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.
Found in a 1969 documentation, this circuit consists of a multi-oscillator with neon lamps....
This high-performance circuit was found in the 1982 SGS Linear Integrated Circuits Manual. The...
We have in the diagram another way to make the null adjustment of an operational amplifier. In...