Check out this cool, but short, video I found on Geek O System where a “Afrotech forumer” (is that a cult?) made a functioning computer touchpad out of paper and pencil graphite.
There isn’t much to explain here. It just uses pencil graphite on paper as a kind of two dimensional potentiometer. Four voltage dividers between 5v, 2M ohm resistors, the paper, and my grounded finger feeds signals from each corner into an Arduino. The Arduino does some insufficient math and spits out mouse coordinates that are not linear.
Of course, so simple. That’s how I thought it worked to, what with the ohms and stuff. Check out the video below.