Abstract: A valve system for distributing particulate materials includes a housing with a number of outlets. A distributor is rotated inside the housing by motor so that its outlet registers in succession with each housing outlet. A position sensor detects registration of the distributor outlet with each housing outlet and comprises a set of spaced-apart optical transmitters, a corresponding set of optical detectors and an encoding disk mounted for rotation between the transmitters and detectors. The encoding disk has a number of circumferentially spaced transmission zones, one corresponding to each housing outlet. Each transmission zone has apertures adapted to place a unique set of the transmitters and detectors in communication thereby causing the detectors to generate a unique signal. The encoding disk is rotated such that a particular transmission zone registers between the transmitters and detectors whenever the distributor outlet registers with the corresponding housing outlet.