This is a 3-digit, base-10 register for an electromechanical computer I'm working on. It is stepper-driven, and works like a 3-digit counter, counting from 000-999 before rolling over. When all three digit wheels read '0-0-0', 3 reed switches will close and a circuit will be completed to detect the condition.

1. Print out 3 copies of the digit wheels, 2 of the 8-tooth gears, and 1 copy of the drive gear.

2. Print out the gear leveler and reed switch holder.

3. Procure two 5/32 brass rods to mount the gear columns on, as well as three 1/8" cube rare-earth magnets, and three reed switches (I got mine from sparkfun.com).

4. Embed the magnets in the digit wheels, and mount the reed switches on the holder. Wire the reed switches in series.

5. Mount everything together, and glue the drive gear to the stepper motor shaft so that it can drive everything.

Update: Boom! Added STL versions of the files. Download, print and enjoy. I'd love to hear feedback/improvement/suggestions!