Abstract: An internal combustion engine spark ignition system includes a transducer and a transducer circuit which coact to produce a rectangular pulse for each spark required. The leading edge of this pulse controls an input transistor which controls an output transistor through an intermediate transistor. The output transistor in turn controls an ignition circuit controlling the current through an ignition coil. The coupling between the intermediate and output transistors includes a resistance chain to points in which two timing capacitors are connected. At high speeds the circuit provides a fixed coil current duty cycle with a relatively short off time, whereas, at low speeds the duty cycle is determined by the length of the pulses from the transducer circuit giving a relatively long off time.