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Building a Pressure-Sensitive, Multi-Point TouchScreen?
sonamchauhan asks: "I'd like advice on building (yes, building) my own low-res touchscreen. The reasons for 'build' instead of 'buy' are: 1) to have it sense pressure (pressure sensing is quite expensive) and 2) to have it sense multiple points of contact simultaneously (which is a useful thing). Back in 1985, researchers at U.Toronto built (PDF file) a touch-tablet (not a touch-screen) that fulfilled both requirements (pressure-sensitive and multi-touch) and used only basic electronics: lots of diodes, A/D convertors, etc. Some 17 years later, it should be possible to build a touch-screen using the same techniques (possibly using layers of transparent conductive and insulating paint for the sensor paths.) Any comments? Some other links: a Microsoft paper (PDF file) describing a touch sensor painted onto a mouse, a basic FAQ on current touchscreen technology, and a slashdot thread that discussed building touchscreens (these links don't address pressure or multi-touch though)."
First, is it ok to use a stylus? If so, you're problem just got easier - you simply use the same technology that wacom uses, except you'll need to track several pens instead of one.
I'm betting, though, that you want a real 'touch' screen which registers anything hitting the surface.
Here's what you do: Use a transparent metalization process to put squares of transparant, conducting (but resistive) metal on two opposing pieces of plastic. Take a wire from each square and use a microprocessor with a lot of i/o (switched A/D converters, most likely) to detect which metal plates are being pushed together. The size of the plates determines resolution.
That's the easy, no brainer way.
The cool way (with infinite resolution ) is to use the normal method modified a bit. Normally you have two sheets covered with resistive material, one which has conductors on the vertical sides and one with conductors on horizontal sides. Increase the conductors (say, four shorter conductors on each side, and make them points instead of lines).
Send a signal to one conductor, and listen to all the other conductors for the wave front of the returning signal. Through a ton of signal processing (and sending signals from the others and measuring the response) you'll be able to detect an arbitrary number of points on the screen. You'll need to do a lot more processing to measure surface area (pressure), but you'll get there eventually.
Alternately, you get rid of it entirely and make a video system that can sense the position and movement of your hand and get rid of the 'touchyfeely' altogether. This (IMHO) would be easier and faster to develop.
-Adam