This circuit, from a 303 Circuits from the 90s, uses a telephone pickup coil as a sensor to capture power grid noise, thus making it possible to locate electrical wiring.
We have dozens of similar projects in articles and circuits on this site. This is from Radio...
This circuit with an unusual configuration was found in the January 1974 issue of Radio...
This interesting circuit from 1967 documentation uses three series lamps to regulate the voltage...