Capacitance Proximity Sensor

Discussion in 'General Electronics Chat' started by KCHARROIS, Jan 2, 2014.

  1. KCHARROIS

    Thread Starter Member

    Jun 29, 2012
    292
    1
    Hello,

    Just want an opinion on whether the idea is ludicrous or not. So I'd like to take a femto farad capacitance meter which there are circuits already available online and use it as a proximity that can detect change from up to 6 inches away. A square inch copper plate will be the sensing capacitor.

    Thanks
     
  2. #12

    Expert

    Nov 30, 2010
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  3. eeabe

    Member

    Nov 30, 2013
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    In my opinion, not ludicrous. A while back I was working with a very high impedance circuit that could sense hand movement nearby. It was not the intended purpose in my case, but it showed that it was possible. I think it might take some interesting filtering and logic to deal with the signal size and other noise sources.
     
  4. alfacliff

    Well-Known Member

    Dec 13, 2013
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    look up circuit for theramin, a musical instrument that produces tones determined by distance of a hand to a probe. an oscilator has one side of the capacitance brought out to a probe, and another oscilator is hetrodyned with it to produce varying tones determined by the distance of the hand.
    cliff
     
  5. KCHARROIS

    Thread Starter Member

    Jun 29, 2012
    292
    1
    I guess its not completely ludicrous but definitely a challenge. I'll give it my best shot.

    Thanks
     
  6. #12

    Expert

    Nov 30, 2010
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    I'm completely serious about doing the math to find the expected capacitance change. I don't see how you can proceed without knowing how much you are trying to measure. I expect the answer is in the picofarad range.
     
  7. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    No, not ludicrous at all... but perhaps overkill.

    *ANY* circuit that depends on some capacitor value to operate can act as the basis of a capacitive sensor. One such circuit is the ubiquitous '555 timer. I used one of those back when I was working on a projective capacitance sensor.

    My sensor only had to see a half inch ahead to sense water. Here is a post of some of my early findings. I have the test sensor around here somewhere... think it's in the shed, and with a foot of snow coming in tonight it may be months before I make it back out there again to confirm what I remember.

    It occurs to me to maximize the effect of the fringe field over the direct (directly between the plates) one may simple remove the direct plate like so:

    [​IMG]

    What the drawback may be is in the original version the back plate also acts as a shield: it did not seem to matter a bit what went behind the back plate, just what went in front of the small plate.
     
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