anti theft security for car audios

Posted on Feb 5, 2014

When 12V DC from the car battery is applied to the gadget (as indicated byLED1) through switch S1, the circuit goesinto standby mode. LED insideoptocoupler IC1 is lit as its cathode terminalis grounded via the car audio (amplifier)body. As a result, the output atpin 3 of gate N1 goes low and disablesthe rest of the circuit. Whenever an attempt is made

anti theft security for car audios
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to removethe car audio from its mounting bycutting its connecting wires, theoptocoupler immediately turns off, as itsLED cathode terminal is hanging. As aresult, the oscillator circuit built aroundgates N2 and N3 is enabled and it controlsthe on`/off` timings of the relay viatransistor T2. (Relay contacts can be usedto energise an emergency beeper, indicator, car horns, etc, as desired. ) Different values of capacitor C2 givedifferent on`/off` timings for relay RL1 tobe on`/off`. With 100 F we get approximately5 seconds as on` and 5 seconds asoff` time. However, when one switches off the ignitionkey, the supply to the car audio isalso disconnected. Thus the output of gateN4 jumps to a high` state and it providesa differentiated short pulse to forward biastransistor T1 for a short duration. (Thecombination of capacitor C1 and resistorR5 acts as the differentiating circuit. ) This on` periodof buzzer can bevaried by changingthe values ofcapacitor C1and/or resistorR5. After construction, fix theLED and buzzerin dashboard asper your requirementandhide switch S1in a suitable location.

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