Simple non-contact voltage tester using transistors

Last updated on May 14th, 2022 at 11:14 am

This is a simple non-contact voltage tester using transistors next to one circuit that use all-transistor as the main, it is a cheap and easy build. Many friends like the Non-contact AC Detector Voltage. It uses CD4060 as the main, some people say that Can not find it.

Today I would like to introduce another interesting cycle, because of the transistor. It is certainly easy to buy. And also a simple circuit intact.

How does circuit work

As Figure 1 is a complete circuit diagram of this project. We see that and are happy since it is simple for all.

Simple Non-Contact AC Mains Voltage Detector

Figure 1 the complete simple Non-contact AC mains voltage detector.

We use the main principle with a very high gain amplifier by three transistors Q1, Q2, Q3 are 2N3904 NPN type with popular to use in many circuits. They have about 8,000 gain so they have a very high sensitivity, when we take some wire about 10 cm connect to the base of Q1 it will act as an RF antenna.

Recommended: Learn transistor circuit works here

As you well know that when an AC-line flow to any wires it will have The electromagnetic field spread out.

Thus when we take the antenna or probe near any wires will have This signal will be induced to expand the force which it will have 60Hz or 50HZ as our main frequency. Then we use the simple buzzer circuit to indicate hum tone as AC main frequency and there is also an LED display (LED1) flashing.

The simple buzzer please read Make simple electronic buzzer.

The component list
Q1-Q4: 2N3904, 0.4A 40V NPN transistors, 4 pcs.
Q5: 2N3906, 0.4A 40V PNP transistors, 1 pcs.
0.25 watts Resistors
R1: 1M, 1 pcs.
R2: 100K, 1 pcs.
R3: 56K, 1 pcs.
R4: 1K, 1 pcs.
R5: 100 ohms, 1 pcs.
C1: 0.047uF 50V, Ceramic capacitors, 1 pcs.
C2: 10uF 25V, Electrolytic capacitors, 1 pcs.
LED1: Color as you want, 1 pcs.
Speaker 8 ohms, 1 pcs.
S1: SPDT switches on-off, 1 pcs.
9 volts battery

How to build an application

The prototype we assemble on small perforated board size

In this project we can assemble them on a perforated PC board as Figure 2. We do not need to set it since no potentiometer, just press S1 to turn on then already to use so well.

Figure 2 The prototype we assemble on a small perforated board size.

Remember Do not use probe or antenna touch to AC-main directly. This circuit will damage and we will also danger from high voltage.

In the video below we will see when to take this tool near the electromagnetic field such as the Main, Lamp, monitor, Microwave, laptop, computer and etc. LED1 will flash and we hear a hum tone sound.

I hope this tool will help you save your body and money it is really easy for you.

You also may like circuit


I always try to make Electronics Learning Easy.


  1. What A Handy Dandy Little Tester To Have..It Could Very Well Save Your Life One Day!!!!

  2. walusimbi frank

    thank you very much for these great projects

  3. Dip

    I find your ckt. very useful for safety(AC) but it would be good to have a similar one for DC(600V) Also.
    can you provide that ckt. for DC also?
    Thank you in advance.

  4. Ruben McMahon

    Excellent. Absolutely loved reading and very helpful at the same time 🙂

  5. Manoj Dhali

    Respected Sir/Madam I wandering for a long time this type of circuit making on it a normal vero board, Sorry to say I make it 2 time properly but it’s extremely failed. I will try it one more time, let’s see what happen. Thanks for the idea.

    • Hello Manoj Dhali,
      It is a circuit built with fun. We will notice different levels of tones. When the antenna is placed near the AC power cord.
      Low gain transistors may not run well.

      I hope you try to build other circuits. These circuits are simple. but increases our learning well.


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