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Water as touch surface

  1. Dec 8, 2016 #1
    an idea for my new project
    construction - a cubical bowl of tap water , 2 sheet electrodes for 1 pair (y-axis pair)of opposite face(square wave 10Hz 10vpp), 2 sheet electrodes for the other pair (x-axis pair)(similar square wave lags by half cycle )
    left hand touches adc pin. right hand touches water surface.10v drops linearly with normal distance from pulse electrode
    process adc input for realtime x-y coordinates of touch (maybe z also if added 3rd pair electrodes at surface and floor)
    so water surface can be used as touch pad .
    will this work?
     
  2. jcsd
  3. Dec 8, 2016 #2

    berkeman

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    The 2-D version could work. What is important about the water/liquid to make this work?
     
  4. Dec 9, 2016 #3

    Baluncore

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    With your system the body will pick up lots of interference and electrostatic discharge that will get into the ADC.

    There are similar 2D positioning systems that inject signal current or noise into a point on a resistive sheet. The edges of the sheet are tied to virtual earth with op-amps that convert each edge current to a voltage. Those four voltages are a function of the sheet x–y position where the current is being injected.
     
  5. Dec 9, 2016 #4
    will this work with water in place of resistive sheet
     
  6. Dec 9, 2016 #5

    Baluncore

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    Yes, but the water should be shallow.
    You may need to add some salt to the water to make it more conductive.
     
  7. Dec 10, 2016 #6
    adding salt to water will cause electrolysis , so it disrupts uniform composition of water.
    voltage drop will not vary linearly with normal distance from pulse electrode
    perhaps something else can increase the conductivity of the water
     
  8. Dec 10, 2016 #7

    Baluncore

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    Then use an AC probe signal, or maybe a DC voltage below the disassociation voltage of NaCl in H2O.
     
  9. Dec 10, 2016 #8
    yeah, at high frequency ac 1kHz salt water must behave more or less like resistor due to negligible capacitive reactance.
    but how can i sense ac voltage and convert it to digital?
     
  10. Dec 10, 2016 #9

    Baluncore

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    How you do it will depend on why you need to do it.
    What are you really trying to do ?
     
  11. Dec 10, 2016 #10
    what did you mean by
     
  12. Dec 10, 2016 #11

    Baluncore

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    Can you explain why you think capacitive reactance would be negligible.
    What is the dielectric constant of water ?

    Do you need to sense the voltage at a point on the sheet, or do you need to inject a current into the sheet at a point?

    How will you place electrodes at the edges of the sheet ?
    What will you do where they meet at the corners ?

    Unless you can write a specification you are only dreaming.
     
  13. Dec 14, 2016 #12
    80.4
     
  14. Dec 14, 2016 #13
    alright , it wont
     
  15. Dec 14, 2016 #14
    I need to sense the voltage at a point on the sheet.
    copper wires attached to sheet at its edge
    the sheets are separated at the corners
     
  16. Dec 14, 2016 #15

    Baluncore

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    That would suggest that capacitive reactance would be 80.4 times greater than with air between the electrodes.

    If you freeze the water it will be much less, IIRC about 3.5, but will the ice be conductive?
    See; Dielectric properties of brine-wetted snow on first-year sea ice.
    https://cartel.recherche.usherbrooke.ca/Documents/Langlois_Alex/Geldsetzer_etal_2009.pdf

    What conductive liquids are there that have low dielectric constant and are safe?
     
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