Abstract: Signals from a coil producing pulses in response to flux variations produced by a rotary member driven by an engine crankshaft are applied in push-pull through series resistors to the inverting and non-inverting inputs of a comparator. A variable voltage divider for the input signals is provided by a pair of complementary transistors respectively connecting the comparator inputs to opposite poles of a fixed voltage supply. The comparator pulses charge a capacitor through a transistor, the capacitor being periodically discharged through a resistor, once per crankshaft revolution. An increase of capacitor voltage beyond a very small amount progressively lowers the resistances provided by the transistors connected to the comparator inputs, acting on one of them simply through a transistor and on the other through a transistor and a current mirror circuit.