Quote Originally Posted by turbokinetic View Post
The NPN - PNP connection allows the PIC to switch +12V on the high side, to keep the power to the circuit on, after the power path from ignition switched 12V throuth the indicator lamp is no longer available. I use the signal through the light bulb to tell it when the ignition is turned ON, as well as being able to ground the lightbulb and cause it to illuminate.
I used a Bosch relay on a 71 chevy to power an HEI ignition and the relay would hold down to 3volts which was low enough to allow engine to run when switched off due to receiving power through the alternator light, had to kill engine in reverse gear, where the backup lights would load it enough to cutoff relay.