Non-contact AC Voltage detector - DIY

Electricity can cause serious injuries or even death which is why safety is our first concern whenever working with electricity or electrical devices. If you are working with an AC supply box or electrical devices that involve AC supply then you must verify that there is no AC voltage remaining even after you turned the supply off. And that is where this non-contact AC voltage detector comes in handy.

Components Required:-
1. 2N3904 NPN-Transistor  x3
2. Resistor 1M ohm, 100K ohms, 220 ohms
3. LED bulb
4. Switch
5. 9v Battery 
6. Connection wires


A conductor carrying an AC voltage produces a varying magnetic field around it. This Non-contact AC voltage detector is equipped with an antenna(copper wire) which is energized by the phenomena of electromagnetic induction, which produces a small amount of current in it.

This device uses 3 NPN-Transistors in order to detect the AC voltage. A transistor has 3 terminals - Collector, emitter, and base. Collector to emitter current is controlled by the base current. When there is no base current, then there will be no current through the collector to emitter. So a transistor acts as a switch.

The gain of the 2N3904 transistor is 200, the gain of a transistor is the ratio of collector current to base current. Now if we connect the output of the first transistor to the second and then the output of the second to the third, then the total will be 8x10^6 (200x200x200=8000000). Therefore a very small amount of current will be able to turn on the switch.

Circuit Schematics-

Non Contact AC Voltage Detector

Place the components on the PCB/Breadboard and connect them as shown in the above circuit schematic.

To make the antenna use a 10-15 cm long copper wire.
Connect the 9v battery as shown in the circuit above and turn ON the switch.
Now the device is ready to use.


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