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 80's documentation using an NE5514, operating at the time. The font...
This circuit generates sinusoidal signals between 300 and 3000 Hz. We find a 303 Circuits from the 90's....
This circuit from a 2003 documentation, has a protection system that disarms the power supply if...