Abstract: A dust detector ring assembly for detecting extremely small quantities of particulate contamination in air being taken into the carburetor of an engine is disclosed. The assembly includes a series of infrared light emitting diodes alternately interposed with a like series of infrared light detecting diodes and arranged in a circle around an interior surface portion of a frame defining a passageway of circular cross-section therethrough. A light beam generated by each of the emitters is directed against a different one of the detectors so that the light beams intersect one another at a common point in the passageway. The sensitivity of the assembly is thus greatest at the common point of intersection of the light beams and decreases proportionally moving away from the intersection point toward the defining surface of the passageway. A flexible circuit board is formed in a circle and is connected to the emitters and detectors. A flat circuit board is connected to the circular circuit board.