Understanding the Technology Behind Touch Screens

[fargoth]

ok, I've searched google and howstuffworks for it so far...
the explanations I am getting are too fuzzy, i don't understand how the X-Y coordinates are determined through each mechanism (ive read so far about resistive, capacitive and acoustic).

can anyone here give me a reference or a thorough explanation about this technology?

 

[berkeman]

I searched Amazon.com's book section for touchscreen info, and got a lot of book hits. You might see if your local library carries a good textbook on display and touchscreen technology:

https://www.amazon.com/exec/obidos/...&field-keywords=touchscreen&tag=pfamazon01-20

I've worked with several touchscreen technologies, so if you have a specific question, maybe I can answer it.

 

[dlgoff]

I've seen touch screens that have light beams from an array of leds to photo-detectors that the finger breaks.

Regards

 

[luckycharms]

By far the most common technology is Indium Tin Oxide (ITO). ITO is a transparent conductor. This allows a thin film of it to be applied to a glass surface (say a screen) but still allow the image to transmit through. The typical installation consists of two ITO films separatated by tiny bumps that act as spacers. When you press the screen, you alter the connection between the two layers which is measured via a resistance or capacitance change. The electrodes are organized in some sort of grid so that coordinates can be communicated and mapped to the underlying image.

ITO is everywhere, in Palm Pilots, phones, ATMs, etc and it's widespread use has driven indium prices up sharply. There is even some mild concern that the world could run low on indium.

 

[fargoth]

thank you all very much, i'll hunt for books next week and ask more specific questions.

meanwhile i want to ask this one:
i understand there is a voltage gradient between the two layers of indium so that when they touch charges move off of one to the other.
now, how i understand it, the layers are connected to electrodes (say from the top and from the left), so if id touch the top-right corner id get the strongest current from the right part of the top array of electordes and from the top part of the left array of electrodes, and that's how the screen knows i touched the top-right corner, am i right?

im having difficulties imagening it, because i thought the current should be the same from all directions toward the point where the layers touch... hmm as i write this i think i realized that my imagination may be true for vacuum, but when traveling inside a conductor the current is not the same for every path because the resistance of each path is different.

i really shouldn't post when I am tired... hope i don't sound too confused

 

[berkeman]

In that example, you would typically have 4 injecting conductors (one along each edge of one ITO layer) and 4 receiving conductors (one along each edge of the other ITO layer). You would time multiplex resistance measurements between different source and receiving conductors to figure out the coordinates of the cross-over connection between the two resistive layers. You can probably do it with just 2 conductors per sheet, placed along adjacent edges on one sheet, and the opposite adjacent edges on the other sheet. But that wouldn't be as accurate as using 4 conductors per sheet...