Older project, but I really like this one- I pulled the knobs off of an Etch A Sketch and attached it to two stepper motors which in turn are attached to the parallel port of a computer. This gives one the ability to control the plotter’s movements with a standard mouse or analog joystick (more intuitive interfaces than the knobs). Having a computer in the loop also allows one to record what movements are done, so one can edit them and play them back. This robotic device is also capable of erasing the Etch A Sketch by flipping the whole contraption upside down and tilting it back and forth. The assembly has been carefully counterbalanced so that it only takes a tiny motor to flip it. A pair of mercury switches let the computer know which way is up, so that the board can be rotated to the correct positions. Link.