Here are some photos of the internals and wiring of my home made stir plate. It's a pretty simple build even for beginners, I've never even soldered before (not successfully anyway) and I got it to work on my first try. It's made with 2 neodymium magnets which are glued to the center hub on an 80mm PC cooling fan, a project enclosure, and some various parts from RadioShack. I saw a lot of people online making these that were just twisting wires together, and using wire-nuts to insulate connections. But I wanted it to last so I decided to solder all of the connections. I had never had much luck with soldering so I found some videos online that explained how to do it, and from that I managed to pull it together. I soldered everything before I put it in the case, and after doing so I realized that the power jack had to be inserted from the outside. So I had to cut the wires, then install the jack, and re-solder the wires back together, no biggie just a beginners mistake. Anyway speaking of power jacks, I also saw a lot of other people's designs they cut the power plug off of an AC adapter and hard wired it into the controls. I had a few ac adapters laying around but they also went with other devices so I didn't want to eliminate my ability to use them with the device they belonged too. So I installed the power jack which allowed me to do this and it also gave me the ability to use other adapters with different voltages to vary speed beyond what the rheostat can provide.
As you've already read I have a batch of barley wine going in it's 2nd week in the secondary fermenter once it is racked to bottles I plan on harvesting some of the yeast form the carboy to rinse and save for future brews. My plan is that since the secondary fermenter doesn't generally have as heavy of a yeast deposit as the primary I'm planning to gather what yeast I can and use my new stir plate to make a starter just to get the cell count up, then put it in the fridge to hibernate so I can dip into this culture for future brews.
Ok so enough of my yammering. . . . . on with the pictures:
Below is a view of the front of the stirrer. The case came from RadioShack along with most of the parts. It is the 6x9x3 enclosure Catalog #: 270-1809. The power switch is Catalog #: 275-711, and the Rheostat (Speed Control) is Catalog #: 271-265, with the control knob being Catalog #: 274-402. The material on top of it is just an old mouse pad that I cut and glued in place with the rubber side up to provide a non-slip surface and to help level the surface with the protruding bolt heads. It also helps absorb some of the vibration and control some of the noise. I also cut squares from the mouse pad and glued them to the bottom of the case for the same purposes. By the way those bolts are temporary, they're just what I had around the house. I do plan to replace them with ones that sit flush with the surface eventually.
This picture is an overview of the open case showing the wiring. I harvested the fan from an old computer power supply that I had lying around, all the wire came from it also. You can also see that I used plastic soda bottle caps for spacers to mount the fan to the top of the enclosure. You can't see the magnets but they are a pair of 3/8"x3/8" round neodymium magnets which are glued as far apart as I could get them on the center hub of the fan. I had to pick these up at an industrial magnet store (Northwest Magnet) for $3, Radioshack didn't have any that were strong enough at the time.
The next picture is of the back of the case showing the power jack with the adapter plugged in. You can see it gives it a nice finished look along with providing the flexibility of being able to use different AC adapters.
So that's pretty much it there's not much to it. If you are even slightly mechanically inclined you can most likely make one too. It cost me around $15 to make and I used as many improvised/recycled parts as possible to keep cost down. My only advice is be careful with the neodymium magnets, those little bastards are strong and can hurt if you get a finger, or skin pinched between them when they're flying together.