ANTI-THEFT SECURITY FOR CAR AUDIOS
T.K. HAREENDRAN
EDI DWIV S.C.
T
his 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
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
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.
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 approxi-
mately 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
