Audio-controlled mains switch

C G Mangold 89 audio-controlled mains switch EE 110 July/August 1986 It is often useful for audio or video equipment to be switched off auto- matically after there has been no in- put signal for a while. The function of the on-off switch in such equipment is then taken over by switch Sz in the accompanying diagram. It remains, however, poss- ible to switch off manually by means of Si. Automatic switch-off occurs after there has been no input signal for about 2 minutes: this delay makes it possible for a new record or cassette to be placed in the relevant machine. The audio input to the proposed cir- cuit may be taken from the output of the relevant TV set, amplifier, or whatever. The input earth is held at +6 V with respect to the circuit earth by potential divider RI-R2-R3-R4. The two 741s function as comparators: the output of ICI goes high when the in- put signal is greater than + 50 mV, whereas the output of IC2 goes high when the input signal becomes more negative than -50 ...