Wednesday, January 20, 2010

Building my magnetic stir plate

Back in November I went out to learn more about brewing beer. Among the demonstrations, one guy showed us a magnetic stirrer he made using a computer fan and a magnet. He was using it to stir up a yeast culture. In theory, this helps the yeast grow and thrive more than just letting them sit. Since starting a healthy yeast culture is an important step in making wine and mead, I thought I could use something like this for myself. Plus, I love a good workbench project.

The principle is very simple: place a magnetic stir bar (a small plastic coated magnet) in a container, and then place a spinning magnet underneath the container. This will spin the stir bar, and thus stir the mixture. I started researching, and ordered some small stir bars. I tried using some salvaged hard drive magnets, but I didn't like the results after my initial testing. After that I bought some smaller stir bars from eBay, and it turns out the seller of those stir bars runs the website stirstarters.com, whom I must give much of the credit for the design and my inspiration. While he sells his own stir plates, he also understands the DIY culture, and provides details about his design including a schematic for the speed control. With this information, I set out to make my own with one major difference: I didn't want to have to keep track of another AC adapter. I just wanted to run AC wall current right into the device for simplicity.

To get power to my stir plate, I would need an AC adapter to convert 120V AC to 12V DC, and it needed to be small enough to fit inside my project. What I found was a wall adapter that converted to a cigarette lighter plug like the one in your car. These are often sold as cell phone accessories, and with some searching I was able to find them very cheap online. I removed the "car plug" and the prongs that go into the wall, and replaced them with wires to connect into my project. This gave me 12 volts DC for powering the stir plate.

To actually get the AC power into the box, I salvaged an AC inlet plug and power cord from an old computer power supply. The plug mounted nicely onto the project box, and looks pretty nice I think. I also used a molex plug and wires to carry the 12 volts DC out of the AC adapter.

The speed control circuit was fairly simple to build, with the exception of the voltage regulator which I put on backwards the first time due to a poor diagram on the package. Thankfully it didn't cause any damage, just a confused engineer. I was able to find all the parts I needed at the local Radio Shack, except for the potentiometer, which had an uncommon resistance value, so I ordered that and the project box from Jameco. Had I not already bought my parts from Radio Shack, I would have gotten everything at Jameco. They have good selection and good prices. I mounted all of it on a proto circuit board to keep things relatively tidy, and added a power indicator LED to the circuit because I had some laying around... and LEDs are just fun.

The stirrer itself is made up of an old computer fan I pulled from an old project, a plumbing adapter from Home Depot which happened to be the perfect height, and some super strong button magnets from Ax-Man Surplus. This whole assembly is held together with epoxy, and thanks to some careful planning and assembly, is pretty well balanced when it spins.

The completed stir plate works quite well. I can just plug it in, turn on the power switch, and adjust the speed. I haven't tried it out yet on a yeast culture, dissolving some bentonite, or anything else yet... but it can stir water like a champ! I tested it out on a 1-gallon jug and got a little vortex, so this looks like it will be a useful tool to have on hand.

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