This small circuit switch off the equipment in the audio rack when sound has not produced for some time.
The circuit is actuated by pressing S1 which causes the capacitor C1 to be charged. Next, the output of opamp IC1B goes high, and the audio equipment is powered from the mains via solid-state relay ISO1.
The LINE OUT signal from the audio power amplifier is fed to the input of the circuit via connector K1. Opamp IC1 is set up to function as a signal detector with a trigger threshold of about 50mV. Note that the ground potential of the audio amplifier is raised to about +4.5V in the auto power off circuit by means of R1 , R2 and R3. When the audio signal is greater than 50 mV, (i.e. 4.05 V with respect to the circuit ground) the output of IC1A goes high and transistor T1 starts to conduct. Consequently C1 is charged rapidly, so that ISO1 continues to conduct and power the equipment.
Auto Power off for Audio Equipment Circuit Diagram:
In the absence of an audio input signal, C1 is discharged slowly via R5 and R6. Opamp IC1B toggles and the equipment is switched off via ISO1 when the capacitor voltage drops below the voltage set with P1 at the inverting input. It should be noted that the solid-state relay specified here has a maximum current rating of 1.5A. When heavier loads are to be switched, it is recommended to use a conventional relay.
Since the relay outputs and the transformer primary are connected to the mains, great care should be taken to ensure the electrical insulation. For reasons of safety, the circuit is best fitted in a mains adapter enclosure with a moulded mains plug. The two mains connections in the enclosure must be made with properly rated and secured screw terminal blocks. The output is connected to a mains cable with a 4 or 5-way distribution board.
The delay before the equipment is switched off will depend on time needed to rewind a tape, change a compact disc or record etc. To adjust the delay, connect a 100 kΩ resistor across R5. This reduces the actual delay by a factor of about 10. Turn P1 fully in the direction of R7, press the START button and wait for the desired delay (divided by 10) to elapse, whereupon P1 must be adjusted until the output of IC1B goes high. Then remove the 100 kΩ resistor, press START again and time the actual delay. IF necessary, rea-djust the preset.