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This simple alarm can be used to protect
powered car accessories (such as driving lights)
from theft. It’s self-arming, uses just a handful
of parts and can be easily integrated with
existing car alarms. Alternatively, it can be used
as a standalone unit with its own 12V siren.
By RICK WALTERS
One night recently, an acquaintance
had a pair of quite expensive driving
lights stolen from the front of his
4-wheel drive while it was parked
in his driveway. For the thief, it was
almost too easy – just unplug (or cut)
the leads, undo a couple of mounting
nuts with a shifting spanner and off
you go. It’s that quick!
This circuit is designed to protect
your expensive driving lights and at
the same time, give the prospective
thief quite a scare. As soon as the
power lead to the light is cut or discon
nected, either the car alarm will be
set off or, if you don’t have an alarm,
a very loud siren will be triggered. If
that doesn’t scare the “low-life” away,
nothing will.
One important feature of this unit
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is that it’s automatically armed each
time you turn the ignition off. After
all, a thief detector isn’t much use if
you forget to turn it on. Similarly, the
unit is automatically disarmed when
the ignition is switched on.
Of course, this also means that the
unit isn’t foolproof – if a thief “hotwires” the car, the alarm won’t sound
but then you’d have much bigger
problems than just a pair of stolen
driving lights.
How it works
Fig.1 shows the circuit details. As
you can see, there is very little to it.
The most complicated part is the circuitry involving IC1, which is used
to mute the alarm after a short time
to comply with noise pollution laws.
We have provided protection for up
to four lights and these are connected
to diodes D1-D4 via connector CON1.
Basically, D1-D4 function as an OR
gate. Normally, their anodes (A) are
pulled low by their respective driving
light filaments and so their commoned
cathodes (K) are also pulled low via an
associated 100kΩ resistor.
As a result, transistor Q1’s base is
also low and so this transistor and
relay RLY1 are off.
Now consider the situation if one
of the driving lights is disconnected.
When that happens, the anode of
the corresponding diode in the OR
gate is pulled to +12V via a 10kΩ
resistor which means that the diode
is now forward biased. As a result,
the commoned cathodes are pulled
to about +11.4V and so Q1 now turns
on (assuming that Q2 is also on) and
activates relay RLY1.
Relay RLY1 is a double-pole double
throw (DPDT) unit. As shown, its normally open (NO) contacts are connected to pins 2 & 7 of CON2. When the
relay is activated, pin 2 is switched to
+12V while pin 7 is pulled to ground.
These outputs can be used to trigger
the high-going or low-going inputs of
an existing car alarm.
September 2002 73
Fig.1: the circuit uses diode OR gate D1-D4 to drive transistor Q1, which in turn
drives a relay (RLY1). IC1 functions as a timer and this automatically shuts the
alarm off after 90 seconds by turning off Q2.
Alternatively, an external 12V siren
can be connected between pins 2 & 7
of CON2 (or connected between pin 2
and ground).
Transistor Q2 (a PNP type) is included to ensure that the alarm remains
off while the vehicle is being driven.
As shown, this transistor is connected
in series between Q1’s emitter and
ground. Normally, Q2’s base is pulled
low via a 10kΩ resistor and so this
transistor is biased on.
However, when the engine is started, Q2’s base is pulled high via the
accessories line (pin 5 of CON2) and
diode D6. This turns Q2 off and so Q1
and RLY1 are also off and the alarm is
disabled. Turning off the ignition then
automatically “arms” the circuit again.
Note that if Q2 were not included,
the alarm would sound each time the
driving lights were turned on.
Alarm timeout
IC1, a 14-stage binary counter, provides the timeout function. Normally,
pin 12 (Reset) of this IC is held high
74 Silicon Chip
via LED 1 and the 1kΩ resistor to the
+12V rail. As a result, IC1 is held in
the reset condition and its operation
is inhibited.
When the alarm is triggered, Q1 and
Q2 are both on and so Q1’s collector is
pulled down close to 0V (ie, ground).
This in turn pulls the anode of LED1
low via diode D10, thus releasing the
high on IC1’s reset pin. Instead, the
reset pin is now pulled down to 0V
via a 100kΩ resistor and so IC1 now
begins operating.
The RC network on pins 9 & 10 sets
the frequency of the internal oscillator, while the selected binary output
determines the alarm period. In this
The parts are all installed on a
small PC board with screw-terminal
blocks at either end. Note that the
final version differs slightly from
this prototype unit.
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Fig.2: follow this diagram carefully when installing the parts on the PC board. Note that you must only fit as many
input diodes (D1-D4) as you have lights to protect; eg, if you just have two driving lights, fit diodes D1-D2 only.
case, we have used the Q14 output at
pin 3, which means that IC1 divides
by 214 (ie, 16,384).
Since we want the alarm to operate
for about 90 seconds, this means that
the oscillator frequency should be
16,384/90 = 182Hz. This frequency is
set by the 33nF capacitor and 150kΩ
resistor on pins 9 & 10.
At the end of the 90s timing period,
pin 3 (Q14) of IC1 goes high and this
pulls Q2’s base high via D8. As a result,
Q2, Q1 and relay RLY1 all turn off and
the alarm stops.
But that’s not all the high Q14 output does – three other events also take
place. First, it turns on transistor Q3
via a 47kΩ resistor and this holds IC1’s
Reset (pin 12) low. Second, it pulls
pin 11 high via D7, which stops the
oscillator. And third, because Q3 is on,
LED1 lights to show that the alarm has
been activated – ie, LED1 functions as
a “tamper” indicator.
When the wiring to the light has
been reconnected, Q3 is turned off by
pressing the Reset switch (S1). This
releases the low on pin 12 and rearms
the circuit.
Power for the circuit is derived
from the car’s battery (via a 1A inline fuse – see construction). Diode
D9 provides reverse polarity protection, while a 10Ω resistor and 100µF
electrolytic capacitor provide supply
decoupling. In addition, zener diode
ZD1 is included to protect the IC from
high-voltage spikes on the supply line
(eg, when other equipment switches
on and off).
Finally, diode D5 protects transistor Q1 by quenching the back
EMF generated each time the relay
switches off.
Building it
Building it is easy since all the parts
are mounted on a PC board measuring
109 x 48mm (code 03109021). Fig.2
shows the parts layout.
Begin by carefully checking your
etched PC board against the published
pattern (Fig.3). That done, install the
wire link, followed by the resistors,
zener diode ZD1 and diodes D1-D10.
Make sure that the diodes are all correctly oriented.
Note that we have shown diode
D4 dotted on both the circuit and the
overlay. This diode should be left out
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September 2002 75
Parts List
1 PC board, code 03109021,
109mm x 48mm
1 DPDT mini PC-mount relay,
Jaycar SY4061 (or equivalent)
3 2-way PC-mount screw
terminal blocks (5mm pin
spacing)
2 3-way PC-mount screw
terminal blocks (5mm pin
spacing)
1 pushbutton switch (S1) to suit
Semiconductors
1 4060 14-stage binary counter/
divider (IC1)
2 PN100 NPN transistors
(Q1,Q3)
1 PN200 PNP transistor (Q2)
1 red LED (LED1)
8 1N914 small signal diodes
(D1-D4,D6-8,D10)
2 1N4004 1A diodes (D5,D9)
1 1N4745 16V 1W zener diode
(ZD1)
Capacitors
1 100µF 25VW PC electrolytic
1 33nF MKT polyester
Resistors (0.25W, 1%)
1 220kΩ
5 10kΩ
1 150kΩ
2 1kΩ
2 100kΩ
1 10Ω
1 47kΩ
We used an IC socket on the prototype but
recommend that you solder the IC directly
to the board. Note that the final PC board
has been slightly modified.
to unused inputs, the circuit will false
alarm.
The remaining parts can now be
installed on the PC board. These include the 33nF and 100µF capacitors,
transistors Q1-Q3, the screw terminal
connectors, IC1 and the relay. Make
sure that IC1 is installed with pin 1
adjacent to D7 and note that transistor
Q2 in a PN200 PNP type.
90 seconds, it should drop out and the
LED should illuminate.
Of course, if you had connected a
siren between pins 2 and 8 of CON2,
it would have sounded for 90 seconds.
However, it’s unlikely you would wish
to experience this pleasure!
Troubleshooting
The most likely reason for it not to
work is that LED1 has been installed
backwards. Other likely possibilities
include poor or missed solder joints,
solder bridges (especially between
the IC pins) and parts installed with
incorrect polarity.
Having a diode connected to an
unused input on CON1 will also cause
problems.
Testing
unless you have a fourth driving light
(or spot light) to connect to pin 4 of
CON1. Similarly, diode D3 should be
omitted if you don’t have a spot light.
In most cases, there will only be
two driving lights to protect, so only
diodes D1 and D2 should be fitted. In
short, only fit as many input diodes as
you have lights to protect – ie, if you
have two driving lights, fit only diodes
D1-D2. If you have diodes connected
Testing is best carried out on the
workbench. First, link the active input terminals on CON1 together (ie,
those with diodes) and run a wire to
pin 8 of CON2. That done, connect
the tamper LED (LED1) as shown
in Fig.2, then connect a 12V power
supply to CON2 (positive to pin 1,
negative to pin 8).
Initially, nothing should happen
and the current drawn should be
only a couple of milliamps. Now cut
the wire between the input terminals
to ground, to simulate an attempted
theft. You should immediately hear
the relay click in and then, after about
Installation
Once the circuit is working correctly, it can be installed in the vehicle.
And that’s easier said than done
because, depend
ing on where the
driving light relay is mounted, you
may have to run some leads through
the firewall.
Table 1: Resistor Colour Codes
No.
1
1
2
1
5
2
1
76 Silicon Chip
Value
220kΩ
150kΩ
100kΩ
47kΩ
10kΩ
1kΩ
10Ω
4-Band Code (1%)
red red yellow brown
brown green yellow brown
brown black yellow brown
yellow violet orange brown
brown black orange brown
brown black red brown
brown black black brown
5-Band Code (1%)
red red black orange brown
brown green black orange brown
brown black black orange brown
yellow violet black red brown
brown black black red brown
brown black black brown brown
brown black black gold brown
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How can you tell whether or not
the driving light relay is a single-pole
or double-pole type? Simple, with
the power off, disconnect one of the
driving light leads from the relay, then
check the resistance between its relay
terminal and earth.
If the reading is open-circuit, the relay is a double-pole type. Conversely, if
you get a reading of just a few ohms, it
means that there is a path back through
the other driving light and so the relay
is a single-pole type.
Fig.3: this is the full-size etching pattern for the PC board. Check your
board by comparing it with this pattern before installing any of the parts.
Generally, the best place to mount
the unit will be close to the fusebox/
relay box. This will enable you to
easily pick up power and run the
input leads to the driving light relay
contacts.
We’ll leave it up to you as to how
the board is protected. Typically, it
could be wrapped in foam rubber and
secured with cable ties. Alternatively,
the board will fit inside a standard
130 x 70 x 40 plastic case, which is
available from most suppli
ers. The
tamper LED and Reset switch should
be mounted in an accessible location
on the dashboard and connected to
CON2 via flying leads.
Pin 1 of CON2 must connect to an
unswitched battery positive terminal.
An unused position on the fusebox is a
good place to pick up this connection
but be sure to fit a 1A in-line fuse. In
most cases, it will be just a matter of
buying a fuse to suit your car’s fusebox. Note that all connections should
be run using automotive cable and
connectors.
The car radio supply line is a good
place to pick up the accessories feed.
Again, this can be picked up at the
fusebox.
Driving light connections
If the driving-light relay has double-pole contacts (ie, one set of contacts for each driving light), the leads
from CON1 can be wired directly to
the relay. Just be sure to connect each
input to the driving light side of its
contact.
However, if the driving light relay
only has a single pole, you cannot wire
CON1 to the relay contacts. That’s
because cutting the leads to one light
would still leave a circuit back through
the commoned relay contact and the
remaining light – and that’s just what
we don’t want.
There is a way around this however, and that’s to run the leads from
CON1 directly to the driving lights
themselves. In fact, you have to make
the connection to each light inside
the lamp housing itself (so that the
thief has to cut the wire). It really
doesn’t matter which side of the lamp
filament you connect to – either side
will do.
Siren
In order for the unit to trigger an
existing car alarm, you will need to
connect one of the switched outputs
to an appro
priate alarm input terminal – ie, either connect the +12V
Switched output to a high-going
input trigger terminal, or the Earth
Switched output to a low-going trigger terminal.
For example, if you car’s alarm is
triggered when a door opens and the
courtesy light switch is in the ground
circuit, then the Earth Switched
output can be connected across this
switch.
Alternatively, you can use a separate
12V DC siren. If you fit this behind
the grille close to the items you are
protecting, it should frighten daylight
out of any thief.
Finally, note that this circuit will
also sound as soon as you turn the
engine off if the filament in one of the
lamps “blows” – ie, it can also function
as a “blown filament indicator”. Of
course, that’s assuming that you have
wired CON1 so that the lamp filaments
are in-circuit.
However, this feature will be disabled if you connect to the “earthy”
side of the lamp filaments inside the
SC
lamp housing.
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September 2002 77
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