This circuit makes use of a low impedance microphone. The relay is according to the supply voltage which can be between 9 and 15 V. The operational gain is approximately 100.
We find this circuit in an American 80's documentation. It makes use of IC sensor which is mounted...
We found this circuit in a 2005 Linear Technology manual. The circuit makes use of the LT6101 and...
We found this circuit in an English magazine from 1976. The magazine no longer exists, but just...