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.

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.

Parts List

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

Video Clip:

Feedback from one of the avid followers of this website:

Am a civil engineer by profession based in Bangalore. Am in the construction industry for the last 20 yrs, have a manufacturing unit for modular kitchens.

Here is my requirement to automate the dust collector on or off for three different CNC based machines.

The company does not allow me to physically tap into any electrical but allows me to use a non contact voltage detector.

So I need to process the output of the non-contact voltage detector through IC LM324 and trigger a 12v relay which will switch the dust collector on or off.
The dust collector load is 7.5 hp 3 phase.

I would like to sense the voltage of the conveyor motor of the machine which is 3 phase Ac, 50 htz, 4amp. When this conveyor motor comes alive I would like the dust collector to come on and vice versa.

I have attached the photo of the motor and the specifications in my next mail. This motor has a MPCB which has a 24v control voltage triggering the mpcb. I intend to have a MPCB for my dust collector motor as well.

Kindly let me know if you need further specifications/requirements for the same.

Circuit Diagram

The complete circuit for the above application can be witnessed in the following diagram.

The first design is a relatively easier one using only transistors. The second one is using 4 opamps of LM324. Both are designed to activate a relay in response to an AC phase detection, non contact.

Another Very Simple Mains AC Hum Detector Circuit using IC 4011

The hum receiver is made up of a single COS/MOS IC consisting four NAND - gates (CD 4011). The four gates are connected in series to form signal amplifier like configuration.

The first gate (N1) detects the 220 V or 120 V AC hum radiated by the mains grid electrical line. You must take care not to keep the NAND gate inputs far away from various other sources of RF interference su ch as amplifier outputs, etc. A copper wire with a length of 2 to 3 cm will be adequate to serve like an antenna for picking up the 50 Hz or 60 Hz hum and to process the signal into a correspnding level of square wave output.

The output may show a risetime of about 20 ns at the output of gate N4. Based on the circumstances, one or two gates can often be eliminated. The current consumption of the complete CD 4011 IC is extremely minimal hence a 4.5 V battery employed as the power supply may be equivalent to almost the normal shelf life of th battery.

AC Mains Wiring Finder

The next circuit describes a straightforward way of finding conductors which carry alternating current or AC mains. A 100mH pick-up coil with a which is utilized as a detector coil.

A current-carrying conductor generates a magnetic field and holds a minute voltage in L₁, which is amplified through opamps A₁ and A₂.

Capacitors C₂ to C₅ occupy a value that makes sure maximum amplification in A₁ and A₂ with signals about 50 Hz. Throughout the positive half-waves of the AC network, D₁ stays lit.

About Swagatam

I am an electronic engineer (dipIETE ), hobbyist, inventor, schematic/PCB designer, manufacturer. I am also the founder of the website: https://www.homemade-circuits.com/, where I love sharing my innovative circuit ideas and tutorials.
If you have any circuit related query, you may interact through comments, I'll be most happy to help!