As I understand it, a non-contact voltage tester works by creating a capacitive connection from a power line to ground, via the detection circuit and the operator's hand/body. This connection passes a tiny amount of current which the detection circuit can pick up.

I'd like to use my Arduino to detect the presence of mains voltage (120v) in a wire without actually connecting to the conductor (for safety and isolation reasons). Because there may not be any current in the wire, current transformers/hall effect sensors would not work.

Is it possible to use a similar approach to the non-contact voltage testers to detect the voltage? I couldn't find any schematics online for commercial testers to see and understand how they work.

  • \$\begingroup\$ The only commercial NCV circuits I can find (e.g. in US patent 6653826 B2) rely on the capacitance of a human operator wielding them. Unless your Arduino is to be hand-held, these may not be useful. \$\endgroup\$ Dec 27, 2013 at 21:50
  • \$\begingroup\$ You might want to consider a home made "opto isolator", LED night lite combined with a photo detector. \$\endgroup\$ Dec 27, 2013 at 22:36

2 Answers 2


I ended up going with a circuit based on the design here. With my modifications, it seems to work well, and does not pick up other RF emissions as false positives.


simulate this circuit – Schematic created using CircuitLab

The antenna is a 4-inch length of solid 22 gauge wire. Out is high until voltage is detected. D2/C1/R1 filter out the 60Hz pulses from the 4011 output, so that the output stays constant.

  • \$\begingroup\$ I image you needed to tie the "ground" on this circuit to true ground (ie a ground prong on an outlet), right? I am thinking of doing something similar, but to detect 220, so I am thinking of using an differential amp solution, but think I would need to tie the sense circuit "ground" to true ground. \$\endgroup\$
    – nPn
    Dec 7, 2017 at 12:48

You don't need to use an arduino at all. All you need to detect AC voltage without contact is a simple set of 3 transistors (circuit here). I don't know if this is how commercial detectors work, but it seems to be just as effective. If you do want the signal to be available to an arduino, you could probably just hook the wires for the LED to a digital input pin. Alternatively, you could be super-careful and run it thorough an optocoupler first.


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