Clever use of kd-28 as voice-activated switch circuit diagram

As shown in the picture is a practical voice-activated switch . A1 is connected as a conventional amplifier to amplify the audio control signal picked up by microphone B. The amplified signal is doubled and rectified by C5, VD1, and VD2 to charge the capacitor C6. When the voice control signal is large enough, the charging voltage is as high as the level of pin 7 of A2 . At this time, the amplifier A2 outputs a high level similar to that used by the op amp. , relay K is powered on, and its normally open contact is closed to connect the controlled electrical appliance. R7 and VD3 make amplifier A2 self-locking, so pin 5 of A2 always outputs high level to ensure the relay's pull-in state. SB is the release button. When SB is pressed, low level is input to pin 8 of A2, A2 flips, pin 5 outputs low level , and relay K is released. The potentiometer RP is used to adjust the sound control sensitivity of the circuit. When selecting relay K, it should be noted that its rated operating voltage should be about 2V lower than the power supply voltage Vcc.

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