Non Contact AC Phase Fault Detector

The circuit discussed in this article is of a non-contact mains AC field detector which displays the presence of a mains AC field from a distance of more an 6 inches.

Locating Fault in AC Lines without Physical Contact

The circuit can be used for locating faults in house wiring without the need of making physical contact with the inner conductor of the wire and becomes useful in locating the breaks in a wire by pin pointing the area where the AC mains may be blocked due to a breakage.

The circuit is basically high gain non inverting amplifier which is configured using a few opamps and a few other inexpensive passive electronic components.
Just a couple of opamps have been incorporated here from the IC 324 for the required operations.

contact less AC mains phase finder circuit

Design Description

Looking at the figure we notice the following things:

The non-inverting input of the IC is grounded making the sensitivity of the configuration to the maximum.

Similarly a feed back loop created by connecting the output of the opamps to the inverting input helps to increase the gain of the set up many folds.

The input is applied to the inverting input 2 of the IC through a blocking capacitor.

The signals entering via the antenna is quickly picked up by the opamp inverting input and sent to the

preceding circuit for the required processing and amplification.

It may be interesting to note that the sensitivity of the design can be simply varied by changing the value of the feedback resistor R1, for maximum sensiticity this resistor can be omitted.

However this can make the circuit a bit unstable and might provide false results.

Second Series opamp Amplifier Function

The next stage includes another identical amplifier which is just the repetition of the previous input stage.

This stage has been included in order to make the response of the circuit instant and so that the circuit is able to pick even the slightest of RF or the AC field within a certain range.

In case the circuit is intended to be used for detecting mains phase only at touching proximity, the sensitivity may be reduced to the required levels or the second stage may be excluded from the design.

The LED connected at the output is used for displaying the presence of the AC field; an illuminated LED identifies the presence of the field while no light from it provides the opposite conclusion.

By connecting a 1V FSD moving coil meter at the output, the device can be used to detect and measure the average strength of the AC mains present in that particular vicinity.

no contact mains 220V 120V phase detector circuit

Parts List

R1 = 2M2,R2 = 100K,R3 = 1K,C1 = 0.01uFA1, A2 = IC 324

Video Clip:

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pangestu said…
how can I adapt this circuit for other voltage & frequency? which part should I change?

Is R2 must stay at 100K even if the voltage & frequency to be detected is not 220v 50Hz?
Swagatam said…
Sorry, you cannot modify this circuit for detecting specific frequencies, it will detect all strong RFs irrespective of voltage and freq.
pangestu said…
Thank you for your response,

I'm stil wondering what is the purpose of R2 there?
Shouldn't it be in between capacitor C1 and inverting input pin 2?
Swagatam said…
R1/R2 both determine the sensitivity of the circuit, R2 is optimally set and should not be disturbed, only R1 should be tweaked for adjusting the sensitivity levels of the circuit.
pangestu said…
sorry if I'm such a noob, I'm just trying to understand your circuit in the deep sir...

I find your circuit is interesting, but I still unable to completely understand the inner working of it.

I have compared your circuit with the resistor configurations in
but none of those fits perfectly with your resistor configuration.

In the link there is no configuration that shows both of the inputs are grounded, while in your configuration they are grounded. That's why I'm confused sir...
Swagatam said…
I know the design is weird and difficult to understand but it works only in this why, if you try alter anything, the circuit would just refuse to work efficiently.

It resembles an open loop differential amplifier kind of configuration where the inverting responds to the negative AC from the antenna and amplifies it appropriately with the aid of the feed back resistor.
lou said…
My led light comes on as soon as I plug in the battery
John said…
Hi Swagatam, I tried to make this project but I cant seem to make it work. Can you please upload a schematic (with the IC symbols etc). Thanks in advance
Swagatam said…
Hi John, you just have follow the pin numbers of the IC as given in the diagram and assemble it exactly as is.

if you can tell me what's specifically happening with your circuit, I would try to troubleshoot it accordingly.
John said…
I have uploaded my breadboard photo (
As soon as i power up, the LED is dimly lit and when i touch the antenna with my hand, it goes off. And there is no change when I bring the antenna near an AC line.
Swagatam said…
I am afraid breadboarding probably will not work for this design since the circuit is very sensitive and the long wires on the breadboard might upset it.

please refer to the prototype image which was successfully built by me, see how the components are placed so compactly on the tiny PCB by soldering them.

it will need to be built exactly in this way
Swagatam said… the way the 1K resistors indicated in your breadboard image are all 100k, and you have used 560k as the limiting resistor for the LED, not sure from where you got this value that's huge and incorrect
John said…
I will try it on a perf board and let you know if anything is wrong.

Regarding the 1k resistors, I labelled them wrong. They should be 100k as indicated in your parts list (R2).

About the 560ohm resistor, i calculated it with an app (I use ElectroDroid app from play store) and determined that thats what i need to get around 10mA across the LED from a 7.2v supply (please let me know if this is incorrect)

Last, i was using a wall wart (AC-DC adapter) as my power supply. Just had a thought if this could interfere with the circuit considering how sensistive it is. Will try a 9v battery.
Swagatam said…
560 ohms is OK, but you have shown 560 K ohms in the image which is incorrect.

AC/DC adapter will not work, you'll have to use a battery.

most probably your circuit is not working because of the adapter....
Swagatam said…
OK, no problem.
Nélio Abreu said…
Hi Swag,
Post this comment in a wrong circuit (in Spectrum Analyzer).
What are de specifications of the antenna? A Wire? Size? Type?
Best Regards.
Nélio Abreu
Swag said…
Hi Neilio, No problems!

The antenna is simply a 3 to 6 inches long flexible wire, any thin flexible wire will do.

Please see the video to get a correct idea of the wire length and type
Nélio Abreu said…
Hi Swag,
I notice that this circuit uses a 324 IC (LM324). This is a Quad Ampop. Can I use a Dual Ampop?
Best Regards.
Swag said…
Hi Neilio,
In my prototype I had used LM324 IC, but any other dual opamp should also work, such as LM358 etc

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