then transistor 16 becomes conductive again accordingly transistor 17 is conductive during the time interval t 1 and non conductive during the time interval t 2 the output voltage waveform at the output 26 of the schmitt trigger 15 is designated b in fig2 a corresponding to low engine speed and is designated b 39 in fig2 b corresponding to high engine speed by comparing the voltage waveforms and b and b 39 it will be evident that the ratio of the conductive time of transistor 17 to the non conductive time thereof t 1 t 2 increases with rising engine speed if this output voltage were used to directly control the conductivity of electronic switch 41 then as the engine speed increased the fraction of an ignition cycle during which current flows through the primary 42 of ignition transformer 43 would decrease this is undesirable because at high engine speeds the current in primary winding 42 would not have time to build up to a value high enough to result in generation of a sufficiently high ignition voltage surge when the current in the primary 42 is interrupted in this embodiment of the present invention the control stage 27 is provided to overcome this difficulty during the time t 2 that output transistor 17 of schmitt trigger 15 is non conductive capacitor 31 charges up via charging resistor 33 towards the 6 8 volts potential at circuit junction 7 the diode 30 isolates the capacitor 31 during the charging up of the latter so that the driving voltage of the schmitt trigger 15 will be uninfluenced by the voltage across the.