Quote Originally Posted by firebird77clone View Post
Coils produce spark when they momentarily ground through the points.
NPN refers to transistor type. NPN output in negative PNP output is positive
Actually, the spark occurs when the points open. This collapses the magnetic field in the coils primary windings and induces an voltage output of the coil secondary windings. In HEI, Electronic controls turn on / off the flow of electricity to induce the voltage in the coil. Modern Tachs can handle either type of ignition. Old tachs used a separate control box to convert the coil pulses into a usable / controlable signal.