This circuit consists of an electrostatic sensor that can check if a cable is energized by simply approaching it. The circuit is from an Electronics Now from December 1998.
Found in documentation from the 90s, this circuit is based on operational amplifiers of the time....
This oscillator, which can be used as a signal injector, was found in a Popular Electronics from the 90s....
This traditional oscillator uses two complementary transistors to produce low frequency signals....