This circuit uses a lamp driven by the Hertz loop and measures its brightness with ordinary photovoltaic cells. The circuit is from a Radio Electronics from February 1964.
This circuit simulates an inductor. Or operational amplifier of old type, but a configuration is...
This circuit for the logic control of the 555 in a power supply was found in the October 1989...
This traditional configuration from a 1980 publication shows how a reed relay is used to control a 240...