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Non-Contact AC Line Detector using BC547

In this electronic project, I will show you a homemade Non-contact AC line detector, where the detector/tester will light up and make noise when it comes close to a hot (live) wire, even one that's covered in plastic insulation. It is the safest way to make sure the power is off without touching any wires.

Let's make it!

Components List

The following components are needed to make this detector:
  • Q1, Q2, Q3: BC547 NPN Transistor
  • R1: 220Ω
  • R2: 100KΩ
  • R3: 1MΩ
  • D1: 3mm Red LED
  • BZ1: +5V Buzzer
  • B1: 9V Battery
  • 2mm Copper wire
  • Connecting wires

Here, you can use any basic NPN Transistor for this project like BC548, 2N2222, 2N3904, etc.

Circuit Diagram

Schematic of non contact ac line detector or tester using bc547 circuit shown below.

Circuit Explanation

The working principle of the Non-contact AC line detector circuit is based on simple transistors basics i.e. Amplification process.

As we know, an AC carrying conductor always produces a small magnetic field with an oscillation around it. The circuit based on 3 different stages where the weak a.c. signal oscillation became strong and at the end it will able to run a LED as well as the buzzer.

When the detector circuit gets d.c. power, the circuit is not active initialy. After taking the circuit Antenna (make with small piece of copper wire) near the 3 Pin Outlet or close to a hot (live) wire, then the Base of the Q3 transistor catches some signal and it became conductive.

But, the signal which is coming form the Q3 is not too much enough to light up a LED as well as powered the Buzzer. Therefore, current flows from R3 resistor to Q2 transistor's Base pin helps to became conductive and boosting the input signal.

As a result, the incoming current flows from the R2 resistor to Q1 transistor's base and the input signal became so high that it can light up a LED as well as powered the Buzzer.

Please handle the Detector circuit with care, if you mistakenly give or tuch the high raw voltage into the sensor antenna as a input signal then it will blow up the AC Line Detector Circuit immediately without any indication.


  1. This is a good DIY project. I'll make it!

  2. Can i use bld123d transistor to make non contact tester ac tester insted bc547


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