Counting number of turns

[DjScribbles]

It depends on what your purpose is:

As others have suggested, the standard approach in many cases is encoders; there are other types available as well, there were some magnetic encoders in First choice this year, and I found a few of these in our electronics boxes (we didn't order them that I know of, so they may have come in the kit this year or last), which if you can get them sourced from the EU may be much easier to get.

If you are measuring wheel speed, you can also build a hall effect sensor quite easily with some magnets, a few discrete electical components and one latching hall effect sensor like this (see the datasheet on sparkfun and build the typical 3-wire circuit). These work well in high velocity measurements like a shooter wheel, but not for high precision stuff like drive trains or arms where a single revolution is a big movement.

If you need to control the position of an arm, a simple potentiometer (ideally one with 10Kohms of resistance, definitely more than 1Kohm), the poteniometer can tell you the exact position of an object (you need to convert the voltage to degrees manually though) but they aren't typically built to withstand high speeds (even continuous potentiometers would not be a good idea for anything that spins over a 120rpm or so)