Abstract: A position detector for generating output signals indicative of changes in the position of a cyclically moving object such as the pointer on a selected dial of a meter comprising a snap action toggle mechanism, drive means responsive to motion of the moving object for operating the toggle through one complete cycle for each cycle of motion of the object, and a photo coupler device for sensing the instantaneous position of the toggle. The coupler is positioned as a stop for the toggle at one limit of its travel, and the toggle reflects light from the coupler back to the coupler when it is at its other, opposite, limit position. When the toggle is at the one limit it covers the emitting and receiving faces of the coupler, thereby positively inhibiting it so that it cannot produce a positive output signal.