https://bellsbeer.com/store/products/stirstarter-%2d-yeast-starter-stir-plate.html
inexpensive and it works great. Highly recommend this one.
I'm going to go against the grain here and recommend AGAINST building one. I built one, and it didn't work. The power supply was not strong enough to turn the fan with the stir bar in place (although it would turn the fan alone). After I replaced the power supply with a strong one, it spun fine, but the stir bar wouldn't spin in the flask - it would just jump around a little, then be flung off to the side of the flask and sit there.
After all the cutting, drilling, wiring, breaking the magnet out of an old hard drive, I got discouraged and broke down and just bought one. Save yourself the frustration.
If you're not mechanically or electrically inclined at all you could buy one. I considered buying one but I needed 4 so that become cost ineffective. I am building one from components I purchased plus parts laying around the house for $97.
See, this is the line of thought I completely don't get... You can buy a StirStarter for $45, shipped. Everything's done.
1. The stir bar needs to be at the geometric center of the fan to stir on the center of its length. If its off center it will get thrown when it encounters the resistance of the liquid.
2. The magnets (if you choose to use hard drive magnets which a lot of people do) need to be separated from the fan motor to prevent it from locking down the motor coils. That will also greatly reduce the amount of current required to get the bar turning.
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