I thought a unipolar stepper was easy: hook it up to a BJT and drive the base with your uC pin. If the voltage is still too much, use a darlington pair. If that's still not enough, hook a couple darlington pairs in parallel. Either I got something wrong, or underestimated how big 12V is. I have a ULN2803 and a Howard Industries 1-19-4200 (12V, 75 ohm, 160 mA, unipolar) stepper. Each pin of the Propeller is (directly) plugged into a pair of pins on the ULN2803 (P16 -> P1 & P5, P17 -> P2 & 6, etc) and each wire of the of the stepper is plugged into a pair of pins to match. The ULN2803's ground is connected to ground, and VCC to 3.3V. I then connected the common lead of the stepper to 5V and I can move the stepper very slowly. Great... now I've proven that my wiring and software is good. Time to step it up! I tried going straight to 12 V and that was not such a good idea. The 5V regulator got really hot and I might have burned a prop chip. I swapped it with a spare and now all seems good again (I can drive the stepper again with 5V), but that leaves me a bit confused. It's been way too long since my EE classes in college, so I don't remember how to do the math on this. Do I need something bigger than a ULN2803 to drive this stepper?