It is often useful for audio or video equipment to be switched off automatically
after there has been no input signal for a while.
The function of the on-off switch in such equipment is then taken over by switch
S2 in the accompanying diagram. It remains, however, possible to switch off manually
by means of S1. 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 circuit 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 R1-R2-R3-R4. The two 741s function as comparators:
the output of ICI goes high when the input signal is greater than +50 mV, whereas the
output of IC2 goes high when the input signal becomes more negative than -50 mV.
Resistors R6, R7, and R8 form an OR gate that drives transistor T1. If the output of
either IC1 or IC2 is logic 1, T1 conducts.
The 555 operates as a retriggerable monostable, whose period is determined by R10 and
C1. The device is triggered when its pin 2 is earthed by the closing of S2. Its
output, pin 3, then remains high for 1 to 2 minutes, depending on the leakage current
of the 555. The monostable resets itself as soon as the potential across C1 exceeds a
certain value. As long as there is an input signal to the circuit, T1 conducts and
C1 remains uncharged. As soon as the audio signal ceases, T1 switches off, and C1
charges until the potential across it is sufficient to reset the 555. The monostable
may also be reset by closing S1, which connects pin 6 of the 555 to +12 V.
When IC3 is reset, C1 is discharged via its pin 7. Resistor R11 serves as protection,
because without it T1 could short-circuit the supply lines.
When the output of IC3 goes high, T2 conducts, the relay is energized, and the relay
contacts switch on the mains voltage as appropriate. To counter the induced potential
when the relay contacts close, which could damage T2, diode D1 has been connected in
parallel with the relay coil.