Well ladies and gents, this is not an actual arduino project I am working on ATM. I am currently making a home brewed mag stirrer; mag plate; mag mixer. What ever YOU would like to call it. If you do not know what this is YouTube "magnetic stirrer"
Its for science projects.
Anyway. I have two "huge" 190mm computer fans with magnets secured to them. Build a rather awesome enclosure for it and all. Even added LED illumination from below with on the fly swappable LED colors. Just in case you are working with something that reacts or doesn't react to a certain color spectrum you can swap LEDs.
I have legit stir bars that I have purchased from the net.
Suffice it to say IMHO this thing rocks. The issue IS that a real mag mixer can go from aprox 60rpm up to 1500rpm or so.
My current set up goes from 800rpm to 1300rpm. It comes up to speed "too fast" and the stir bars tend to fly off and bounce around. Plus I would like to have a better range of rpm. I'm comfortable with the high end (1300rpm) but would like more low end.
I'm controlling the rpms via the pot that came with the pc case fan.
Can someone please recommend a resistor value. Or other means to give me more "control" over the speed of the fan via the pot control knob that came with the fan and I have implemented?
I would greatly appreciate any input. If anyone really cares about this project I would be happy to give more details and or provide pics. If that is of any interest.
P.S. the fan (and everything else) is all running off 12v dc (obviously?). 
TIA