The FreeDS boards are designed specifically for the project and are a variant of Ultra Low Cost (V2) generally running at 10Hz.

There are old versions (V1) that are of the Low Cost type that work at 500Hz.

The image below corresponds to an unassembled Official FreeDS PCB.

The formula to calculate the resistance of the base of the transistor (Rb), would:

Rb ≈ 9 / Iv

Iv would be the Fan current expressed in Amps.