Anti-theft security circuit diagram of car audio system based on CD4093

This is a circuit diagram for car audio system anti-theft security, which can effectively protect and protect your expensive car audio system from being stolen. This simple circuit design is based on the popular CMOSNAND chip CD4093.

When the circuit is turned on through switch S1, the indicator LED1 will light up and the circuit status will be in standby mode. The LED inside the optocoupler IC1 lights up because the cathode terminal is connected through the car audio (amplifier) ​​body. As a result, the output of pin 3 of gate N1 goes low and disables the rest of the circuit.

Whenever I try to remove the car stereo from its mount by cutting the connecting wires, the optocoupler shuts down immediately because its LED cathode terminal is left floating. As a result, the oscillator circuit built around gates N2 and N3 is activated and manages the "on"/"off" timing of the relay through transistor T2. (If desired, the relay contacts can be used to power emergency buzzers, indicator lights, car horns, etc.)

Change the value of capacitor C2 to get a different "on"/"off" timing, making relay RL1 "on"/"off". For 100uF we get about 5 seconds "on" time and 5 seconds "off" time. You can conduct your own experiments if needed.

The N4 gate and its related components form a self-test circuit. Normally, both of its inputs are in the "high" state. However, when one turns off the ignition key, the power to the car stereo is also disconnected. Therefore, the output of gate N4 jumps to the "high" state and provides a differential short pulse to forward biased transistor T1 for a short period of time. (The combination of capacitor C1 and resistor R5 acts as a differential circuit.)

Therefore, the buzzer at the collector terminal of T1 will emit a short beep to announce that the safety circuit is intact. The period of this "on" loop can be changed by changing the value of capacitor C1 and/or resistor R5.

After the construction is completed, fix the LED and buzzer on the dashboard according to your requirements, and hide the switch S1 in a convenient place. Then connect lead A to the car stereo's body (not the body) and lead B to its positive lead terminal. Power the circuit directly from the car battery.

WARNING: This design only works with negative grounded car stereos.