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Touch screen issue!

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Hello,

I have a small 4 wire resistive touch screen approx. 2" diaogonal. It believe that the outer edge two wires of the FPC cable are "+". Where as the two other wires are the return signals each in respect with their "+" wire. I am applying 3.0VDC at the "+" wires and trying to read the output voltage on the return wires as I move an object pressing on the touch screen.

The problem is, I am reading a steady 3.0VDC on the return wires no matter where I touch on the touch screen.

Has anyone done this before? If anyone can help, it would be very appreciated.
Thanks

R
 

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  • TouchScrren.jpg
    TouchScrren.jpg
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hello again,

I read a document late last night at :

https://www.electro-tech-online.com/custompdfs/2011/10/AN10675.pdf

I tried a few things and got it to work. I tried the "x" axis reading it via the "Y+" output. So I have made a schamatic on how I got it to work, please view attachment "Touch Screen Bias".

Also, the "Y-" can't always just stay open. It also has to be tied to ground when we want to measure the y axis via the "X+" output. So I would need the PIC to control the "Y-" to be at "Open" state at one instance and "grounded" state in another instance. I am Planning to do the circuit outlined in attachement called "touch screen transistor". I have not tried this though... I am not that good at electronics. I was wondering if you can take a look as to weather or not I should proceed this way. I was planning to use a 2n2222 with a 1K ohm resistor!

Your help would be very appreciated. Thanks.

PS. The "touch screen transistor" schematic is just for the X measurement, I would obvioulsy do the same transistor layout for
"X-" !

r
 

Attachments

  • TouchScreen_Bias.jpg
    TouchScreen_Bias.jpg
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  • TouchScreen_Transistor.jpg
    TouchScreen_Transistor.jpg
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Basically you can think of a touch screen as 2 variable resistors which you must read twice, one time for the x axis and one time for the y axis.

e.g. you have 5 volts across the x axis (resistor) and nothing on the y axis (it is an output when the x axis is powered). Depending on where you touch it (make contact with the variable resistor) you will get a different voltage (different part of the resistor) directly corresponding with the x position of the touch.

You then 'high impedance' the x axis (set to input) and power the y axis and do the same process again to give you the y position.

I suggest you use an ADS7843. They are so easy to use and do a lot of the work for you. At least read the datasheet, very informative on touch screens.
 
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