Timing circuit with audio control

Circuit working principle: It consists of two parts: audio control circuit and timing indication control circuit
. IC . , Icz, IC3, VT3, VT4 and other components form an audio signal control switch
circuit. When there is an audio signal input at the audio input terminal XS2, the circuit is connected via lC. After amplification,
the photocoupler IC: works. Pins 2 and 6 of IC3 always maintain a low level (
below 1/3 Vcc), pin 3 always outputs a high level, VT3 is cut off, and has no effect on the conduction of VT4.
VT4 is turned on, K. Work, its contact point K,. , closed to protect itself, K, one: closed so that the socket xs
has power and the audio circuit can work normally. When there is no audio signal , IC: There is no working voltage
and is cut off, and the power supply passes through R6 and redundant P. For C, charging, when the voltage of pins 2 and 6 of IC3 rises
to greater than 3/3 Vcc (about 10 rrun), the output state of IC flips, pin 3 outputs
low level, VT3 is turned on, VT4 is turned off, K. When the power loss contact is disconnected, the power supply of the audio equipment
is cut off, realizing the automatic shutdown function with a delay of 10 minutes without signal.