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.
Found in old American documentation, this circuit is current as it uses the 555 to modulate an...
We found this circuit in an old documentation from 1979. It is a logic level tester that is...
Found in the April 1978 issue of Radio Electronics magazine, this circuit is intended to be...