Non Contact Voltage Detector




About: Part software developer, part maker.

In this Instructable I’ll show you how you can build a non contact voltage detector for checking of live power wires.

Tools and materials used (Affiliate links):



Prototype PCBs:

Soldering Iron:

Solder Wire:

Step 1: Transistor Operation

A transistor is a device that can be used in two basic operations, as an electronic switch or as an amplifier. Depending on the current we apply to it’s base, it can control a much larger current through the collector and emitter path with a typical multiplication of around 200 times. This is called the transistor gain.

By connecting the output of one transistor to the base of another, we can multiply this gain to now get an amplification of 40 000 times. By building a circuit with three such stages we can create a device that is capable of detecting even the tiniest charges and movement of electricity.

Step 2: Assemble the Components

To begin, take a piece of perfboard with at list five to six rows of holes. I’m using this 2 by 8 cm board that I’ve purchased online.

Place the first transistor on the first row of holes and the second one, one row apart. Additionally, move the second transistor one hole up so its emitter aligns with the base of the first transistor. Same as with the second transistor, the third one is placed one row apart with its emitter being aligned with the base of the second transistor.

All three of the resistors connect to the collectors of the transistors and the values are all marked on the schematic.

Check out the full schematic on EasyEDA:

The LED is connected with it’s negative side to the resistor on the first transistor and its positive side is then commoned with the resistors.

Step 3: Add the Antenna

The sensor for the circuit will be this copper rod that I’ve soldered on the top of the PCB and it is then connected with a piece of wire to the base of the third transistor.
The entire circuit is powered with a 9v battery and as soon as you bring it close to the wires connected to the mains voltage, the LED starts flickering.

While this is a nice indicator for isolated cables, it should never be used to test bare wires. For testing open contacts, only use proper isolated measuring equipment.

Step 4: Experiment With 4th Stage

As a final experiment I wanted to see how the detector will react with one more stage so I soldered one more transistor with a 5MOhm resistor so we now got 4 stages in total of amplification. The detector is a bit more sensitive now where it can detect the wire a bit further apart than before. Additionally it is also more sensitive to false triggering to a static electricity as well.

Step 5: Enjoy

I hope that this Instructable was educational and interesting so I suggest to follow me and don’t forget to subscribe to my YouTube channel.

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    7 Discussions


    Reply 4 months ago

    Sometimes, but it is not that reliable as it also reacts on static charge, walls can trigger it even if there are no cables in them.


    4 months ago

    I see these type circuits on here from time to time. And yes there are actually extremely high gain circuits. But you need to also tell people that just because such a circuit doesn't indicate voltage on a line, doesn't necessarily mean there is not any voltage present. Any failure in the circuit, be it battery low or a broken run in the circuit will not indicate voltage present and if the person touches that circuit , they could be in for a severe shock or worst. Safety has to come first.

    With that all stated, nice high gain setup. Most any NPN transistor will work and even a Darlington Transistor can as well. Thumbs Up!

    1 reply

    Reply 4 months ago

    Yes, even the professional ones are not reliable.
    Might be OK to say "watch out", but I still wouldn't rely on them - they give false positives and false negatives.
    A metal detector might be a better bet.


    Tip 4 months ago

    The copper wire sensor should be insulated. Professional voltage detectors have insulated probes, so this one also should be. This allows insertion into outlets to detect AC voltages. It will not reduce sensitivity.

    Add a warning label "AC only" because it may not detect high DC voltages, unless the detector circuit is modified. by adding a ground reference wire. to the circuit under test.


    4 months ago

    Only for alternative currents, doesn't detect continuous voltages.