Ideally, the sensor should produce a
pulse for each engine revolution and
this is most easily achieved with the
aid of an inductor (coil) to pick up the
ignition pulses inductively. Since the
voltage in the ignition pulses is fairly
high, it suffices to construct the coil
from 10–20 turns of insulated circuit
wire around the spark-plug cable.
While the voltage level of the ignition
pulses is fairly high, their shape
varies appreciably. Therefore, the sensor
is followed by a pulse shaper to
transform the ignition pulses into stable,
uniform count pulses. This ensures
that random variations in the width
and amplitude of the ignition pulses
do not affect the readout.