Interested in building your own stir plate? It's a tinkerer's dream and easily accomplished as a weekend project. Using parts accessible to most, a functional stir plate can be constructed in just a few hours. The parts you'll need to gather are:

· Computer fan

Most computer fans are of the 12 VDC variety. Any size will do, I use an 80 mm fan

· Rare earth magnets

These can be harvested from dead hard disk drives. A good source if you need to purchase your own is www.magnet4less.com

· Stir bar

You don't need a giant stir bar! A 1" bar works fine. Single bars are available on eBay or get bags of ten from www.stirbars.com

· Flask

Get a 1- or 2-liter flask, a good place to buy is www.cynmar.com

· Electronics for voltage control : potentiometer, 2,000 ohms ; LM317 voltage regulator ; resistor, 330 ohms ; capacitor, 0.1 mfd

Radio Shack is a good place to get single piece parts like this. Since I buy lots of these parts, I go to www.mouser.com. I have seen some designs that use just a pot for speed control. I don't recommend this. At low speeds, you are using the potentiometer to drop the full voltage from the power supply which could be several watts. Most pots are designed to dissipate ¼ to ½ watt. They won't last long operating like this.

· Plug in power adaptor

This adaptor can be anywhere from 9 to 12 volts DC, capable of delivering at least 200 mA continuously. The stir plates I build draw about 100 mA. The current requirements of other fan motors may be larger, but 200 mA should do.

· Stub of 1” PVC pipe

Use this as a spacer between the drive magnets and the hub of the fan. Attaching the drive magnets directly to the fan hub will interfere with fan operation and torque. A ¾” PVC coupler also works well.

· Enclosure

Any box made of plastic or non-ferrous metal will work but it should be around 2" deep so you can mount the fan to the bottom of the enclosure instead of trying to “hang” it from the cover with long machine screws.

Tips and hints for construction and operation:

* It's important for the drive magnets be as close to the stir bar as possible for good coupling. This is where the PVC spacer serves as a useful way to stand off the magnets from the fan hub and get them as close to the lid of the enclosure as possible. Measure and cut the spacer carefully, and you'll get the drive magnets up close where they belong.
* You don't need to develop a “Wizard of Oz” vortex in your starter to get good results. A 1" or 2" dimple in the liquid is all that's needed. The deep vortex you see in some of the YouTube videos is fun to watch, but not necessary.
* The distance between the poles of the drive magnets should be as close to the dimensions of the stir bar as possible. If you are using a 1" stir bar, the poles of the drive magnets should be about 1" apart. This is easy if you use two button magnets like I do. You can adjust the distance between the two button magnets for best stir bar stability.
* Don't fill your flask all the way up to the neck with starter wort. Aeration is important for yeast propagation, so try to keep the surface area of the starter wort high. Fill a 2-liter flask with no more than 1.5 liters of wort, for a 1-liter flask, use no more than 750 mL of wort.
* Don't use a stopper and airlock on your flask as you stir. This defeats the whole purpose of aeration. Just keep a loose foil cap over the neck of the flask to keep dust borne bacteria out as much as possible.