Vibration / Sound Activated Switch

Someone asked me for a circuit for vibration or sound activated switched in a forum. I had drawn this quickly by hand.

The sound is detected by the electret microphone then is amplified by the PN2222 NPN transistor. Then the amplified signal is DC converted by the 2 1n4148 diodes.

A comparator is then used to detect the right voltage set by the potentiometer at the inverting input. A small hysteresis is added. The last NPN transistor is the one to drive a relay and with another 1n4148 diode to protect it from the reverse EMF generated by the relay coil when the transistor goes off.