This circuit was obtained from a National Semiconductor manual from the 1990s. The circuit employs a pair of matched transistors, and the indicator is analog.
In this circuit the capacitor under test is charged at switch position A and then discharged...
This is a common circuit in the 1990s, when experimental FM transmitters were in vogue. The...
Using a piezoelectric sensor of the time, this circuit was found in the February 1994 issue of...