Three digit electromechanical Counter / Register 2.0

by chris, published

Three digit electromechanical Counter / Register 2.0 by chris Jan 16, 2012

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This is version 2.0 of 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.

This one uses fewer parts and has at least been slightly tested. It also has larger digit surfaces for better visibility, and comes with printable labels!


  1. Print out one copy of each piece, and one additional copy of the 8-tooth partial gear.

  2. Mount 3 reed switches (http://www.sparkfun.com/products/10601) in reed_holder_leveler. These should be wired in-series (I basically clamped the holder piece between the 3 switches and a bit of proto-board).

  3. This design assumes you have a 1/8" piece of wood between the stepper motor and the reed_holder_leveler piece, and that the motor is attached at a 45 degree angle relative to the front of the counter. Attach reed_holder_leveler to stepper motor / frame with an M3 bolt, so that it sticks up from the rear of the counter. Add an additional M3 bolt to the front of the motor to secure it.

  4. Assuming that the wood frame also has a cut-out around the motor shaft such that it can lay flush against the wood, attach 2 4-40 bolts to a mounting hub (http://www.sparkfun.com/products/10006) so that they are sticking up vertically. Attach the hub to the shaft such that the back of the bolts is almost touching the motor surface.

  5. Install a 1/8" magnet (http://www.sparkfun.com/products/8644) in the face of each digit wheel. Orientation really matters for some reason, so experimentally figure out which orientation of the magnet trips the reed switch at the longest distance. Press-fit should work, but adding a bit of glue will ensure the magnets don't fall out.

  6. Print out the digit_wheel_label file, and cut out the labels. When you attach the labels, the "0" face should be directly opposite the face with the magnet embedded in it. I used scotch tape to attach them.

  7. Install the "counter_gear_lowest." The digit surface should be at the bottom, and it should fit snugly onto the mounting-hub/motor-shaft.

  8. Cut a 5/32" brass tube such that it can rest on top of the motor shaft and also reach the top of the reed-switch holder.

  9. Insert counter_gear_bottom into counter_gear_top. Rest it on top of counter_gear_lowest such that the middle holes are aligned. 20-tooth gear should be on the bottom.

  10. Rest counter_gear_highest on top such that the 20-tooth gear is on the bottom.

  11. Insert the brass tube you cut in step 8 so that it holds all 3 digit wheels in place, but they can all move independently of one another.

  12. Cut another short segment of 5/32" brass rod so that it fits in the whole length of the reed switch holder. Insert the two 8-tooth partial gears such that they rest on top of the 'digit' layer and sit between the gears. Insert the rod to hold them in place.

  13. Your counter should be complete! All three reed switches should trip when the counter reads "0-0-0," and the circuit will complete.

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Thanks for sharing! I found a great website if you are looking for more ideas: www.directindustry.com/industrial-manufacturer/counter-61022.html

I did the design in solidworks, but the shapes are pretty simple - any 3D CAD tool would probably be fine.

This is really amazing. Exactly the kind of thing that makes 3D printing so exciting.

What program did you use to design the pieces?