I made this stir plate to stir a yeast starter which is needed to make high gravity beers. It works great for what I need it to do, and I'm sure it would work for whatever you needed to stir.
What you will need:
1 Computer Fan (I used a fan from an old power supply)
1 Power Source (some form of 12v DC power. I used a power adapter from an old router)
1 Rare Earth Magnet (I used a magnet from an old hard drive http://www.instructables.com/id/Pulling-apart-a-desktop-hard-drive-to-get-rare-ear/all/ )
2 Rheostat - 25 Ohm (if you can find a 100 Ohm I would recommend that in place of the two 25 Ohm's)
1 Project Enclosure (I used 8"x5.75"x3")
1 Stir Bar (get a set of different sizes so you can figure out which one works best for you)
4 Bolts (I used #10-32 x 2" and had to drill out the holes in the fan. You can get smaller ones if you prefer not to use a drill)
4 Rubber Bushings
Soldering Iron / Solder
Step 1: Glue Magnet To Fan
I used gorilla glue to first glue a washer and then a magnet to the fan. Be sure to center the magnet as best as possible. If you're using two separate magnets, be sure they are the same distance from the center of the fans rotation. The magnet need to be pretty strong, so use rare earth magnets. It takes several hour for the glue to dry, so while you are working on the rest of the project set this aside and let it dry.
Step 2: Drill Holes In Project Enclosure
Now you will want to drill out the holes for the bolts that hold the fan and for the switch and rheostat(s). Make sure the holes for the fan mounts align with the holes in the fan casing.
Step 3: Mount Bolts and Controls To Enclosure
Next you will want to mount the bolts to the enclosure. Use a screwdriver and tighten a nut to the inside of the enclosure. Go ahead and test and see if the fan is able to slide down over the bolts. Make any adjustments to ensure the bolts slide fairly easily through the holes in the fan casing. Also go ahead and mount the switch and rheostat(s) to the project enclosure.
Step 4: Make Wiring Connections
Go ahead and wire the system using the wiring schematic provided. You will want to take the positive (red) side of the power supply and solder it to one side of the switch. The negative (black) side of the power supply needs to be connected to the negative side of the fan. Next, take a small piece of jumper wire and solder it to the center position of the switch. Solder the other end of the wire to the center position of the rheostat. Take another piece of jumper wire and solder it to the left side (looking at the face of the rheostat) of the rheostat. If you are using two rheostats then you will want to solder this wire to the center position of the second rheostat. If using only one rheostat, then solder the jumper wire from the first rheostat to the positive fan wire.
Step 5: Mount Fan To Project Enclosure
Now you will want to mount the fan to the bolts you secured earlier. Before you slide the fan over the bolts, first place a second nut on the bolts. This will allow you to adjust the height of the fan to the top of the project enclosure. Next place a washer over that but and then a rubber gasket over the washer. The rubber gasket will help reduce the vibration of the spinning fan. It will take some experimenting with the distance between the fan and the enclosure and the size of the stir bar you are using. You want the stir bar to be close enough that the magnet has a good grab on it, but not too strong of a grab that it actual prevents the fan from spinning once turned on. 1 1/2" from the enclosure to the top of the fan worked good for me.
Step 6: Add Rubber Pads
You will want to add rubber legs to the bottom of the project enclosure to keep vibrations to a minimum. I had some extra pads so I placed them on the top of the enclosure, so my flask can sit on them. It may take a few adjustments before you dial it in, but once you get it you can sit back and enjoy.