An ignition circuit for internal combustion engines Includes a resonant oscillation circuit (4) comprising a capacitor (3) In parallel with a coil (2a) of a sensor core (2b). The core is disposed adjacent an engine driven rotary metallic member (1) having a plurality of projections (1a-1d) spaced around its periphery and equal to the number of engine cylinders. The circuit resonates unless a projection lies opposite the core to suppress oscillations by eddy current loss. The oscillation signal amplitude is monitored by a threshold detector (5) whose switching output controls the supply of power to an ignition coil (10). This minimizes the ignition timing lag increase as a function of engine speed, and attendantly any engine power loss.