Apparatus for measuring angular displacement of an object in which an alternating potential source is applied to the electrodes of a light source, such as a gaseous discharge light, resulting in alternating illuminations in the medium, e.g., gas, about the respective electrodes, which are detected by photodetector means. The light source and the photodetector means are mounted such that the distances between the photodetector and the respective electrodes vary proportionately with any rotation of the object whose angular displacement is being monitored. The output signals from the photodetector means are separated and compared and, in response thereto, a signal is generated indicative of the angular displacement of the object.
Two, adjacent, but circumferentially displaced, photodetectors radially positioned exterior to a rotating body sense the changes in the intensity of light impinging on the detectors emanating from a defined source on the periphery of the rotating body. The sequentially changing outputs of the photodetectors are compared in a comparator circuit which provides a triangular wave shape output signal having either a sharp leading edge or a sharp trailing edge depending upon the sequence of activation of the photodetectors by the rotating light signal. The triangular wave is differentiated providing either positive going spike pulses or negative going spike pulses depending on whether the rotating body was moving clockwise or counterclockwise. The output of the differentiator is displayed on an oscilloscope to visually indicate by the presence of either positive or negative going spike pulses the direction of rotation of the rotating body.
