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EMF Meter Project

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

  1. Mr RB

    Mr RB Well-Known Member

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    Ok humour aside, I've got a little "telephone amplifier" that i got in the 1970's its just a little audio amp and a suction cup coil plugged into it. It was sold for hearing impaired etc to stick it on the telephone handset and amplify the voice picked up from the dynamic earpiece in the handset.

    I used it quite a few times to "listen" to bad light switches arcing, carbon resistors in tube amps failing, dodgy capacitors etc. It was a handy little gadget and you got me wondering now where it got stored in all my tons (literally) of old electrical crap.

    I'm sure the pickup coil had a lot of turns because it would pick up 50Hz sine fields etc from transformers.
     
  2. kpatz

    kpatz New Member

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    More cowbell... er... turns... should I just buy a bulk spool of like 10 miles of 40 gauge wire, stick an AM core in the chuck of my drill and just let 'er rip?

    I do have the unit assembled, and it works, though it isn't perfect. One issue is that it doesn't respond well to low frequencies such as 60hz hum. I may redesign the input stage in phase 2. It also picks itself up to some degree, I added a shield between the boards and the coil inside the box which helps but it isn't perfect. Maybe that's why commercial EMF meters don't have microcontrollers in them. ;)
     
  3. Sceadwian

    Sceadwian Banned

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    Yeah they're usually pretty basic analog devices, which belies their cost. I've never made one before so I don't know what the optimal number of turns would be, did you count how many you have on the coil you're using? I wouldn't buy 10 miles, yer not gonna get that much on an am loopstick core and you probably wouldn't wanna try anyways. Also give some thought to feeding the output of the amplifier stage into an audio amplifier so that you can hear it too, personally I don't think the little VU style meter is worth a damn. The human ear can pick out modulated EMF pretty well. Use the right coil and you can pick up passing cars, can just about guaruntee you'll pick up lightning from many miles away. ULF transmitions and sferics is a modest hobby for some folks, just don't call it a ghost hunter and people will take you a lot more seriously.
     
  4. dave

    Dave New Member

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  5. kpatz

    kpatz New Member

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    I forget how many turns I used, but it's well over 100. Maybe over 200. It was a pretty small doughnut I used though, if I used a ferrite stick I could get a lot more turns on it. As long as the stick is short enough to fit into the project box. I need to post new pics now that I have it stuffed in the box.

    I guess it all boils down to... if it's in "antenna mode" (one end of coil floating), what length of wire is needed to pick up frequencies below 60hz. If it's in "coil mode" (one end of coil connected to circuit ground), then the amp would just pick up anything inducted in the coil by EM fields, and more turns = more gain. So far I haven't had much luck with picking anything up in "coil mode", but I'll play around on a breadboard socket and build another amp stage circuit so I can get to the bottom of it.

    I do have a headphone jack so I can "listen" to the signals. That's how I know it isn't responding well to 60hz. I can hear it pretty loud in the headphones before the meter responds. It probably has something to do with the filter stage.

    Basically, the unit consists of an op-amp input stage with a 4M7 feedback resistor, coupled via capacitor to an NPN transistor which further amplifies the signal. The bias on this transistor is adjustable via a trimpot, to get the optimum sensitivity without causing falses. The headphone jack comes off this transistor, and it is also fed to a RC filter circuit (w/another transistor) to convert the AC signal into a steady DC level to feed into the comparator stage. The comparator (another op-amp) has the filtered signal from the analog section going into one input, and a filtered PWM output from the uC on the other. The software in the uC uses successive approximation to determine the level of the signal and lights the LED display appropriately, and beeps an attached piezo speaker.

    I did put the amplifier/filter section on a separate board from the uC so I can "upgrade" it with a better design down the road.
     
    Last edited: Jun 1, 2009
  6. Sceadwian

    Sceadwian Banned

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    As an antenna? It won't work. 60hz has a wavelength of 3000 miles, so even a 1/4 wavelength antenna isn't practical. That's why most AM radios use loopstick antenna's, the only practical way to couple to the RF field is through the magnetic portion, which just requires lots of turns. I'm a dunce when it comes to filters so I have no idea what the cuttoff frequency of your amplifier is. If you want to hear or measure frequencies that low you're going to most likley have to make sure the amplifier is DC coupled not AC.

    Not sure where you live but I live in a relativly large apartment complex and the 60hz hum here is horrible. Like I said I just used a random spool of wires rigged as a big coil directly into my sound card, no amplification, if I attach a high impedence set of headphones directly to the coil I bet I could have heard it directly off the coil.
     
    Last edited: Jun 1, 2009
  7. blueroomelectronics

    blueroomelectronics Well-Known Member

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  8. tcmtech

    tcmtech Well-Known Member Most Helpful Member

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    If your looking for a premade super coil take an old neon sign transformer apart. The secondary coils are made of several thousand turns of hair like copper wire.

    Then get a role of general purpose tie wire or a small roll of MIG welder wire. Both are made of iron and will work well for low frequency magnetic field sensing.

    Just cut the iron wire to a length longer than your coil is and bundle it up to make a core.
     
  9. kpatz

    kpatz New Member

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    I picked up a 100uH RF choke and another 200 foot spool of 30awg enamel magnet wire at Radio Shack.

    I'll remove the existing wire from the choke and use the choke's ferrite core, winding as much of the 30awg on it as I can. I'll come up with a McGyver-ish rig to wind the coil. Should be fun.

    Then I'll play with amps on a breadboard to get the gain and frequency response I'm looking for. And eventually, I'll come up with the "version 1.1" update for my meter. :)
     
    Last edited: Jun 1, 2009
  10. Sceadwian

    Sceadwian Banned

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    ewww radioshack really rakes you over the coals on price for wire. Mind you I bought my magnet wire there too <smirk>

    What kind of RF choke is it? You should just visit a thrift store and go to their electronics section and pick up a clock radio, I'll wager there's a loopstick antenna in there and it's gonna make a better core. I've never seen the internals but if you pop open a commercial EMF meter I almost guarantee what you'll find inside is very similar. I get all my best parts from thrift stores. Someone's always donating 20 year stereo equipment old PC parts printers scanners keyboards clock radios all sorts of goodies. I got 20 slid pots and a huge number of good discrets from an old equalizer
     
  11. tcmtech

    tcmtech Well-Known Member Most Helpful Member

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    I love scrounging the thrift stores for organ donor equipment too!
    I have have picked up some high end equipment for a few bucks because the store personnel had no idea what it was other than someone died and the family donated all his junk to them to get a tax write off!
    Garage sales are great too! Lawn a garden machinery have great little engines to destroy in the name of scientific research on alternative fuels!
     
  12. kpatz

    kpatz New Member

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    Here's the pics of the K-II EMF meter innards. I don't see a coil at all. Can anyone decipher the circuits? ;)

    [​IMG]
     
  13. blueroomelectronics

    blueroomelectronics Well-Known Member

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    Well the coil is on the PCB. Since it's a ghost detector it could be almost anything. Probably just a re-badged wall stud detector. Damn those haunted nails.
    [​IMG]
     
  14. tcmtech

    tcmtech Well-Known Member Most Helpful Member

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    could be a dedicated Micro controller programed to just make the LEDs randomly 'do ghostly detection stuff'.

    Stud finders either work as tiny metal detectors and need a a pair of coils to work with or use some type of capacitive sensing effect I think.

    SO what type of signals do ghosts give off anyway? radio, thermal, magnetic, photonic, smell, sound? inquiring minds want to know! :p
     
    Last edited: Jun 1, 2009
  15. blueroomelectronics

    blueroomelectronics Well-Known Member

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  16. Sceadwian

    Sceadwian Banned

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    The 'coil' is plainly visible. The four parallel lines across the top and bottom under the main IC which is probable just a cheap VU meter.
     
  17. kpatz

    kpatz New Member

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    At one time I had a better pic of the K-II innards and the IC appeared to be a LM324 quad op amp (of course I can't find that pic now). They're probably wired up as comparators to light the LEDs at different signal levels.

    You think those 4 traces are the "coil"? I think they're just circuit traces to other components on the board.

    The K-II wasn't designed to be a ghost detector, even though some ghost hunters (both real and fraudulent) do use them. The K-II was intended as a device to detect EM fields, such as those coming from electrical wires. Here's a link to the manufacturer's web page (no ghost hunting content here): Safe Range K-II EMF Meter

    What I find interesting about the K-II is it detects the ELF range down to 50Hz.

    As for ghosts, AFAIK they can manipulate electrical fields to do their ghostly things. These manipulations can be picked up by an EMF meter. My hunch is their manipulations would be in the ELF range down to near-DC frequencies.

    But, back to my project. No more ghost talk to rile up the skeptics. I attached pics so you guys can see the end result. Yes, messy I know, I'm not that good with a dremel, yet.

    I'm also starting work on "Revision 2". I wound the "Coil from Hell" last night. Almost 400 feet of 30-gauge magnet wire on a ferrite core. The thing is almost an inch around. Some informal tests with an op-amp and a pair of headphones doesn't show much of a gain increase over the original circuit, but I am still working on the op-amp circuit. Sort of doing the "Op-amps for Complete Morons" google course at this point. ;) My goal is get it to have good gain down to a few hertz.

    UPDATE: Here's another pic of the K-II innards. In this pic you can read the markings on the IC, and it definitely appears to be a LM324.

    [​IMG]
     

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  18. blueroomelectronics

    blueroomelectronics Well-Known Member

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    Why would a ghost emit an EM field?
     
  19. Mr RB

    Mr RB Well-Known Member

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    It's a side effect of all that ectoplasmic energy. ;)

    You can build an audio based ghost detector... It picks up the specific sound of "Wooooooooooo! Wooooooooooooooo!"
     
  20. kpatz

    kpatz New Member

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    Well, ghosts are *made* of energy, so it seems logical that they would be able to manipulate energy such as EM fields.

    But enough ghost talk... I want to talk circuits! Low-frequency amplifier circuits!
     
  21. blueroomelectronics

    blueroomelectronics Well-Known Member

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    Really? Do you have any proof? I thought ghosts were stuff of fables and children's stories, pure conjecture so you might as well say ghosts are made of baked beans or cabbages. And I wonder who figured out the frequencies of these ghosts. How do they know the phantoms aren't 2.4GHz, my iPhone can find those ghosts.
     

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