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Engine
ImmobiliserMk.2
Protect your car from
theft with the . . .
This basic engine Immobiliser kills the ignition
if a thief tries to steal your car. Fit it to your car
as cheap insurance and peace of mind. If a thief
tries to start your car, the engine will repeatedly
stall and he will move on to an easier target.
By JOHN CLARKE
While many modern cars include a
comprehensive anti-theft system with
ignition disable, central locking and
rolling code entry, older vehicles or
the less expensive models do not have
this protection. The lack of engine
immobilisation renders the vehicle
more susceptible to theft, particularly
24 Silicon Chip
for older style vehicles, some of which
can be entered, started and driven
away in just a few seconds.
You can improve the odds against
your vehicle being stolen simply by
adding some form of engine immobilisation. Whether it is a hidden
switch which breaks the points signal
from the distributor, or a more fancy
method, the inclusion makes it more
difficult for a thief to start the engine.
But if the ignition system can be
“hot wired” to effectively bypass the
immobilisation wiring then it will be
worse than useless.
This Engine Immobiliser shorts
out the switching transistor or points
which control the ignition coil. It
does not produce a permanent short
because it is switched on and off at a
slow rate. The engine can be started
with the Immobiliser in action but it
will only run for about two seconds
and then switch off. The engine can
then be restarted only to stall again.
If the thief persists, the engine will
continue to start, only to stall again
and after several tries he is likely to
decide that the car is not worth the
trouble.
On the other hand, if the thief decides to lift the bonnet to investigate
further, it is important that the wire
from the Immobiliser to the ignition
coil is well hidden. Naturally, the
switch to turn the Immobiliser on
and off must be well concealed or
camouflaged to look like one of the
accessory switches, other
wise the
whole subterfuge will be for nothing.
Killing the ignition
In effect, a switch is placed in parallel with the car’s points or the ignition switching transistor, as shown in
Fig.1 & Fig.2. Each time the Engine
Immobiliser switches on, it effectively
shorts out the points or the switching
transistor and prevents the coil from
producing any sparks.
By shorting out the points or
switching transistor and diverting the
coil current for just a brief period, no
damage can result to the coil. But the
ignition coil could be easily burnt out
if the coil current was continuously
diverted, as it would be if the ignition
was permanently disabled by a simple
switch.
Now have a look at the circuit of the
Engine Immobiliser in Fig.3. It uses
a high voltage Darlington transistor
(Q1) which is connected in parallel
with the points or the ignition transistor. Q1 can switch the coil current
of several amps and can withstand
the high voltages normally developed
when the ignition system is functioning normally.
This circuit is quite similar to the
Fig.1: when fitted to a car with conventional ignition, the Immobiliser
effectively shunts the points and stops the coil from producing spark
voltage.
Fig.2: when fitted to a car with electronic ignition or an engine
management system, the Immobiliser shunts the main switching
transistor. This does no damage because the coil current is
intermittently diverted through the Immobiliser.
original Engine Immobiliser which we
featured in the December 1995 issue
of SILICON CHIP but there are some
important differences which we will
mention later in this article.
IC1, a 555 timer, is connected to
operate as an astable oscillator. It is
powered from the ignition circuit of
Fig.3: the circuit
consists of a 555 timer
which cycles the
transistors on and off
to periodically shunt
the ignition and hence
stall the engine.
December 1998 25
started and will just as surely stall
each time.
One important feature, which may
not be immediately obvious, is that the
Immobiliser does not do any damage
to the car’s ignition system if the thief
leaves the car stalled and hot-wired.
The Immobiliser will continue its cycle of 0.7s on and 2.3s off indefinitely
but no damage should result apart
from the possibility of the battery
becoming discharged.
One other minor point is that when
power is first applied to the Immobiliser circuit, as when the ignition is
first switched on, pin 3 of IC1 will be
high and so Q1 will be on, pulling
the negative side of the coil low and
thus preventing any sparks from being delivered for about one second.
However, most cars need to be cranked
for at least a second to start them so
there is really no noticeable effect on
starting the car.
Power for the Immobiliser comes
from the ignition switch and the enable
switch S1. It is fed via diode D2 which
protects transistors Q2 & Q3 against
reverse connection of the supply
while the associated 0.1µF capacitor
decouples the supply from hash. IC1
is protected from voltage transients
with the 16V zener diode ZD1, together
with the series 10Ω resistor and 100µF
decoupling capacitor.
Fig.4: the component overlay for the PC board. Note that the zener diodes must
be installed the right way around otherwise the circuit won’t work. If the 3W
zeners are installed the wrong way around they could be burnt out by the coil
current when the circuit is connected up.
the voltage across the
capacitor rises above
+8V (ie, 2/3 of the posi
tive supply), pin 3 of
IC1 goes low. The 10µF
capacitor is then discharged to about +4V
via the 330kΩ resistor
connected between
pins 6 & 7 and pin 3
goes high again.
The cycle then continues with pin 3 being switched high for
about 0.7 seconds and
low for 2.3 seconds.
Each time pin 3 of
IC1 is high, Q3, Q2 &
Q1 are switched on
This photo shows the keypad version of the Engine
and so the ignition
Immobiliser, to be published next month. The keypad
coil is prevented from
circuit board mounts above the Engine Immobiliser
producing its normal
board in a standard plastic case.
primary voltage and
the engine will be
the vehicle via the enable switch, S1. stalled. This 0.7s on-time for Q2 is
Initially, when power is first applied,
quite sufficient to stall the engine and
pin 3 of IC1 goes high. The 10µF cameans that there is no chance of any
pacitor at pin 2 is then charged via the damage to the ignition system.
100kΩ resistor and diode D1. When
So the engine can be repeatedly
Construction
The Engine Immobiliser circuit is
accommodated on a PC board measuring 106 x 60mm and coded 05412981.
The component overlay for the board
is shown in Fig.4.
Before discussing the construction
details, we need to mention a number
of differences between this version
of the circuit and that originally published in December 1995. The first
and most obvious difference is that
this Mk.2 version uses the MJH10012
which is the plastic version of the
Table 1: Resistor Colour Codes
No.
1
1
1
1
1
1
26 Silicon Chip
Value
330kΩ
100kΩ
4.7kΩ
1kΩ
82Ω 5W
10Ω
4-Band Code (1%)
orange orange yellow brown
brown black yellow brown
yellow violet red brown
brown black red brown
not applicable
brown black black brown
5-Band Code (1%)
orange orange black orange brown
brown black black orange brown
yellow violet black brown brown
brown black black brown brown
not applicable
brown black black gold brown
Parts List
Fig.5: this is the actual size artwork for the PC board. Check your board
carefully before installing any of the parts.
MJ10012 TO-3 power transistor.
An alternative transistor which may
be supplied in some kits is the BU941P
(manufactured by 57 Microelectronics). While the plastic Darlington high
voltage transistor should be cheaper it
does require a small heatsink.
The second point of difference is
that there is provision on the board
for another transistor and this will
be used in a keypad-operated Engine
Immobiliser to be published next
month. The version being published
this month has the virtue of simplic
ity; next month’s version offers more
bells and whistles and the security of
a keypad to disable it.
Now that we’ve got those points out
of the way, we can discuss assembly of
the board. You can begin construction
by checking the PC board for shorts
between tracks, breaks in the pattern
or undrilled holes.
You will need to fit PC stakes at
the external wiring points (four) and
then insert the links using the tinned
copper wire.
The resistors can be installed next
and you can use the colour codes in
Table 1 as a guide to selecting each
value. Alternatively, you can use a
digital multimeter to measure each
resistor before it is soldered in.
The diodes can go in next, taking
care with the polarity of each. Make
sure that you use the 1N914 or 1N4148
type for D1 and 1N4004 for D2. The
16V zener diode ZD1 is quite small
and may be marked 1N4745 while the
four 75V 3W zeners (which may be
marked 1N5374) are quite a lot larger.
Transistors Q2 & Q3 are positioned
as shown but make sure you don’t get
them swapped around; Q2 is a BC327
while Q3 is a BC337. Transistor Q1
is mounted on a small heatsink and
secured with an M3 screw and nut to
the PC board.
Next, insert the 555 IC and the three
capacitors, making sure that the IC
and the electrolytic capacitors are
installed the right way around. The
0.1µF capacitor may be marked as
100n or 104, being the IEC and EIA
codes, respectively.
Testing
To test the circuit, connect it to a
12V DC supply or battery. There is no
need to connect a coil to the collector
of Q1. Connect your multimeter, set to
measure 12V DC, to check the voltage
at pin 3 of IC1. You can do this most
conveniently by connecting to the
4.7kΩ base resistor for Q3.
Now apply power and check that
pin 3 goes high immediately and then
drops low after about a second. It
should then stay low for 2.3 seconds
or thereabouts, then go high for 0.7s
and so on. You can then check the
sequence at the collector of Q3 and
the collector of Q2. Q3 will invert the
voltage from pin of IC1 and Q2 will
invert it back again.
Finally, you can verify that the high
voltage transistor Q1 comes on by
measuring the resistance between its
emitter and collector. The transistor
will be on when the resistance is low
and off when its resistance is high.
If the circuit operates properly you
are now ready to install it into your
vehicle. The board can be housed in
several ways. It can be mounted in a
standard plastic case measuring 130
x 67 x 43mm or it could be sheathed
in heatshrink tubing.
1 plastic case, 130 x 67 x 43mm
1 PC board, code 05412981,
106 x 60mm
4 PC stakes
1 mini heatsink, 19 x 19 x 9.5mm
1 M3 x 9mm screw
1 M3 nut
1 1m length of heavy duty black
automotive hookup wire
1 1m length of heavy duty red
automotive hookup wire
1 1m length of light duty red
automotive hookup wire
1 1m length of heavy duty yellow
automotive hookup wire
1 150mm length of hookup wire
Semiconductors
1 555 timer (IC1)
1 MJH10012, BU941P power
Darlington transistor (Q1)
1 BC327 PNP transistor (Q2)
1 BC337 NPN transistors (Q3)
1 16V 1W zener diode (ZD1)
4 75V 3W zener diodes (ZD2-5)
1 IN4148, 1N914 diode (D1)
1 1N4004 1A diode (D2)
Capacitors
1 100µF 16VW PC electrolytic
1 10µF 16VW PC electrolytic
1 0.1µF MKT polyester
Resistors (0.25W, 1%)
1 330kΩ
1 1kΩ
1 100kΩ
1 82Ω 5W
1 4.7kΩ
1 10Ω
Miscellaneous
Automotive connectors, solder.
Find a suitable position under the
dashboard to mount the unit and then
locate the fused side of the ignition
circuit and the fused side of the battery supply.
The wiring to these points should
be made using automotive connectors. Also you will need a chassis
point to connect the ground supply
of the circuit to the battery negative
terminal. This can be an existing
screw in the bodywork or a separate
self-tapping screw which secures the
eyelet terminal for the ground lead in
place. The connection to the ignition
coil should be made with an eyelet
terminal. Try to conceal this wire as
SC
much as possible.
December 1998 27
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