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Home > Archive > Electrical Engineering > May 2007 > hacking a computer touchpad to return 2 resistances?
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hacking a computer touchpad to return 2 resistances?
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| On 18 May 2007 10:57:24 -0700, Mad Scientist Jr
<usenet_daughter@yahoo.com> wrote:
>Can someone explain how to hack a computer touchpad such as this
>
>http://www.newegg.com/Product/Produ...N82E16826152013
>http://www.newegg.com/Product/Produ...ouchpad&x=0&y=0
>http://en.wikipedia.org/wiki/Touchpad
>
>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
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.
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| Andrew Smallshaw 2007-05-18, 5:25 pm |
| On 2007-05-18, Mad Scientist Jr <usenet_daughter@yahoo.com> wrote:
> Can someone explain how to hack a computer touchpad such as this
>
> http://www.newegg.com/Product/Produ...N82E16826152013
> http://www.newegg.com/Product/Produ...ouchpad&x=0&y=0
> http://en.wikipedia.org/wiki/Touchpad
>
> 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?
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.
--
Andrew Smallshaw
andrews@sdf.lonestar.org
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| Mad Scientist Jr wrote:
> Can someone explain how to hack a computer touchpad such as this
>
> http://www.newegg.com/Product/Produ...N82E16826152013
> http://www.newegg.com/Product/Produ...ouchpad&x=0&y=0
> http://en.wikipedia.org/wiki/Touchpad
>
> 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
>
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.
http://www.digikey.com/scripts/DkSe...=410632&Site=US
I think a joystick with potentiometers on the X and Y axis, is
a *lot* less work...
Paul
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