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.
This test circuit was found in the PE Hobbyst Handbook in a 1990s edition. The circuit tests the...
We found this circuit in GE's 1977 transistors manual. Transistors admit equivalents according to the...
Here is a very useful circuit that can be expanded, including the use of more integrated circuits...