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Build electronic projects that are for your automobile.
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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 .
V
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
^ ^ ^ ^ Secondary
Figure 1.9 The ignit ion coi l
I Sparking plugs winding ^ J g n M o n wteh ^ | I
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,
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 | 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*
_
>
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 * 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 . 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 I I field for coil ^ - / /
( 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 - - ~ " ^ \ ^ ^ / / || jj-i s y s t e m u n d e r rT 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 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 !
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
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
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ag
ne
tic
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ase
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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
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3
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Co
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Auto electronics projects
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
' M i s s i n g ' r e f e r e n c e
t o o t h
O u t p u t c o m p a r e t o g g l e s p i n at
d e s i r e d e n g i n e a n g l e
T o o t h . s i g n a l | [_
P u l s e a c c u m .
v a l u e I n p u t c a p t u r e i n t e r r u p t
r o u t i n e d e t e c t s m i s s i n g t o o t h & r e s e t s p u l s e a c c u m u l a t o r
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 C 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
Auto electronics projects 00
IC2q,
IC3b
TR
6
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dro
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Microcontrollers
99
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ure
3.1
0
Co
nti
nu
ed
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
.
10 20 30 40 100 % Slip
Fully locked
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
he
el
4
Bro
ke
1 i
I1 c
yli
nd
er
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Clo
ck
m
on
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ck
Figu
re 3
.12
Bloc
k di
agra
m o
f an
elec
tron
ic A
BS s
yste
m
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
C 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.
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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.
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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 .
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Car battery monitor
Construction
Component pos i t ions and printed c i rcu i t board t rack
layout is shown in
Recommended