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 circuit uses a photo-transistor to trigger a circuit sourcing a square signal for the output. The...
This power control circuit was found in General Electric documentation from 1969. The table...
This circuit, from the manual of the integrated circuit used, is a bass and treble control for...