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EMF Meter Project

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by kpatz, May 29, 2009.

  1. kpatz

    kpatz New Member

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    Hi folks... I've been developing a homemade EMF meter with LED bargraph display, and am looking for some tips on tweaking the EMF sensing circuit.

    I don't have a schematic for my actual circuit handy (circuit's at home, I'm at work), but it's based on this one:

    [​IMG]

    For my first couple questions, look at the left side of the circuit (sensing coil and op-amp). When I wired mine the way shown in the schematic, it wouldn't pick up anything. I didn't have a 1mH radial inductor handy, so I fashioned my own using 30ga enamel wire and a round core (100 turns or so, give or take). But, if I disconnect the side of the coil going to the + input on the op amp in the schematic, leaving it free floating, then I can pick up fields. So, in essence, it's acting more as an antenna than an inductor. Any thoughts on this? Maybe I need a better coil?

    My 2nd question: the circuit is a LOT more sensitive if I connect it to an earth ground, such as when I connect the grounding clip on my oscilloscope when taking readings. Any thoughts on this? Since I intend this device to be portable, I can't be earthing it all the time.

    At present I'm using a TL082 op-amp, but I just got my hands on some LMC6482 CMOS rail-to-rail op amps. Do you think that one would work better for this? I went with the rail-to-rail so I can get a better range in the comparator portion in my circuit, not in the schematic above. I am using a Basic Stamp 2 feeding one side of a comparator with a filtered PWM signal, and the other side is a filtered and amplified version of the EMF signal, and the uC uses successive approximation to determine the signal level and it lights up the bargraph display and beeps a piezo transducer based on its measurements.

    I'll post a more complete schematic this weekend when time permits, but thought I'd post what I have in case anyone has suggestions for making it better!

    Thanks,
    KJP
     
  2. MikeMl

    MikeMl Well-Known Member Most Helpful Member

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    100mH is a lot of turns. I suspect that the coil you made only has a few 10s of uH.

    Any low input-bias-current opamp would work. Rail to rail is not a requirement. \

    What frequency range are you internested in? The circuit as drawn is only capable of responding to a few 100 kHz.
     
    Last edited: May 29, 2009
  3. kpatz

    kpatz New Member

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    So, more turns then? Threading that thin wire through a 3/4" doughnut killed my hands after a while. I suppose I can go hunt down a pre-made coil... but then there's online ordering and shipping. I don't have anything to measure inductance either, but I'm sure the mH/uH value doesn't matter as much as how many turns, right?

    Maybe cut a doughnut in half with a hacksaw, then coil the wire on one half, and then glue the halves back together would work ok? Does the doughnut have to be intact to work properly? What if I cut a notch out to aid in winding?
     
    Last edited: May 29, 2009
  4. dave

    Dave New Member

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  5. MikeMl

    MikeMl Well-Known Member Most Helpful Member

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    I think you want an open coil, not one wound on a toroid. Toroids are self shielding, and the only way to induce a magnetic flux in them is by passing current through a winding. Since you are trying to intercept a magnetic field that happens by, you want an air-core coil, or a solenoid wound on a ferrite rod.
     
  6. tcmtech

    tcmtech Well-Known Member Most Helpful Member

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    where is the 100 mh coming from? I only see a 1 mh in the schematic.
     
  7. kpatz

    kpatz New Member

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    He probably meant to say 1mH.

    So, what if I don't have a ferrite rod handy? Any common household items that would work? Maybe a nail? Ok, that brings back old memories of making electromagnets out of iron nails, some wire and a battery. ;) Or I could run to RadioShack and pick up the (I think) 100uH ferrite choke, and just wind a new coil on that core.

    If an air core will work, I could just use the remaining spool of enamel wire as the coil.

    As for the toroid, I only have one side of the doughnut wound with wire, so that should defeat the "self shielding" you would normally get, right? I figure this would make the pickup less directional.

    I think I'll try cutting a notch out of a doughnut (I have a bunch of them) and then spend some quality time winding it and see how it works.
     
    Last edited: May 29, 2009
  8. Sceadwian

    Sceadwian Banned

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    The frequency range of this is going to be very limited.
     
  9. kpatz

    kpatz New Member

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    Ok, any suggestions on how to improve this? Limited in terms of low frequencies, or high? I'm not real concerned with high frequencies (above a few tens of kilohertz) anyway, but if I can have it able to detect very low frequencies (down to a few Hz or so) that would be ideal.
     
    Last edited: May 29, 2009
  10. Sceadwian

    Sceadwian Banned

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    I'm not so good in the analog department. Someone else should be able to give you an idea of it's bandwidth. The range is going to be pretty limited though, and in a house you're you're going to pick up almost nothing but 60hz hum from mains pollution.

    I got bored one day and made an ELF receiver, which was nothing more than a spool of about 500 feet of CAT3 cable with each wire fed into the next to form a giant magnet coil. Plugs right into the sound card. The 60hz hum was so bad in my house it was unusable.

    What do you plan on doing with it? They don't serve much of a function.
     
    Last edited: May 29, 2009
  11. kpatz

    kpatz New Member

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    I've gotten into ghost hunting, and ghost hunters often use EMF meters in looking for paranormal activity. And being the geek I am, instead of buying one (like the K-II meter), I decided to build my own.

    As the circuit exists now, it can pick up noise from various devices in my house. The strongest emitters seem to be CFL light bulbs, which can peg my breadboard prototype from 2 feet away. Other devices, I have to put the coil right on them for it to pick them up. So I'm still fine tuning the thing.
     
  12. tcmtech

    tcmtech Well-Known Member Most Helpful Member

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    Ghost hunting?!?
    Do you have any idea what kind of grief you just unleashed on yourself now?:eek: :D

    If you find my long since dead great great great uncle ask him to contact me so I can find out where he buried all the money he had!

    Or if you get one of the big scientific electrical inventors send him over her we all want to have a chat with him! ;):D

    It will get worse from here on! :eek:
     
  13. Sceadwian

    Sceadwian Banned

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    You might as well build a meter that shows random blips and ticks and have a button installed on the back that causes the meter needle to shoot up, cause that's what "Ghost hunters" do.
    If you believe the "Ghost Hunter" shows you see on TV this is NOT the forum you want to be on.
     
  14. tcmtech

    tcmtech Well-Known Member Most Helpful Member

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    I dont think they use an actual button. Professional con men are fairly clever. I think a polarity sensitive magnetic read switch and a magnetic ring would be far more practical.
    Anyone could pick it up and not find any external 'make ghostesses' control with a magnetic triggering system.

    If your going to do it professionally let me know. I have a bunch of non conventional methods to make things seem unnatural or real that are not. ;)

    If you really want to make some good money go into the fake alternative energy systems! I'll sell you an HHO push mower engine that appears to run on water! only $1000! :D:)
    Really! you can even put water in the gas tank and it will still run! Cash up front and then I show you how! Its very obvious how to do it if you know the layout of a Briggs and straton push mower engine. :)
     
    Last edited: May 29, 2009
  15. kpatz

    kpatz New Member

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    Ha ha guys... I'm not doing it professionally... nor am I attempting to con anyone or make myself famous on TV. It's just one of my dozen-odd hobbies I partake in, just like electronics. ;)

    I could do like the guys on TV and buy a K-II but I thought, hey, I know the difference between a transistor and a resistor, I'll build my own. :)

    Aside from ghost hunting, EMF meters can be handy for identifying live wires inside walls as well. You know, before you drill a hole and hit a live 120V line, making your eyeballs light up like a Christmas tree...
     
  16. Sceadwian

    Sceadwian Banned

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    I just don't see how they're useful for finding ghosts, considering no one's every found one with one =) And they sell commercial products that can find electrical lines behind a wall, that's assuming you can't find the electrical blueprints for the building.

    All that being said, try using the loop antenna from an old AM radio. You may need something with even more turns than that to get an decent reception. Maybe a few AM antenna coils in series. Maybe 3 in an inverted U configuration.
     
  17. kpatz

    kpatz New Member

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    Here's a pic of the project as it currently exists. I still have a couple components to add (power LED and headphone jack), then I'll start stuffing everything into a project box.

    I ended up snapping a ferrite doughnut in half and wrapping the heck out of it with enamel wire. That, plus switching to a rail-to-rail op-amp for the preamp and comparator gives me some extra sensitivity.
     

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  18. Mr RB

    Mr RB Well-Known Member

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    You're doing it all wrong man!

    You need the proper big nuclear resonance powered backpack with the big gun that shoots the lightning bolts and puts the ghost in the little cube box so you can take it back to safety at Ghostbuster's HQ.

    Your equipment won't do it! You'll just get yourself slimed.
    ;)
     
  19. kpatz

    kpatz New Member

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    That's my next project. You see, I need the ghost *finding* device before I build the devices to capture and hold them. Remember the gadget they were using in the library near the beginning of the movie?

    BTW, don't cross the streams.

    But, back on topic: my project as it presently stands can pick up a CFL bulb from 3-4 feet away. ;)
     
    Last edited: May 31, 2009
  20. tcmtech

    tcmtech Well-Known Member Most Helpful Member

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    If the conditions are right I can pick up a 13 watt CFL from over a mile away! Eyes beat RF detector any day! :p
    IF I were to get some of those umm 'medicinal herbs' some former co workers used I bet I could see ghosts too! Or at least think I am! :eek:
     
  21. Sceadwian

    Sceadwian Banned

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    CFL's run at a pretty high frequency, 10-100khz. I think all you really need to do is really step up the coil turns. That looks like red magnet wire which I'm guessing is 30 gauge, try something smaller like maybe 40 gauge it's not gonna cary a lot of current, wrap the coil around the core of an AM loopstick antenna (they're easy to strip) The length of wire you need is REALLY long though. 4X or more what you're using.
     
    Last edited: May 31, 2009

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