I had recently purchased a pair of 5V stepper motors and a motor driver shield from the fine folks at adafruit industries which seemed perfect for spinning the knobs. The question was how to connect the steppers to the knobs. As it happens I had also just bought a gear set from American Science & Surplus on a lark and they fit perfectly on the Etch A Sketch shafts and the stepper shafts. So I simply needed a way to line up the steppers over the knob gears.
There are few problems you’ll encounter in life that can’t be resolved with the judicious application of a laser beam. I drew up a set of parts in Corel Draw and popped in to TechShop to cut them out of acrylic with their Epilog laser cutter.
I cut most of the parts out of clear acrylic except for the side guides which you can see in blue acrylic. The side guides hold the Etch A Sketch in place so it doesn’t move around laterally.