Homebrew DIY: Building a Stir Plate

Undoubtedly one of the best side-benefits of homebrewing beer is the ability to acquire Science Jars and put them to legitimate use. Other than brewing beer or cooking crystal meth, there are very few legitimate reasons to collect Science Jars — unless you're cool enough to be an actual Scientist or work in a Sciencetorium. (Sidenote: some people have an odd habit of referring to Science Jars by the layman's term 'Erlenmeyer Flask.' These people are not "in the know." Calling a Science Jar an 'Erlenmeyer Flask' will get you laughed at by any serious Scientist.)

A Science Jar just sitting on a table, holding precious starter yeast, is one thing. But a Science Jar sitting on a stir plate, operating under some mechanical wizardry, with a vortex inside? It's no surprise that homebrewers have a reputation as some of the coolest, sexiest people around. Among the various other benefits of a stir plate (secondary to "vortexes," but nonetheless important) is the ability to continuously aerate your starter wort, leading to happy, productive yeast. With a stir plate, you are much more likely to be pitching an aggressive, energized starter, and therefore more likely to end up with a well-attenuated beer free of off-flavors. So of course you want to build a stir plate. And you've come to the right place.

Fig. 1

Before we begin, I must offer a DISCLAIMER: do not work on live circuits. Make sure your power supply is not plugged into the wall until you're done and ready to test the whole thing out. I am not an electrician nor particularly experienced in working with electronics; I'm just a guy from The Internet who successfully built a stir-plates, so please do not do anything stupid while following this guide. Now. With that out of the way, the first step, of course, is to collect all the pieces. (The full parts list is at the bottom of the post). Many of the parts necessary for this project can be salvaged from around your house or office.

Rare earth magnets, for example, are found in all desktop hard drives. Once you've got your hard drive, though, you're still going to need to pry the magnets from its cold, dead hands. And I mean "pry" — this is actually one of the most challenging aspects of the whole project. Most people probably don't have a Torx screwdriver sitting around the house — the kind where the head looks like an *asterisk  — but you can pick one up for a couple bucks at a hardware store. Dismantle your hard drive until you can see the reflective plate. Check out Fig. 1 for the location of the magnets — there should be two, and you'll have to pry out the metal bars that they're attached to.

With that accomplished (Fig. 2), you still have to remove the magnets from the bars — I accomplished this by grasping each end (of the bar, not the magnet) with pliers and slowly bending them until there was a gap under the magnet. From there, it was a simple matter to wedge a knife underneath and pop them off.

Fig. 2

If you have two rare earth magnets from your hard drive, stack them on top of each other in the center of a 2-inch metal washer. You'll want to center them in the middle of the washer or else the stir bar will just get "thrown" when you turn it on, and even then, you may need to adjust their position later until you get it right. I don't see a need to glue the magnets, being that they're magnets and all. However, you will need to glue the metal washer to the fan.

Next, drill your holes. There's not much I can do to guide you here — you kind of have to feel it out, make sure you have drill-bits big enough for the power switch and potentiometer, and line up the holes for the fan. For the potentiometer hole, a 3/8" drill bit served, but for the power switch, I had to widen the hole a bit with a Dremel. Finally, drill a small hole in the back of the box for the power cord.

I used a Long Pointy Object (the pointed end of my metal thermometer, in this case) to indent the four spots where the bolts go through the fan holes and the lid of the box. The 2.5 inch 10/32 bolts are a tight fit through the fan holes, and didn't appear to fit at first, but just keep on screwin'. Before fixing the fan to the case for good, pop two nuts onto the bolts (between the fan and the lid), then a washer. This will provide enough clearance between the fan and the lid for the magnets to spin.

Now you should be ready to wire her up. Get your cords ready. Cut off the "phone" end of the phone charger, and strip about a 1/2 inch from the ends of the wires from both the charger and the computer fan. My fan had both red and black wires — standard — but also a yellow wire that may have been for an LED light or speed control. After some testing, I realized I could simply tape this yellow wire up in a bundle and keep it out of the way. My phone charger, for whatever reason, had a yellow and red wire, with the yellow wire taking the place of black. Most likely, you will only have to deal with black and red wires.

Power switch wiring

Take the black cable from your computer fan and the black cable (yellow, in my case) from your phone charger / power supply, then twist the exposed wire together and push it through a female quick disconnect. (Obviously, you'll want to have pulled the power supply wire through the hole in the back of the box, at this point). Connect this quick disconnect to the “Earth” male connector on the power switch (the switch should have an identifying diagram). Next, take the red cable from the power supply, slide it into a quick disconnect, and attach it to the power switch on the terminal labeled “Supply.”

The third and last terminal on your power switch should be the one labeled “Load.” Get some spare wire, about half a foot or so, and strip off the insulation from both ends, then attach a disconnect to each end. Attach one side of the spare wire onto the “Load” terminal of your power switch, and the other end to the center pin of the potentiometer.

Potentiometer wiring.

Finally, take the red cable of the computer fan and add a disconnect (this should be the only remaining cable not attached to anything, at this point). Attach this disconnect onto the right side pin on the potentiometer — the pin that's furthest "down" in the above image.

Now that everything is wired, you should be good to turn it on and watch the pretty magnets spin. Obviously, you should test things out with a Science Jar full of water before you try it with an actual starter. You want to make sure your stir bar is going to stay seated — if it doesn't, the stir plate is useless until you re-calibrate it. As I said, I've had the best luck with two magnets positioned on top of one other in the direct center of the fan / washer. Even then, you'll have to slowly move the Science Jar around above the magnet until the stir bar is locked in — turn it on only after the bar is in position.

Then, my friend, you should have your vortex.

Parts List-

-Enclosure. I used a cheap wooden cigar box. A cigar box is probably the cheapest / easiest route, but you could use a project enclosure from RadioShack, or even an external hard drive case.
-cell phone charger. I had a number of these around the house from old phones. Anything in the 5-12 volt DC range should work. The charger I used for this build was 5 volt, though it's possible this will turn out to be too weak for larger starters. 6v to 9v is probably ideal.
-computer fan. 12 volts DC
-hard drive magnets. I collected two of these from an old hard drive. You can also buy rare earth magnets online.
-2-inch diameter steel washer
-four (4) 2.5-inch 10/32 bolts

-four (4) washers to fit the 10/32 bolts

-twelve (12) 10/32 nuts

-6-8 inches of scrap wire
-25 ohm 3 watt Potentiometer (Rheostat) - Catalog # 271-265 from RadioShack
-12 Volt DC/30Amp Rocker Switch with LED - Catalog # 275-018 from RadioShack
-Silver Tone Knob - Catalog # 274-424 from RadioShack
-1/4" Fully Insulated Quick Disconnects - Catalog # 64-3131 from RadioShack

With the parts I was able to collect for free, I don't believe it cost more than $15 at most to build this.