3.5" TFT Breakout Touchscreen Unresponsive
Moderators: adafruit_support_bill, adafruit
Please be positive and constructive with your questions and comments.
- dnicolay
- Posts: 2
- Joined: Sat Feb 04, 2023 12:02 pm
3.5" TFT Breakout Touchscreen Unresponsive
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.
- adafruit_support_mike
- Posts: 67446
- Joined: Thu Feb 11, 2010 2:51 pm
Re: 3.5" TFT Breakout Touchscreen Unresponsive
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.
- dnicolay
- Posts: 2
- Joined: Sat Feb 04, 2023 12:02 pm
Re: 3.5" TFT Breakout Touchscreen Unresponsive
Thanks for the tutorial. Very helpful.
Please be positive and constructive with your questions and comments.