To start I needed find a mechanism for spinning and after consulting the internet I decided that a case fan out of my old computer was the perfect device. Once I removed it from my old case and stripped off the molex connector. I identified the power cables while it was connected to the molex; the grounds on a molex connector are on the inside. Before investing in any of the other components I wanted to make sure that I had a working system so I attached my recycled 12v power supply to the fan and tested it; this created a circuit at full power.
(read more after the break)
In following with the internet guide I knew that I would need a rheostat and a toggle switch; these items are available at most Radioshacks. Radioshack has cut back its stock over the years but they still carry a useful, if small, assortment of small components. The next part of this project was going to be finding a water tight enclosure for my project. I went with a project box also from Radioshack.
From Radio Shack I needed: A Rheostat(3.99), a Switch(3.39), and a Large Project Box(7.39). My Radioshack total came to 14.77.
A stop at ace hardware would complete the shopping spree; I needed a large washer(.50), a package of small magnets(2.99), 4x 3inch machine screws(.48), 12 nuts(1.54), and super glue(2.50). This brought my ace hardware total to 8.01 and my overall total to 22.78.
Later I found out I'd need a 10 dollar stir bar, this brought my grand total to 32.78; these cost less than 10 dollars online but I wasn't willing to wait, savings of $5 or so dollars could have been had by tacking this onto my next Midwest Supplies order.
To assemble your stir plate:
(1) mount the washer to the center of the fan, and glue the magnets to the washer. Use the fan as a guide for where to drill your holes in the lid of your project box, drill one hole, insert a screw to hold the fan still while you drill the a hole in opposite corner, insert a second screw to secure the fan, and then you're set to do the final two screws. Space the fan with the nuts so that the magnets are near but not too near the top of the box; because the top is plastic it may warp slightly, and there is play on the fan head. If you hear grinding once you start it up, like magnets swirling on plastic, or the fan is immobilized, try increasing this gap.
(2) With the fan in place wire it up logically: Power in => Switch => Rheostat => Fan => Power out. This is a good time to run the wires in through the back of the box so that you don't have to disconnect and reconnect them later. Solder if you feel an urge but I just wrapped the exposed connections with some black electrical tape. Finish this step by gluing the wires to the bottom of the box so they don't travel into the fan.
(3) Mount the switch and the Rheostat through the front of the box.
(4) Put everything into the box and close it.
Below are some pictures and a video of the whole thing:
If I had it to do all over again and I wanted to spend a bit more money I could have gotten very expensive rare earth magnets on the internet for around 25 dollars after shipping. These come in multi packs with more magnets then this project requires and there is always a chance they'd mess with the fan even through the metal backing of the mounting washer. The 10 lbs of pull strength on the 1/2 inch by 1/4 inch magnets though can not be beat. Good luck throwing that stir bar. I may try this if my stir plate can not stir a 4 liter starter once I get that size flask. I would also be interested in trying out some of the higher end case fans for computers, I know some of them get 2500 or so RPMs and come with speed controls. All in all though I am happy with my home made stir plate and if it can stir a giant starter I won't mess with it.