Four permanent magnets are disposed at predetermined angular intervals equal to an angle of 36* on a circular magnetic substrate by having the S pole faces of the first and fourth magnets and the N pole faces of the second and third magnets fixed to the substrate on a circle concentric to it. With the substrate rotated, a reed switch above it faces successively the first and second magnets to close the switch, the second and third magnets to open the switch, and the third and fourth magnets to again close the switch to permit pulses to be generated.