Range mark position control employing optical encoding

An optical encoder device for use with a variable range mark display. An operator rotatable cylindrically shaped encoder wheel with longitudinal slits is panel mounted. Two light-emitting diodes are positioned outside the wheel opposite two phototransistors located inside the wheel. The light-emitting diode-phototransistor pairs are spaced at a forty-five degree angle from one another from the center of the wheel. Rotation of the wheel interrupts the light path between the light-emitting diodes and phototransistors producing two output signals. For one direction of rotation the first signal leads the second while for the other direction of rotation the first lags the second. A circuit is disclosed which determines from the output signals the direction of rotation and produces a count indicative of the amount of rotation.