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.
However, you do it, the key is detecting a very small magnetic field. Honestly, this is one of those things that are educational, but not very practical when you can pick up a pen-style probe for well under $10 (Harbor Freight’s entry is about $6). Still, the circuits are very educational, especially if you take the time to work out how they do what they do.
You don’t see it as often, but you can even get noncontact voltage probes made to connect to an oscilloscope. It would be interesting to experiment with variations of some of these circuits interfaced to a scope.