Laser cut gear on the motor, and a ring gear on the lens that was originally designed to be used with a follow focus unit. The motor is a special type of servo that can turn 2.5 full rotations.
A servo is just a DC motor that knows its position, so you can tell it to go to, say, 120 degrees. Normal DC motors you just say GO at some speed, not go to some place. However standard RC servos usually only rotate 180 degrees. I needed one that would turn more rotations so we could gear down its rotations and turn each step of the servo into a much smaller step of the focus/zoom ring.
We put slots in the base plate of the device so we could:
a) Use this on lenses of multiple lenses and
b) Move the motor forward or backward depending on if we wanted to actuate the zoom or focus ring