I think I understand the basics of capacitive touch screen, i.e. when one or more fingers touches the panel it creates a potential difference at that location drawing current. But how do you go about locating the position of contact? From what I have gathered, there are wires thinner than human hair that forms a grid on the panel. I was looking at the wiring grid on my keyboard recently, and considering the accuracy of the touch screen on my phone there must be thousands of these wires for both x and y. But obviously ICs only have so many input pins. So how does it work?
Electrical – How is the location of a finger or more detected on a capacitive touch panel
touch-paneltouchscreen
Related Solutions
I've been wanting to find out info about this subject my self and also don't have much in the way of electronics background. I believe that Raiden wants to create his own DIY fine point stylus like the Nota stylus (http://hex3.co/products/nota) which is described as electronically activated. There are also others which are coming to market in the near future.
As far as I understand they work by increasing the capacitance of the stylus point. So I've looked into what can be done to increase capacitance of the stylus.
Formulas
C = Q/V OR Capacitance = Charge / Voltage
C= εS/D OR Capacitance = (Dielectric Constance X Surface Area) / Distance between plates
If i understand correctly when you touch the screen you make a sort of capacitor due to the layer of insulating film /glass and the change in voltage at that point is the detected. (May be completely wrong about this). So in the 2nd equation if you reduce the surface area you will reduce the capacitance. So in order to increase the capacitance we need to change the dielectric constant of the material being used.
Here is an example of a fine point stylus that uses foam and water https://www.youtube.com/watch?v=alrweIVt2RQ . The Constant of water is ~80 compared to rubber of ~3 and i believe that is why this example of a fine point stylus works. Unfortunately it is inconsistent and water doesn't play nice with touch screens. I would theorise that using a material with a high constant would help reduce the size of a stylus point. For example Graphite has a constant of ~ 36.
My other thoughts are to try using an electronic circuit to add voltage to the stylus point. But thats well out of my range of understanding.
It's hard for me to imagine that your 1 big sheet of metal would work as a touch pad for 3 main reasons
- The (x, y) extraction from the measurement. Having only one big capacitor won't be accurate enough, and that's why touch panel on the maket place have a grid of capacitors. This is the first problem you are trying to solve now.
- The touch tracking. It's a fundamental need to detect the position of a touch, but not enough. To build a touch pad you'll also need to follow the touch movement, manage the case of a possible "not desired" second touch, handle the "tip switch" in other words when the touch is leaving the touch pad... and many others.
- The touch driver. To make your customized touch pad as pointing device, you'll need your OS to understand data from your touch pad. You'll build you own protocol, or used a USB standard for example, which requires a huge work IMHO.
Although, I congratulate you for your effort, and I'm just adding one remark
Designs for capacitive touch screens typically involve a "grid" of sensors that give x y values, but if I am not mistaken, this is only needed to register multiple touches.
No, not really. In an other post that have nothing to deal with yours, I've added 2 pictures that a touch screen controller (with image processros inside) is using as primary data. You would notice how a grid is used to capture the touched area. Imagine a big big finger touching the screen.
It would be seen on many "touch pixels", then an algorithm like a centroid detection, will compute the center (x, y) of the touch area. That's not only for multi-touch.
Best Answer
A capacitive touch screen measures capacity/resonant frequency of "flat plate" senslrs spread across the screen. Your finger touching the screen actually doesn't cause current to flow (that would be a resistive touch screen), but changes the capacitance between 2 microscopic plates.
There are many thousands (actually, several millions on modern displays) of capacitive sensors on each screen. To handle all of those wires, there is a separate MCU (processor) that is part of the 'touch sensor device.' That MCU goes through and checks each 'cell'/'element' in series, then sends information on which one(s) are being 'touched' out on a (usually) serial interface, with several dozen, to a few thousand, updates per second.
See the Wikipedia Article, and related links from that article, for further reading/information.