Abstract: An engine driven rotor has pairs of arms of different lengths passing by the stator core teeth with respectively different clearances. The stator teeth carry pick-up coils that are connected in series-aiding relation. Although the stator teeth are uniformly spaced, the rotor arms, that are equal in number to the stator teeth, have the longer pair angularly offset from equiangular relation to the shorter pair in such a way that when the rotor moves in its normal direction, a weak pulse is followed by a stronger one and this pattern is repeated four times per revolution. The output of the coils is supplied to a threshold switch, which will be triggered on the second of the pair of pulses at low rotation speeds of the rotor and on the first of the pair of pulses at high speeds.