Using a 555, this configuration appears elsewhere on this site. This is from American documentation from the 90s. The indicator can be an LED or some device controlled by a relay.
This configuration uses the 4 gates of a 7401 integrated circuit. The capacitors determine the...
We found this circuit in a documentation from the 70s. The original text was kept. The oscillator...
This circuit was found in the September 1974 issue of Radio Electronics magazine. It consists of a...