Sonic remote control light switch

The power circuit is composed of VD5 ~ VD9, c8, C9, etc. After connecting to the 220V AC power, both ends of c8 can output
a DC voltage of about 12V to power the entire machine. VT3 and VT4 form a bistable circuit. Due to the action of capacitor C7, VT3 is in a cut-off state when power is first applied
. VT4 is turned on, its collector outputs low level, VT5 is cut off, the relay K does not act,
its normally open contact kl is opened, and the lighting lamp H does not light up.
    If we send a sound wave signal, the piezoelectric ceramic transducer B converts the sound wave signal into a corresponding electrical signal and
sends it to the frequency selective amplifier VTI for amplification. If the frequency of the sound wave electrical signal is within the frequency selection range of L and CI, This signal is
output by the collector and added to the base of the negative pulse generator Arf2 via C2. The collector of VT2 outputs a negative pulse trigger signal,
forcing the bistable circuit to flip, and VT3 changes from the off state to the on state. , VT4 changes from the on state to the off state, and
its collector outputs a high level, causing VT5 to quickly conduct, the relay K is electrically closed, its normally open contact kl is closed, and the lamp H
lights up. If you need to turn off the light, you only need to send another sonic signal, the bistable circuit flips again, VT3 is cut off,
VT4 is turned on, the relay K is released, and the light H goes out.