The hot wire indicator is a fat pencil shaped thing that senses the field around the wire to decide whether to turn the LED from green to red. No connection required.

 

I have a stud finder (used to locate studs inside a wall) that indicates when AC voltage is present. It doesn't appear to
require that the circuit be loaded. They're sold in most all home improvement and hardware stores.

 

I have a stud finder (used to locate studs inside a wall) that indicates when AC voltage is present. It doesn't appear to
require that the circuit be loaded. They're sold in most all home improvement and hardware stores.

A stud finder relies on magnetic detection of nails or gyproc screws etc, there is also a capacitive version that detect a change in density (stud).
Max.

 

A stud finder relies on magnetic detection of nails or gyproc screws etc, there is also a capacitive version that detect a change in density (stud).
Max.

Yes, a simple stud finder works as you describe. The multi-tool version is both a stud finder and mains power indicator. The reason it is important, is to make sure the user does not drill, screw or pound through a live wire.

 

The LED would be powered from the source, It would not require its own batteries, unlike, the mains power indicator refereed to by Ronald Bridges and #a12. Here, I presume that, this is the case with the multi-tool version.

To maximize the power transfer from the power cord, which acts as a radiator of electromagnetic energy, the indicator circuit would have to be tuned to the frequency of the mains, just like a radio.

blocco a spirale idea also has merit