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.
The circuit shown is from an old European publication. In it we have the way to activate a relay...
We found this circuit in a European publication from the 1990s. Transistors are still common types...
This circuit was found in an American documentation and can be added to common amplifiers of any...