How to get current flowing through the darlington transistor as a switch for a stepper motor?
Here is a modified version of a TIP120 running from a buffer impulsed by Arduino. I want to credit the original design here



NOTE:  the protection diodes around the Darlington transistor are not necessary because they are included in the chip.

15 to 5 voltage use a Voltage regulator setup .  I do not recommend using the 5 V source from the Arduino unless you are using a 9 Meg supply to power  Arduino and NOT depend solemnly in the the standard computer USB connection)