Geneva mechanism

A Geneva mechanism is used in a film projector to step the film quickly, holding it still, then stepping it again, etcetera. Here is a compact implementation.

The dotted circles indicate the mount, and the spacing. So there is a separation of 5 units between the axes of the wheel and the driver, and the pivot is 3 units above that, 2 units from the left. The cam on the arm (at the left) is one of those symmetric round length 2 cylinders. You should mount things so that the two 'eggs' can move the pivot arm (to free up the rotation just in time), and so that the 'beak' (with the two small washers) can grip between the gap in the big wheel to perform the actual quarter turn.

Happy turning, it feels great and is highly relaxing!