Projected Capacitance Touchscreen, confused by how the capacitance is formed.

  1. Hello,

    I have a question thats really bothering me. So I've been reading about projected capacitance touchscreens (the kind thats in so many cell phones, the iphone etc) and I don't understand what the physical property of my finger is that allows the screen to function. From what I've been reading, which is really hazy incomplete information, I am kind of stumped. I've read that any conductor should be able to work but when I place a penny (copper-plated) on the screen and move it with eraser on the back of a pencil, I get nothing. If I am touching the penny with my finger though it works.

    The description on wiki does not seem ...complete or seemingly correct in some instances including this specific sentence
    "Bringing a finger or conductive stylus close to the surface of the sensor changes the local electrostatic field. " ...since I can't seem to make that work.

    I have several questions but I would like to see the response to this before I ask them. So yeah if anyone can explain in a technical manner what property my finger has over the penny that allows the screen to register the input I would appreciate it.

    I speculate that my finger is holding a charge and thats the difference but I rubbed the penny on the cat haha trying to put some charge on it to get it to work that way and I got nothing.

    Thank you,
  2. jcsd
  3. mgb_phys

    mgb_phys 8,809
    Science Advisor
    Homework Helper

    Your finger is earthed.
    The penny simply rises to the electric field of the screen - you + the Earth resists changing potential, so a potential difference exists which the screen can sense

  4. Interesting...

    But my finger is not technically earthed, of course so is it then that my body is just a much larger conductor than the penny relative to the charge on the screen so I can hold a much larger amount of charge without changing my potential?
  5. mgb_phys

    mgb_phys 8,809
    Science Advisor
    Homework Helper

    Your finger is earthed, unless you are wearing very good shoes.
    Even if it wasn't, yes you are still a large enough source not to change potential much
  6. sophiecentaur

    sophiecentaur 14,324
    Science Advisor
    Gold Member

    Your finger doesn't need to be earthed. But your body has an appreciable capacitance to earth (i.e. the case of the device you are holding or is on your lap etc. . This is in series with the small capacitance between your finger and the particular area of the screen and this constitutes a loop with a lower impedance than any other path involving other parts of the screen.
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