Here is how to get the TIP31 transistor into a darlington configuration so that it has half a chance of saturating and driving the stepper motor winding. Note that you will need one of these circuits per winding on the stepper motor.

It may also be necessary to put each of the TIP31 transisors on a heat sink. In a darlington configuration the TIP31 transistor will saturate to a voltage drop from collector to emitter of about 1.2 volts. At 1.5 amps of current (if that is the winding limited current of the stepper motor you will be asking the power transistor to dissapate about 1.8 watts. (This level of power demands at least a small heat sink say 1.5" x 1.0" x .75".)

Michael Karas