Tri-Field Meter Part 2

I couldn’t find the old schematic for this project, so here is the very basics of the high gain OP AMP (LM741) and the audio amplifier (LM386):

This doesn’t have the fancy stuff like the LED based output or the external audio jack or the low/hi gain switch, but it should get you started. The probes are connected to a telephone pickup coil (can be purchased at radio shack) or any other coil. Make sure you use shielded cable for the probe.

To make the +9 -9 battery just take two 9V battaries and connect the + side of one to the – side of the other, that will be ground, and then you will have a +9V and a -9V terminals.

Also you will have to bypass the battery using a couple 10uF capacitors. Just connect the +9 and -9 terminals to ground using the capacitors (this is not shown in the schematic).

Once I have more time I will reverse the design to provide a full schematic of the whole device.

Electromagnetic Field Pollution!

Have you ever wondered what the electrical activity around your house sounds like? Once you have listened to all the EM pollution around, you really wonder about all these waves.

I got some recordings of a few EMF sources around the house:

Quartz Watch EM Pulse

Even a small quartz watch gives out large pulses each time the seconds hand moves.

Neon Lamp Ballast EMF

Incandescent Light-bulb

Sony DVD/VCR LED Display EMF

Refrigerator LED Display EMF (or something more)

The fridge LED display really is strong, I could pick it up several meters away from it. I wonder if the source of the radiation is more than the display. The oscillations that sound like a lightsaber are due to my moving of the probe around the area.

Microwave Oven LED Display EMF

900Mhz Wireless phone in base EMF

When I moved the probe over the area with the phone the signal changes and you can hear static and a solid pulse every second or so. The base just gives out a more high pitched sound also with a periodic oscillation.

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8 Responses to “ Tri-Field Meter Part 2 ”

  1. tell me what frq. range it detect?? can it work below 10 hz??

  2. hello,
    what kind of probe sensor do you use?


  3. Sanmay, because it has audio output the range is as wide as human hearing at least. I suspect there is waveform distortion on certain frequencies, but this is irrelevant since we are not interested in that. So yes it works perfectly below 10 hz. In fact you can hear here in the Quartz Watch EM Pulse audio a pulse that is exactly 1 Hz.


    The sensor for the EM radiation is simply a coil, I have tried different coils with different responses. A simple telephone pickup coil from Radio Shack works fine, but anything else can be used, from a hand made 10″ wide copper wire coil, to one stripped from an old earphone.

  4. So this Tri-Field Meter basically amplifies the relatively low frequencies that are radiating. When I first started reading I was thinking this could be an good poor man’s sniffer probe for high frequencies (>1MHz, 10MHz or even 100MHz). Obviously for that you couldn’t just amplify the received frequencies, and you would need tuned antennas for frequency ranges of desire. Interesting project.

  5. Bret, yes, it is basically an amplifier with a coil probe. The one I made, for which I have yet to post the schematics, has a bit more work into it. I added a charge probe using a FET and a Hall effect sensor to have magnetic and electric field readings independently.

    I don’t have spec sheets with me right now but it is possible to obtain OP amps with high bandwidth. There are some that go into the GHz range. In addition to that you will need to construct the device keeping in mind stray capacitance and inductance effects of the high frequency fields. So don’t build on a breadboard or perfboard :) The best would be to use surface mount devices on an etched/milled board, but non-SMD construction should hold up to 100MHz.

    Now the problem becomes: how to do listen to that? Those frequencies are beyond human hearing range, so we are out of luck. One possibility is to use a chopper (clipper) to turn the signal into a pseudo digital signal and then use a frequency divider on it (chip or software on a fast microprocessor).

    Another possibility is to hook up a low pass filter (simple enough), and hook the output to a visual output mechanism like an LED or a volt-meter (galvanometer or digital). With a 2nd or 3rd order low pass filter to cut off below 1MHz, the LED would only light up for the higher frequencies. If you build notch filters then you can “listen” to specific frequencies of your choice.

    Yet another thing is to build a frequency to voltage converter and hook the output to an LED or a voltmeter. You may still have to add the filters as above if you have many frequencies at once.

    Finally you could divide the frequency using a Miller Frequency Divider, but that is more expensive.

    No matter what the choice, it is indeed a larger project.

  6. Hello Sir,

    I am working on detecting electric field lines from 50 Hz power transmission lines

    Can i get your new circuit for detecting Electric fields emitting from it

    Pls reply soon

  7. Ashutosh,

    If you want to detect the electric field from 50Hz power lines, you can simply use the meter as above. You are not trying to detect charge (which is what the electric field part does).

    You will be detecting the electromagnetic radiation (the electric and the magnetic fields are coupled) from the power lines through the coil sensor. If you want to take measurements, you could substitute the speaker for a meter (or add a meter in parallel), then by controlling the distance from the sensor to the power lines you could make a plot of the strength of the field.

  8. do you have any pictures of the the part used for the probe? I am trying to do a similar project using air inductor for the probe but it is not working. Not so sure if what I consider the telephone coil is what you are talking about, so could you show me a picture please?

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