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 RF monitor or field strength meter can use instruments from 100 uA to 1 mA full scale. The...
This amplifier for automotive use was found in the SGS manual of linear integrated circuits of...
This SCR circuit can operate on frequencies from 1 to 20 kHz. We found it in an American...