You’ve probably seen probes that detect live wires in, for example, home wiring, without having to actually probe the wire. These are sometimes used to test strings of Christmas lights, too. We’ve even seen the sensors built into a voltmeter. [Crazy Couple] has a few do-it-yourself versions that can do the job. You can see the circuits in the video below.
A contactless probe picks up the changing magnetic field around an unshielded wire with an AC voltage on it. Current doesn’t have to be flowing since it picks up the voltage (for example, you can detect voltage on a switch that is turned off or a Christmas tree light that is burned out. There are several different circuits using chips ranging from a CMOS IC to a 555. There’s also a version with three bipolar transistors.