3.5" TFT Breakout Touchscreen Unresponsive

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dnicolay
 
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3.5" TFT Breakout Touchscreen Unresponsive

Post by dnicolay »

I'm looking at the resistive touch screen on Adafruit 3.5 TFT breakout board. The display section is working as expected. However, the touch screen does not seem to respond. The X+ to X- resistance measures 294 ohms and the Y+ to Y- measures 555 ohms. Pressing on the display does not change either of the resistance values significantly. I can measure only a few ohms change with a very firm press. The protective screen has been removed. What is the normal resistance change range associated with pressing on the display? Is this behavior an indication of a defective touch screen?Thanks.

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adafruit_support_mike
 
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Re: 3.5" TFT Breakout Touchscreen Unresponsive

Post by adafruit_support_mike »

dnicolay wrote: Sat Feb 04, 2023 12:20 pm What is the normal resistance change range associated with pressing on the display? Is this behavior an indication of a defective touch screen?
The end-to-end resistance doesn't change.

A 4-pin resistive touchscreen is basically two force-sensitive resistors sitting face to face. Pressing on a point connects the middle of one to the middle of the other.

It's technically possible to measure positions by measuring the resistances:

- X+ to Y+
- Y+ to X-
- X- to Y-
- Y- to X+

then doing some resistor network conversion. The math for that is chunky though, and there's an easier alternative:

1: Apply voltage between X+ and X-, then measure the voltage at Y+
2: Apply voltage between Y+ and Y- and measure the voltage at X+

In either case, you're measuring the voltage at the center of a voltage divider through some additional resistance. The current to the ADC is negligible, so there's almost no voltage drop across the extra resistance. The ratio of the voltage on Y+ to the voltage between X+ and X- will be proportional to the location of the touch.

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dnicolay
 
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Re: 3.5" TFT Breakout Touchscreen Unresponsive

Post by dnicolay »

Thanks for the tutorial. Very helpful.

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