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 appeared in Radio Elecronics magazine in 1968. The operating circuit is a...
This 22 V symmetrical amplifier provides close to 15 W of output in the bridged configuration. Integrated...
This detector with ultra-low deviation was found in an American documentation from the 90s....