Man, that circuit confused the **** out of me for a few minutes there until I realized what they were doing. Funny enough it's the same way those LDD drivers dim
You think I'd know this one by now...
You should be fine Dom. The BC549c and BC547 are basically direct substitutes. The difference between your diodes and his are his are fast recovery diodes, while yours are general purpose. I'd have to go back and look again I think the diodes are just being used as protection in case there is any inductance from the fans. I think the general purpose should be fine because you aren't doing any high-frequency switching.