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 is a voltmeter with high input resistance. The transistor is germanium. We found it in an...
Found in old American documentation, the components used are still common. The circuit directly...
This circuit was found in an Electronics World magazine from 1990. The circuit can be mounted...