Designed for an IR interruptor sensor  attached to your chassis or hot-glued to the side of the motor. Might well also work with a reflective sensor, but I haven't tried that. You can go into the openscad file and change the number of teeth/slots, but how much positional accuracy are you going to get with this kind of setup anyway. 

The slots in the face of the disk should be a press fit with the spokes of the wheel. If not, adjust the openscad or use a dab of hot glue.