Car audio system anti-theft circuit

The anti-theft circuit of the car audio system is shown in the figure. Its 12V DC power supply is taken from the battery in the car. When the switch S1 is closed, the light-emitting diode in the photocoupler IC1 is lit, and the internal photosensitive transistor is turned on by the light. IC1 outputs a high level to the input end of the NAND gate, and the NAND gate 1 outputs a low level. The low level is added to the input end of the NAND gate 2, pin ⑤, to control the low-frequency oscillator composed of the NAND gate 2 and the NAND gate 3 to stop oscillating, so that the entire circuit is in a state of alert and standby. When the thief cuts off the audio connection line, the negative connection of the photocoupler to the internal light-emitting diode is also cut off, leaving pin ② of IC1 floating, the internal light-emitting diode stops working immediately, pin ④ of IC1 outputs a low level, NAND gate 1 outputs a 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: K1's normally open contact K1-1 also closes and closes from time to time, controlling the audio alarm device to sound an alarm. The loud alarm will attract people's attention.