran-
s is tors ; the direct ion of the secondary pulse determines
which pair of plugs will rece ive the current via the diode
matrix, and the t rans i s to rs will no doubt be under the
cont ro l of an engine management computer .
A variation will use two ignition co i l s , a lso with floating
open-ended s econdary windings but terminated straight
to a spark plug at each end. Again the relevant pair of
p is tons move toge ther but their valve timing is 180° out
of phase, so that while one is on its compress ion s t roke,
the o ther is on its exhaust s t roke .
In actual fact motorcyc les have featured duplicated com-
plete ignition sys tems , and the wasted spark t echn ique
for many years , and it is only a quest ion of t ime before
m o t o r c a r s fo l low su i t and b e c o m e e q u a l l y
d is t r ibutor less .
70
3 Microcontrollers
The microcontro l le r is the workhorse of the modern elec-tronics industry. That s ta tement may be strong, but it is
not an exaggerat ion, for it is becoming increasingly diffi-
cul t to p u r c h a s e any s ign i f ican t p i e c e of e l e c t r o n i c
hardware that does not conta in one or more of t hese
complex ICs.
A microcont ro l le r (μΟ), o therwise known as a single chip
mic rocompute r unit or MCU, is effectively a comple te
compute r cont ro l sys tem integrated onto a single chip
of s i l icon. Referring to Figure 3.1 the main functional
b locks of the mic rocon t ro l l e r are:
• m i c r o p r o c e s s o r co re : with opt imised inst ruct ion
se t for real t ime cont ro l ,
71
Auto electronics projects
R O M - 1 0 2 4 b y t e s
S e l f c h e c k R O M - 2 4 0 b y t e s
R A M - 6 4 b y t e s
R S T C P U
c o n t r o l
A r i t h m e t i c
l o g i c u n i t
( A L U )
M 6 8 H C 0 5 C P U
C P U r e g i s t e r s
A c c u m u l a t o r
I n d e x r e g i s t e r
0 0 0 0 0 1 1 S t a c k p o i n t e r
0 0 0 P r o g r a m c o u n t e r
C o n d c o d e s 1 1 1 H I N Z C
O s c i l l a t o r D i v i d e
b y 2
W a t c h d o g a n d
i l l e g a l a d d r e s s d e t e c t
P O W E R
Figure 3.1 MC68HC05J1 MCU block diagram, showing the
basic functional blocks common to all microcontrollers
72
Microcontrollers
it
σ I Q
P A 7
P A 6
P A 5
P A 4
Ρ A 3
P A 2
PA1
P A O
ö Q
P B 5
P B 4
P B 3
P B 2
P B 1
P B O
1 5 S t a g e
m u l t i f u n c t i o n
t i m e r s y s t e m
Figure 3.1 Continued
73
Auto electronics projects
• memory; usually ROM to contain the cont ro l pro-
gram p lus RAM to ho ld v a r i a b l e s dur ing p r o g r a m
execut ion,
• I/O and on-chip per ipherals ; t hese allow the MCU
to communica te with the hardware of the real world ap-
plicat ion that it is control l ing. T h e s e per ipherals range
from simple digital input/output ( I /O) por ts to complex
analogue-to-digital (A-to-D) and digital-to-analogue (D-to-
A) conver te r s and t imer sys t ems . Tab le 3.1 l ists some of
t h e peripherals t h a t a r e a v a i l a b l e on c u r r e n t
microcont ro l le r families.
Microcont ro l le r s are avai lable in a range of complexi-
t i e s and power (and t h e r e f o r e p r i c e ) , making them
sui table for a very wide range of appl icat ions where they
can rep lace s tandard logic or more complex microproc-
e s so r based solut ions . The advantages of the MCU over
t h e s e t rad i t iona l so lu t i ons a re , r e d u c e d ch ip coun t ,
which br ings c o s t ; r e l i ab i l i ty and s ize b o n u s e s ; and
greater flexibility for the designer — allowing easy modi-
fications to the functionality of the appl icat ion via the
software. T h e s e advantages coupled with the dev ices '
relat ively low cos t ( typical ly from SO.75 in high volume)
have led to m i c r o c o n t r o l l e r s be ing used in a g rea t
breadth of appl ica t ions . With a few excep t ions such as
industrial cont ro l , t hese MCU appl icat ions can be split
into two groups; automotive and consumer .
Tab le 3.2 gives a non-exhaust ive list of mic rocont ro l l e r
appl ica t ions in t he se two a reas . The intent ion of this
chap te r is to give the reader some more insight into a
few of t he a u t o m o t i v e a p p l i c a t i o n s tha t depend on
microcont ro l l e r s , and to highlight the proper t ies of par-
t icular MCUs that make them sui table for each d iscussed
applicat ion.
74
Microcontrollers
MCU peripheral Function
Digital I/O port
Timer
Serial port
VFD port
LCD port
A-to-D
P W M or D-to-A
Watchdog tinner
EEPROM —
in addition to ROM
PLL
RTC
Wake-up port
DTMF
OSD
The basic hardware used by the CPU to access the
outside world (read switches, drive LEDs, etc.).
One off the most common and useful MCU
peripherals — allows timing tasks to be
accomplished while the CPU does something else.
Both synchronous and asynchronous ports are
available allowing fast serial communications over
short or long distances respectively.
Special high voltage output port for driving
vacuum fluorescent displays.
Special low voltage output port for driving LCD
displays. Usually includes multiplexing for large
displays.
Analogue-to-digital converter used to read a
variety of sensors, etc.
A pulse width modulated output that can be
filtered to produce a programmable analogue
voltage, thus acting as a digital-to-analogue
converter.
A special type of timer that guards against CPU
errors and resulting software runaway.
Re-programmable memory that can be used for
calibration purposes or for a non-volatile data store.
Phase locked loop. Used in tuner applications such as
TV and radio.
Real time clock. Special timer designed to count in
real time, i.e. seconds, minutes and hours.
Modified digital I/O port that can generate CPU
interrupts when an input signal changes.
Dual-tone multi-frequency generator, used in tone
dialling telephone applications.
On screen display. A character generator for showing
messages on a TV screen.
Table 3.1 Commonly available on-chip microcontrol ler
peripherals
75
Auto electronics projects
76
The automotive industry is widely recognised by semi-
c o n d u c t o r manu fac tu r e r s as be ing the p e r f o r m a n c e
driver of the mic rocon t ro l l e r market . Originally using
mic rocon t ro l l e r s with 4 and 8-bit buses , the automot ive
des igner ' s ques t for more p rocess ing power for some
appl ica t ions , such its engine management , has pushed
the semiconductor industry into designing first 16-bit and
now 32-bit MCUs. Some ca r s being designed today have
more process ing power under the bonnet than an aver-
age PC!
A well recognised trend in the automotive industry is to
in t roduce new features on up-market ca rs and then mi-
gra te them down on to the i r mass market v e h i c l e s as
rel iabi l i ty and user a c c e p t a n c e are proven, and c o s t s
come down. This explains why many of the features avail-
able on today 's ca r s ( such as e lec t r i c windows) were
yes te rday only avai lable on expens ive luxury models .
However, in many c a s e s t h e s e sys t ems are using yes ter -
day's dumb t echnology and many of the mic rocon t ro l l e r
app l i ca t ions of T a b l e 3.2 a re st i l l the domain of up-
market veh ic les . As the t echno logy migration t rend and
green legislation cont inue, this si tuation will change and
wi th in a few y e a r s al l c a r s will c o n t a i n m o r e
mic rocon t ro l l e r s than wheels! See Figure 3.2.
Interfacing MCUs in the automotive environment
T h e r e is a fundamenta l p r o b l e m with us ing m i c r o -
cont ro l le rs , or digital logic in general , in an automobi le ;
the vehic le e lec t r ica l sys tem is invariably 12 V and logi-
cal devices work at around 5 V, and would be severe ly
Microcontrollers
Automotive Consumer
Engine management
Alarm system
Anti lock braking
Central locking
Trip computer
Dashboard
Electric w indows
ln-car entertainment
Act ive suspension
Multiplexed wiring
Seat adjustment
Electric mirrors
Television
Microwave oven
Telephone
Video cassette recorder
Washing machine
Remote control system
Toys
Fridges and freezers
Alarm system
Radio
Compact disc player
Satell ite receiver
Table 3.2 Typical microcontrol ler applications
Engine lanagement
Dashboard
A.B.S
Alarm system
Central locking
Multiplexed wiring
Active suspension
Figure 3.2 Soon an average car wi l l contain more
microcontrol lers than wheels!
77
Auto electronics projects
damaged if c o n n e c t e d d i rec t ly to a 12 V sys tem. This
means that a supply for the MCU must be derived from
the 12 V supply using a regulator c i rcui t , and that all
inputs to the device must be buffered from the 12 V world
around it. The MCU is a lso incapable of d i rect ly driving
automotive loads, so that external drive c i rcui t s must
be employed to interface the logic outputs to the 12 V
loads. The si tuation is actual ly even worse than this ini-
tial s ta tement implies; the automotive environment is one
of the harshes t known, with ex t remes of temperature and
the sys tem voltage varying cons iderab ly depending on
the condi t ion of the ba t t e ry and whe ther the veh ic le
engine is being cranked (when the vol tage drops consid-
e rab ly ) . The biggest problem however , is the ignition
ci rcui t . When the ignition coil swi tches , large vol tage
impulses (50 to 100 V) can be genera ted on both rails of
the ent i re e lec t r ica l sys tem. Although of shor t duration,
t hese pulses would spell d isas ter for a logic c ircui t in-
put. For th is r e a son great c a r e must be taken when
designing protect ion ci rcui ts for the e lec t ronic hardware
in ca r s . Despite these problems and the assoc ia ted c o s t s
to counte r them, the outlay is justified due to the ben-
efits brought by e l e c t r o n i c s and m i c r o c o n t r o l l e r s , in
particular to the automobile . In the following discuss ions
and examples , the pro tec t ion and drive c i rcui t s may not
always be shown for simplici ty, but the reader should
be aware that these precaut ions have to be taken in all
automotive microcont ro l l e r appl ica t ions .
Electric windows
This is one of the most common electrical goodies to be
fitted to many cars . Figure 3.3 shows the traditional dumb
78
Microcontrollers
Door frame
ι ι
Figure 3.3 Conventional electr ic window c i rcu i t (duplicated for
other doors)
e lec t r ic window circui t that is in common use today. The
swi tches direct ly cont ro l the supply current to the mo-
tors , thus propelling the window in the desired direction.
When the window r eaches the end of its t ravel there is
no cut out, instead the motor simply stal ls and the cur-
rent is limited to a value that does not damage the motor
windings. You can obse rve this by trying to ra ise both
c losed windows in a car when the engine is idling the
engine r.p.m. will drop appreciably due to the heavy load-
ing on the a l ternator . Although this sys tem works quite
well, it does have a couple of problems. The first of these
is quite a major safety conce rn and s tems from the fact
that to deal with icy windows or a dirty mechanism a
powerful motor is deployed. The problem is that if an
obs t ruc t ion is p laced in the way of a c losing window the
motor will exer t a great deal of force before it s tal ls ; that
obs t ruc t ion could be a chi ld 's neck. The second prob-
lem is more of an annoyance than a real problem and it
conce rns the amount of t ime that the driver must keep
his finger on a small but ton to fully open or c lo se the
window.
79
Auto electronics projects
Central locking
That great innovation for the wet Bri t ish c l imate , cen-
tral locking, has t radi t ionally been opera ted via a switch
in the lock mechanism of the front doors , but in recen t
years a new development has made this feature even
80
Both these problems are solved by the intelligent MCU
based system, shown in Figure 3.4. Here the swi tches and
sensor s are connec ted to inputs of the MCU and it in turn
con t ro l s the motors via output por ts that switch exter-
nal dr ivers . The senso r s inform the mic rocon t ro l l e r that
the window has reached the end of its travel and the MCU
can s top the motors . This posi t ional feedback along with
the current s ense means that the MCU can immediately
de tec t when an obs t ruc t ion o ther than the end-stop has
caused the motor to slow or stall ins tead. In t he se c a s e s
the MCU can now take evasive act ion by stopping and
reversing the direct ion of the window for a couple of
inches thus releasing the obs t ruc t ion . The MCU also al-
lows the option of one-touch open or c lose , e i ther via an
additional button, or by counting how long the normal
button is held for — e.g. if the button is p ressed for more
than 2 s econds then the MCU assumes a full motion of
the window is required. Although these features could
be implemented using logic cont ro l , the integrat ion and
very low cos t of a s imple MCU such as the MC68HC05J1
from Motorola make it the ideal c h o i c e . This device is
supplied in a small 20-pin package and has only 1 Κ of
ROM onboard to s to re the program, along with the CPU
and a simple t imer (Figure 3 .1 ) . However, t hese limited
features linked with low cos t make it the ideal device for
displacing c lumsy logic solut ions .
Microcontrollers
81
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Auto electronics projects
more des i rable — remote cent ra l locking. In this set-up
a remote key uses a t ransmiss ion by radio, or more com-
monly infra-red ( IR) , to ac t iva te the cent ra l locking from
a wide angle and cons ide rab le d is tance from the vehi-
cle — Figure 3.5 shows the s chema t i c of such a sys tem.
The t ransmi t te r uses e i ther a very bas i c microcont ro l le r
or, more commonly, a dedicated logic device such as the
MC145026 IC. Instead of using a keypad to de termine
which code to transmit , the device has its inputs fixed
in the factory, into a cer ta in combinat ion of logic levels ,
so that it will always transmit the same code . The number
of inputs allow a large number of different codes to be
configured — just like the number of levers in a padlock.
Although matched pairs of t r ansmi t t e r s / r ece ive r s could
be employed in this applicat ion, the logis t ics of keeping
t rack of which key belongs to which car during produc-
tion are obviously difficult, never mind how you would
handle an owner losing his key and request ing a replace-
ment! For these reasons , intel l igence is employed in the
rece iver to allow it to be cus tomised after product ion.
The microcont ro l le r chosen for the job will include some
on board programmable non-volatile memory (EPROM
or EEPROM) that can be used to s tore the codes of match-
ing t ransmit ters . This customising of the receiver is often
performed by the dealer , just before the new owner gets
his car . The memory size of the MCU allows for several
key codes , so that multiple keys can be used by different
family members . Secure software can be employed to
prevent someone from trying to cyc le through all the
valid codes for the t ransmit ter type until the co r r ec t one
is found. In its s implest form this could just involve ig-
noring incoming IR codes , for a couple of s econds , after
an invalid code has been received — with so many codes
8 2
Microcontrollers
83
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Auto electronics projects
to cyc le through, this would make the j ob overly t ime
consuming for the potential intruder.
S ince the rece iver must remain powered up at all t imes,
low power consumpt ion is of vital impor tance . For this
reason the MCU will invariably be a CMOS device, with a
specia l low power sleep or stop mode, where the power
consumpt ion will be in the order of microamps . Any in-
coming signal will wake the MCU, via the interrupt pin,
and it will r e c e i v e the c o d e and o p e r a t e the locking
m e c h a n i s m ( e i t h e r s o l e n o i d or m o t o r d r i v e n ) , if it
ma tches one of the valid codes s tored in its memory. A
s u i t a b l e d e v i c e for t h i s a p p l i c a t i o n would b e t h e
MC68HC05P8, which is a c lo se family member to the pre-
viously d iscussed J l device. Its distinguishing feature for
r e m o t e c e n t r a l l ock ing is t h e 32 b y t e s of o n b o a r d
EEPR0M that can be used to s to re several t ransmi t te r
key codes .
Engine management
Engine management in this con tex t means having com-
plete cont ro l over an engine 's ignition timing and fuel
mixture on a cycle-by-cycle bas i s . The t rend in increas-
ing engine management performance has been driven by
the tightening of emissions regulations around the world.
This is the real performance end of the microcont ro l l e r
market, and it has been respons ib le for the growth in
complexi ty of the on-chip t imer sys tems for, as we
will s e e , engine managemen t invo lves a lot of t ime-
cr i t ica l tasks . Before discussing where mic rocon t ro l l e r s
fit into this applicat ion, a brief explanat ion of what is
involved in engine management and how it has been tack-
led in the past would be beneficial .
8 4
Microcontrollers
Figure 3.6 shows the four s tages of a comple te cyc l e of a
four - s t roke in t e rna l c o m b u s t i o n eng ine . In t h e f irst
s t roke , the piston is travell ing downwards with the inlet
valve open, thus drawing in the air/fuel mixture from the
inlet manifold. In the s econd s t roke , the piston r i ses with
both valves c losed , t he reby compress ing the mixture.
As the piston r eaches the top of its t ravel ( top dead cen-
t re or t . d . c ) , the spark plug is fired to ignite the mixture.
The third s t roke is the combus t ion /power s t roke , when
the cyl inder delivers its power; the rapidly combust ing
mixture b e c o m e s very hot and the result ing rapid in-
c r e a s e in p ressure drives the piston down the cyl inder.
In the final s t roke, the piston t ravels upwards again, with
the exhaus t valve open, thus expell ing the remaining
burnt gases . The piston is then ready to s tar t its next
downward intake s t roke , and so ini t iate ano the r four
s t roke cyc le .
The problem for the automot ive designer is that to max-
imise the power and fuel consumption of an engine (while
minimising its pollutants) , the timing of the ignition spark
and the rat io of the air/fuel mixture must vary according
O p e n C l o s e d C l o s e d C l o s e d C l o s e d C l o s e d C l o s e d O p e n
1: I n t a k e 2 : C o m p r e s s i o n 3 : C o m b u s t i o n / 4 : E x h a u s t P o w e r
Figure 3.6 The four strokes of the internal combustion engine
85
Auto electronics projects
to a number of fac tors . The most significant of t hese fac-
tors are engine speed, t empera ture and engine load. The
job of engine management is to cont ro l the ignition and
fuelling of the engine, keeping it as c lo se to its ideal op-
erating condi t ions as poss ib le .
Ignition
On average it takes 2 ms to comple te the combus t ion
p roces s after the ignition spark has been fired. S ince the
aim is to have maximum pressure in the cyl inder just
after the piston has passed the top of its s t roke ( too early
and the pressure would inhibit the pistons upward travel;
too late and power is wasted in the downward s t roke ) , it
is nece s sa ry to fire the spark before the piston r eaches
t .d.c. It is cus tomary to represen t this ignition point as
the number of engine degrees before top dead cen t re
( b . t . d . c ) . As the piston t ravels faster and faster with in-
c reas ing engine speed , and b e c a u s e the c o m b u s t i o n
p roces s takes the same length of t ime, a fixed firing an-
gle for the spark would resul t in maximum p re s su re
occurr ing further and further into the downward s t roke,
so wasting power and increasing fuel consumpt ion. For
this reason the ignition point must be advanced (more
degrees b. t .d.c.) with increasing engine speed. Tradition-
ally th is has been a c c o m p l i s h e d with the centr ifugal
advance mechanism in the dis t r ibutor .
Another factor which influences the ignition timing is the
engine load, which can be shown to be proport ional to
the amount of air inducted by the engine. Historically
this factor was taken into accoun t by connec t ing a pipe
from the inlet manifold to the distr ibutor advance mecha-
8 6
Microcontrollers
nism ( the vacuum a d v a n c e ) . This mechan ica l sys t em
(which has remained virtually unchanged for many years )
is re l iable , but only allows crude cont ro l of the ignition
timing, result ing in compromises in the engine perform-
ance and great difficulty in reaching today 's emiss ion
regulat ions.
Mixture control
For an engine to run well, a specif ic air/fuel rat io must
be maintained. The theore t i ca l rat io of fuel to air for
comple te combust ion (and therefore maximum economy
and lowest emis s ions ) is jus t under 1:15 in weight (or
a l ternat ively 1 L of fuel for every 10,000 L of air in vol-
ume) . In p rac t i ce , maximum fuel e conomy is obta ined
with around 20% e x c e s s air, while maximum power is
ob ta ined with approx imate ly 10% air sho r t age . S ince
engines normally run at part-load, the fuel sys tem is de-
s igned for maximum e c o n o m y at th i s point and the
mixture will be r icher at idle and maximum power. The
task of the ca rbure t to r , or fuel inject ion sys tem, is to
produce the bes t mixture for the engine under its cur-
r e n t o p e r a t i n g c o n d i t i o n s . T h e s i m p l e m e c h a n i c a l
ca rbure t to r again compromises the fuel mixture under
different condi t ions and the t rend today is towards e lec-
t ronic fuel inject ion, where a p rec i se amount of fuel can
be del ivered to the individual cyl inders .
B e c a u s e ignition timing and fuel mixture are both de-
pendent on the same var iables (engine speed, load and
t empera tu re ) , it makes a lot of s e n s e to combine the
cont ro l of both into a single unit — the so-cal led e lec-
t ronic engine management system. With its ability to read
87
Auto electronics projects
sensor s , perform high-speed ca lcula t ions and measure
time, the microcont ro l le r is the ideal device for engine
management .
Figure 3.7 shows a b lock diagram of a typical sys tem
based on a high-performance mic rocon t ro l l e r . Engine
speed and angle are both obta ined from a single induc-
tive senso r that genera tes e lec t r ica l pulses when tee th
on the flywheel pass by. To provide a re ference point
for determining the engine angle, one or more tee th are
omit ted from the flywheel, thus producing a pulse pe-
riod twice or more than the normal. Alternatively, but
less common, an ext ra tooth may be present resulting in
two pulses each of half the normal period. This means
that to determine engine speed and angular posit ion, the
microcont ro l l e r must perform two bas ic tasks :
• it must de tec t the miss ing /ex t ra too th and then
count tee th to determine the engine angle,
• it must t rack the t ime between adjacent tee th , and
from this ca lcu la te the current engine speed.
As there are typical ly 30 to 60 tee th on the engine fly-
wheel and a typical engine has to be designed for an
8000 r.p.m. maximum, the pulse period from the flywheel
sensor can be less than 125 μ 8 . Clearly then, if the μC used software loops to count the periods of the incom-
ing pulses, there would be very little process ing time left
to use the data obtained, even if it didn't have o ther sig-
nals to measure as well. For this reason independent
t imer sys tems on board the microcontrol ler have evolved
to lessen the load on the CPU. T h e s e t imers use an input
capture mechan ism to time tag incoming pulse edges
against a free running counter t imebase , and then inter-
8 8
Microcontrollers
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rupt the CPU to tell it to read the captured t ime. The
following sec t ion i l lustrates how the t imer sys tem inter-
a c t s wi th t h e CPU on M o t o r o l a ' s M68HC11
microcont ro l l e r in order to determine engine speed and
angle.
The diagram in Figure 3.8 shows a simplified b lock dia-
gram of t h e T i m e r and Pu l se A c c u m u l a t o r s y s t e m s
o n b o a r d the M o t o r o l a M68HC11 MCU, a pa r t i cu la r ly
popular device for current engine management solut ions.
The hear t of the t imer sys tem is a 16-bit free-running
counter that the CPU can read via two 8-bit regis ters ,
TCNTHI and TCNTLO. The main purpose of the counte r
is to ac t as a t imebase for the input capture and output
compare functions. The input capture function allows a
t ransi t ion on an external pin to be timestamped by latch-
ing the value of the free-running counte r at the t ime of
the t ransi t ion. The CPU can then read the la tch at a later
t ime and get an exac t r ecord of when the t ransi t ion oc-
curred.
The output compare , or match, function is the inverse
of the input capture ; it allows the CPU to schedu le a
change in the s ta te of an output pin, at a p rec i se t ime in
the future, by writing a value into the 16-bit compare
register . When that value is matched by the incrementing
free-running counter , the output will change s ta te . The
M68HC11 has various combina t ions of input capture and
output compare pins avai lable on several of its family
members . The pulse accumula tor is an 8-bit counter that
is c locked by a specif ied t ransi t ion on an external input
pin. The CPU can write any value into the counter , and
can read it at any t ime. The pulse accumula tor can gen-
era te an interrupt to the CPU when it overflows.
90
Microcontrollers
The t imer module and pulse accumula tor can be used in
a number of ways to determine engine speed and angle,
and to genera te the n e c e s s a r y output pulses . The fol-
lowing is one such method.
The condi t ioned signal from the flywheel s enso r is con-
nected to both the pulse accumula tor input pin and input
capture pin. Both the input capture pin and the pulse
accumula tor pin are configured to de tec t a rising edge,
and the input capture interrupt is enabled so that the
CPU will be interrupted on every pulse rising edge. The
interrupt software routine will read the captured value
of the free-running t imer, s to re it and then sub t rac t the
last captured value, to obtain the tooth pulse period in
t imer counts . S ince the number of tee th are known, the
engine speed can easi ly be derived from the pulse pe-
riod. S ince the period of every pulse is measured, the
interrupt software can identify the longer period assoc i -
ated with the missing tooth angular re ference . At this
point it can c lear the pulse accumula tor , which will then
s tar t count ing pulses ( t e e t h ) . S ince each tooth cor re -
sponds to a number of engine degrees , the value of the
pulse accumula tor is a represen ta t ion of the engine an-
gle.
To genera te one of the output pulses (e.g. for an injec-
to r ) , the input capture interrupt software can c h e c k the
pulse accumula tor against the desired tooth count (mi-
nus 1 or 2, to allow for interrupt l a t enc i e s ) . When the
pu lse a c c u m u l a t o r m a t c h e s th i s va lue , t he CPU can
schedule the s tar t edge of the pulse by reading the free-
runn ing c o u n t e r , add ing an of f se t and wr i t ing t h e
resul tant value into the output compare regis ter of the
desired pin. The offset is a value in t imer counts that
co r re sponds to a number of engine degrees at the cur-
91
Auto electronics projects
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rent speed . This number of engine degrees is the differ-
e n c e b e t w e e n t h e a n g l e m a t c h e d to t h e p u l s e
accumula tor value, and the exac t number of engine de-
grees at which the pulse must begin — s e e Figure 3.9.
The o ther input parameters of Figure 3.7 are measured
using an analogue-to-digital (A-to-D) conver te r , which is
usually integrated on-chip as part of the microcont ro l le r .
As previously mentioned, the air inducted by the engine
can be used as a measure of the engine load. A value for
this is obta ined, via a vane device in the air intake that
opera tes a potent iometer , or a l ternat ively via a hot-wire
sensor . The lambda senso r is a fairly recen t addition to
engine management s y s t e m s prompted by increas ing
anti-pollution regulations that have led to the use of cata-
lytic conve r t e r s . The ca ta ly t ic conve r t e r is a de l ica te
ob jec t and very str ingent control of the engine emiss ions
must be obta ined if the ca ta ly t ic conver te r is to opera te
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In the a b o v e e x a m p l e a n o u t p u t p u l s e m u s t b e g e n e r a t e d s t a r t i n g at a n e n g i n e a n g l e c o r r e s p o n d i n g to 5 t e e t h p l u s a bit ( a s e a c h t o o t h is a n u m b e r o f d e g r e e s ) f r o m the r e f e r e n c e m a r k
Figure 3.9 An example of output pulse timing
94
Microcontrollers
efficiently. The lambda senso r is bas ica l ly a hot plati-
num/ce ramic dev ice that p roduces an output vol tage
which var ies , depending on the oxygen con ten t of the
gas it is surrounded by. By insert ing such a s enso r into
the exhaust manifold, it is poss ib le to determine the air/
fuel compos i t ion current ly being burned in an engine.
This effectively t ransforms the engine management sys-
tem, from an open-loop control sys tem into a closed-loop
one, where def ic iencies in the desired output ( c o r r e c t
air/fuel mixture) can be de tec ted and the input var iables
(ignition timing/fuel quant i ty) adjusted to compensa te .
This means that much c lose r control of the exhaust emis-
s i o n s can b e m a i n t a i n e d , he lp ing to m a x i m i s e t h e
e f f ic iency of the c a t a l y t i c c o n v e r t e r mounted down-
s t ream in the exhaust sys tem.
Having m e a s u r e d all t h e s e p a r a m e t e r s , t h e
microcont ro l l e r must de termine the cor responding out-
puts — i.e. the timing of the spark ignition pulses , and
the t iming/duration of the pulses which fire the fuel in-
j e c t o r s . Th i s is a c h i e v e d by a c c e s s i n g the so -ca l l ed
engine maps t h a t a r e s t o r e d in t h e m e m o r y of t h e
microcont ro l le r . T h e s e maps are, in fact, t ab les of data
that hold the ignition and fuelling cha rac t e r i s t i c s of a
par t icular engine type against a number of input vari-
ab les . B e c a u s e it is impract ica l to try and s to re all the
pos s ib l e c o m b i n a t i o n s of output t iming ve r sus input
cha rac t e r i s t i c s , a number of points are held in the map
table , and the μC must then perform an ar i thmet ic cal-
culat ion to in terpolate between the two c lo se s t points
given, to the exac t input condi t ions obta ined from the
various s enso r s .
As there are a number of var iables to be taken into con-
siderat ion, t he se interpolat ion ca lcu la t ions are complex
95
Auto electronics projects
and require a lot of p rocess ing power to be comple ted
quickly, in t ime to set up the output timings for the next
engine cyc le . This is the reason why 16 and now 32-bit
mic rocon t ro l l e r s are replacing older 8-bit sys tems for
engine management . They allow more complex calcula-
t ions to be comple ted quickly so that c lo se r cont ro l can
be maintained on a cycle-by-cycle bas i s .
When the microcont ro l l e r has obta ined the desired out-
put t imings, it must actual ly genera te the pulses to fire
the spark plugs and in jec tors . This is done via the out-
put match facility of the t imer sys tem, where the CPU
writes a value into a specia l regis ter . When the value of
the incrementing t imer-counter r eaches the same value
as that in the regis ter , the hardware of the t imer sys tem
automat ical ly changes the output pin s ta te to a desired
level. This mechanism allows very accu ra t e p lacement
of the various pulses required in the engine cyc le , as we
have seen from the descript ion of the Motorola M68HC11.
The method desc r ibed above, using the input capture
and output match t imer functions, is used in virtually all
of today 's production engine management sys tems . How-
ever, this sys tem is not perfect as the CPU still has to
respond to a large number of interrupts generated by
the t imer, thus slowing down its cont ro l ca lcu la t ions .
This interrupt overhead has se t the performance limits
of today 's sys tems , and so a new approach will be re-
quired for the even more complex cont ro l a lgori thms
required for tomorrow's emiss ion regulat ions.
Motorola has been the first microcontrol ler manufacturer
to address this problem by introducing the innovative
MC68332 device . Not only does this device have a pow-
erful 32-bit 68000-based CPU, but is unlike any o ther
9 6
Microcontrollers
microcontro l le r in that it a lso has a second on-board CPU
d e d i c a t e d to con t ro l l i ng t imer func t ions . Th i s T ime
Process ing Unit, or TPU, is in effect a mic rocont ro l l e r
within a microcont ro l le r ! The TPU is used to handle al-
mos t all of the in te r rup t s a s s o c i a t e d with the t imer
channels , thus freeing the main CPU to spend more t ime
on complex cont ro l ca lcu la t ions . At sui table points in
the cont ro l cyc le , the main CPU obta ins new input read-
ings from the TPU and presen ts new data for the TPU to
ca lcu la te and schedule the output pulse t imings.
Vehicle alarms
The huge inc rease in car-related c r imes in the 1980 /90s
has been paral leled by an equally large inc rease in the
demand for car a larms. Originally based on simple logic
c i r c u i t s and t r i g g e r e d d i r e c t l y from i n t e r i o r l ight
swi tches , the complexi ty of a larms has grown to try and
match the skill of the potent ia l in t ruder . Figure 3.10
shows the s c h e m a t i c of a typical soph i s t i ca t ed MCU-
based alarm sys t em. Using a m i c r o c o n t r o l l e r in th is
application provides a great deal of sophis t icat ion within
a very low componen t count , allowing the alarm to be
small and thus easi ly concea led .
An MCU chosen for this j ob should have a low power mode s ince the alarm must be powered up for long peri-
ods of t ime without the engine running. It should also be
poss ib le to wake the device from this mode via several
sou rces , so that a number of c i rcu i t s can trigger the de-
vice into sounding the alarm. A simple 8 or 16-bit on-chip
t imer is a lso des i rable to t ime the output audio/visual
warning p u l s e s , and to r e s e t t h e a la rm af ter it has
9 7
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Auto electronics projects
sounded for a se t t ime — this is a legal requirement . The
t imer can also be used to arm the alarm after a defined
period, if it is not armed via a remote cont ro l .
A.B.S.
The increased performance of everyday ca r s , along with
their increasing numbers (and therefore greater densi ty
on the roads ) , has resul ted in a cont inual improvement
in braking performance. This t rend has included the pro-
gress ion from all-drum braking, drum/disc braking and
vent i la ted d isc /drums, through to the all-disc braking
sys tems found on today 's higher per formance ca r s . The
most recen t improvement has been the introduct ion of
ABS.
The Antilock Brake System does not i tself inc rease the
braking capac i ty of the vehic le , but improves safety by
maintaining optimum braking effort under all condi t ions .
It does this by preventing the vehic le wheels from lock-
ing, due to over -app l ica t ion of the b r akes , and thus
maintains s teerabi l i ty and reduces stopping d i s tances
when braking on difficult surfaces such as ice .
ABS allows shor te r s topping d i s tances than with locked
wheels , due to the friction or mu-slip cha rac t e r i s t i c of
the tyre-to-road interface; as a wheel brakes , it slips rela-
tive to the road surface producing a friction force . A
typical mu-slip curve is dep ic ted in Figure 3 .11 . This
shows that peak friction occu r s at about 10 to 20% slip,
and then falls to approximately 30% of this value at 100%
slip ( locked whee l ) .
100
Microcontrollers
mu 0.5H
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Figure 3.11 A typical mu-slip characterist ic for the tyre-to-road
interface
The aim of the ABS sys tem is to cont ro l the braking force
so as to s top the slip for any wheel exceeding this opti-
mum value by more than an a c c e p t a b l e window.
At t h e h e a r t of al l ABS s y s t e m s ( e x c e p t t h e a l l -
mechan ica l sys tem implemented by Lucas ) is an e lec -
t r o n i c c o n t r o l unit (ECU) b a s e d a round a powerful
microcont ro l le r . Figure 3.12 shows a b lock diagram of
such a sys tem. The solenoid valves that form part of the
hydraulic modulator allow cont ro l of the p ressure avail-
able to the individual wheel brake cylinders, independent
of the force supplied by the driver via the brake pedal.
T h e s e three-way valves can connec t the brake cyl inders
to:
• the normal master cylinder circuit , so that the brak-
ing pressure will be di rect ly cont ro l led by the driver,
• the return pump and accumula to r in the hydraulic
modulator , so that the p ressure in the brake cyl inders
will fall as the fluid returns to the mas te r cylinder,
101
Auto electronics projects
102
r-----------
, W
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el
4
Bro
ke
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er
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BS s
yste
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Microcontrollers
• nei ther of the above two c i rcui ts , thus isolating the
brake cyl inder so that the p ressure will be maintained
at the value immediately preceding the move to this po-
sit ion.
The cont ro l for t hese valves is supplied via drive cir-
cui ts from the output por ts of the microcont ro l le r .
T h e b a s i s for all e l e c t r o n i c ABS s y s t e m s is t h e
mic rocon t ro l l e r ' s abil i ty to determine the speeds of the
individual wheels (al though some front-wheel drive ve-
h i c l e s s h a r e a c o m m o n s p e e d s e n s o r for b o t h r ea r
whee l s ) . It does this via an inductive senso r and too thed
ring that produce an output waveform, the f requency of
which represen t s the speed of the wheel . This arrange-
ment is a lmos t ident ica l to the engine speed s e n s o r
d i scussed earl ier , excep t that s ince no angular posi t ion
information is required the re are no missing or ex t ra
teeth. It follows from this that the explanation previously
given on determining engine speed a lso applies to deter-
mining wheel speeds in an ABS sys tem.
In this ca se , there are around 50 to 100 tee th on the en-
coder ring, and this could result in a pulse f requency of
6000 Hz when the vehic le is travelling well in e x c e s s of
100 mph. As the re can be a speed senso r on each of the
4 wheels, a total of 24,000 pulse edges have to be resolved
every second . The solenoid valves in an ABS sys tem typi-
ca l l y have a r e s p o n s e t ime of 10 to 20 ms , and t h e
mic rocon t ro l l e r must be able to sample the inputs at
least twice that often, to reso lve lock-ups in 5 to 10 ms.
Put another way, the mic rocon t ro l l e r must be able to
de te rmine 4 independent wheel speeds from 6000 Hz
103
Auto electronics projects
signals within a 5 ms window, and still have t ime to carry
out p rocess ing on this data to determine the new valve
s t a tes . T h e s e str ingent timing requi rements mean that
ABS sys tems are the domain of high performance 16-bit
mic rocon t ro l l e r s that can respond quickly to interrupts
from the t imer sys tem which is capturing the speed sen-
sor edges .
So far it has been s ta ted that the mic rocon t ro l l e r in an
ABS sys tem must prevent the wheel-slip value from ex-
ceeding the optimum, and we have d i scussed how the
μC measures the wheel speeds (angular ve loc i ty ) . How-
ever, it may not be c lea r how these wheel speeds are
related to the slip values that the sys tem is at tempting
to cont ro l . The slip of any wheel can be defined as the
difference between the angular ve loc i ty of the slipping
and non-slipping wheels , divided by the angular ve loc-
ity of the non-sl ipping wheel . Th i s makes s e n s e and
sounds quite simple, but for one problem; how to find a
non-slipping wheel? The ABS algorithm s e a r c h e s for the
fastest spinning wheel and uses this as the re fe rence for
calculat ing the slip values of the o ther wheels . If the slip
value of a wheel is greater than the peak friction value
by a cer ta in margin ( i .e . the wheel is heading towards a
locked condi t ion) , then an ABS cont ro l cyc le is execu ted
on that wheel .
First the mic rocon t ro l l e r will i so la te the wheel brake
cyl inder from the brake c i rcui t to prevent further pres-
s u r e i n c r e a s e . It will t h e n r e c h e c k t h e s l ip and
acce le ra t ion values to determine if the wheel is still de-
celerat ing, and whether the slip value is still exceeding
the desired value. If so , then the valve posi t ion is moved
104
Microcontrollers
momentar i ly to the return posi t ion, reducing the brak-
ing effort on that wheel . This pulsed re lease of p ressure
is cont inued until the mic rocon t ro l l e r de tec t s that the
wheel acce le ra t ion is posi t ive, at which point it s tops
reducing the pressure , and r e c o n n e c t s the wheel cylin-
der to the b rake c i rcu i t to prevent o v e r s h o o t of the
acce le ra t ion . This ent i re cont ro l cyc le of holding/reduc-
ing/ increasing brake pressure is repea ted until the slip
value for the wheel has been brought back into the ac-
cep tab le window.
This is obviously a simplified explanat ion of how ABS
works and the algori thms are in fact very complex and
will vary from one ABS implementat ion to another . When
you remember that this algori thm must be execu ted on
all wheels in jus t a few mil l i seconds , it is not surprising
that ABS is among the most demanding mic rocon t ro l l e r
appl ica t ions .
An important point worth discussing about ABS is that it
is one of the most safety cr i t ica l p r o c e s s o r appl icat ions
in ex i s t ence . The c o n s e q u e n c e s of a faulty ABS sys tem
could be potent ial ly d isas t rous if the brakes were pre-
vented from operating, or were applied er roneously . For
this reason ABS manufacturers take great ca re in the
safety a spec t s of the sys tem design. It is not uncommon
for two identical mic rocon t ro l l e r s to be implemented,
running the same software in parallel and cont inual ly
checking each o ther via a communica t ion pro tocol for
any e r roneous operat ion.
Another solution to this problem is to have a s impler
( lower c o s t ) s lave ( that ac t s as a watch-dog for the
main ABS m i c r o c o n t r o l l e r . Th i s s lave dev i ce is pro-
105
Auto electronics projects
grammed to monitor the major act iv i t ies of the mas te r
and it has the abil i ty to shut down the ABS sys tem if
a fault is de tec ted , thus revert ing full braking cont ro l to
the driver.
A subjec t worth mentioning here is t ract ion control . Trac-
tion cont ro l is a fairly recen t development and can be
thought of as applying ABS in reverse . The idea of t rac-
tion cont ro l is to prevent wheel-slip due to e x c e s s power
on loose surfaces by applying a braking force to the slip-
ping w h e e l ( n o t e t h a t t r a c t i o n c o n t r o l is o n l y
implemented on the driven whee l s ) . This feature is a
natural progress ion for ABS, as all the n e c e s s a r y com-
ponents and measurements required for t rac t ion cont ro l
are inherent in the ABS sys tem — excep t some means of
applying a braking force when the driver is not depress-
ing the b rake pedal . Th i s is usual ly a c h i e v e d via an
e lec t r i c pump arrangement .
With the cons ide rab le improvement in safety provided
by ABS, there can be little doubt that the next few years
will s ee this sys tem becoming more popular, poss ib ly
becoming a s tandard feature on all but the lowest co s t
ca r s .
The future
Hopefully this chap te r will have given the reader some
insight into the fascinating and challenging appl icat ions
for mic rocon t ro l l e r s in automotive appl ica t ions . It has ,
of course , been imposs ib le to cove r all of the applica-
t ions l isted earl ier in this chapter , or even to cover some
106
Microcontrollers
of t hose we have in great t echn ica l depth (engine man-
agement or ABS themse lves could each fill a text book) ,
but the se lec t ion chosen has shown just how varied in
complexi ty the automot ive mic rocon t ro l l e r applicat ion
can be .
As a finishing thought, it may be worth pondering what
t h e fu ture h o l d s for e l e c t r o n i c s , and p a r t i c u l a r l y
mic rocon t ro l l e r s , in ca r s .
Perhaps the next major advance, one which all the ma-
j o r v e h i c l e manufac tu re r s and s t anda rds b o d i e s a re
working on, is the multiplexed wiring system. As the e lec-
t r i ca l c o n t e n t of v e h i c l e s e s c a l a t e s even higher , the
weight and cos t of all the in te rconnec t ing cab le s is be-
coming a major concern , and the number of e lec t r ica l
c o n n e c t o r s poses a rel iabil i ty problem — most veh ic le
breakdowns are due to e lec t r ica l faults. The concep t of
the mult iplexed wiring sys tem is to use a very high per-
f o r m a n c e s e r i a l c o m m u n i c a t i o n s n e t w o r k b e t w e e n
intelligent and semi-intelligent modules s ta t ioned at stra-
tegic points around the veh ic le . This means that only
power and the serial link need be dis t r ibuted about the
car — all the loads have shor t connec t ions to the near-
est intell igent sub-module.
The poss ib i l i t ies of this sys tem are enormous; the en-
gine management sys tem could talk to the e l ec t ron i c
gearbox cont ro l le r and to the ABS/ t rac t ion cont ro l sys-
tem. No longer would turning on your l ights s imply
connec t power direct ly to the bulb — it would signal one
unit to send a command to another unit, instruct ing it to
turn on the bulb using a Smart Power device .
107
Auto electronics projects
This scenar io is not fantasy, it is going to happen and
because the microcont ro l le r has a p lace at the hear t of
every one of t hese intelligent modules, it is safe to say
that its future in the automotive market is very secu re
indeed.
108
4 Car battery monitor
Any number of things from a faulty a l te rnator to left-on
headlights (or s idel ights , even) can result in a flat bat-
tery — and the first you are likely to know about it is
when you turn the key one morning and the car won't
start! This car ba t te ry monitor is a useful little unit de-
signed to warn you in advance by displaying the bat tery ' s
s ta te of charge with a row of ten LEDs.
The moni tor consumes a miser ly 20 mA (it would take
2000 hours to d ischarge a 40 Ah ba t t e ry ) , so it can be
left permanent ly connec t ed to the ba t te ry if required.
Alternatively, it could be connec ted to the ancillaries side
of the ignition switch.
109
Auto electronics projects
The car ba t tery monitor will even reveal faults like a slip-
ping fan-belt: a problem which prevents the ba t te ry from
charging proper ly , but l eaves the da shboa rd ba t t e ry
warning light off. It will even show how the ba t t e ry is
handling the s t renuous work of s tar t ing the ca r (did you
know it takes some twenty minutes of driving to put back
what a five-second s tar t takes ou t? ) .
Circuit
The heart of the monitor circuit (Figure 4 .1) is the LM3914
bar-graph driver IC, used to drive a row of red, orange
and green LEDs which toge ther indicate a magnitude of
the ba t te ry charge vol tage in ten s teps , approximately
V2 V each s tep from 9 V to 14 V. The IC conta ins an input
buffer, a potential divider chain, compara to r s , and an
accura t e 1.2 V reference source . Logic is a lso included
which gives the c h o i c e of bar or dot-mode operat ion —
the la t ter is used in this appl icat ion. The compara to r
causes the LEDs to light at 0.12 V intervals of the input
voltage. TRI ac t s as an amplified diode, raising the lower
end of the divider chain and the negative terminal of the
re ference source (ICI pins 4 and 8 ) to 1.9 V. The upper
end of the chain at ICI pin 6 is connec t ed to a re ference
sou rce output vol tage of approximate ly 3.1 V from pin 7.
The potential divider formed by R l and RV1 a t tenuates
the supply voltage and produces the signal input to the
compara tor , such that a supply range of 9 - 1 4 V covers
the span of the divider chain and is indicated over the
whole of the ten segment LED display. The LED bright-
ness is held cons tan t by an internal cons tan t current
source .
110
Car battery monitor
Construction
Component pos i t ions and printed c i rcu i t board t rack
layout is shown in Figure 4.2. Construct ion of the projec t
is s traightforward: first fit the res i s to r s R l to R3 (so lder
and crop as you go) ; next insert the two prese t s , then fit
both printed c i rcui t board pins from the t rack side us-
ing a hot soldering iron to push them home. Now fit the
IC socke t and t rans i s to r T R I . Note that the t rans i s tor
package is not the same as the legend on the PCB — see
Figure 4.1 for pinout detai ls . Next, identify the polari ty
of each LED. Hold the LED with the ca thode towards you
( the ca thode is the shor t e r lead and adjacent to the flat
on the lower side of the package) , then with the aid of
long-nosed pliers bend the leads downwards through 90
degrees at a point approximate ly 5 mm from the body
TR1 B C 5 4 8
TOP view
H t
i e. c i
LM3914
PIN VIEW
D1-3 red D4-7 orang* 08-10 green
TIL 209
RV1
BOARD OUTLINE
Figure 4.1 Ci rcu i t diagram
111
Auto electronics projects
Figure 4.2 PCB
( s ee Figure 4 . 3 ) . Each LED is inser ted from the compo-
nent side of the PCB then soldered in posi t ion to c r ea t e
a line of LEDs at the same d is tance from the edge of the
PCB. Fit in the following order: D1-D3 red, D4-D7 orange,
D8-D10 green. Lastly insert ICI into its socke t .
112
Car battery monitor
Figure 4.3 LED lead forming
The next j o b is to drill the holes in the box. Cover the
box with masking tape, as this helps with marking out
the holes and prevents sc ra tch ing the box, and it a lso
provides a non-slip surface to prevent the drill bit mov-
ing. See Figure 4.4, for hole posi t ions . After having drilled
all the holes , the PCB can be fitted into the box using
two M3 χ 16 mm sc rews , with two M3 χ V4 inch space r s
under the PCB to posi t ion it at the c o r r e c t height, and
the PCB secured with M3 nuts ( s ee Figure 4 . 5 ) . The zip
wire should now be soldered to the veropins ; fit the Ρ
clip to the zip wire and secure it to the M3 χ 16 mm screw
using a s econd M3 nut. Having fitted the zip wire, insert
the fuse holder in the posi t ive (+) supply line as c lo se to
the ba t te ry as poss ib le ; s ee Figure 4.6 for a s sembly in-
s t ruc t ions . The fuse is included to pro tec t the ba t te ry
wiring in the event of a shor t c i rcui t . The unit is now
comple te and ready for ca l ibra t ion.
113
Auto electronics projects
Zip wire exit hole'
12 ,
γ — I
Bock
20
(or
PCB mounting
(insiovTbox)
Hole dato: A - # 3mm Β *• # 3.5mm C « 0 4.5mm
Figure 4.4 Box dr i l l ing details
12
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In mm
IS
Back Track Component
side side
Lid Screws
Figure 4.5 Box and PCB assembly
<-Solder C U M to wires - »
Λ Λ - φ H=»HHHH |frJ«=fttttttt
t t Fuse Spring
Figure 4.6 Fuse holder assembly
114
Car battery monitor
Calibration
Connect the ba t te ry moni tor to a fully charged bat tery ,
or preferably use a var iable vol tage bench PSU set to
14 V. If this is not poss ib le c o n n e c t a ba t te ry charger to
the charged ba t te ry and switch it on to ensure that a
real 14 V level is achieved (maximum output from a ca r ' s
charg ing/ba t te ry sys tem while running is 14 V, not 12 V ) .
Take note — Take note — Take note — Take note
Connecting the battery monitor to the supply the
wrong way round will result in permanent damage
to ICI!
Set your mult imeter to the 2 V range, connec t the com-
mon (b l ack ) lead to 0 V, and the posi t ive ( red) lead to
pin 8 of ICI . Using a screwdriver , adjust RV2 until the
vol tage on the mult imeter reads 1.9 V. Remove the me-
ter leads, then adjust RV1 until the top end green LED
lights. The ba t te ry moni tor is now ca l ibra ted . All that is
left to do now is to sc rew the back cover on to the box,
and fit it into your car . Quickst ick pads are supplied with
the kit to mount the box onto the dashboard if required,
and r emember to s e c u r e the wiring away from hot or
moving parts using cab le t ies (order code B F 9 1 Y ) as ac-
c i d e n t s c an be e x p e n s i v e if no t d a n g e r o u s . Happy
motoring!
115
Auto electronics projects
T a k e n o t e — T a k e n o t e — T a k e n o t e - T a k e n o t e
I n c e r t a i n i n s t a n c e s t h e r e i s a n a p p a r e n t p o s -
s i b i l i t y o f d a m a g e t o t h e L M 3 9 1 4 I C i n s o m e
v e h i c l e s , d u e t o h i g h v o l t a g e s p i k e s b e i n g
p r e s e n t o n t h e s u p p l y l i n e * T h e o p t i o n a l 1 5 V
z e n e r d i o d e m a y b e f i t t e d a c r o s s t h e s u p p l y
p r i n t e d c i r c u i t b o a r d p i n s a s s h o w n i n F i g u r e 4 . 7
t o m a k e s u r e t h i s d o e s n o t o c c u r .
Figure 4.7 Adding zener diode protection to the module
L O O O O O O o> m no Γι% Γ\Λ ηκ ne 2D1 D2 D3 D4 D5 D6
PCB
116
Car battery monitor
Car battery monitor parts list
Resistors — 0.6 W 1% metal film
117
RI 27 k 1 (M27K)
R2 lk2 1 (M1K2)
R3 15 k 1 (M15K)
RV1 10 k hor e n d prese t 1 (UH03D)
RV2 47 k hor e n d prese t 1 (UH05F)
Semiconductors
D l - 3 mini LED red 3 (WL32K)
D4-7 mini LED orange 4 (WL34M)
D 8 - 1 0 mini LED Green 3 (WL33L)
T R I BC548 1 (QB73Q)
ICI LM3914 1 (WQ41U)
Miscellaneous
batt mon PCB 1 (GA19V)
DIL socke t 18-pin 1 (HQ76H)
box 301 1 (LL12N)
zip wire 3 mtrs (XR39N)
Ρ clip 78 in 1 ( JH21X)
M3 χ V4 in s p a c e r 1 pkt (FG33L)
M3 χ 16 mm sc rew 1 pkt ( JD16S)
M3 nut 1 pkt (BF58N)
quicks t ick pad 1 strp (HB22Y)
Auto electronics projects
in-line fuse holder
1 74in 100 mA fuse
c o n s t r u c t o r s ' guide
1 mm PCB pins
instruct ion leaflet
15 V 1.3 W zener diode
(RX51F)
( W R 0 8 J )
(XH79L)
pkt (FL24B)
(XK10L)
(QF57M)
All of the above avai lable as a kit
ba t te ry moni tor kit (LK42V)
118
5 Car digital tachometer
In t hese days of ever-higher motoring c o s t s the unit de-
sc r ibed here will help the driver to change gear at the
most advantageous point to save fuel and extend engine
life. Anyone using a car to tow a t rai ler or caravan will
a lso benefit by being able to make the bes t use of the
torque avai lable from the engine.
Conventional t achomete r s give a display of engine speed
on a mil l iammeter, usually with a s ca l e of about 270° a rc .
Pulses produced by the act ion of the con tac t b reakers
are integrated and fed to the meter to give an analogue
display of engine revolutions. The disadvantages are that
(1 ) an average reading is displayed, which can easi ly lag
behind rapid speed changes , and ( 2 ) meters tend to be
somewhat fragile.
119
Auto electronics projects
The t achomete r descr ibed here ove rcomes both of these
disadvantages by counting pulses and displaying engine
revolut ions over a very shor t t ime, as the digital display
is cont inuously updated. Two digits display the number
of revolut ions χ 100, hence a display of 99 would cor re-
spond to 9,900 r.p.m. On the o ther hand, as the unit only
has a two digit display, the reading could be in er ror by
as much as 100 r.p.m. compared with true engine speed.
The unit is designed for negative earth ca r s , and if you
are not sure of the polari ty of your car , a glance at the
owners manual or even at the ba t te ry connec t ions will
tell you.
Circuit
The comple te c i rcui t is shown in Figure 5 .1 . Pulses pro-
duced by the make-and-break action of the engine contac t
breaker points are fed to ICla , which is a dual Schmit t
t r igger m o n o s t a b l e , via a r e s i s t o r / c a p a c i t o r ne twork
composed of R l , R2 and CI . This network helps to smooth
out any high vol tage spikes which may be present on the
con tac t breaker pulses . The zener diode ZD1 limits the
input pulse at IC la to 4.7 vol ts , to avoid any damage to
the device . To prevent any false triggering due to con-
tact point bounce (produced when the points do not open
and c lose c leanly) the monos tab le period is se t to 3 mil-
l i seconds by R3 and C2, after which it is ready to be
retr iggered by the next pulse, and this t ime period al-
lows for the maximum count for a 4-stroke, 4-cylinder
engine of 10,000 r.p.m. — a speed not often at tained on
normal road cars! The maximum count of 10,000 r.p.m.
(- 100 r.p.m.) co r responds to 20 ,000 pulses /minute and
the t ime for 1 pulse is 60 /20 ,000 s econds or 3 ms. A high
120
Car digital tachometer
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Auto electronics projects
engine speed would the re fo re not al low enough t ime
between pulses for triggering the monos tab le . This de-
sign is for 4-cylinder ca r s only and anyone using it on a 6
or 8-cylinder car would have to modify the count period
accordingly, or use a compensa t ion factor on the read-
ings — not easy while driving!
The output pulses from ICla , pin 12, are fed to the count
input, pin 11, of IC2. This is a 4 digit coun te r with both
latch and rese t . It drives a mult iplexed 2 digit display
directly, with t rans is tors TRI and TR2 select ing the digit,
and res i s to r s R 4 - R 1 0 limiting the segment current . The
counte r requires latch pulses in order to give sens ib le
readings and these are provided by IC3, TR3 and their
a s soc ia t ed componen t s . IC3 is the ever useful 555 , used
as an osc i l la tor whose f requency is cont ro l led by RV1.
The osci l la tor output waveform, arranged such that there
is a long high and a shor t low period, is inverted by TR3
so that a shor t high period is achieved . This shor t pulse
is used to cont ro l the latch of the coun te r device IC2, so
that when this input goes high the information in the
counter is transferred to the internal latch and displayed.
The trailing edge of this shor t pulse is used to tr igger
the monos tab le IClb , whose output pulse is used to re-
set the counter so that it will s tart counting from 00 again.
The use of a separa te monos tab le to rese t the counter
ensures that the rese t pulse always occurs after the latch
pulse and a true reading displayed.
B e c a u s e the supply vol tage of a car , nominally 1 2 - 1 4 V,
varies between these limits with engine speed, integrated
circui t IC4 is used to derive a regulated 5 V supply from
this , which is then used to supply ICI , IC2 and IC3 and is
important for the s tabi l i ty of the osc i l la tor ( IC3) . Diode
D2 and capac i to r C5 remove any noise on the supply.
122
Car digital tachometer
Construction
T h e Digital T a c h o m e t e r is cons t ruc t ed on two PCBs; the
main board and the display board. The display board is
mounted at 90° to the main board by being soldered to
Veropins , and this holds the display so that it can be
viewed through the cut-out display window at the end of
the ca se . Figure 5.2 shows the cons t ruc t iona l detai ls .
First j ob , however, is to build up the main printed cir-
cuit board. Referring to Figure 5.3, begin by fitting the
smal les t componen t s first. Check the polari ty of C3, C5,
and the direct ion of Dl, D2 before fitting, then work your
way through the componen t s by size fitting C5, the larg-
est , last . Insert the ICs into the appropr ia te socke t s only
Hain Board
Figure 5.2 Preparing the veropins for attaching main and
display PCBs together
123
Auto electronics projects
Figure 5.3 Digi ta l tachometer main PCB and legend layout
after all o ther cons t ruc t ion of the t achome te r module is
comple ted , taking the usual precaut ions with CMOS de-
v ices . Note that the negative end of C5 must be c lo se to
the PCB or you may find that adjusting RV1 is difficult
during cal ibrat ion!
Display board
Refer to Figure 5.4. First mount res i s to r s R4, R5, R9, RIO,
and the veropins from the component s ide, being care-
ful not to s t r ip the pads off the PCB in the p rocess ! A hot
soldering iron will help to push the pins home. Don't
124
Car digital tachometer
forget to fit the wire link ( this can be made from an off-
cut from a res is tor , or with single-strand wire) . Solder
and crop the res i s to r s and the wire link. Next fit the dis-
play to the PCB; pins 1 to 9 are on the side where the
decimal point will be found, and pins 1 and 18 are marked
on the PCB. Solder and c rop pins 1 to 18. Now measure
the required length for the display board pins by offer-
ing the display board up to the main PCB, 3 - 4 mm is
about right; s ee Figure 5.2. If the pins are too shor t , the
connec t ions won't be mechanica l ly s trong. After trim-
ming the pins down you can solder the display board to
the main PCB. The pins on the display board also require
soldering, and if this has a l ready been done, you may
find that the two boards do not marry snugly to each
other .
Figure 5.4 D isp lay board layout and legend
Use m a i n s c o n n e c t i o n wi re for p o w e r s u p p l y and
sc reened wire for the input signal so ldered to the three
pins. Sc reened cab le is used to s top the emiss ion of RF
interference, and the outer screening must be connec ted
to earth, preferably at the HT coil end. Label the func-
tion of each wire at the end that will connec t to the car
e l ec t r i c s . If you are going to use the optional box, the
front panel of the c a s e is rep laced by a p iece of red filter
125
Auto electronics projects
cut to size (using the original panel as a t empla te ) with a
pair of s c i s s o r s or craft knife. This s lo ts neat ly into the
case , which is moulded in two sec t ions . As you may have
not iced, there is no method of mounting the t acho mod-
ule into the suggested box, so the a l ternat ive is to use
qu icks t ick pads. The sugges ted box a lso needs to be
modified by removing part of the ba t te ry compar tment ;
only the top and front part i t ions of this need to be re-
moved, the s ides will help to keep the PCB centra l in the
box, and the sc rew holes must remain intact or e lse the
box cannot be fastened toge ther ( s e e Figure 5 .5 ) .
12mm, 30mm
Cut out shaded area of battery compartment wall to leave box fixing holes.
This will allow the PCB, pins and large capacitor
to pass through.
Figure 5.5 Box modification details
126
Car digital tachometer
Setting up
One advantage of a digital t a chome te r over an analogue
type is the ease of setting-up and cal ibra t ion. Only one
adjustment to RV1 needs to be made and, barring acc i -
dents , will prevail for the life of the unit. This set t ing
ensures that the osc i l la tor runs at the c o r r e c t f requency
and the method of cal ibrat ion depends on the equipment
available. Calibration against another t achomete r is pos-
s i b l e , s e t t i ng RV1 to give a d i sp lay of 30 when the
s tandard t achome te r reads 3000 r.p.m. If you have ac-
c e s s to a signal genera tor , se t the frequency to 100 Hz
(s ine or squarewave) and the output level to maximum
(more than 4.7 V ) . Connect this signal to the I/P pin on
the PCB, and again this will s imulate an ignition pulse
train input of 3000 r.p.m.
Alternatively cal ibra t ion can be carr ied out against the
mains frequency by using a t ransformer and bridge rec-
tifier to provide a 100 Hz signal as shown in Figure 5.6,
Signal to Tachometer
OV
Figure 5.6 Mains frequency doubler for cal ibrat ion
127
Auto electronics projects
and a ba t te ry charger is very effective in this role . In
e i ther c a s e RV1 is adjusted to give a display reading of
30 . Calibration should include a tes t run for up to half an
hour or so for warming up and s tabi l isa t ion, whereafter
it might be noted that RV1 requires further fine tuning.
When you are satisfied with the ca l ibra t ion of the coun-
ter, RV1 should be fixed in posi t ion with wax, paint or
nail varnish.
Fitting the unit into the car
After cal ibrat ion, the unit is ready to be fitted to the car .
It is imposs ib le to give detai led ins t ruc t ions for every
car but the following notes may be helpful.
• it is a good idea to try the unit in various posi t ions
for bes t readabil i ty, using adhes ive tape, until you are
satisfied,
• having decided on the bes t posi t ion use double-
sided tape, adhesive pads or two p ieces of velcro- tape,
one glued to the unit and one glued to the car dashboard .
All of t hese methods , of course , mean that the unit can
be removed easi ly and the dashboard c leaned and left
unmarked,
• al ternatively, use self-tapping sc rews through one
half of the c a s e into the dashboard . This works well, but
unless you can util ise exist ing sc rew holes you will be
left with holes in the dashboard if you decide to remove
the unit.
128
Car digital tachometer
The th ree leads must pass into the engine compar tment
and it is important that they are p ro tec ted by a rubber
or plas t ic grommet . It may be poss ib le to squeeze them
through an exist ing cab le entry or you may have to drill
a new hole, but e i ther way make sure they are p ro tec ted .
Connect ion to the car e l ec t r i c s is fairly straightforward;
the t acho input lead is connec t ed to the CB terminal of
the HT coil , which can be identified by the lead from the
points and dis t r ibutor to the HT coi l . CB s tands for con-tact breaker, often marked with a ( - ) minus sign. The
supply would bes t be taken from the ignition switch via
a 100 mA fuse, so that the unit is swi tched off when the
ca r is not running. The eas ies t way of doing this would
be to follow the o ther lead from the HT coil (marked with
a (+) plus s ign) up to the bal las t r e s i s to r (if f i t ted), and
make the connec t ion to the o the r s ide of it, s e e Figure
5.7.
Take note — Take note — Take note — Take note
Not all ignition systems are the same so consult
your workshop manual before attempting to fit
the tacho. Also please remember that a car en-
gine compartment is a hazardous area — never
attempt to fit the tachof or anything else for
that matter, while the engine is running! Also,
secure all cables away from hot or moving parts !
Anchor them to existing wiring looms using ca-
ble ties.
129
Auto electronics projects
To starter motor
solenoid switch
Γ 2 ί
Tachometer +/N connection here
Contact To breaker
distributor points
Tachometer CB connection here
Ballast resistor
Figure 5.7 Connecting tachometer to a typical ignit ion system
with contact breaker
130
Car digital tachometer
Car digital tachometer parts list
Resistors — All 0.6 W 1% metal film
R l , 2 , 5 6 0 Ω
R3 100 k
R 4 - 1 0 150 Ω
R H 390 k
R12 1 k
R 1 3 - 1 5 10 k
RV1 100 k ver t encl p rese t
2 (M560R)
1 (M100K)
7 (M150R)
1 (M390K)
1 (M1K)
3 (Ml OK)
1 (UH19V)
Capacitors
CI 100 nF polyes te r 1 (BX76H)
C2 47 nF poly layer 1 (WW37S)
C3 1 μΡ 35 V tantalum 1 (WW60Q)
C4 2n2F mylar 1 (WW16S)
C5 1000 μΡ 16 V axial 1 (FB82D)
C6 10 nF 50 V disc 1 (ΒΧ00Α)
Semiconductors
ICI 74LS221
IC2 74C925
IC3 NE555
IC4 LM78L05ACZ
TR1-3 BC549
ZD1 BZY88C4V7
D2 1N4001
DY3 DD display type C
1 ( Y F 8 6 T )
1 ( Q Y 0 8 J )
1 (QH66W)
1 (QL26D)
3 (QQ15R)
1 (QH06G)
1 (QL73Q)
1 ( B Y 6 8 Y )
131
Auto electronics projects
Miscellaneous
8-pin DIL socke t
16-pin DIL socke t
dig t acho main PCB
dig t acho display PCB
1 mm PCB pin
red display filter
cab le grommet
twin mains DS black
cab le single black
quickst ick pads
c o n s t u c t o r s ' guide
instruct ion leaflet
1 ( B L 1 7 T )
2 (BL19V)
1 (GA26D)
1 (GA27E)
1 pkt (FL24B)
1 (FR34M)
1 (LR47B)
3 mtrs ( X R 4 7 B )
3 mtrs (XR12N)
1 (HB22Y)
1 (XH79L)
1 (XK03D)
All of the above available as a kit
car digital t a chome te r kit 1 (LK79L)
Optional (not in kit)
small remote cont ro l box 1 (LH90X)
in-line fuse holder 1 (RX51F)
1 74 in 100 mA fuse 1 (WR08J )
ve lc ro mounts as reqd (FE45Y)
cab le tie-wrap 100 as reqd ( B F 9 1 Y )
132
6 Car lights-on warning indicator
If your car is not fitted with some kind of lights-on warn-
ing, the c h a n c e s are that you will at some t ime (if you
have not a l ready done so!) leave your lights swi tched
on. Murphy's law d ic ta tes that when you do so , your
a b s e n c e from the car will be of sufficient duration to
ensure that the ba t t e ry will be well and truly flat. Of
cou r se Murphy, not con ten t to do things by halves , will
ensure that it happens when you are late for some im-
portant occas ion and that the re is no one e lse around to
give you a push or a jump start!
Modern ca rs further aggravate the si tuat ion as many of
them, being fitted with e l ec t ron ic ignition or e l ec t ron ic
engine management sys tems, just plain refuse to be push-
star ted!
133
Auto electronics projects
It is amazing that such mechanica l ly advanced cars of-
ten do not have a lights-on warning indicator of some
kind. To i l lustrate this , the pro to type was instal led in a
2.0 li tre petrol- inject ion Ford Sier ra Es ta te — despi te
being a Ghia, there was no lights-on warning device!
Various warning devices are avai lable, however, some
b e c o m e a nu i sance b e c a u s e they sound cont inuous ly
when the lights are de l ibera te ly left on. For ins tance ,
while the driver is waiting in the car at night, with the
engine swi tched off.
Some more sophis t i ca ted devices will not sound if the
lights are swi tched on again after the ignition has been
switched off, i.e. for parking lights. However, this fails
to warn the driver if he inadvertent ly knocks the light
switch on when leaving the car — as could be the c a s e
with many ca r s having the light switch stalk on the driv-
er ' s door side of the s teer ing column.
This lights-on warning indicator will emit a c lear ly audi-
ble buzzing sound when the car lights are left on, the
ign i t ion s w i t c h is tu rned-of f and t h e d r i v e r ' s d o o r
opened. In this manner the buzzer will only sound when
the driver is genuinely about to leave the car .
Now that you are thoroughly convinced that for the sake
of a few pounds, you need not be caught out in the fu-
ture, why not build this handy a c c e s s o r y (which the
manufacturer should have included as s tandard) and fit
it into your car? Enterprising readers may wish to offer
this add-on to friends, re la t ives and neighbours for a
sui table fee (don ' t forget to tell the tax man!). A personal
tale of woe and the a s su rance that, " / V e got one and it has stopped me from getting caught out againF is sure
to win a few favourable r e sponses .
134
Car lights-on warning indicator
Circuit description and operation
The circui t of the lights-on warning indicator is very sim-
ple , as can be s e e n from Figure 6 . 1 . However , it is
worthwhile to know how the c i rcui t ope ra tes as this will
help, should problems occur .
P I of the unit is connec t ed to the sidelight c i rcui t of the
car and provides power to the circuit only when the lights
are swi tched on. The sidelight c i rcui t is live when e i ther
s idel ights or headlights are swi tched on.
P2 is connec t ed to the a c c e s s o r y c i rcui t and when the
ignition switch is off, P2 is pulled low via res i s to r R3 (P3
is c o n n e c t e d to OV) . Diode Dl is forward b iased and
turns on t rans i s to r T R I via res i s to r R2. Note that the
internal r e s i s t ance of a c c e s s o r i e s ( i .e . r ad io -casse t t e )
may be sufficiently low to make the connec t ion to P3
unnecessary; this can be determined by experimentat ion.
P6 is connec t ed to the dr iver 's door switch, thus when
the door is opened, a comple te path to 0 V is provided
by the door switch, allowing the buzzer to sound.
P I
ο
J L LN4001 ^ R3 ^
Γ Ρ" P3 P6
Figure 6.1 Ci rcu i t diagram
135
Auto electronics projects
When the ignition switch is on, P2 is pulled high, reverse
biasing diode D l . Res is tor R3 ensures that t rans is tor TRI
is held in the off s ta te . The posi t ive supply to buzzer
BZ1 is removed and thus prevents it from sounding, re-
gardless of whether the driver 's door is open or shut.
When the lights are off and the car doors are c losed , the
polari ty of the supply to the unit is effectively reversed .
Diode D2 prevents damage to the circui t under this con-
dition.
Construction
Assembly of the unit is s implici ty itself. Referring to Fig-
ure 6.2, it is advised that the PCB pins are fitted first,
followed by the res i s to r s and the diodes and finally the
t rans is tor . Make sure that the t rans i s to r is fitted fairly
c lo se to the PCB otherwise the PCB will not fit into the
ca se .
Next solder the buzzer 's wires to the PCB pins, red (+V)
to P4, black ( - V ) to P5. Attach the connec t ing wires to
the PCB pins and label the free ends so that you can iden-
tify the wires after the PCB has been fitted into the case !
The PCB simply lies in the case , the wires protruding
through the aper ture provided. Screw the c a s e toge ther
Figure 6.2 PCB legend and track
136
Car lights-on warning indicator
and affix the buzzer onto the lid of the c a s e using one of
the double-sided adhes ive pads. The o ther pads can be
placed onto the unders ide of the c a s e ready for fitting
into the car .
Although it is unlikely that there will be any problems
with the unit, it is advisable to tes t it before fitting into
the car . It is eas ie r to take remedial ac t ion on the work
bench than underneath the car dashboard! Using a 9 to
14 V supply ( i .e . PP3-sized bat tery , ba t te ry el iminator ,
e t c . ) connec t P3 and P6 to 0 V, then connec t PI to +V,
the buzzer should sound. Connect P2 to +V as well, this
should s i l ence the buzzer.
Refer to Figures 6.3, 6.4, 6.5 and 6.6. It is n e c e s s a r y to
gain a c c e s s to the ca r ' s wiring, which will undoubtedly
involve removing the unders ide of the dashboard , trim
Installation
D r i v e r s d o o r s w i t c h
i n l i n e f u s e
• C h a s s i s c o n n e c t i o n
m a y n o t b e r e q u i r e d
S e e t e x t
Figure 6.3 PCB connections
137
Auto electronics projects
panels , e t c . It is advisable to refer to a workshop manual,
e.g. of the Haynes variety; if you do not have one, e i ther
buy one — as it will be useful anyway, or borrow one
from your local library. A workshop manual will a lso help
you to ascer ta in the c o r r e c t wires to connec t to — oth-
erwise it will be a c a s e of t racing the c o r r e c t wires with
a mult imeter .
Take note - Take note - Take note - Take note
Disconnect the car battery before making con-
nections to the wiring* Connections to existing
wiring can be made using snap lock connectors or
terminal blocks of adequate current rating —
remember the lights-on unit draws very little
current, but two 55 W headlamp bulbs draw con-siderably more ! Ensure that the new wiring will
not become entangled with any controls, espe-
cially the brake pedal and steering column. To
prevent short circuits, make sure that all con-
nections are properly insulated, use adhesive
electrical tape.
Connect P I , via a fuse and fuseholder, to a point in the
wiring which b e c o m e s l ive when t h e s i de l i gh t s a re
swi tched on (Figure 6 .4 ) .
Connect P6, to the driver 's door switch (Figure 6 .5 ) . To
prevent o ther doors from operat ing the buzzer, install
an MR751 diode in se r ies with the wire to the cour t e sy
light.
138
Car lights-on warning indicator
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Auto electronics projects
Connect P2 to a point in the wiring which b e c o m e s live
when the ignition switch is turned to accessory, i.e. +V
supply to the radio (Figure 6 .6 ) . Alternatively, if there is
no accessory posit ion, connec t P2 to a point in the wir-
ing which becomes live when the ignition switch is turned
to ignition.
Connect P3 to the ca r ' s chass i s (0 V) or to a point in the
wiring which is permanently connec ted to the ca r ' s chas-
sis . Note that this connec t ion may be unnecessa ry if the
internal r e s i s t ance of any a c c e s s o r y is sufficiently low.
This may be ascer ta ined by test ing the unit with P3 left
unconnec ted and all a c c e s s o r i e s switched off. If in doubt
connec t P3 as previously desc r ibed .
Double-check connec t ions , r e connec t the car bat tery .
COURTESY LIGHT
+ 1 2 V Permanent
supply O -Fuse
- 0 — o ^ c Door
Of f r N.C. ι Ο η Ο η
Drivers I — ° °~ door switch
Other d o o r switches
4 Γ
Cut wire and insert diode
^ i ( M R 7 5 1 )
To P6 of Lights on PCB
Figure 6.5 Typical ignit ion switch c i rcu i t and connections
140
Car lights-on warning indicator
IGNITION SWITCH
fc Permanent (~"^+12V supply
Start To starter solenoid
Fuse Ignition Ignition switched 5 + 12V supply
V Off
Accessory To P2 of lights on PCB
Car 12V
Battery
I JCTJ Radio
Figure 6.6 Typical ignit ion switch c i rcu i t and connections
Φ switch lights on, leave ignition swi tched off and
open the driver 's door; the buzzer should sound,
• with the driver 's door shut, opening any other door
should not cause the buzzer to sound,
• with the ignition swi tched to accessory or ignition, opening the driver 's door should not cause the buzzer
to sound,
• with lights turned off, the buzzer should not sound
with any combina t ion of ignition swi tch pos i t ions or
doors open or c losed .
Assuming the unit is working cor rec t ly , refit underside
of dashboard and trim panels . Happy motoring.
Testing
141
Auto electronics projects
Lights-on warning parts list
Resistors — all 0.6 W 1% metal film
R l 3k9 1 (M3K9)
R2 10 k 1 (M10K)
R3 100 k 1 (M100K)
Semiconductors
Dl 1N4148
D2 1N4001
T R I BC327
MR751
1 (QL80B)
1 (QL73Q)
1 (QB66W)
1 (YH96E)
Miscellaneous
BZl 12 V buzzer
P l - 6 1 mm PCB pins
1 V4 in in-line fuseholder
1 74in 100 mA fuse
PCB
mini box and base
quickst ick pads
instruct ion leaflet
c o n s t r u c t o r s ' guide
All the above available as a kit
lights-on warning kit
1 (FL40T)
1 pkt (FL24B)
1 (RX51F)
1 ( W R 0 8 J )
1 (GE88V)
1 ( J X 5 6 L )
1 strp (HB22Y)
1 (XT11M)
1 (XH79L)
1 (LP 7 7 J )
Optional (not in kit)
16/0.2 wire as req (FA26D-
FA36P)
snap lock cab le c o n n e c t o r as req ( J R 8 8 V )
5 A terminal b lock as req (HF01B)
142
7 Courtesy lights extender
How many t imes have you got into your car of a night
only to find the ignition switch has gone for a walk around
the dashboard , as your a imless efforts to s tar t the ca r
only result in the ignition key gouging severa l grooves
into the p las t ic?
This pro jec t keeps the interior light on after the car door
has been c l o s e d , allowing t ime to find keys , ignit ion
switch, or even your way out of the garage!
Circuit description
Figure 7.1 shows the circui t diagram of the cour tesy light
extender. PI and P3 connec t directly ac ross a door switch
143
Auto electronics projects
PI
o-
TR1 TIP122
D Rl 4k7 Dl
1N4148
R2 560k
D2
9 1N4148
4k7 1
TR2 MPSA14
9 cb ci •i 47uF
P3 O-
Figure 7.1 Ci rcu i t diagram
control l ing the inter ior light and P2, to a sou rce that has
power while the ignition is on.
With a door open, PI and P3 are effectively shorted, caus-
ing capac i to r CI to d ischarge through diode D l . As soon
as all doors are c losed , as capac i to r CI is d ischarged,
t rans i s tor TR2 is turned off. Res i s to r R l pulls the base
of T R I high, turning it on and causing current to flow
through the cour tesy light. Capaci tor CI now s ta r t s to
charge through res i s tor R2 until t rans is tor TR2 turns on,
pulling the base of t rans i s tor TRI low and halting the
flow of current through the inter ior light. B e c a u s e ca-
paci tor CI charges through res i s to r R2 and the cour tesy
light, it can be seen that the wattage of the inter ior light
plays quite an important role in the t ime delay given.
144
Courtesy lights extender
Figure 7.2 shows typical t ime delays given at var ious
values of R2 for 5, 10, 15 and 20 watt cou r t e sy l ights.
If, during the t ime delay given by the unit, the ignition is
turned on, capac i to r CI charges up very quickly through
res i s to r R3, turning the inter ior light off a lmost immedi-
ately. This avoids the poss ibi l i ty of driving away at night
with the cour t e sy light still on.
Construction
The pos tage s tamp sized printed c i rcui t board is of the
high quality, single-sided glass fibre type, s e e Figure 7.3.
The s e q u e n c e in which the componen t s are fitted is not
cr i t ical ; however, the following ins t ruc t ions will be of
use in making these tasks as straightforward as possi-
ble.
145
50-1 1 j , 1 1 1 1 1 j 1
0" ^^^ —ι » ι ι ι ι ι ι ι ι ι ι ι ι — τ- I t \ I I I I I I ' I I I
0 100k 200k 300k 400k 500k 600k 700k 800k 900k 1M R2/0
Figure 7.2 Graph of R2 against time delay for various wattage
courtesy lamps
Auto electronics projects
Insert and solder the PCB pins using a hot soldering iron.
If the pins are heated, very little p ressure is required to
press them into posi t ion. Once in place , the pins may
then be soldered. It is now easier to start with the smaller
components , such as the res is tors , work upwards in size,
and t rans i s tor TRI is fitted last .
The diodes should be inser ted such that the band at one
end of the diode co r re sponds with the white b lock on
the PCB legend. When fitting the e lec t ro ly t ic capac i to r ,
it is essent ia l that the co r r ec t polari ty is obse rved . The
negative lead of the capac i to r , which is usually marked
by a full-length s t r ipe and a negative ( - ) symbol , should
be inser ted away from the hole marked with a posi t ive
(+) sign on the PCB legend. Insert and solder the two
t rans i s tors , matching the shape of each c a s e to its out-
line on the legend.
Lastly, so lder lengths of wire to the veropins and mount
the PCB inside the box, as shown in Figure 7.4.
Installation
The cour t e sy light ex tender is ex t remely simple to fit.
However, for someone who is not familiar with automo-
tive e lec t r ica l installation it is advised that they seek the
advice of a qualified person before proceeding.
The re are two methods of switching the inter ior cour-
tesy light:
146
Courtesy lights extender
Figure 7.3 PCB legend and track
î Figure 7.4 Fit t ing unit into box
Take note — Take note — Take note — Take note
When carrying out any form of electrical work on
a vehicle always disconnect the battery and
never work inside the engine compartment with
the engine running!
147
Auto electronics projects
• door swi tches are fitted to the 0 V side of the cour-
tesy light, for instal lat ion follow Figures 7 .5(a) or 7 .5 (b ) ,
• door swi tches are fitted breaking the +12 V supply
to the cour tesy light, for installation follow Figures 7 .5 (c )
or 7 .5(d) .
In its s implest configuration, the unit c o n n e c t s direct ly
ac ros s a door switch; PI connec t ing to the more posi-
tive side of the switch and P3 to the more negative.
If ignition override is required then P2 must be connec ted
to a sou rce that has power while the ignition is on (for
example, + SW terminal of the ignition co i l ) . If no easy
connec t ion can be made to the ignition circui t then P2
can be connec t ed into the accessory c i rcui t .
As the comple te unit is small and unobtrus ive it can eas-
ily be mounted inside a door-post , behind an exist ing
door switch. The box can be held in p lace using a self-
adhesive pad (such as HB22Y) or bol ted down using the
two mounting holes provided in the b a s e of the box .
Check behind panels before drilling any holes and en-
sure that no wiring harness or o ther componen t s are
located behind panels that would otherwise be damaged.
Min Typ Max Units
Operating voltage 10 12 15 V
Quiescent current at 12 V 3 mA
Maximum switching current 5 A
Table 7.1 Specif icat ion of prototype
148
Courtesy lights extender
Courtesy +12V
ü3
ht
Fueebox ® I I I I ι lp1 .
y S S J Extender
Courtesy
+ 1 2v LÎQht To ignition switch From QO * j · » 1 P1 (or other source of
Fueebox r-—if-1—ι +12V when engine
(b) P2 {
« running). / / / / o.S°°u Extender
+ 12V From f f 1 1 1 P 1
Fueebox ι „ 'r' ι
Courtesy Ught
β »ιΓ / / / / Extender
( ο m l _ V J L Courtesy ® Light
+12V To ignition swttch From • · 1 τ 1 P1 ( o r other source of
Fueebox r - — i f ' ι +12V when engine
°Ϊ5|Γ P ? "running).
<d) 1 1 1 1 _ r JL Courtesy ® Ught
Figure 7.5 Λ*Γ
(a) Simple connection for vehicles with negative switched
courtesy l ight
(b) Connection for negative switched courtesy l ight with ignit ion
override
(c) Simple connection for vehicles with posit ive switched
courtesy l ight
(d) Connection for positive switched courtesy l ight with ignit ion
override
149
Auto electronics projects
Courtesy light parts list
Resistors — All 0.6 W 1% metal film
R l , 3 4k7 2 (M4K7)
R2 560 k 1 (M560K)
Capacitors
Cl 47 μΡ 16 V minelec t 1 ( Y Y 3 7 S )
Semiconductors
T R I TIP122
TR2 MPSA14
Dl ,2 1N4148
1 (WQ73Q)
1 (QH60Q)
2 (QL80B)
Miscellaneous
PI ,2 ,3 1 mm PCB pins 1 pkt (FL24B)
PC board 1 (GE81C)
min box and b a s e 1 ( J X 5 6 L )
cour t e sy light leaflet 1 (XK96E)
c o n s t r u c t o r s ' guide 1 (XH79L)
All of the above are avai lable as a kit
cou r t e sy light kit 1 (LP66W)
Optional (not in kit)
16/0.2 b lack hook up wire 1 (FA26D)
16/0.2 red hook up wire 1 (FA33L)
150
8 Car audio switched-mode psu
Take note — Take note — Take note — Take note
This project requires a fair degree of expertise and pa-
tience to build. Please read through the article thoroughly
before undertaking construction. The power supply design
described in the following article is intended to power two
Maplin 50 W bipolar amplifiers and a stereo pre-amplifier.
It is possible for the power supply to be used to supply
other amplifiers, however, it is outside the scope of this
article to detail the necessary modifications . The supply
is capable of delivering instantaneous power levels much
higher than the continuous rating, which is ideal for au-
dio applications where the peak current requirement, due
to transients, is much higher than the average current re-
quirement. Higher levels of power may be drawn as long as
the average power is maintained at 120 W. The figure of 120 W
is based on maintaining a heatsink temperature at less than
65°C.
151
Auto electronics projects
For many years the motor is t has not been able to ben-
efit from hi-fi quality sound while travelling in the car .
For the long-distance traveller , bus iness execut ive or hi-
fi buff o n - t h e - m o v e , t h e c a r is a far from idea l
env i ronmen t for l i s ten ing to mus ic ; th i s is due to a
number of r easons . First, the ca r ' s inter ior is designed
for conveying passengers and not for ideal locat ion of
c o n v e n t i o n a l box design l o u d s p e a k e r s . S e c o n d , the
sound replay/receiving equipment has to be miniatur-
i s e d and c a p a b l e of o p e r a t i o n in a v e r y h a r s h
environment. Dashboard temperatures often exceed 60°C
in hot weather (yes , even in the English cl imate!) and
fall to several degrees below zero in cold weather . Vi-
brat ion and humidity also add to the s t r e s s e s that the
equipment must endure. Third, the low, noisy and some-
what v a r i a b l e supp ly vo l t age makes life even m o r e
difficult for the e lec t ron ic c i rcui t ry .
The environmental and size problems of the car envi-
ronment have largely been solved by c lever ly designed
equipment . Car loudspeakers are opt imised for opera-
tion in rear parcel she lves and door panels instead of
conven t iona l sea led or por ted e n c l o s u r e s . Car radio,
ca s se t t e , CD (compac t d i sc ) and DAT (digital audio tape)
equipment is very compac t . Such equipment is designed
for e i ther mounting in the dashboard /cen t re conso le or
remote mounting in the boot or under a seat , with just
the cont ro l s loca ted within the driver 's easy reach .
It is however, the third point that is the main reason for
this projec t , the vehic le e lec t r ica l supply. The 12 V e lec-
tr ical sys tem is far from ideal when it c o m e s to powering
audio amplifiers. The e lec t r ica l sys tem itself, although
general ly referred to as being 12 V, usually opera tes at
152
Car audio switched-mode psu
around 1 3 - 1 4 V when the engine is running. By conven-
tion, the vol tage when the engine is running is assumed
to be 13.8 V.
A singled-ended amplifier operat ing from a supply volt-
age of this ( low) level is capab le of delivering around
7 W r.m.s. into a 4 Ω load. If a BTL (br idge tied load)
amplifier is used the power output can be increased to
around 22 W r.m.s. into a 4 Ω load. Most high power ra-
d i o / c a s s e t t e players have an output power of around
22 W r.m.s., regardless of how many watts the advert is-
ing b rochures boast!
For hi-fi quali ty sound reproduct ion in a car it is neces -
sary to have the capabi l i ty of higher power levels . This
not being required for blowing out the windows (although
often used as such by drivers of ageing Ford Cort inas
with pink fluffy dice hanging from the inter ior rear-view
mir ror ) , but simply b e c a u s e a high power amplifier op-
erating at modes t power levels will in t roduce far less
dis tor t ion and handle t rans ien ts far be t t e r than a me-
dium power amplifier running a lmost flat out. This is
espec ia l ly true if the sound sou rce is CD, where the dy-
namic range of the recording is often very wide.
The re are two ways in which the output power can be
increased , by e i ther decreas ing the loudspeaker imped-
a n c e o r i n c r e a s i n g t h e s u p p l y v o l t a g e . T h e main
disadvantage of the former method is that ca r speakers
are not commonly produced with impedances below 4 Ω
and that power losses in cab le s are increased . The lat-
ter method of increasing the supply voltage is commonly
used in high power car boosters and in hi-fi ca r audio
amplifiers — this is the method that is desc r ibed here .
153
Auto electronics projects
Circuit description
Figure 8.1 shows a b lock diagram representa t ion of the
power supply and Figure 8.2 shows the full c i rcui t dia-
gram.
The supply input to the power supply is via PI (+V) and
P2 (0 V ) . The power supply is connec t ed direct ly to the
vehic le ba t te ry via high current cab les , therefore the off-
board supply fuse FS1 is e s sen t i a l in c a s e of a fault
causing a shor t c i rcui t d i rect ly a c r o s s the bat tery . Re-
mo te power swi tch ing is a c h i e v e d by T R I , RL1 and
a s s o c i a t e d c o m p o n e n t s . T h e con t ro l input P3, when
taken to +V, b i a s e s T R I on and o p e r a t e s RL1 , thus
powering-up the rest of the supply. LD1 serves to indi-
ca t e power on. The cont ro l signal is provided by the
electric aerial output found on most radio-casset te units.
Diode Dl clamps the voltage spike produced by RL1 when
it de-energises . Diode D2 provides polari ty pro tec t ion
by blowing fuse FS2 and preventing the remote power
switch from operating.
Capaci tors CI , C2, C3, C4, C5 and toroid LI form the in-
put ji-filter, the output of which supplies the push-pull
output s tage. The power MOSFETs are arranged in two
pairs (TR2 and TR3) and (TR4 and T R 5 ) , each driving
one half of the t ransformer primary. Res i s to r R8 and ca-
p a c i t o r C6 form a s n u b b e r ne twork to i n c r e a s e t he
rise-time of switching spikes . Components ZD1, D3 and
TR2 and ZD2, D4 and TR5 form an ac t ive spike clamp,
employed to pro tec t the MOSFETs' d ra in / source junc-
t ions from high voltage switching spikes . This opera tes
by feeding the spike back into the gate of the relevant
154
Car audio switched-mode psu
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Auto electronics projects
Figure 8.2 C i rcu i t diagram of the switching PSU
156
Car audio switched-mode psu
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Auto electronics projects
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Car audio switched-mode psu
MOSFET thus turning it on and clamping the spike. Gate
res i s to r s R4 to R7 help to ba lance current flow through
each MOSFET pair and a lso help to r educe switching
noise .
T l is a s tep up t ransformer compris ing six windings, two
connec t ed to form a cen t re tapped primary winding and
four are connec ted in two pairs to form two cent re tapped
secondary windings.
Components R36, C27 and C28 form a simple R-C filter
for ICI which a t tenuates supply borne noise . C29 and
R20 se t the soft-start t ime period for ICI . At switch on,
C29 is d ischarged and IC l ' s outputs are inhibited. As C29
charges via R20, the pulse width of the PWM drive sig-
nals are allowed to increase from zero. D14 prevents ICl ' s
soft-start input from being pulled negative at switch-off
and a lso se rves to d ischarge C29 more quickly. TR6 dis-
charges C29 and inhibits IC l ' s outputs in r e sponse to a
thermal shutdown condi t ion or a s tandby input ( low)
from P19. D18 and D13 form a d i sc re te AND circui t . When
the shutdown condition and s tandby inputs are removed,
TR6 allows C29 to charge again and the power supply
res ta r t s .
Res i s to r R21 and capac i t o r C31 se t the osc i l l a to r fre-
q u e n c y , P24 may be used to m o n i t o r t h e o s c i l l a t o r
waveform. Care should be exe rc i sed to ensure that this
pin is not sub jec t to undue capac i t ive loading, o therwise
the osc i l l a tor f requency will shift.
Res i s to rs R17, R18, R19 toge ther with capac i to r C30 form
a phase se lec t ive network that se t s the gain of the over-
vol tage amplifier. Phase compensa t ion is n e c e s s a r y to
ensure good loop stabi l i ty , o therwise the power supply
159
Auto electronics projects
could break into osci l la t ion. Res i s to rs R15 and R16 form
a potential divider which is used to apply over vol tage
feedback to ICI, with the values as shown, the maximum
output vol tage is ±30 V.
T r a n s i s t o r s TR7 to TRIO and a s s o c i a t e d c o m p o n e n t s
form two high speed driver c i rcui ts which are able to
charge and discharge the gate capac i t ance of each of the
MOSFETs very quickly. Circuit operat ion for one of the
(two ident ica l ) drivers is as follows: R23 is the pull-up
res is tor for the open co l lec tor output of ICI (pin 8 ) . When
pin 8 goes low (output on) TR7 is b iased on by R25 (C34
se rves to inc rease switching speed) , D15 conduc t s and
TR2, TR3 turn on quickly. At this t ime TR8 is swi tched
off. When ICI pin 8 goes high (off) TR7 swi tches off and
TR8 base is pulled low; as the gates of TR2 and TR3 are
charged to a posit ive potential , D15 is reverse biased and
TR8 conduc t s . This act ion rapidly swi tches off TR2 and
TR3.
Integrated c i rcui t IC2 is a compara to r with its inputs
connec t ed to two potential dividers. Res i s to r s R31 and
R32 form a reference potential divider and thermis tor
TH1 and R30 form a tempera ture sensing network. R33
and D17 provide a large degree of hys te res i s when the
output changes s ta te . Normally the output from IC2 (pin
7) is high and the vol tage on pin 2 is around l/ 2 supply.
The vol tage on pin 3 is dependent on the r e s i s t ance of
TH1, governed by the heats ink tempera ture with which
it is in con tac t . As the tempera ture of the heats ink r ises ,
the r e s i s t ance of TH1 reduces and the vol tage on pin 3
inc rease . When the vol tage on pin 3 exceeds the vol tage
on pin 2, the output of IC2 goes low. LD2 il luminates in-
dicating thermal shutdown and the power supply shuts
160
Car audio switched-mode psu
down. At this point D17 conduc t s , this adds R33 to the
lower half of the re ference divider reducing the refer-
ence potential on pin 2 to around V3 supply (ignoring
D17 vol tage drop and sa tura ted output vol tage of IC2).
The vol tage on pin 3 will now have to fall below V3 sup-
ply before the circui t will rese t and the supply allowed
to res tar t . Correspondingly the r e s i s t ance of TH1 will
have to r ise and its t empera ture fall before supply op-
erat ion is resumed. With the circui t values as shown, the
trip tempera ture is 80°C and the rese t t empera ture is
60°C.
Diodes D5 to D8 form a bridge rect if ier (main output) ,
the devices used are high speed types , essent ia l for use
in switch mode appl ica t ions . Capaci tors C7 to CIO help
to reduce t rans ients and switching noise . Components
C i l , C12, L2, L3, C13, C14, C15 and C16 form jt-filter net-
works for the main outputs . Res i s to r s R9 and RIO serve
to provide a minimum load lor the power supply and also
d ischarge the filter capac i to r s quickly after switch-off.
Fuses FS3 to FS6 provide pro tec t ion against shor t cir-
cui ts and over loads . Posi t ive 30 V outputs are avai lable
from P4, 5, 6 and 7. Negative 30 V outputs are avai lable
from P12, 13, 14 and 15. Pins 8, 9, 10 and 11 provide a
zero volt return.
Diodes D9 to D12 form a s econd bridge rect if ier (auxil-
iary output) , again high speed types are used. Capaci tors
C17 to C20 help to reduce t rans ients and switching noise.
Capaci tors C21 and C22 are the reservoi r capac i to r s for
the auxiliary output. Res i s to r s R l l and R12 serve the
same purpose as R9 and RIO in the main output c ircui t ry .
Voltage regulators RG1 and RG2 regulate the supply rails
and a t tenuate switching noise on the auxiliary output.
161
Auto electronics projects
Capaci tors C23, C24, C25 and C26 are decoupling capaci -
tors and ensure supply s tabi l i ty . Posi t ive and negative
12 V auxiliary outputs are avai lable on P16 and P18 re-
spect ively . P I 7 provides a 0 V return.
Construction
The PCB is of the single-sided glass fibre type, with a
printed legend to ass is t insert ion of the componen t s . To
inc rease the current rating of some of the t racks it is
n e c e s s a r y to tin the exposed areas of t rack on the un-
ders ide of the PCB. T h e s e t racks will be c lear ly seen as
they are not covered by the so lder res is t layer. Tinning
of the t racks should actual ly be the final a s sembly task.
Removal of misplaced componen t s can be very difficult,
espec ia l ly on a densely populated board such as this , so
p lease double check component type, value and orienta-
tion (where appropr ia te ) before insert ing and soldering
the component .
Referring to the following const ruct ional notes , the parts
list and Figure 8.3, begin const ruct ion. It is recommended
tha t t he following c o n s t r u c t i o n o rde r is adhe red to
c lose ly , o therwise it will be found ex t remely difficult, to
fit some of the componen t s .
Start by insert ing the th ree 22 SWG wire links, t hese are
indicated on the PCB by a single s t ra ight line and an
adjacent LK mark.
Next insert the 1N4148 signal diodes, ensuring co r r ec t
or ientat ion.
162
Car audio switched-mode psu
Insert 0.6 W metal film res i s to r s , but do not insert the
3 W wire wound res i s to r s at this s tage .
Bend and insert the four 16SWG wire links, t he se are
indicated on the PCB by a single s t ra ight line and an
adjacent LK number.
Next insert the 1N4001 diodes and the two 39 V zener
diodes .
Referring to Figure 8.4, loose ly fit the M3 power input
connec t ion hardware and solder the M3 nuts to the PCB
pads.
Insert the polystyrene capaci tors and the ceramic capaci-
tors .
Next insert the DIL socke t s , but do not insert the ICs at
this s tage.
Insert the 45 PCB pins into the holes for TR2 to TR5 and
D5 to D8; and pos i t ions marked with a c i rc le and a Ρ number. Do not insert pins into posi t ions marked with a
c i rc le and a W number.
Next insert the fuse c l ips . You should find that by care-
fully bending over the two legs on the t rack side of the
PCB before soldering, the fuse cl ips will remain straight .
Insert the BC337 and BC559 t rans is tors , ensuring co r r ec t
or ientat ion.
Next insert the tantalum capac i to r s , ensuring that the
co r r e c t vol tage rating capac i to r is inser ted in the cor-
r ec t loca t ion . Tanta lum c a p a c i t o r s are polar i sed and
must be co r r ec t l y or ienta ted, the plus (+) sign on the
body must be inser ted into the hole neares t that marked
with a plus sign.
163
Auto electronics projects
Figure 8.3 PCB track and legend
164
Car audio switched-mode psu
Figure 8.3 Continued
165
Auto electronics projects
M 3 I s o s h a k e
M 3 S o l d e r T a g
/ i M 3 N u t !
(Ttn S o l d e r
P C B
M 3 N u t t o P C B p a d .
Figure 8.4 Power input connection assembly
Form the l eadouts of the B Y W 9 8 rec t i f ie r d iodes , as
shown in Figure 8.5 and insert t hese into the PCB. En-
sure that the ca thode lead, which is indicated by a band
around the componen t body is inser ted into the hole
neares t that marked with a k sign.
C a t h o d e
b a n d
Figure 8.5 Lead formation for BYW98 rect i f iers
166
Car audio switched-mode psu
Insert the 0.1 μΡ poly layer capac i to r s and the small e lec-
t r o l y t i c c a p a c i t o r s . T h e e l e c t r o l y t i c c a p a c i t o r s a re
polar ised and must be co r r ec t l y or ienta ted, the negative
( - ) s t r ipe on the capac i to r can must be inser ted into the
hole furthest away from the hole marked with a plus (+)
sign.
Drill the heats ink as shown in Figure 8.6. Form the leads
of the BUZ11 MOSFETs and the BYW80 rectifiers as shown
in Figures 8.7 and 8.8. Assemble the heats ink a s sembly
81 m m
3 . 5 m m D i a .
51 m m
Γ Τ
E X I S T I N G H O L E S - U N U S E D E X I S T I N G ^ H O L E S -D R I L L — O U T T O D R I L L - O U T T O
3 . 5 m m D i a . 3 . 5 m m D i a .
Figure 8.6 Heatsink dr i l l ing information
Figure 8.7 Lead formation for BUZ11 MOSFETs
167
Auto electronics projects
Figure 8 .8 Lead formation for B Y W 8 0 r e c t i f i e r s
using the M2.5 hardware as shown in Figure 8.9 and Photo
8.1. Solder the leadouts of the t rans i s to rs and rect if iers
to the PCB pins. Referring to Figures 8.10 and 8.11 and
Photo 8.1, connec t the sc reened cab le to the thermis tor
and the PCB pins, use heat shrink sleeving where neces -
sary to avoid shor t c i rcu i t s . Glue the thermis tor to the
heats ink using some epoxy resin. Hold the thermis tor in
place whilst the resin se t s .
M2.5 χ 12mm Bolt
Heatsink
Insulat ing b u s h
BUZ1 1
c
Insulat ing w a s h e r
M2.5 Nut
Figure 8 . 9 Assembly of heatsink components
168
Car audio switched-mode psu
Phote 8.1 Close-up of heatsink assembly
Twist a n d s o l d e r c o n n e c t i o n s
s c r e e n C P 16 Heat—shr ink T h e r m i s t o r
P C B Pins
Figure 8.10 Thermistor connection
169
Auto electronics projects
Figure 8.11 Wiring to switching PSU
Insert the two regulators , ensuring that the c o r r e c t type
is fitted in the co r rec t locat ion and that the package lines
up with the outline marked on the legend. Ensure that
the two metal t abs do not touch, see Photo 8.2.
Referring to Figures 8.12 and 8.13 extend the leadouts of
the 3 W res i s to rs and axial inductors and insert t hese
into the PCB, see Photo 8.2.
Referring to Figure 8.14 wind 2l/2 turns of two lengths of
16 SWG EC wire wound bifilar (s ide by s ide) around the
toroid co re . Prepare the ends of the EC wire to facil i tate
soldering and insert this inductor into the PCB at the
170
R R R R P P P P © ™ 1
I II 11 ~|I I I II 11 ΙΓΊ P26
Main s^J1-*) L J \P27 Outputs ( V ] C K LeJ^ ' _J
to power >—»* V - > * r _ I _a β P « C
. Γ^ρ^ <B Q J^T^K^^ Thermal shut down
H o £ Tp P " 9 | P 3 & 2 " | d P o w e r 0 n
+ 12V - 1 2 V Standby Π 15A OV
, nP
ut LJFuse
Auxiliary Oscillator I er ι ι Outputs test OV 13.8V
to pre—amp point » , > O n / O f f Battery control supply Input Input
Car audio switched-mode psu
Photo 8.2 Close-up of the regulators, resistors and inductors
posit ion marked L I , s ee Photo 8.3. It is helpful to smear
the windings and toroid co re with si l icon rubber sealant
to prevent the a s sembly from rattling.
C u t o f f
e x c e s s w i r e
W r a p w i r e a r o u n d
l e a d o u t Sc s o l d e r
2 0 S W G
T C w i r e
Figure 8.12 Extending leadouts of axial inductors
171
Auto electronics projects
T C w i r e
Figure 8.13 Extending leadouts of axial resistors
2 x 2 / 2 t u r n s o f 1 6 S W G
E C w i r e w o u n d b i f i l a r
o n F X 4 - 0 5 4 t o r r o i d
S m e a r s i l i c o n
r u b b e r s e a l a n t
o v e r w i r e s a n d
t o r r o i d t o h o l d
i n p l a c e
R e m o v e e n a m e l f r o m
w i r e e n d s u s i n g
e m e r y p a p e r t o
f a c i l i t a t e s o l d e r i n g
Figure 8.14 Li winding information
172
Car audio switched-mode psu
Photo 8.3 Close-up of the toroid inductor f i t ted into the PCB
Next insert the power relay.
Insert the large SMPS e lec t ro ly t ic capac i to r s , ensuring
that the co r r ec t vol tage rating capac i to r s are fitted in
the c o r r e c t loca t ions and are c o r r e c t l y or ien ta ted as
previously desc r ibed .
Referring to Figure 8.15 wind the t ransformer , this is
p robably the most difficult part of the cons t ruc t ion pro-
cedure and should not be rushed. Note that the diagrams
do not figuratively show the required number of turns
per layer. When winding the t ransformer take ca re not
to over s t r e s s the bobb in o the rwi se pins may break
173
Auto electronics projects
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Car audio switched-mode psu
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Auto electronics projects
Photo 8.4 The component parts of the ferr i te transformer
Cover each layer with a single layer of masking tape.
Starting at pins 13 and 11, wind bifilar two 9 turn windings
of 18SWG EC wire first, finish at pins 3 and 5 respec-
tively, as shown in Figure 8 .15 (a ) .
S ta r t ing at pins 16 and 15, wind bif i lar two 26 turn
windings of 20 SWG EC wire in two layers ; first wind
bifilar 13 turns, as in Figure 8 .15 (b ) . Wind bifilar a fur-
ther 13 turns and finish at pins 10 and 9 respect ively , as
in Figure 8 . 1 5 ( c ) , note the wires c ros s over. Check for
cont inui ty between pins 16 and 10, and 15 and 9.
176
off — use pliers to carefully bend the wire around the
pins. It will be nece s sa ry to remove the enamel coat ing
from the wire to allow soldering, emery paper is ideal
for this . Photo 8.4 shows an exploded view of the com-
ponent parts of the t ransformer .
Car audio switched-mode psu
Starting at pins 1 and 2, wind bifilar two 15 turn windings
of 20 SWG EC wire in two layers; first wind bifilar 13 turns,
as shown in Figure 8 .15(d) . Wind bifilar a further 2 turns
and finish at pins 7 and 8 r e s p e c t i v e l y , as in Figure
8 .15 (e ) , note the wires c r o s s over . Check for cont inui ty
between pins 1 and 7, and 2 and 8.
Solder all of the leadouts to the t ransformer bobbin pins,
fit the c o r e s and clip into p lace the sprung s tee l co re
re ta iners .
Insert the t ransformer into the PCB, ensuring that pin 1
aligns with the number 1 on the PCB.
Referring to Figure 8.11 and using the 32 /0 .2 power con-
nect ion wire, link Wnumber holes ; W l to W l , W2 to W2,
W3 to W3.
Finally tin the exposed lengths of PCB t racks with a thick layer of solder . Take ca re not to splash solder e l sewhere
which may cause shor t c i rcu i t s .
Double-check your work and remove e x c e s s flux from the
underside of the PCB using PCB c leaner . Photo 8.5 shows
the assembled PCB.
Connect the two LEDs to the PCB via lengths of insulated
wire as shown in Figures 8.11 and 8.16.
Testing
Figure 8.11 shows the locat ion of the input and output
connec t ions referred to in this sec t ion .
177
Auto electronics projects
Photo 8.5 The assembled PCB
Fit ICI, IC2 and the fuses.
Using a mult imeter on a sui table r e s i s t ance range, meas-
ure the r e s i s t ance between FS7 and P2, the r e s i s t ance
should be greater than 2 kQ. Check also the r e s i s t ance
between FS2 and P2, the r e s i s t ance should be greater
than 2 kQ. If significantly lower readings than s ta ted are
measured, r echeck all of your work as there is likely to
be a shor t c ircui t or a misplaced component .
178
Car audio switched-mode psu
F l a t o n p a c k a g e
S h o r t l e a d
C a t h o d e A n o d e
C P 3 2 H e a t - s h r i n k s l e e v i n g
\
S l i d e o v e r c o n n e c t i o n s a n d s h r i n k .
R e s i s t o r
T w i s t a n d s o l d e r Λ
c o n n e c t i o n s
C a t h o d e
A n o d e
Figure 8.16 LED leadout ident i f icat ion and connections
Connect a 12 V supply capab le of delivering 5 A to the
input pins PI (+V) and P2 (0 V) via a 5 A fuse (for FS1)
and a mult imeter on 5 A or higher range. The qu iescen t
current should be less than 1 mA.
Link P3 and PI with light duty wire, whereupon the relay
should energise and the power-on LED (LD1) should illu-
minate. The current indicated on the meter should be
approx ima te ly 400 mA. If an o s c i l l o s c o p e and /or fre-
quency counte r are avai lable, then t he se may be used to
confirm that a 50 kHz (approximate ly) sawtooth wave-
form is avai lable on P24. Avoid undue capac i t ive loading
o therwise the f requency of the osc i l l a tor will be shifted.
179
Auto electronics projects
Unlink P3 and P I , d i sconnec t the supply and d i sconnec t
the mult imeter .
Reconnec t the supply and relink P3 and P I . Measure the
voltage on the output pins, using a suitable voltage range.
P4 to P7 should read +30 V with r e spec t to P8. Pins P12
to P15 should read - 3 0 V with r e spec t to P8. P16 should
read +12 V with r e s p e c t to P17 and P18 should read
- 1 2 V with r e spec t to P17.
The thermal shutdown circui t may be tes ted by carefully
hea t ing t h e t h e r m i s t o r with a h a i r d r y e r . When the
thermis tor r eaches a tempera ture of approximately 80°C
the thermal shutdown LED (LD2) will i l luminate and the
power supply will shutdown, this can be confirmed by
measuring one of the supply vol tage outputs . When the
thermis tor temperature drops to approximately 60°C the
power supply will res tar t and the thermal shutdown LED
will extinguish.
This comple tes test ing of the power supply.
As previously s ta ted, the power supply is specif ical ly
intended for use with two Maplin 50 W bipolar power
amplifiers. In most appl icat ions the audio output power
a t ta inable from these amplifiers when used in conjunc-
t ion with t h i s p o w e r s u p p l y s h o u l d b e m o r e than
sufficient for in-car use. However the purist may wish to
use separa te power suppl ies for each amplifier to in-
c r ea se the power available per channel . Similarly, if a
single channel subwoofer amplifier is required, a single
amplifier may be driven from one power supply.
It is strongly recommended that the power supply is fully
cased and provided with an additional external heatsink,
type 2E is suggested. Metal c a s e s are ideal for this pur-
180
Car audio switched-mode psu
pose, and a lso provide a degree of shielding against ra-
diated radio frequency emiss ions . The audio amplifiers
may also be housed in the same c a s e , which could be
convenient ly mounted in the ca r boot or under a seat .
The audio amplifiers should a lso be heats inked, again
type 2E is suggested.
To connec t the 50 W bipolar amplifiers to the power sup-
ply, t r e a t t h e s w i t c h e d - m o d e p o w e r s u p p l y as a
convent ional power supply (as shown in the amplifier
cons t ruc t iona l de ta i l s ) and c o n n e c t accord ing ly (HT1
and HT2 are posi t ive, HT3 and HT4 are negat ive) . Refer
to Figure 8.11 for connec t ions to the power supply. The
amplifier set-up procedures should be followed in the
same way as for the convent ional power supply. Connec-
t ions from the power supply to the amplifiers should be
made using 32/0 .2 wire.
Take note — Take note — Take note — Take note
Loudspeakers should be suitably rated for high
power use. Beware — many car loudspeakers are
given misleadingly high power ratings, so try
and find out what the true r.m.s. ratings are
before you use any loudspeaker. Often car loud-
speaker ratings are given in peak power or total
peak power, so be prepared to divide the rating
by 1.414 or even 2.828! Loudspeaker wiring
should also be sufficiently rated for the pur-
pose .
181
Auto electronics projects
Connect ions from the power supply to the car e lec t r ica l
sys tem should be made using very heavy duty cab le . It
is advisable to connec t the power supply direct ly to the
car ba t te ry via its own in-line fuse at the car ba t te ry end.
Assuming a negative earth car , the chas s i s may be used
to provide the 0 V connec t ion , which saves on wire.
Take note — Take note - Take note - Take note
It should be pointed out that excessive sound
pressure levels may lead to long term, irrevers-
ible hearing problems · High levels of sound may
also blot out other external sounds, which could
be dangerous when on the move · Please use common
sense when using a high power in-car entertain-
ment system.
Input: Input current (P 0= 116 W ) : Output power: Outputs Main: Auxiliary: Continuous output current ±30 V ±12 V Efficiency: Thermal shut-down temperature: Thermal shut-down hysteresis: Standby input: Remote switch-on input: Thermal shut-down output: Input noise (P 0 = 120 W ) : Output noise (P 0 = 120 W ) Main: Auxiliary: Switching frequency: Converter mode:
11 to 15 V d.o, nominally 13.8 V 10.7 A (Vs = 11.3 V) 120 W continuous, see note below
±30 V ±12 V
2 + 2 A 50 mA + 50 mA >90% 80°C 20°C Active low Active high Active low 140 mV
60 mV 40 mV 25 kHz Push-pull
Table 8.1 Specif icat ion of Prototype
182
Car audio switched-mode psu
Car audio switched-mode PSU parts list
Resistors — All 0.6 W 1% metal film (unless specified)
R l 6k8 1 (M6K8)
R2 68 k 1 (M68K)
R3.35 lk2 2 (M1K2)
R4,5,6,7 56 Ω 4 (M56R)
R8.36 1 0 Ω 2 (Ml OR)
R9.10 1 k 3 W 2 ( W I K )
R11.12 470R 3 W 2 (W470R)
R13,17 ,22 ,
23 ,26 ,27 ,
28 ,29 ,34 1 k 9 (M I K )
R14 ,19 ,21 ,
31 ,32 ,33 1 0 k 6 (Ml OK)
R15 24 k 1 (M24K)
R16,20 ,
24 ,25 4k7 4 (M4K7)
R18 1 M 1 (M1M)
R30 3k3 1 (M3K3)
TH1 15 k bead the rmis to r 1 (FX22Y)
Capacitors
C l , 5 , 1 5 ,
16 ,25 ,26 ,
28 ,30 100 nF polyes te r
C2.13.14 220 μ Ρ 5 0 ν 8 Μ Ρ δ
C3,4 , l 1,12 1000 μΡ 50 V SMPS
C6.31 2n2F 1% polys tyrene
C7,8 ,9 ,10,17,
18,19,20 560 pF ce ramic
8
3
4
2
(BX76H)
( J L 5 1 F )
( JL57M)
( B X 6 0 Q )
8 (WX65V)
183
Auto electronics projects
C21.22 1000 μΡ 25 V SMPS 2
C23.24 10 μΡ 25 V tantalum 2
C27 100 μΡ 25 V PC e lec t 1
C29 22 μΡ 25 V PC e lec t 1
C32 10 μΡ 16 V tantalum 1
C33,34 150 pF polys tyrene 2
Dl ,2 1N4001 2
D3.4.13,
14,15,16,
17,18 1N4148 8
D5,6,7,8 BYW80-150 4
D9.10,
11,12 BYW98-150 4
ZD1,2 39 V BZX61C/BZX85C 2
T R I BC337 1
TR2,3 ,4 ,5 BUZ 11 4
TR6,7 ,8 ,
9,10 BC559 5
LD1,2 Red LED 2
RG1 μΑ78121Κ: 1
RG2 μΑ79121Κ: 1
ICI TL494 1
IC2 LM311 1
Miscellaneous
LI FX4054 ferrite toroid 1
L2,3 3 A RF suppressor 2
T l ETD39 ferrite co re 2
ETD39 former 1
ETD39 clip 2
RL1 12 V 16 A relay 1
( JL56L)
(WW69A)
(FF11M)
(FF06G)
(WW68Y)
(BX29G)
(QL73Q)
(QL80B)
(UK63T)
(UK65V)
(QF67X)
( Q B 6 8 Y )
(UJ33L)
(QQ18U)
(WL27E)
(QL32K)
(WQ93B)
(RA85G)
(QY09K)
( J R 8 4 F )
(HW06G)
( JR81C)
( JR82D)
( J R 8 3 E )
(YX99H)
184
Car audio switched-mode psu
FSl
FS2.7
FS3,4 ,5 ,6
15 A 1 V 4in AS fuse 1 (UK13P)
100 mA 20 mm QB fuse 2 (WR00A)
2 A 20 mm AS fuse 4 (WR20W)
174·η chass i s fuse holder 1 (RX50E)
fuse clip 12 (WH49D)
6mm M3 isobol t 1 pkt (BF51F)
12 mm M2.5 i sobol t 1 pkt (BF55K)
M3 isonut 1 pkt (BF58N)
M2.5 isonut 1 pkt ( B F 5 9 P )
M3 i soshake 1 pkt ( B F 4 4 X )
M2.5 i soshake 1 pkt ( B F 4 5 Y )
M3 isotag 1 pkt (LR64U)
TO220 insulator 8 (QY45Y)
T0220 bush long 1 pkt (UL69A)
50 W heats ink 1 (HQ69A)
16-pin DIL skt 1 (BL19V)
8-pin DIL skt 1 ( B L 1 7 T )
1 mm PCB pins 1 pkt (FL24B)
PCB 1 (GE61R)
0.9 mm 20 SWG TC wire 1 (BL13P)
1.6 mm 16 SWG TC wire 1 ( B L U M )
3202 green wire 1 mtr (XR35Q)
1.6 mm 16 SWG EC wire 1 ( B L 2 4 B )
1.25 mm 18 SWG EC wire 1 (BL25C)
0.71 mm 22 SWG EC wire 1 (BL27E)
lapped pair 1 m t r ( X R 2 0 W )
CP 32 heat shrink 1 mtr (BF88V)
CP 16 heat shrink 1 mtr ( B F 8 6 T )
c o n s t r u c t o r s ' guide 1 (XH79L)
ins t ruct ion leaflet 1 (XK50E)
fast-setting adhes ive 1 (FL45Y)
All of the above are avai lable as a kit
switched-mode PSU kit 1 (LP39N)
185
Auto electronics projects
Optional (not in kit)
car fuse holder
15 A 1 V4 in AS fuse
HC wire b lack
HC wire red
32/0 .2 wire red
32 /0 .2 wire b lack
32 /0 .2 wire blue
zip wire
50 W power amp
2E heats ink
1 (RX51F)
1 (UK13P)
as r eq (XR57M)
as req (XR59P)
as req (XR36P)
as req (XR32K)
as req (XR33L)
as req (XR39N)
2 (LW35Q)
2 (HQ70M)
186
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