IIRC, when a stepper is labelled 12 volts, it means that, at rated current, the motor winding drops 12 volts. This means that you need some headroom, namely, a higher supply, so that you can control that current with some accuracy.

You can drive a stepper with a high-side driver, often a mosfet, or with a low-side driver, which can easily be a bipolar transistor in series with a current-limiting resistor. This latter configuration relies on the fact that a bipolar transistor has an easily determined emitter voltage, with respect to the base driver voltage, and that emitter voltage, when applied to a limiting resistor, sets the current in that emitter, hence, the collector, hence, the motor winding. None of this will function properly if the sum of the required voltages exceeds the available supply. It's not unusual or unthinkable that your supply might be 24 or even 48 volts. In such a case, the base resistor to which I referred in my earlier post will, to some extent, protect the MCU, but a series schottky diode(Vf~0.25 volts) might be desirable as well, to protect the MCU in the event of a driver short.

The ULN2003 is a darlington array with diodes to discharge the emitter. Note that there's no Vcc connection. It's just darlington transistors with those protection diodes. Being darlingtons, the transistors lose about 1.4 volts or so of the voltage they're switching. You must allow for that in addition to the motor winding voltage.

RE