Anti-Theft Security For Car Audios

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This small circuit, based on popular CMOS NAND
chip CD4093, can be effectively used for protecting your expensive car
audio system against theft. When 12V DC from the car battery is applied
to the gadget (as indicated by LED1) through switch S1, the circuit goes
into standby mode. LED inside optocoupler IC1
is lit as its cathode terminal is grounded via the car audio
(amplifier) body. As a result, the output at pin 3 of gate N1 goes low
and disables the rest of the circuit. Whenever an attempt is made to
remove the car audio from its mounting by cutting its connecting wires,
the optocoupler immediately turns off, as its LED
cathode terminal is hanging. As a result, the oscillator circuit built
around gates N2 and N3 is enabled and it controls the ‘on’/‘off’ timings
of the relay via transistor T2. (Relay contacts can be used to energise
an emergency beeper, indicator, car horns, etc, as desired.)

Different values of capacitor C2 give different ‘on’/‘off’ timings
for relay RL1 to be ‘on’/‘off’. With 100µF we get approximately 5
seconds as ‘on’ and 5 seconds as ‘off’ time. Gate N4, with its
associated components, forms a self-testing circuit. Normally, both of
its inputs are in ‘high’ state. However, when one switches off the
ignition key, the supply to the car audio is also disconnected. Thus the
output of gate N4 jumps to a ‘high’ state and it provides a
differentiated short pulse to forward bias transistor T1 for a short
duration. (The combination of capacitor C1 and resistor R5 acts as the
differentiating circuit.)As a result, buzzer in the collector terminal
of T1 beeps for a short duration to announce that to announce that the
security circuit is intact. This ‘on’ period of buzzer can be varied by
changing the values of capacitor C1 and/or resistor R5. After
construction, fix the LED and buzzer in
dashboard as per your requirement and hide switch S1 in a suitable
location. Then connect lead A to the body of car stereo (not to the body
of vehicle) and lead B to its positive lead terminal. Take power supply
for the circuit from the car battery directly.

Caution.
This design is meant for car audios with negative ground only.