How to turn a standard computer fan from eBay or an old computer into a Stir Plate.
Screw in 4 woodscrews which are marginally thicker than the locating pins.
Get yourself some magnetic adhesive metal strip cut to length and attach to rear of fan.
Buy some small extra strong magnets from eBay and place at either end of your strip, try and ensure they are equidistant from the centre.
I have stacked 2 to improve magnetism because of the monster stir bar, but I tried a pair on their own with a smaller bar and they were fine.
Get yourself a variable transformer and connect it to wires of your fan.
Now the tricky bit, it my case I used a piece of Perspex but thin timber or any thin sheet of plastic would do and rest on top ensuring you have a uniform gap.
Some Ribena in to illustrate vortex, remembering that the trick is not to beat the crap out of it, just to keep your liquid yeast in suspension.
9v setting for smaller starters, 12v for larger ones.
Bigger distance you have between your magnets and stir bar will lead to stir bar flying off, use the screws to get to the correct height.