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.
We found this circuit in an old documentation. Its purpose is to turn on warning lights when the...
This percussion circuit was found in an Electronics World of 1968. The transistor may be general...
We found this transmitter circuit with a single chip in the Motorola Linear Databook of 1990. The...