Abstract: A rotary position sensor connected to the butterfly valve of an internal combustion engine, the sensor having a dual magnet structure interconnected by a pole piece having a generally C-shaped cross section forming a varying dimension air gap. The magnets, the pole piece and the air gap define a closed magnetic circuit. A Hall effect sensor is fixedly mounted in the air gap and is exposed to a well defined but varying magnetic field. Through the use of specific magnetic materials and sloping, curved surfaces, a precise yet tolerance friendly magnetic circuit is produced so that the sensor produces signals as a function of the angular positions of the magnets.