The back of the projector is where the shutter and the main drive shaft are. This is where the motor will be mounted.

Shutter

I technically don’t need to keep the shutter from the Bolex 18-5 projector, but it provides a nice place to mount the magnet so that it will pass by a reed switch in order to let the Raspberry Pi know that the frame is in position and ready to be digitized.

This shutter is a marvel of engineering, and I was a bit sad to destroy it. It has a mechanism that allows it to have 9 blades at slow-motion speed (5 fps) and then “fold up” at normal speeds (18 fps) and become a 3-bladed shutter.

Nevertheless, I pried it apart and discarded everything but the  inner ring that attaches to the main shaft. I reinstalled this along with the main pulley.

Motor

Now we come to the most troublesome part of the project so far — how to connect the NEMA 17 stepper motor. This is the motor that will be controlled by the Raspberry Pi to advance the film frame-by-frame. Attaching the stepper shaft directly to the projector shaft seems like a non-started.
At this point I’m thinking the best approach will be to find a pulley that will fit on the 5mm shaft of the stepper motor, then use a belt (or O-ring) to drive the original pulley.

I’m having a lot of trouble finding a suitable part online, though. If anyone has any suggestions, I would be very grateful.

Update: I got a suggestion to 3D print the pulley as well, so I’m going to that a try.