The timing cam is advanced for switching purposes by the large reciprocating pawl acting on the ratchet teeth molded on the timing cam. When the program requires a timed period during which conditions remain the same the ratchet teeth are spaced so the pawl cannot pick up the next tooth until the cam is advanced by the small pawl acting on the separate ratchet provided with small, closely spaced teeth. This ratchet has a pinion gear driving the ring gear molded inside the timing cam. The steps imparted to the timing cam by the small pawl and ratchet are very small and consume appreciable time--the length of time taken to position the next tooth for engagement by the large pawl being determined by the space between the large teeth.