Sound activated relay

  
The device remains dormat (in an off condition) until some sound causes it to tum on. The input stage is a 741 operational amplifier connected as a noninverting follower audio amplifier. Gain is approximately 100. To increase gain raise the value of R2
Sound activated relay - schematic

The amplified signal is rectified and filtered to a dc level by R4. Then R5 is set to the audio level desired to activate the relay.




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