Abstract: Means for automatically positioning light sensing means relative to a linear series of light sensitive areas, e.g., apertures, on a longitudinally moving conveyor belt which is monitored by the sensing means. Rollers are used to receive the side edges of the belt, such rollers being freely rotatable so as to reduce the tendency of the belt edges to fray or otherwise degrade. The light sensing means and rollers are both mounted on a suspended frame means which moves transversely in response to transverse movements of the belt, whereby the sensing means is always aligned with the linear series of light sensitive areas on the belt.