This circuit was found in documentation from the 80s. The components are from the time, but the configuration can be adapted to use modern components.
This sophisticated audio voltmeter circuit was published in the March 1966 Radio Electronics...
This immobilizer circuit for the car was found in the Radio Electronics magazine of December 1976....
This 50 Hz rejector or notch filter was found in a 1975 documentation. Other frequencies can be...