DIY Non-contact Low Cost AC Voltage Detector


Electric shocks can cause a lot of injury, especially when maintaining and repairing power lines and pylons. It is difficult to isolate and check for voltage in a wire without cutting it. When dealing with such situations, a non-contact voltage detector may come in handy to ensure that voltage is not present before performing any tasks related to electrical system repair. Also, it is always advisable to make sure there is no voltage being supplied before troubleshooting an electronic device at home. DIY Non-Contact Low Cost AC Voltage Detector can help! It uses minimal resources and performs very well when used for such purposes. Whether identifying live or distinguishing line from neutral, low cost AC voltage detectors are available. You can also check the broken wire detector circuit with CD4069, it also works very similar to our circuit.


AC Voltage Detector

1. Components of the AC Voltage Detector

Components needed for our FireWire Detector project, the components needed for this module are listed below and you can find them at your local hobby store.

1.1 3 NPN transistor (BC 547/2N2222)

1.2 220 ohm resistors

1.3 9 volt battery

1.4 Light Emitting Diodes (LEDs)

1.5 buzzer

1.6 copper wire

2. Circuit Diagram and Connection of AC Voltage Detector

Due to the small number of components, the production of the live wire detection circuit is not very complicated. It uses different components including 3 NPN transistors, LED, 220 ohm resistor, 9V battery and a copper wire. The antenna is connected to the base of a first NPN transistor (Q1), the emitter of the first transistor (Q1) is connected to the base of the second emitter (Q2), forming a Darlington pair (further explained). Connect the emitter of the transistor ( Q2 ) to the base of the transistor ( Q3 ) again, then make it act as a switch and help the LED glow. The collector areas of transistors Q1 and Q2 are shorted and connected directly to the positive terminal of the 9V battery, while the collector of transistor Q3 is connected to the LED (D1) cathode (negative) terminal and the anode (positive) terminal is connected to one leg of a 220 ohm resistor ( R1 ), the other end is also connected to the positive pole of the 9V battery.

3. Operation of AC Voltage Detector

The copper wire is twisted to act as an antenna, which detects any electromagnetic induction around it and produces a very low signal. After that, a series of transistors come into play. The signal from the antenna may enter the base of transistor Q1 in nanoamps, where Ic = β × Ib. Since the Beta ( β ) value is very large (around 110-800), this in turn gives us a larger collector to emitter current as output. The process repeats again, with the emitter of transistor Q1 going to the base of transistor Q2. Therefore, further increase the current level Beta (β) times. Transistor Q3 acts as a switch and turns on the LED and buzzer when current is supplied to the base terminal of transistor Q3.

Tip: The sensitivity of the module can be changed by increasing or decreasing the size of the antenna coil.

Requires multiple transistors (Darlington pairs)

Darlington pairs are also known as super-alpha circuits. It is an arrangement of standard bijunction transistors (NPN and PNP) with their bases and emitters connected to increase the sensitivity and gain of the transistor. For our live wire detector project we can put them to use to increase the gain and generate more current to switch the third transistor Q3. Output gain is simply the Beta (β) of a single transistor multiplied by the input current supplied to it. Therefore, we get a higher gain output.

Output current: β1 × β2 × input current

Since two identical NPN transistors are used to form a Darlington pair, so that Beta1(β1)/hfe1 and Beta2(β2)/hfe2 are equal, the current gain will be:

β1 = β2

Ic = (β1 + (β1×β2) + β2) × Ib

Ic = (β^2) × Ib (negligible because 2β is very small)

4. Testing of the AC Voltage Detector

The DIY AC Voltage Detector Module is powered by a 9V battery and ready to detect any AC voltage. Move the module and antenna along the live wire and you can see the LED light up and the buzzer start beeping, as shown in the picture below, our circuit is placed near the live wire that powers the RPS.

The antenna on the circuit board detected an electromagnetic signal, so it was a live wire and should be handled with care.

Trial Sample
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