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high resistance switch question

Discussion in 'Electronic Design' started by Hal Leemux, Aug 6, 2004.

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  1. Hal Leemux

    Hal Leemux Guest

    HI guys,

    I'm looking for a bit of newbie advice re. switching circuits.

    Basically I'm trying to create a book which can tell which page it is open
    on. One of the ways I'm trying is by using conductive paper for the pages -
    using each page as a simple switch so when two pages aren't touching the
    page knows its open. Crude I know...but it should be enough for what I
    need. The problems is that without using black carbon paper I've only
    managed to source anti-static paper with a very high resistance.

    My question is, is there a way I can still use this low level of
    conductivity to form a simple switch?

    Any comments would be gratefully appreciated as I am a little out of my
    depth!

    Thanks in advance,

    Hal Leemux



    Paper:

    http://www.alxtechnical.com/products/wbesdpad.htm

    @ 50% RH is 5x106
    @ 20% RH is 6x109


    I don't quite understand how to read an area resistance but I know that it
    doesn't even show on my multimeter! (I assume the above means at 50%
    humidity the resistance is 10 to the power 6 ohms??? so 10m??)
     
  2. I read in sci.electronics.design that Hal Leemux <hal_leemuxNOSPAMPLEASE
    For a uniform sheet of conducting material, the resistance from one side
    to the other of a square of any dimensions is the same. For any
    rectangle, R = Sl/w, where S is the area resistivity, l is the length of
    the conducting path and w is the width of the path. If l = w, R = S, for
    all l or w.

    You forgot the '5x', so it's 50 Mohms; 10 m is 10 milliohms. You can
    certainly detect 50 Mohms using a FET or a FET-input op-amp. You main
    problem will be keeping mains frequency and a lot of other stuff out of
    you circuit. A 100 nF capacitor directly across the input from your
    pages would be a good start.

    6 Gohms at 20% RH is quite a bit more difficult; you probably can't use
    a printed board and surface leakage across components and wiring will be
    significant.

    Have you thought of printing conducting strips on the page margins close
    to the spine? You can get much lower resistances, although the lower the
    resistance the more costly the ink is. But you would then be using
    ordinary paper.
     
  3. Ban

    Ban Guest

    Leemux,
    I had an idea about your task. If you print on each paper a small circle
    with conducting paint and then have on each side below the row of circles a
    coil that you exite at a certain frequency, the induced losses by the
    eddy-current of those printed conductors would be bigger with a lot of
    pages, so the difference between the 2 coils would be a possibility to
    determine the opened page. with a T-shaped core you might even shield the
    bottom from metallic desks. I do not know if it can be done.
     
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