Schematic of "Fire on Open" version.
how it works. While the points are closed, Q3 is turned off which
means that the collector is at 12 Volts. When the points open, Q3
turns on, and it's collector goes to ground. The transition from
12 Volts to ground is coupled through C2 and R2 to the base of Q1and
briefly turns Q1 off as C2 discharges through R2 and D1. This
causes the collector of Q1 to rise to 12 Volts for a millisecond or two
then, when C2 has discharged, R3 turns Q1 back on, making it's
collector return to ground. The short 12 Volt pulse turns Q2 on
and, when the pulse goes away, the coil fires.