Tortilla Pad: A touch pad made out of a tortilla or wet lumber

I’ve been trying to build a touchpad that appears to be different than those I see online.

Normal touchpads, both capacitive and resistive, have multiple layers, so they can be operated with a plain finger or stylus.  This is good, but costs a non-trivial amount of money.

I want to build one with a single layer of simple resistive material (a material with electrical resistance in the 100-100000 ohm range, see here for a list), with a probe connected to an analog voltage sensor (metal pen with a wire on it OR the finger of someone holding said pen).  The advantage would be that you can make ANY resistive material into a touchscreen, regardless of size, just by putting four conductors in the corners hooked up to a microcontroller.

My first prototype was with a fresh tortilla, and I’m very sorry I didn’t get a picture of it.  It looked dumb as hell…I cut a rectangle out of it, hooked alligator clips to the corners, and toggled voltage at the corners with Arduino data pins, reading the voltage of a probe placed on the surface with an analog pin.  It worked okay, but then stopped working when the tortilla dried out, and re-wetting it didn’t work. (I later found out from a food scientist friend that the reason re-wetting it didn’t work was that the water needs to be entangled in the starch molecules to get the electrical conductivity effect — heating the wet tortilla may have helped.)

I needed a new sort-of-conductive material to use.  Metal was out, since it is TOO conductive.  A chunk of steel has resistance so low I can barely measure it — assume 0.1 ohms corner to corner.  If I put 5V across it, that would be:

V = I * R
5 = I * (0.1)
50 = I

50 amps!  The arduino pins are rated for 40 mA (0.04 A), so that’s no good – the chip would either fry or shut down, depending on if the over-current protection works.  Worse, how much power is that?

P = I * V
P = 50 * 5
P = 250

250 watts, enough to fry the wires or heat the metal, even if the chip didn’t die.  So how much resistance do I need to be safe, let’s say 20 mA over 5V:

V = I * R
5 = 0.02 * R
R = 250

250 ohms minimum.  However, if it’s much more than a few megaohms (1000000 Ω), there will be so little current I won’t be able to measure the voltage accurately. So I need a pretty mediocre conductor…something between 10^2 and 10^7 ohms.  I found a list of materials with different resistances on Wikipedia.  The ρ figues are in Ω·m units, indicating that resistance depends on the dimensions of material used (see that page for details), but I just had to get the right order of magnitude.  In the key range I needed, I saw a bunch of esoteric materials I don’t have, plus one thing that I do have, or rather could make easily:

Wood (damp): 1×10^3 to 1×10^4

So I got out a chunk of particle board from the parts bin, wetted it down with a damp rag, screwed four wires down in the corners, drew a grid, and got started, and it actually worked…as long as I kept it wet.

Further, when I say it “works”, I mean I get meaningful coordinates out of it, but I have some physical and mathematical problems.  Let me run down the setup:

My code flips the X and Y pins shown above high and low to measure X and Y coordinates:

Here’s a pic of the values I get when I trace the grid drawn on the board, graphed with a Processing program:

So it “works”, sort of, but I have two problems:

1. Messed up coordinates: I get 2D values that correlate to where the probe is, but they aren’t nice rectangular coordinates — I need help on the math to turn these readings into real XY coordinates.  I know the basics (resistors in parallel, voltage divider basics), but the solution involves solving a system of equations, and I get stuck.  Help?

2. My “resisitive surface” sucks. My first prototype was a damn tortilla, and it only worked until it dried out.  Now I’m using wet particleboard, which is very inconsistent (~30kOhm on the bottom X axis, ~50kOhm on the top X axis, etc.).  It also constantly needs to be rewatered to stay conductive. What’s a good, cheap resistive surface I can get?  I need it to be between 500Ω-1MΩ end-to-end.

I’m excited to get this working, because it’s incredibly cheap and made of just one simple material instead of layers.  If I can find a resistive paint, I could make whole walls into touch pads.

If anyone wants to weigh in, I’m discussing the project on the Arduino forums here.

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