LiamOSM

The 3 control lines are used to send data to the shift register. Then the shift register actually does the multiplexing using its 8 outputs.

A hall-effect sensor would be a big improvement over a reed switch. Once you overcame the limit of having a mechanical switch, it could theoretically accelerate until the air resistance (drag) equals the force applied by the electromagnet, causing a net force of zero and therefore no acceleration. That is, assuming there wasn't any other bottleneck component like the speed of the transistor.

I wonder if this would be able to erase the RFID tags found on clothing in stores. If the tag was erased, I wonder if it would still trigger an alarm when passing through the sensor gates at the doors.Just a theoretical question of course.

This Instructable is great! The first gears I modelled used the typical circle-shaped teeth and the meshing was absolutely horrific. I also love the resources you linked to. Great work!