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hacking a computer touchpad to return 2 resistances?

Discussion in 'Electronic Basics' started by Mad Scientist Jr, May 18, 2007.

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  1. Can someone explain how to hack a computer touchpad such as this

    to function as 2 variable resistors - where resistance #1 is
    controlled by putting your finger on the x axis, and resistance #2 is
    controlled by finger on the y axis?

    It isn't as simple as finding the right wires to connect to is it? Or
    has anyone seen a circuit that you can wire a touchpad into and wire
    your circuit into and achieve this result? Would building such a
    circuit be hard?

    Any info appreciated... thanks
  2. kony

    kony Guest

    Definitely not just a couple of wires.
    A touchpad uses a capacitive grid. The onboard electronics
    sense that and turn it into a serialized data stream.

    There is no easy hack to do it. Maybe if you reverse
    engineered enough, taking the IC datasheet but IIRC these
    are proprietary ICs and those datasheets may not be
    available. You can certainly look around for some, one
    making is Alps.
  3. It won't be easy. Those devices key off capacitive rather than
    resistive effects so it isn't a case of ripping circuitry out and
    wiring up to the appropriate points. The best way I can think off
    would be to get a model with PS/2 output. The PS/2 mouse protocol
    is relatively simple and well documented. Decoding the signal
    would be a simple matter for e.g. a PIC microcontroller.

    Are you absolutely sure you need resitances though? PWM output or
    analog voltages would be far easier. What are you hooking it up
    to? Coming up with a voltage-controlled resistor that works as
    required can easily end up as a project in itself.
  4. Paul

    Paul Guest

    Your first step, is finding out the data format coming out on the
    PS/2 connector. Convert the output byte codes for the X and Y
    values, and store them in parallel registers. Connect the registers
    to digital to analog converters. The output at this point is a
    voltage. The voltage may be suitable for driving something
    directly (with a little buffering, say).

    Digikey has more than 5000 product listings for DACs, so you
    have plenty to choose from. Here is a DAC chosen purely at random.

    I think a joystick with potentiometers on the X and Y axis, is
    a *lot* less work...

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