Abstract: An improved photosensitive motion detector which detects abrupt changes within a field of view while ignoring gradual changes in ambient lighting therein. In addition, sensitivity to such changes is maintained without regard to the intensity level of the ambient light, and is not sensitive to the "flickering" of fluorescent lamps. Light from said field of view, which can contain an abrupt discontinuity therein, is focussed upon two substantially identical photocells, each of which is in parallel with one of two substantially identical photoresistors, each of which is closely optically coupled to one of two LEDs connected in parallel but in reverse polarity from a common junction to a common potential.