Controlling this stepper motor with NPN transistors
This motor might be a five-wire stepper motor. Such a motor can be driven with four NPN power transistors or four N-channel MOSfets.
You should measure coil resistance with an ohmmeter. Measure resistance between each wire pair. Don't compare wire colour to other motors - colour coding is unreliable. You might see a resistance sequence something like this:


pink-to-blue: 25 ohms
pink-to-black:25 ohms
pink-to-gray: 25 ohms
pink-to-purple: 12 ohms
blue-to-purple: 12 ohms
blue-to black:25 ohms
blue-to-gray: 25 ohms
black-to-gray: 25 ohms
black-to-purple: 12 ohms
gray-to-purple:12 ohms







simulate this circuit &ndash; Schematic created using CircuitLab

 It is apparent that this example has a purple wire that is common to individual coils of pink, blue, black and gray, because its resistance is lowest, about half the others. So pink, blue, black and gray coils are driven with transistor switches to ground, while the purple wire is fed with the positive end of the DC supply.
However, sequencing of the switches to get smooth, proper rotation in either direction will have to be worked out by trial-and-error. Once you have determined which wire is common, you can likely step this motor by hand-switching wires to your supply, and watching which direction the motor steps. Find the sequence that always steps clockwise. Reversing that sequence should step counter-clockwise.
Then replace the switches with transistor switches. Add flyback-protection diodes across each coil too.