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 found in an English publication from the 80's. It triggers a buzzer (with...
The SN72741 integrated circuit is nothing more than the version of Texas Instruments of the known...
This simple circuit connects in external battery to a load when the AC Power line fails. S1 can be...