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How to switch on and off capacitance on a piece of conducting material?

  1. Feb 26, 2015 #1
    I don't know if this is the right way of describing it, but I need to activate an area of capacitance on a touchscreen by applying some current to a piece of conducting material like tinfoil. I am needing to remotely turn pages on a touchscreen ereading device with a microcontroller, and this is one method of doing that. In an earlier post on this form, someone said to use an n channel mosfet, in order to switch the capacitance on or off, or something like that. Based on my limited understanding, a mosfet is used to provide higher voltages and current, but I only need to provide a few milliamps. Should I still consider using one, or is there a better way?
  2. jcsd
  3. Feb 26, 2015 #2
    You can use a fet as a simple switch.
  4. Feb 27, 2015 #3


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    You want a finger replacement? Dividing the screen up into a number of finger sized areas and switching them could give you a very limited resolution for your input. But perhaps that wouldn't matter is you only want to activate button sized regions of the screen
  5. Feb 27, 2015 #4


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    There are a few different technologies; See: http://en.wikipedia.org/wiki/Touchscreen#Capacitive

    The foil screen contacts you need will depend on the topology of the sensor. You may need two or more small electrodes with a gate between them, or you might require a single patch connected through a gate to ground.

    The “gate” technology will probably be a FET of some sort. It will depend on the sensor technology and how you will control the gate.

    Do you know how your system works? Can you trigger it with foil pads and a short wire on a long thin insulated stick?
  6. Feb 27, 2015 #5
    I have a Kindle, but don't know any specifics on the touchscreen except that it is capacitive. I did see a video where a small piece of foil was taped to an iphone and connected to a pushbutton switch. The other terminal of the switch was connected to a larger metal surface, perhaps 8 x 10". I could be wrong, but I would expect that my Kindle should respond to that same type of circuit. I guess I have two problems to solve. First, I need to figure out what kind of ground I would need, such as the best material and how large it needs to be. And then second, what component I should use to open and close the connection between the two conductors (is that what you mean by gate?), whether that would be a relay, fet, or mosfet.

    By the way, what is the difference between a fet and a mosfet? Someone suggested an n channel fet, but when I do a Google search to look for one to buy, all I get are mosfets. Does it really matter?
  7. Feb 27, 2015 #6


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  8. Feb 27, 2015 #7


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    The type of sensor has changed with different versions of the Kindle.

    Last edited by a moderator: May 7, 2017
  9. Mar 1, 2015 #8
    I have a Kindle Paperwhite, so this project idea should work, right?
  10. Mar 1, 2015 #9


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    If the screen involves a grid of horizontal and vertical transparent conductive tracks then you may need to attach a thin horizontal conductive tape to the screen, near a thin vertical tape. When they are electrically connected together they should significantly increase the capacitive coupling and behave as though you are touching the point on the screen where the x tape axis crosses the y tape axis.
  11. Mar 3, 2015 #10
    I'm not sure if the screen uses a grid, but I do know that if I touch the screen with a stylus, it registers the touch. So any kind of capacitive material that is the same size as the stylus tip should work. Is that what you mean?

    I think I understand the basic idea, but I have a couple of additional questions. If I decide to use a mosfet in order to trigger a touch with a microcontroller, which I probably will do, what kinds of materials should I use for conductors? I saw video of someone using the special foam that is used in packaging to protect electronic components, and they connected that foam with copper wire to an exacto knife. When they touched the foam to the screen, it registered a touch. So I guess I could use similar materials, although I won't be using an exacto knife. What about a small aluminum plate of 1 x 1 inch? And then if I put a mosfet in between the conductive foam and the aluminum plate, then I could leave the foam on the screen, and use the microcontroller to open and close the mosfet to simulate touch. Do I have the right idea here?

    One more question. Someone told me that the second plate, in this case, the aluminum plate I mentioned earlier, should also be connected to GND on the microcontroller. Does that make sense? I'm not really understanding why that is important. My microcontroller will not be connected an outlet for power, but will rely upon a battery.
  12. Mar 3, 2015 #11


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    No, to switch it capacitively you must have two fixed electrodes that can be connected extrernally, or use a stylus.

    It is too early to make that decision. You must first experiment.

    No, the idea is that you experiment and try alternatives until you understand what is happenning. The www is no substitute for experimentation.

    Go to a model shop and get some very thin brass shim, (between 5 and 10 thou), from which you can cut tape strips with scissors. You can solder a thin wire to each shim tape, (or just bend the ends up away from the screen), then place it on the screen and test by shorting the thin wires, about an inch away from the screen. Find out how wide and long the tapes need to be when arranged as an "L".

    Alternatively, approach the screen with a brass patch on an insulated stick. Does that trigger the screen? Find the soft material that works best and consider an electromagnetic way such as a dc motor or relay coil to physically touch the screen where and when needed.

    It will depend on the way the system works. Can you find the grid in the screen by experiment?
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