Cleverly designed voice-activated electronic doorbell

 

Transistor V2, potentiometer KP and capacitor C2 form a control circuit, and V1, V3, R2 and C1 form a complementary oscillator. When the switch S is closed and the power is turned on, the power supply charges C2 through C2, the BE junction of V2 and the speaker BL. The large instantaneous charging current causes V2 to saturate and conduct, clamping the collector potential of V3; after the charging of C2 is completed, The power supply provides base current to V2 through KP to maintain the critical saturation state of V2, so that the oscillator does not work. When external sound waves vibrate the speaker cone, an induced voltage is generated at both ends of the speaker. This voltage is added to the emitter of V2, causing V2 to exit the critical saturation zone and enter the amplification zone. The voltage drop between the collector and the emitter of V2 increases. This causes the oscillator to oscillate and BL to emit an audio sound. To release the beep, you can open S and close it again. VI selects 3DG6, 3DG6, B>=80; V3 selects 3AX31, B>=60; V2 selects 3DG6, B>=100. BL uses an 8-meter speaker and installs it on the upper middle part of the inside of the door panel. Visitors can sound the message when they knock on the door.