Here is the situation: I have a 28BYJ-48 motor, a cheap unipolar stepper motor. However, I would like to drive it as a bipolar stepper motor.
Unipolar stepper motors have 5 wires, whereas bipolar ones have only 4 wires. The 5th wire is a common wire joining the coils, creating 4 half-coils that can be enabled independently.
Both types of stepper motors are not driven the same way. Unipolar steppers are simpler to drive since you don't need to reverse the current: the driver only applies the tension to the common wire and sequencially grounds the other wires to power the half-coils. The drawback of course is that they are less powerful because only half of each coil is powered at a given time, so you get half the torque for the same coil length.
Luckily, you can use a unipolar stepper as a bipolar one with twice the torque, provided you have a circuit able to drive it. The modification is indeed rather simple: you need to remove the common wire and cut the link between the coils …