A rotary position detector includes a housing and a light source positioned to emit light rays onto a light detector assembly having a first number of toroidal-sector-shaped light sensors, which are disposed in "A,B" pairs about a motor shaft axis so that each "A" detector is circumferentially positioned between two "B" detectors. A light blocker is affixed for rotation with a motor shaft between the light detector assembly and light source and has a second number of opaque, pie-shaped elements arrayed about the shaft, the second number equal to one-half of the first number. An angular subtense of the light blocker elements is greater than that of a detector element. A signal connection between the light detector elements and a circuit measures a signal from the "A" and "B" detectors relating to an amount of light falling thereon, a difference therebetween related to an angular position of the shaft.