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PICList Thread
'[PICS] Touchscreen'
1996\08\23@114444 by karlq

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>
> Does any one know how can I do a touchscreen?
> I'm planning to use a PIC to do this, Any idea?
>
> Octavio
>
> --
> ========================================================
> Octavio Nogueira
>    e-mail:   spam_OUTnogueiraTakeThisOuTspammandic.com.br
>  homepage: http://ourworld.compuserve.com/homepages/tato
> voice/fax: +55 11 240-6474
> ========================================================

Depends on the type of touchscreen!

Resistive technology is the easiest and cheapest, it has a maximum
usable resolution of 256 x 256 points.

It is basicly two sheets of plastic glued together at the outside
rim but held apart by very tiny glass beats or bumps.

One is coated with a very low resistance transperent coating, (plane A)

the other has a higher resistance of about 80 to 120 ohm depending
on the distance between the outside edges. (plane B)

The actual value doent matter, You are dealing in ratios here.

Use some cmos switches to apply com and +5V to the horizontal outside
edges of plane B and use an 8 bit A/D input to read the voltage from
plane A.

Do the same thing in the other direction and you have your X,Y position.

You have to use a calibration routine match the physical values received
to your screen layout.

Try a company called ELOGRAPHICS INC. Oakridge Ten. for screens.

I used theirs in Bowling Alley Systems about 5 years ago on an 80C31
and they worked fine as long as people dont use sharp objects to poke
at the screen.

Capacitive screens are better in a public application.
They usualy have a solid glass front and cant be damaged as easyly.
They require analog circitry to drive them, but have a minimum
resolution of 1024 x 1024 points.

Karl
.

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