Electrical – What’s the minimum amount of transistors I need to operate a stepper motor

When I read your post the fist time I missed the fact that you've already cut the traces.

From the photo it appears that Node 5 the common 5th wire is brown. The first inductor pair are red-white wires, and the second are yellow-blue. If you reconnect the trace you cut between red-white(around 10 o-clock on the photo) you will have a bipolar config.

I'm not an expert so I can't tell you if this would work, but it sure looks worth a try.

Assuming that the outputs of your brushed drivers are a voltage or current proportional to the commanded speed, you can certainly drive a bipolar stepper with two brush drivers. Let's assume that the output of a driver set for 100 gives exactly the drive current needed by a stepper, and that -100 gives exactly the current needed, but with opposite polarity. Then the two driver channels can be driven like this:

^{simulate this circuit – Schematic created using CircuitLab}

The position of the stepper motor is given by the relative amplitudes of the two phases. In this case, the requirement is that they be 90 degrees apart. In the figure, reading from left to right gives rotation in one direction, while reading from right to left gives rotation in the opposite direction. Always keep track of what step you are at, and only change the drives so as to move one step to the right or left.