This silencer or squelch preamp circuit was found in a documentation for radio amateurs. It completely mutes the circuit when there is no sound picked up by the microphone.
This is the complete circuit of a multimeter using a 50 uA instrument with a 2k coil. The circuit...
Two comparators are used to indicate the status of a 9 V battery. The circuit is from Electronics Now...
This is a possible circuit for the receiver of the previous circuit. Other circuits can be found...