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impersonating a resistive touchpad

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  • impersonating a resistive touchpad

    I'd like to use a µLCD-32PT(GFX) to impersonate a touchpad on an existing system. I.e. replace the existing touchpad with the one built into the 32PT module and spoof it.
    This would allow me to have a screen and touchpad where, at the moment, I have only a resistive touchpad (4 wire). I want to draw on the LCD, following the user on the touchpad - without affecting the existing electronics.
    I also want to simulate the touchpad and drive it in a "replay" mode.
    Any ideas on how to:
    • use the 32PT touchpad as a replacement for the existing one and monitor the XY whilst drawing on the LCD as the user uses the touchpad.
    • OR maybe I should electrically isolate them and write out an analog (PWM) signal on two pins and ignore the touchpad signaling wires. So basically relay the 32PT's touchscreen signals.some other way to do it...?

    Apparently all resistive 4 wire touchscreens work the same way. You turn on one axis of the touchpad and read (with the ADC) the position on its corresponding pair. SO I guess I could take the 32PT's touchpad 4 wire connections and parallel connect them to the other system - replacing the touchpad - but this may load the pad too much for a reliable reading. Not sure of the ADC input impedance.
    Likewise if I can buffer this signal line then I could switch in my DAC output (hopefully PWM the output lines) when I wanted to spoof the signal for replay.
    One thing I do know is that my existing touchpad is probed at 500Hz and 3.3V

    But can I do this in GFX - will it be fast enough - or am I going to have to add another little micro to do this man in the middle job for me.


  • #2

    Unfortunately if you connected the two in parallel (and ingnored the extra resistance created) you would have a fight on your hands as both systems would be turning on the axis at different times and shorting each other out.

    You would need to monitor the existing touchpad (using pins available on the picaso) and when the existing electronics turns on one axis, you need to ADC read (if recording) (or write, if simulating) the other axis.

    So you would need to add a 2 channel AD-DA I2C chip to Picaso.

    Of course it might not work, but there is only one way to find out


    • #3

      Are you saying the picasso output pins are not up to a PWM output ?
      If so then I will need to add a tiny23 or somesuch to do the pwm work...


      • #4

        There's no PWM involved in touch


        • #5

          ESPsupport wrote:
          There's no PWM involved in touch
          What I mean is I can drive it with a DAC or I can send out a pwm signal to drive it.
          E.g. using this method here:


          • #6

            I would have thought PWM would be overkill, and as that article shows, it has to be quite high frequency.


            • #7

     maybe a MAX 5841 I2C DAC 3.3V low power $3.60 in 1K
              - use the touch input on the 32PT - disconnecting the existing touchpad
              - draw on the LCD screen as user moves about touchscreen - recording etc...
              - writeout on the MAX5841 holdng the two XY channel output voltages and ignoring the scanning signals from the existing touchpad.

              Looks promising. Thanks for the advice...
              Now we wait for the 32PTs to be back in stock

              alternate chips:
              DAC101C085 (x2) -
              AD7992 (12bit) -