This Vox or sound activated relay was found in an American documentation from 1975. Equivalent transistors can be used, and the SCR can be a TIC106. The relay is according to the supply voltage.
Obtained from an English publication, this circuit actually needs a current limiting resistor,...
This 2003 circuit serves to digitally show the intensity of a sound captured by a microphone....
Suggested by a Popular Electronics from the 90s, this circuit uses a buzzer to indicate the...