DIY Stir Plate Build

As many home brewers have no doubt experienced, yeast can be one of the more difficult variables to control in the brewing process. Much like an impressionable youngster, yeast requires just the right conditions in order to thrive and therefore become successful in its efforts to produce something of value. There are many environmental factors that can have a lasting effect on your yeast, most commonly known is temperature and of course, proper nutrients – just to name a couple. However, let’s not ignore the fact that a solid foundation for strong, healthy yeast can go just as far in achieving delicious results. In my opinion, the end game as a parent in bringing up a resilient young adult should be that he or she will grow to be resilient enough to contend with the adversities life will throw at them. The reasons for nurturing and bolstering your yeast culture prior to brewing in a non-commercial environment are the same.

Sure, “smack-packs” are a great way to give your yeast an early boost before pitching, and I have and will continue to use them nearly every time I brew. But I’m not entirely convinced that the nutrient pack contained in them is everything you need to give your yeast the wake-up to start fermenting at a mad clip, especially if time is a consideration.

That’s where yeast starters come in. I began making starters, like most home brewers, in growlers and Mason jars with a cup or two of sanitized water and some DME, shaking the bottle vigorously a few times a day in the time leading up to pitching. Inevitably though, I would forget and felt that these passive starters were not only failing to get the job done to my satisfaction, but were just another thing to keep track of.

Then, while brewing at a friend’s house, I discovered stir plates. Yes, the hallmark of any good chemistry student, not to mention the likes of Walter White, was now accessible – except that the prices I was finding online were not exactly cost effective. It was going to be a hard sell to my wife, plus I just wanted something simple and was hesitant out of principle to drop a couple hundred dollars on a “good one”. So, after a little research I decided to take on the task of building one myself with some spare parts that, not only can you find at almost any hobby store including your LHBS, but most likely have in your house as your read this.

For Your DIY Stir Plate Build:

you will need the following supplies:

  • Some kind of box, I found a plastic project box at my local electronics hobby store but you could just as well use an old cigar box or some other container that
    has some considerable depth to it, something close to 2”, can be opened and closed easily and has some way to latch or secure shut
  • A computer fan, preferably the smaller types found in most desktop models
  • A potentiometer, 2K ohms
  • A rocker switch
  • A plug-in power adapter, anywhere from 9 to 12 volts. I used an old one from an Atari gaming system
  • A rare earth magnet, I used a hard drive magnet. You may want to find a couple, they’re kind of fragile. I broke the first one I was trying to use
  • Magnetic stir bar, I found them at my LHBS – buy a couple because you’ll need at least two so that you don’t lose them during pitching (explained later)
  • Large fender washer, try to match the diameter of the computer fan used
  • 1.5” – 2” machine screws, the thinner the better
  • Small diameter plastic tubing, at least an 1/8 of an inch, but slightly wider than the diameter of the machine screws you use
  • Scrap wood
  • Scrap small gauge wire
  • Slide-on insulated terminals
  • Small L-brackets
  • 2, 3” – 6” C-clamps
  • Super glue or Gorilla glue

you will also need the following tools:

  • A pair of wire cutters
  • A pair of crimpers
  • A Phillip’s-head screwdriver
  • Small eye-glass screwdriver with a flat-head bit
  • Dremel tool or drill with small bits

Step 1: Wiring the Guts

diy stir plate guts

wiring the guts

Of course whenever you’re working with electricity you should always be extremely cautious and you should not have the power supply plugged in while working with it. Also, if you have a voltage meter it would be a good idea to test continuity before plugging in for the first time. That being said, with your wire cutters snip off the end of the power supply (the end that plugs into whatever device it’s from) and locate the hot and neutral wires. The neutral wire will most likely have a white strip on it. Cut back some of the insulation from where you snipped and attach a slide-on terminal to each the hot and neutral lines and then secure them by crimping down on the collar of the terminal. Once the terminals are secure (be sure to perform a light tug test) you can slide the hot onto the “power in” post of the rocker switch (on mine it was the left-most post as you look at the back of the rocker switch) and the neutral to the “power out” post (the right-most post). Then take some scrap wire, roughly 2” – 3”, attach a slide-on terminal to each end, and jump from the middle post of the rocker switch to the middle post of the potentiometer.

Now, there should be a wiring harness coming out of the computer fan that you harvested. Locate the hot lead, on mine it was red, strip back some of the insulation and attach a slide-on terminal. Then slide the terminal on to the “power out” post of the potentiometer. I had some trouble with the terminal and ended up simply wrapping the bare end of the red wire around the post as seen in the photo below. Now you’re fan and power controls are completely wired up.

When I built mine, I wired everything first and then mounted the potentiometer and rocker switch to the front of my box. Do whatever you feel most comfortable with. After I wired, I used my Dremel tool to cut a rectangular hole into the front left of my project box and drilled a hole large enough for the potentiometer’s post to fit through on the front right. I had to epoxy around the hole I cut for the rocker switch because I made it a little too big. Get to where you’re comfortable and then at this point it would be a good time to test with live juice. Plug the power supply in and hit the rocker switch. Mine glows green but there are others that use either a different color light or none at all; it’s all up to your preference. Attach the potentiometer’s knob with the small set screw provided. Here you have a chance to get creative. I’ve seen other builds where people glue wine corks to the post instead of using the stock knob. Or maybe you could glue a beer cap on? The sky is the limit. Once the knob is fixed onto the post, turning it with the rocker switch in the “on” position should cause the fan to begin spinning: slowly at first and then more rapidly as you continue turning the knob. If everything works, proceed to the next step.

Step 2: Mounting the Fan

If everything is working properly, and your switch and knob are looking fine, the next step would be to mount the fan inside the box. Stir plates exploit the strength of the rare earth magnet through the lid of the box and the bottom of the vessel you’ll be keeping your starter in. So it’s very important to get it as close to the underside of the top of the box as possible to ensure a solid magnetic engagement. I have mine raised up approximately 1.5” on screws which I’ve passed through small cuts of plastic tubing. I measured to the best of my ability but still had to play around with the length of the plastic tubing. I purchased the tubing at Home Depot, I think it was around 1/8” in diameter, but try to use a size that will fit snugly around the screws you’re using to mount the fan to the bottom of the box. Also, be sure to use tubing that is somewhat rigid so it will help to support the fan in addition to absorbing some of the rattle. Pick a spot that’s as close to the exact center as you can get, this will help the box not to move so much when the fan spins. There should be four holes in the bottom of the fan where it was mounted to the inside of the computer tower (there may have even been screws left over from that that you could use). Mark the spot on the bottom of the box where the screws will go using the computer fan as a guide. Drill the holes and place the screws up from underneath and into the box. Slide on the cuts of plastic tubing, leaving enough of the screw ends exposed to attach to the bottom of the fan, and slide the fan onto the ends of the screws. Tighten evenly from underneath (think of the star method used when changing a car tire).

mounting the magnet to the fan

wiring the fan, and mounting the magnet to the fan

Step 3: Gluing the Washer and Magnet to the Fan

Using super glue or plastic bonding epoxy, fix the fender washer to the top of the fan, then the rare earth magnet to the top of the washer. It would be a good idea to get these as centered as possible as well to avoid any imbalance during operation.

Step 4: Attaching Wooden Skids (Optional)

For my build I wanted to be sure that this things would not rattle itself off the counter top overnight or while I was away and ruin the whole starter. To be sure I could set it and forget it (literally for days at a time) I attached wooden skids to the box using some L- brackets and some scrap 1X3 lumber. The skids not only keep the box off of whatever surface I’ve put it on (keeping the machine screw heads for the fan mount from scratching it up) but they also distribute movement away from the box and give me a point to affix the whole assembly for increased stability. The two C-clamps mentioned in the parts list are for this purpose. I clamp down onto the skids and my counter top and now it’s set to operate for the next few days.

Step 5: Take Your DIY Stir plate For a Spin

Once you’re confident that everything is in working order, close up the box. If you’re using a project box like mine, there should be four small screws included to fix the top and bottom halves of the box together. If you’re using something else, feel free to close it securely however you like.

stir bar on diy stir plate

don’t pour in the stir bar with your yeast starter!

Next, go out and grab yourself some brewer’s yeast and some Dry Malt Extract and some yeast nutrient. I typically get my starter going with the stir plate anywhere from three to five days ahead of brew day. I use about 2 cups of sanitized water, 3/4-cup of light DME and about a ¼ teaspoon of the nutrient;, but use whatever you feel is appropriate for your recipe. Then I boil it all together for approximately 15 minutes, being sure to stir now and again in case any of the DME or nutrient has settled. Once it’s been sanitized and cooled, add the yeast strain you plan to use for brewing and then simply pour the mixture into the Mason jar, drop the magnetic pill in and place the lid with the airlock on the jar. Before starting the fan I always like to make sure the pill is as close to being centered at the bottom of the jar as possible. I do this by placing it on the top of the stir plate box, waiting for the internal rare earth magnet to engage the pill and then slide the jar toward the back edge of the box until I see the pill against the glass. Now I know the pill is caught by the magnet and so then slide the jar back toward the front of the box so that it’s more or less centered on the top surface. With the starter being so muddy, this is about as precise as you’re likely to get. I used water in the picture so you could see what the stir bar looks like. I should mention that the resistor I used for this build is too heavy for the potentiometer, but it does work. I was young and impatient so I’ve never bothered to find the “right” one because my build has worked without fail every time. That being said, it would be worth some testing before you use it the first time to figure out the correct gauge of resistor to use. For me, I just have to finesse the potentiometer knob until the magnetic pill starts to spin. It should spin in place in more or less the exact center of the bottom of the Mason jar. Sometimes mine starts spinning too quickly and rattles wildly around the bottom of the jar, in which case I ramp the speed back down and then up again slowly until I hit that sweet spot. When the magnet pill is spinning the way it should be you will see a slight to considerable dimple on the surface of your starter, you may also hear a sucking noise as air is pulled down into the mixture. Achieving a “Wizard of Oz” type vortex like the one pictured below is not necessary. Seeing a small dimple on the surface of the starter should suffice. You just want to be sure that the pill is spinning in place and keeping your yeasties in motion while also pulling in oxygen to feed them. Once you get that spin though, you can set it and forget it until you’re ready to pitch.

Also worth noting is that the magnetic pill will spin differently depending on how much water you use, and the ratio of water to DME. There is probably a viscosity formula out there that would allow you to dial in the exact amount of water and subsequent ratio to DME, but that is outside the scope of this article (and my brain capacity). On brewing day, once you’re ready to pitch, simply turn off your stir plate and grab that second magnetic pill I told you to buy. Place the second pill on the bottom of the Mason jar as you hold it upright. You should feel the two pills “snap” together through the glass.

Now you can dump the starter right into your chilled wort without worrying about dropping the first pill into your fermenter. If you do, don’t worry – you can always get it back out after fermentation. However, I’ve done this a couple times and have forgotten about the pill being in there and dumped it out with my trub. At any rate, they’re not that expensive.

Happy brewing.

diy stir plate vortex

testing for a vortex. You don’t need one of this caliber to have an effective yeast starter.


21 Responses to “DIY Stir Plate Build”

    • m_cubed on

      The washer simply provides a more stable and comprehensive base for the magnet. The shape of the fan is such that the whole surface of the fan would not be in contact with the magnet.

      I’m only allowed so many pictures for the article, so I had to choose some of the more important ones to support the details of the build.

  1. AkBrew907 on

    Or buy the coolermaster USB fan off amazon for a few bucks and you don’t have to wire anything and already has a speed adjuster.

  2. Twotaureanbrewing on

    Wow, tough crowd here! Its always fun to build something on your own from the ground up and see what your ingenuity conjures! Thanks for the share! And some of the kindly worded tips in the comment section!

    • m_cubed on

      Right? No worries though, the stir plate works and hope the build helps someone else who may be looking for a fun project. Happy brewing.

  3. Singletrack on

    I appreciate the author’s enthusiasm for brewing and DIY. Thank you for submitting an article, and happy brewing!

  4. CU_tony on

    Why build a stir plate when you can but a laboratory grade one from amazon?
    Why use a fan lying around when you can buy a fancier one that has speed controls built in?
    Why make beer when you can buy a case of PBR at the store?

    The answer to all these is because brewers like to make stuff! Thanks for the write up, I just built mine last week and already am in love with it! I cant wait to crank it up again :)

    • m_cubed on

      My answers to those questions would tend to be the same. I love working with my hands and figuring out why things work the way they do. What’s the saying? “If you can’t take it apart, you don’t own it.” Anyway, glad the build helped you add to your brewing arsenal. The stir plate is definitely one of my favorite toys, and one of my prouder moments as a home brewer. Cheers!

  5. MagicMatt on

    Maybe I’m the only one but this seems like overkill to me. L brackets and wooden slats?!?

    I built a stir plate that works flawlessly, and didn’t need several of the items you mentioned. A box that can open and close easily? For what? Why are you opening the stirplate once it’s built? The washer is completely not necessary, as are the wood pieces and L brackets (just get some rubber feet – they should have come with your project box). Same with the screws and “small diameter tubing slightly larger than the screws” – not needed. A little hot glue is all.

    You also don’t need 2 stirbars – any fridge magnet will do. Spending $4 on another stir bar for the sole purpose of holding the one in the jar while you pour seems wasteful.

    And you recommended a smaller diameter computer fan – I found the opposite to be true, as a larger fan will be much quieter at the same RPMs as a smaller fan. I opted for a 120mm fan and it works great.

    IMO this is all you need:

    Project box
    Computer fan
    Rocker switch
    Power cable
    Electrical tape
    Rare-earth magnets
    Hot glue
    Stir bar

    Also the comment about only needing a small dimple is hogwash. You want the biggest funnel you can reasonably achieve without throwing the stirbar. If you don’t have a funnel/vortex, then no air (oxygen) is really getting worked into solution (only at surface area levels, which TBH is not much different then just having the jar sit on a counter and shaken every hour or two). If you’re going through the trouble of building and using a stir plate, why wouldn’t you want to maximize your efficiency? Bigger vortex = more oxygenation = more plentiful and healthier yeast.

  6. psymonkey on

    I finally broke down and bought a stirplate last night. NB had 20% off any purchase, so I scooped up the maelstrom/oxygen wand/2000ml kit for $40 off. I wanted another 2000ml, and to upgrade to the Oxygen wand anyway, so it was a decent deal. Still, it pained me to do it. I’ve built three stir plates and have had an awful time with them. The first was by far the best and for some reason I felt like I had to improve upon it – since then it just seems like it’s always something with them and I’ve found myself so frustrated I finally caved.

    Thanks for the write-up anyway! Don’t worry about the assholes.

  7. Andy Winch on

    Buy a used stir plate of eBay….I got a Cornell for $30. It has a nicely graded speed control, and it also has a heat coil it it to keep the yeasties warm. And it is built like a tank.

  8. Painter on

    In Step 5 you mention using a resistor, but it isn’t on the materials list, nor is it mentioned in the build instructions. What is it for and is it necessary?

  9. HeyStringGuy on

    stubborn me. So far I’ve modified my bathroom fart fan six times, each improvement brings me closer to a working stirplate, every one though throws the flask magnet after a few spins. The ceiling light potentiometer may be a culprit, I’m determined to keep messing with it till it works like it should. Really the only problem is the stupid flask magnet won’t stay centered. oh, good article!

  10. ericbw on

    Is a potentiometer the same as a speed control dial? I wired one into mine because if I go more than half speed, it throws the bar off. Speed is the biggest problem with it not staying centered.

    I have not had luck with Mason jars (the bottom is not flat), so I bought a 1 liter flask.

    For all the haters on here: There’s more than one way to skin a cat, and this is just one man’s journey toward better yeast starters. Yes, HBT needs better editing.


Leave a Reply

  • (will not be published)