In the early days, PNP bipolar transistors were common, but the bulk of circuits you see today use NPN transistors. As [Aaron Danner] points out, many people think PNP transistors are “backwa… ...

Essentially, each pnp transistor compares a reference voltage on the npn emitter with the voltage across the lower limit switch circuit resistors R4, 5 and 6, directing current to the led if the limit ...