This circuit was originally intended for reading movie soundtracks. The circuit is from 1967 and was found in an English publication. The sensor is semiconductor type.
This circuit makes use of a low impedance microphone. The relay is according to the supply voltage...
This circuit was found in an old Radio Electronics, but can still be assembled as the components are...
Any small sound amplifier from 200mW to 1W can be used in this simple circuit. Or circuit of two...