I recently made a stirplate (with massive help from my good friend Nate) for yeast propagation using a computer fan, magnets from an old hard drive, an A/C power supply from a cell phone, a combo switch/potentiometer from Radio Shack, bolts and a washer from Home Depot, and a plastic container from Target. The whole thing cost me maybe $8.
The single hardest part of this project was cracking into an old hard drive. This required lots of small screws to be removed, and some serious prying.
Once you get the magnets, you can start to test your wiring. Take the fan, and glue the washer to the fan, and once the glue sets, stick one of the magnets to it. Take the A/C power supply, and cut the power cord so that you have the wall plug and the wire. Strip the wire ends, and do some (careful) testing with the fan to make sure that it spins in the right direction.
Next, put the potentiometer inline, between the fan and the power supply, and get your connections set so that you can adjust the speed of the fan with the potentiometer. This is important so that you don’t throw the stirbar when you start up the stirplate. Once you know where all your wires go, you can start to assemble.
Take your plastic container, and set the fan in it, with your bolts put through the fan. Adjust the height of the fan by using the nuts on the bolts to raise the height of the fan. Ideally, you want the fan right up next to the lid, without actually touching the lid. Give it some space, as the lid will come down with your sample sitting on it. This will take some testing. Fill your planned sample jar (erlenmeyer flask) with as much water as you think you might use, and put it on the lid, and plug in your power. If your fan can spin freely with your flask sitting on top of the lid, you’re all set. Take off the flask and lid, and glue the fan in place. I also glues the bolts at each corner just to minimize movement.
Drill a hole in the side of your container for the power to come in, and drill a hole on the opposite side for your potentiometer. Run your wires in through what will be the back, and put your potentiometer in the front opening. Re-test all your connections to make sure that the fan still spins the way you expect when you turn on the potentiometer, then glue, sauter and glue some more until nothing moves.
Close the lid, put on your flask, and turn it on with a stirbar inside. It should look like this: