Active EM Field Sensory

so after we all got our magnets... its time to move on.

while i build a microphone amp i noticed that those sneaky little electret mics are prone to electric fields.
with some cheap op-amps i was able to pick op the humming of powerlines in walls (20cm or so), my laptop ( >1meter), and all sorts of other electrical devices. only problem... the human body seems to shield them off quite well.

another idea would be to use active magnetic implants (a receiver coil, the amp circuit, output) to allow much weaker signals to be detected.

time to get the battery charger done... so the 2nd generation implants can be started.


  • edited March 2011
    Excellent, good to see you've turned your attention to the next mod
  • Would a hall effect sensor work in place of a coil? This sort of thing.
  • hall sensors require a rather high constant current, and they require rather big circuits to get a useful signal.
    9mA are a lot. it would drain my targeted battery in less than 10 hours. unless you are interested in picking up static magnetic fields there is no reason to use hall sensors. 
  • edited May 2012
    hello, this is my first post. also i'm not good in english (i think ;p)

    anyway, i will have magnetic implant in about 15 days and i'm thinking to build a cyberwear (maybe a glove) that will sense EMF and will vibrate as an output.

    i found this
    but i want to create something with better EMF sensory and better than magnetic implant..

    i thinking to use the following hardwares:

    and one of the following for EMF sensory:

    3-axis magnetometer

    hall effect sensor

    EM coil
    i will buy electromagnetic wire

    what is the best to use as a sensor? do you have any other ideas?
  • edited May 2012
    first off. sparkfun is a great place — to waste money.
    sampling the parts from the manufactors (usualy free of any charge), or getting them on ebay is a lot cheaper.

    about hall effect sensors. they are designed to work a bit like switches to detect rotations of parts, they require pretty strong magnetic fields.
    a 3-axis magnetometer like the mag3110 will give you 2000 times more sensitive output.

    vibration motor and liion battery aren't a bad choice but totaly overpriced. more than trice the regular market value.

    about sensing EM fields. simply hooking up a wire to an arduino might give you a rough idea. but with just a hand full more parts you can build a detector that's a lot more sensitive.
    for example using a jfet, with the gate wired to ground via 10megohms (preferably more but higher resistance is hard to get hands on) (i'd recommend 2sk170 as input as it is very low noise, in case you want to amplify the signal like crazy). source would be grounded. and drain pulled to your supply voltage 300Ohm resistor. starting from the jfet's drain you can continue to add some amplifier stages.
    a simmilar circuit allows me to detect old crt-tv's within a 10m range around my appartement, clearly able to see each tv-frame and the line-flybacks on my scope.

    last note:electromagnetic wire. that sorta made me smile. pretty much everything that conducts electrons is electromagnetic. any wire will do.

    so. summary:

    a magnetometer is good for a compass. a wire with ADC will mostly catch electric fields, and a coil will get you mostly alternating magnetic ones. the higher the frequency the more they mix.

    edit:fixed a bunch of typos
  • So I'm doing some grindhouse stuff with magnetometers, and it seems like it's time to bump this thread.

    These devices wouldn't be particularly hard to build. I'd like to make them autonomous, if possible, like the nodes they will replace. Ideas anyone? How are the glucose fuel cells coming along?

    If this gets enough attention- and it should, because everyone is getting nodes and these are potentially better- it might be worth doing as a group effort, or a grindhouse side project, or something.
  • I think there are several relatively big hurdles to overcome for this idea to be realised, but I would love to contribute to this project if necessary.
  • @rdb apart from the power source thing, which is more of a sheer cliff than a hurdle, what might they be?
  • rdbrdb
    edited June 2012
    Well, several ideas were discussed in the thread, which one were you referring to exactly?  I assume you mean the active magnetometer implant, correct?
    The power source is probably the easy part (I've personally been looking a bit into building a wireless charger circuit for a tiny LiPo battery; the glucose fuel cell seems less feasible at this point in time).  Depending on the way you want to interface with the body, you'll need to figure out and design a safe electrical stimulation circuit that would work correctly inside the human body, and how to have bioproof electrodes for doing so.  (A vibrating motor has been suggested instead of stimulation, but I'm not sure if that would be feasible as I think that these motors consume quite an amount of current.  Do prove me wrong, though.)

    Then, you would need to find a way to fit a battery, charging circuit, coils, magnetometer, amplification circuit, probably a microcontroller, and various filtering components into a tiny implant.  In theory, it isn't impossible to design it, but in practice this will require access to some more advanced assembly robots (if manufactured in bulk, this is very cheap, but the initial prototyping will be expensive unless you have friends that can do this for you).
    Soldering the components of this size by hand is simply impossible.

    Optionally, you would also need to get an interface working for you to be able to reprogram a microcontroller in the implant over an RF connection, to allow modifications to the way it works, which is quite a desirable feature if you think of it (assuming you don't want to rip yourself open when it turns out that the implant gets quite uncomfortable in certain situations.)

    All of this is certainly possible, though, and as I said, I would love to offer help where I can.
  • inductive power transmittion over short distance is pretty easy, i have yet to test it with off-the-shelf smd parts, but it should work good enough to charge a battery over a few hours/a night.

    as rdb pointed out. building this as non-implantable prototype is a piece of cake. but using the same components (with smaller footprint) and squeeze them into a space as small as an implant pretty much requires assembly under a microscope.
    it can be done. but it takes an awful lot of effort to prototype stuff this smal.

    since i originally posted this more than a year ago now, i build a couple of circuits which picked up EM fields perfectly fine. the only drawback: human skin shields electric fields pretty well.
    luckily, at least magnetic fields are easy to pick up from below the skin. and that can be done at pretty high sensitivity. like detecting a crt-tv's flyback driver from several meters away.
    for me, the hardest part is to figure out a save, reliable and easy to product electrode geometry and it's operating parameters.
  • edited July 2012

    Sorry about the delay in replying, I've been all side tracked.

    I think the vibrating motor would consume a lot more power than the electrodes. It'd be way bigger too… electrodes are probably the smallest way to go about it.

    I'm quite willing to do some self experimentation to get electrode I/O happening. If anyone has existing data, or a place to start, I'd love to hear it. (@ThomasEgi, how are your experiments going?)

    A microcontroller could even be overkill. When I posted before I wasn't thinking of making it much more complicated than Thomas' original suggestion. If you had an analog-output magnetometer all you'd need is an op amp, maybe two or three if you wanted vector sensitivity. Don't know how you'd turn it on and off, and you're right, that would be a useful feature.

    TBH if you were going to use a microcontroller you wouldn't need a very good one. They probably have tiny ones for tiny stuff like this. Maybe use an also-tiny LED to transmit data?

    I assume, perhaps naively, that a functioning active EM implant could be made with the same circuitry density and perhaps size as an arduino pro mini. (probably without the battery and charger coil. Could be possible to integrate them into a circuit that size... I hesitate to say 'make it circular and run the coil around the outside', because that might cook the circuit. If you had a glucose cell, you wouldn't even need those two, probably just a small supercap to smooth out irregularities).

    I'll have to drop this idea if it can't be done at that scale. I'm very much limited by budget.

    Backing the device with, or wrapping the circuit in, diamagnetic foil might improve detection. It might also let you use higher voltages in the charger without wrecking the circuit, and that would let you have a smaller charger too.

  • sry for my long reply-time. i was strolling through the wildniss for a couple of days.

    to get things straight. Electric fields are shielded by the body pretty well. so this turned out to be a bad idea for implants. what you can do, having a simple coil to pick up alternating magnetic fields. then amplifying that in a small analog circuit, then having a microcontroller evaluate the signal and giving in output accordingly.

    microcontrollers are available in tiny little packages. same goes for all other electronic parts. by far biggest one is the battery, and the charging coil (which could serve as receiver-coil for picking up signals at the same time).
  • It's all good. I let it slide and got stuck into other projects. Didn't think about doubling the coil up for both applications, that's a really good idea.

    You can get these battery management IC's from sparkfun that are like, the size of a rice grain. No idea how you'd prototype them but they would be perfect for implanted stuff.
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