RatDoc- I also tried the route with potentiometers and wiring all over the place with no luck. Then I got turned on to this item:
www.amazon.com/ThermaltakeMobileFan (there's 2 sizes, I got the smaller 80 x 80mm). I disassembled it, freed up the speed control knob, mounted the fan on the bottom of a shallow but thick cigar box, drilled a hole through the front of the box, and mounted the relay to the inside of the front box panel so it doesn't spin around as I turn the knob. Then I stuck a steel washer on the top of the fan with some sticky-sided velcro and stuck 6- ½" neodymium magnets (6 for $4 at HD) to the top of the washer, 3 and 3 next to each other. There was no need to glue the magnets on, they are strong enough to stay on at full speed. The flat cord unwinds from the retractable housing (nice!), plugs into a USB adapter, and into the outlet. The fan is on as soon as you plug it in and spins "slowly" at the low setting; as you turn the knob, it spins quite fast at the highest setting. With a 2 liter Erlenmyer flask and stir bar in place, this stir plate creates an amazing vortex that would throw the stir bar off but for the strength of the magnets and their close proximity to the underside of the cigar box top. Of course, the speed can be lowered to barely a dimple at the top of the liquid surface so a true variable speed can be accomplished. At the end of the day, all this can be neatly retracted and stored in the box for easy storage. This is one of my first completed projects, was very satisfying and still cheaper than buying a commercial stir plate. What do you think?
