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 was designed to power the previous circuits of the wireless sound system, but it can...
This circuit is from the documentation of Radio Shack 1975. It can be mounted with 12 V relays,...
This circuit is suggested by the Optoelectronics Sourcebook by Texas Instruments when AC signals...