Who says nothing productive ever came out of doodling? Certainly not the hacker responsible for this fun (and at least somewhat functional) paper-and-pencil touchpad, which takes advantage of the natural conductivity of 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.
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