Hello,
I am attempting to turn a stepper motor with a darlington array. It occurred to me that I made one fatal mistake when I purchased parts in that bipolar steppers require the current to change polarity during the complete cycle. I was thinking it would be simpler to use a bipolar because less wires and I only need to move in one direction and will never need to switch directions.
At first I thought I would only need two outputs from my photon wired to two inputs on the darlington array and then the two corresponding outputs would be connected to the correct wires on the stepper. The driving voltage would be conected to the other end of the stepper coil wires and then the darlington array would have the appropriate ground connection. When I send a high to the darlingon input, my thinking is that it would then sink the current through the coils to ground. I could do this back and forth to pulse the coils. However, I did not realize I need to reverse the coil polarity during the complete cycle.
So, I am thinking I should pick up a different stepper motor with a unipolar connection. I dont think there is any simple way using my darlington array and photon to do this with a bipolar stepper. I am thinking I could with an H-bridge. But the new stepper -while more expensive- should be simpler. Maybe.
Suggestions? 
Thanks!
Brian