Make a Non-Contact Voltage Probe

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.

We’ve seen various types of non-contact voltage and current sensing over the years. We’ve even seen biological voltmeters lately.

10 thoughts on “Make a Non-Contact Voltage Probe

  1. I’m so bad at electronics and general hackadaying stuffs, I can’t figure out if *this one* is an april’s fools. Doesn’t sound like it, but maybe I’m so lost I can’t know ? April’s fools is stressful.

    1. Elon dropped a trap single about Harambe. For real. April fool’s is cancelled. Honestly it’s about time, it’s just become another holiday for brands anyway.

      But back on subject, my favorite tool for this is one of those tone probes you use to trace phone lines in the wall or whatever. Bonus: you can sense so much other stuff besides the usual 60Hz hum. You can kinda tell what’s going on for a lot of things, and the brain processes certain frequencies of information much better as audio instead of imagery. Makes sense. Worth a buy, or if you’re like me, nab one from the junk drawer at work right before you quit.

  2. Fun video I suppose but the theory side incorrectly mixes several concepts. The video and the summary both confuse magnetic fields with electric fields (e-fields). AC voltage detectors sense the changing electric field and are not magnetically coupled. They will work with or without current flowing in the conductor. DC electric field sensors are also a thing, but are more complicated as charge tends to accumulate.

    1. Indeed quote correct (though DC just as easy with high value bleed resistors for most part and/or switching circuit toggle).

      This is why dirt cheap and simple MOSFET circuits can be built very easily and really small too. Simple variation of the encroaching thunder storm detectors which came on the market shortly after first insulated gate MOSFETs appeared which failed way too easily since the input impedance extremely high with tiny capacitance, I recall very early power MOSFETs failing randomly due to the self generated static by even sliding them out of the tube despite full on high grade 3M static controls on bench, wrist/ankle and floor. In one case it was flakey mains ground to lighting enclosure together with induction from over head fluoros generating a 27v AC field across (at) the static mat surface but just enough volts directly under that one long light fitting which exceeded the 20v gate breakdown – easily fixed lowering the static mat’s and wrist groud resistor and fixing the fluoro fitting grounding. Entertaining watching the techs with their manager arguing profusely with the 3M representative about their mats and how many tubes of expensive power MOSFETs they’d lost for making inverters – eeks :D

    2. It is apparent from the schematics in the video, as well as from the footage of breadboarded circuits in operation that these “coils” are actually single-ended. The coupling of the AC line to the circuit is in all three cases capacitive, not inductive (magnetic).

  3. Has anyone seen an ECG using a single probe point? Or maybe two independent probe points? That would make ECG on a smartwatch possible without requiring lots of wiring on the body.

    1. You really need at least two points some distance away from each other for the differential measurement; the range relative to ground varies an order of magnitude more than the signal you’re trying to extract.

      I recommend experimenting first with an armband that could go on the same arm as the watch, but a few inches away, and plot out how the resolution changes for different locations.

      1. I understand that. I have built an analog differential amplifier for ECG many years ago. It is not that easy to get all the noise out.

        That being said, the signal is there to be measured, two independent very sensitive (and with high dynamic range) point like charge measurements could be used, the info shared via radio (like BT) and the differential part could be done via software.

        Maybe easier to construct a starship probe…

        1. To use a radio between them what would do about a reference point? The wire with a consistent impedance between the sensors is what lets you get away with a pseudo-ground as a reference. Without that, you’re going to need a really high quality reference. And what about thermal drift? You don’t have any thermal coupling at all, so they’ll be different temperatures. Just having radios ensures that. How can you have that good of a “common” reference without any coupling to make it “common?” You don’t really want to run wires to Earth from a smart watch, so instead you’ll have to be doing a differential measurement against some voltage reference on each side. Is it realistic to get references that would be close enough, even with thermal differences?

          Certainly possible. Definitely harder than a starship probe; a metal detector circuit might work fine for detecting starships, at least the ones constructed on Earth. An electronically scanned radar is surely easier; people build those at home all the time. And a starship is a bigger target than the soda cans people usually detect.

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