This is the Audio Controlled Mains Switch diagram circuit that is most useful for audio or video equipment, where the device can be turned off automatically when there is no input signal.
This can be handled properly by the ON-OFF S2 switch. But you can still turn it off manually by using the S1 switch. Turn off automatically when there is no input signal for 2 minutes. This will be effectively applied to various hobbyist needs.
The input signal from this circuit can be in the form of TV, smartphone or other audio sources. Ground input is held at + 6V by a potential divider circuit R1, R2, R3, and R4. Two IC 741 functions as a comparison circuit, which IC1 output becomes high when the output signal is greater than +50 millivolt.
Conversely, IC2 output will be high when the input signal becomes more negative than -50mV. The combination circuits of resistors R6, R7 and R8 form like an OR gate which will drive transistor T1. If either IC1 or C2 output logic 1 (high), transistor T1 will be active.
IC 555 works as a monostable multivibrator capable of retriggers, which have periods determined by R10 and C1. This circuit will work when triggered by closing the S2 switch. When active, pin 3 IC555 will have high logic for 1 to 2 minutes depending on the leakage current that accompanies it. A monostable multivibrator will reset itself when the current at C1 exceeds a certain value.
As long as there is an input signal entering the circuit, the transistor T1 will be active and capacitor C1 has no charge. But when the input signal stops, the transistor T1 is inactive, and capacitor C1 is in the condition of charging the charge until the charge is sufficient to trigger IC 555. To reset IC 555 can be done by closing the switch S1.
When IC3 is reset, C1 will automatically be released via C7, Resistor R11 serves to protect transistor T1 from short-circuit on the input channel. When IC3 output logic is high, the transistor T2 will be active and provide current to the relay, so the relay will be active and the relay switch will be closed. Diode D1 serves to protect the relay from the high voltage jumps generated by the relay coil induction.