Car audio system anti-theft circuit diagram

The anti-theft circuit of the car audio system is as shown in the figure. Its 12V DC power supply is taken from the battery in the car. After closing switch S1. The light-emitting diode in the optocoupler IC1 is lit, and the internal phototransistor is turned on due to the light. IC1 outputs a high level to the input end of the NAND gate, and NAND gate 1 outputs a low level. The low level is added to the input terminal ⑤ pin of NAND gate 2 to control the low-frequency oscillator composed of NAND gate 2 and NAND gate 3 to stop vibrating, so that the entire circuit is in an alert and waiting state. When the thief cuts off the audio connection cable, the photocoupler connected to the negative pole of the internal light-emitting diode is also cut off at this time, leaving the ② pin of IC1 floating, the internal light-emitting diode immediately stops working, and the ④ pin of IC1 outputs a low level, and the NAND gate 1 outputs high level, and the low-frequency oscillator composed of NAND gates 2 and 3 starts to oscillate. The oscillation signal it outputs controls VT1 and relay K1 to work intermittently: the normally open contact K1-1 of K1 is also sometimes closed and sometimes closed, controlling The audio alarm device sounds an alarm, and the loud alarm sound will attract people's attention.