A rotary position detector includes a housing having an inner space having a reflective element. A light source emits light rays upwardly. A base supports a light detector assembly having a first number of toroidal-sector-shaped light sensors disposed in pairs about a motor shaft axis, one “A” detector element and one “B” detector element alternately disposed. A light blocker positioned between the light source and the light sensors rotates with the shaft. The light blocker includes a second number of opaque, equal-surface-area elements arrayed about the axis, the second number equal to one-half the first number. A circuit measures a signal from the “A” and “B” detectors relating to an amount of light falling thereon, a difference related to an angular position of the motor shaft.