This simple circuit disconnects equipment rack of your audio in case has not delivered a sound a specified period. Circuit is operated by pressing his S1, which produces loading of C1. Further, switch IC1b AO exit in the state “up” and the audio equipment is powered from the network through semiconductor relay ISO1.
LINE OUT terminal signal of power audio amplifier is applied to the entry circuit through connector K1.
Operational amplifier IC1a is configured to function as a signal detector with release threshold of approximately 50mV. Observe that potential for audio amplifier ground is high in the autodeconnecting circuit at approximately +4.5 V, through R1-R2-R3 resistors. When the audio signal is greater than 50 mV, IC1output goes in H state and T1 transistor starts to conduct. In the absence of audio signal input C1 is slowly discharging through R5 and R6.
Semiconductor relay specified here has a maximum rated current of 1.5A. If it is necessary to switch larger loads, it is recommended to use a conventional relay.
For safety reasons the best solution for mounting circuit is in the carcass of a network adapter. The two connections to the network inside the carcass must be carried out using a connection strip with screw, well designed and properly fixed.
To adjust the delay, connected by a 100 kΩ parallel resistor with the R5. Thus, it reduces the actual timing factor with approximately 10. Rotate P1 completely in the direction of R7, press START button and wait leaking of time desired (divided by 10), then adjust the output P1 until his passing in the state IC1b H. Remove the 100KΩ resistor, press the START button and measure the real timing. If necessary, readjust semipotentiometer.