working principle:
Q1 and Q2 are composed of secondary audio amplifier circuit, audio signal received by the MK1 by C1 coupling to the Q1's base, amplified by the collector directly fed to Q2's base, collector in Q2 get a negative square wave, used to trigger the bistable circuit. R1 C1, the circuit frequency response for high sensitivity limit in the range of about 3kHz. When the power supply is switched on, the bistable circuit state for Q4 cutoff, Q3 saturation, D3 light. When MK1 receives the control signal after two amplifier outputs a negative square wave, after differential processing after a negative sharp pulse through D1 added to the Q3 base, the circuit quickly reversed, D3 was lit. When MK1 received a control signal circuit, and flipped out D3.


Note: electret mic which foot is connected with the outer casing is negative, the light emitting diode and an electrolytic capacitor feet long as cathode (length is short negative) and triode installed in front of a kind of direction, resistance and ceramic capacitors without polarity.