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

[dmehling]

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?

 

[jasonleroy]

You can use a fet as a simple switch.

 

[sophiecentaur]

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

 

[Baluncore]

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?

 

[dmehling]

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?

 

[Baluncore]

There are many different types of FET. They come in two polarities, N and P channel. N is cheaper.
http://en.wikipedia.org/wiki/Field-effect_transistor
MOSFETs are used to switch significant power. http://en.wikipedia.org/wiki/MOSFET

You will not need a relay, there will be a lower power way to influence the capacitive sensors.
First you need to experiment with the foil geometry.

 

[Baluncore]

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

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The touchscreen works as follows (though IR sensor placement may be slightly different for the Kindle Touch):

Essentially there are two extremely thin IR sensors on the top and side of the Reader’s screen. When your finger or the Reader’s stylus interrupts the IR beams at a certain point, the Reader registers the interrupted IR signal as a touch and works from there, but because the beams are so thin, you need to get up close to the screen for it to work, making it act like a touchscreen.

Between the glass surface of the Amazon Kindle Fire and the background of the screen is a sandwich of different layers. The base of this sandwich is the LCD display. The layers closest to the screen are conductive layers of transparent material such as indium tin oxide (ITO). These layers create a capacitance grid. The Kindle Fire generates a weak electric field across this capacitance grid.

When your finger -- or any material capable of altering the electric field -- comes into contact with the screen, the field changes. Your finger actually draws current from the field. It's such a weak electric field that you don't sense it yourself. But the Kindle Fire can sense the changes in the field and map them to a specific spot that corresponds to the display screen. The Kindle Fire's software maps the touch to whatever command you were executing.

The Amazon Paperwhite display also has a higher resolution (221 ppi) and “25 %” greater contrast (whiter whites and blacker blacks), Amazon says. That should make letters on the screen even clearer and sharper. And the display now also utilizes capacitive touch that should make touch input faster and better. Previous generations of the Kindle used an infrared touch technology where infrared sensors where placed at the edge of the bezel. Infrared touch technology is far slower and more inaccurate.

 

[dmehling]

I have a Kindle Paperwhite, so this project idea should work, right?

 

[Baluncore]

I have a Kindle Paperwhite, so this project idea should work, right?

Maybe.

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.

 

[dmehling]

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.

 

[Baluncore]

So any kind of capacitive material that is the same size as the stylus tip should work. Is that what you mean?

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

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?

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

Do I have the right idea here?

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.

Does that make sense? I'm not really understanding why that is important.

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