A work in progress: a magnetic stir plate for building up yeast starters.
First thing is to get a 12 volt adapter and hook it up to a rocker switch. This gives you on/off. Yay, power!
Next you wire up a rheostat to a used hard drive fan and your rocker switch. This allows you to adjust the speed of the fan. Wheeeee!
Now, take a hard drive rare earth magnet and affix it to the spinning side of the fan. Test mount the parts in an enclosure, put the magnetic stir bar in a flask and voila! It magnetically stirs your solution.
See the whirlpool funnel?
Needs a little refinement, but you get the idea. I'll do a more complete post later.