As shown, the circuit by the sound detection circuit and the monostable trigger circuit, the actuator for the relay. VT1 complete input audio signal amplification, when the sig
nal comes, 555 set, K pull. Meanwhile, VT2 conduction, the charge on C2 through the interior of the discharge tube 555 and VT2 immediately discharged. After the sound, VT1, VT2 off, C2 through R2, RP2 charging it monostable circuit transient stability time td l.1 (R2 RP2) C2, delay range from 0.05 to 0.5 seconds, and can be adjusted by RP2 .