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.
Circuit 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.
Circuit Schematic
Parts List
R1 = 2M2,R2 = 100K,R3 = 1K,C1 = 0.01uFA1, A2 = IC 324
Video Clip:
Nélio Abreu says
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 says
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
John says
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 says
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 says
I have uploaded my breadboard photo (http://imgur.com/JQiVPKT).
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 says
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 says
….by 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 says
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 says
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 says
OK, no problem.
lou says
My led light comes on as soon as I plug in the battery
pangestu says
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 says
Sorry, you cannot modify this circuit for detecting specific frequencies, it will detect all strong RFs irrespective of voltage and freq.
pangestu says
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 says
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 says
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
http://en.wikipedia.org/wiki/Operational_amplifier_applications
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 says
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.
Kylie Vl says
im so sorry .. i have a big mistake the circuit have done and work perfect thenx for your job 🙂
Swagatam says
You are welcome…..It happens:)
Mdr Shivkhumar says
PLROL HELP ME TO MAKE 3 PHASE AC 440 VAC 4KW CONTROL THROUGH MC3PHAC IC (28 PIN DIP VERSION) PLEASE SIR I WANT 2 HAVE COMPLETE CIRCUIT & UR VALUABLE GUIDANCE. IF ANYBODY CAN HELP ME TO COUPLE
MC3PHAC WITH 6 MOSFETS OR 6 IGBTS TO RUN 3 PHASE INDUCTION MOTOR FROM 1HP (0.75 KW) TO 7.5 KW.
REGARDS
SHIVKHUMAR
hrd_comfort@yahoo.co.in
BANGALORE INDIA
Swagatam says
The circuit has been provided in the following datasheet:
http://cache.freescale.com/files/microcontrollers/doc/data_sheet/MC3PHAC.pdf
Kylie Vl says
hello! i have do this circuit .. but dont work… why ?
Mdr Shivkhumar says
VERY GOOD APPROACH I WILL TRY OUT THIS THEN COME BACK 2 U.
SHIVKHUMAR
hrd_comfort@yahoo.co.in