A downpipe sensor detects single grains in a downpipe. A transmitting unit and a receiving unit are spaced apart across a measurement field. Light beams emitted by the transmitting unit are guided in the case of free beam path through the downpipe interior to the receiving unit and are attenuated during a passage of a grain. The receiving unit is a line element with a predefined number of receiving elements. The transmitting unit has light-emitting diodes with perforated screens and a reflector element in the form of a right triangular prism. Light is emitted from the diodes transversely to a receiving axis of the receiving unit, bundled via the perforated screens, guided into the reflector element, deflected by total reflection toward a exit surface to form a light band of parallel light beams. The light band illuminates the entire measurement field with even intensity.
