This is a pulley system I designed to allow for a more precise, printed mechanism to operate the eyes of Crow, the sassy robot from Mystery Science Theater 3000.
It works well with my Crow, but if anyone has any issues with the radius on the wheel, you can hack it up, or ask me and I can upload a different size on it. It's a Work In Progress at the moment, input on improving it is welcome! Don't mind the globby glue. I was putting this together to test the mechanisms, not ready for golden pretty paint yet!
Credit for Crow himself goes to Best Brains Inc. The design is mine as an improvement over the styrene-in-dowel system I've seen online.
12/14/2015 - Majorly updated it to address issues I've experienced with the old one that was on here. The rotation wasn't optimal and wouldn't allow him to roll his eyes back properly. I also put a cutout in the main bracket so it can be swapped out without having to cut the string.
I printed these at 0.2mm height, 2-3 shells, 20-30% infill. Everything is hidden anyways so favor structural strength over aesthetics.
Print out A, B2, and C. A should compression fit onto the longer peg of the B2 piece. B2 should fit onto C with a bolt or something to allow it to rotate.
Drill/cut holes in the ping-pong balls for the pegs in A to fit into. The pegs are sized to 38mm (the official size of the eyes), so in theory a 38mm ping-pong ball will have the peg hit the opposite side to help you center it. A larger ball should just tilt to the side when they hit each other since the walls of the ball are so thin.
The puppet string of your choice is then laced through the holes in B2. I like to drape the string over the top groove in B2, and loop it through each of the inner holes, up through the outers, and back down the inner holes to create a sort of knot. You could also take two strings and tie one to each side, it's really your choice and I tried to make it a simple design with multiple options. The strings feed through the C plug which is then glued into the CPVC pipe. My Crow is designed so the C plug goes into a 1/2" CPVC pipe. It is not a tight fit, so a bit of glue keeps it in place (this is so the plug isn't ever too big if you go with a tighter ID pipe).