Abstract: A ferrous disk is rotatably driven by an internal combustion engine shaft. The disk has outer and inner circular rims projecting outwardly from one side thereof. The outer rim has two arcuate notches of predetermined arcuate length and position and the inner rim has three arcuate notches of predetermined arcuate length and position. A permanent magnet is mounted between and radially spaced from two Hall-effect sensor devices in fixed relation to the shaft axis such that the outer rim passes between one Hall-effect device and the magnet and the second rim passes between the second Hall-effect device and the magnet as the shaft is rotated. The notches are so positioned to provide four separate two digit binary outputs to a microcomputer that provides output signals to ignition coil drivers for the spark ignition devices of the engine and provides spark advancement and coil dwell time variation in accordance with shaft rotational speed and other engine operating conditions.