Easy Upgrade Series: Building a DIY Stir Plate for Yeast Starters

Posted by on August 23

We haven’t posted any new DIY projects recently, but if you saw my latest post, then you know that we have several in the works.  This week, I tackled a project that I’ve been thinking about doing for a while, but just haven’t done for one reason or another – building a stir plate.

A stir plate is used to agitate a yeast starter while it ferments.  The reason for this is to keep oxygen getting to the yeast so that they continue in an aerobic state, which promotes more yeast propagation.  In other words, that means that with more yeast exposed to air, they will produce a larger amount of yeast in a shorter amount of time.  Additionally, it keeps the yeast in suspension and in contact with the wort instead of allowing them to flocculate at the bottom of the container.  This keeps them in contact with the wort steadily, which increases their potential.

OK, enough of the science lesson.  You’re convinced, what you need to make better beer are larger, healthier yeast starters.  You could just buy additional packages of yeast each time you brew, but at $6-9 each time, that can get expensive.  Especially for that 10-gallon batch of Imperial Stout you’ve been wanting to brew.  Your brewing software or calculators probably recommend 6 or 7 yeast packages for that.

An easier way to go is to use a stir plate.  This allows you to increase the cell count of a single package so you don’t need to spend as much money on yeast.  Instead of 6 or 7 packages for that Imperial Stout, you’re probably down to 2-4, depending on the size of the starter you use.


  • 12v computer fan (can be salvaged from any old computer, bought on amazon, or purchased at RadioShack for a $10-15 bucks. I found a free one in a computer power supply by the dumpster)
  • 6v power supply (old cell phone chargers are perfect.  Anywhere from 5-7.5v will work.  I am using a 5.1v supply)
  • Rare earth magnet (can be salvaged from a computer hard drive.  For instructions on harvesting one from a hard drive, check out this post.  You can also purchase them online or at RadioShack.  Just don’t get one that’s too tiny, as some of the RadioShack ones are.)
  • Stir Bar (various sizes available. 1 or 1.5-inches is probably best, and run $4-8 at your local homebrew shop, or online)
  • Power toggle switch (optional. You could just plug it in to go, unplug it to stop, but a switch is nice to have.  I used this one from RadioShack)
  • Rheostat (potentiometer) 25ohm 3watt (RadioShack for about $4. Allows you to dial down the speed of the fan to a reasonable stirring speed)
  • Control knob to place on rheostat to control the speed. (Plenty of options in the $2-4 range at RadioShack)
  • A few wires and connectors to hook it all up. (20 gauge wire works great for this, and a sample pack of various electrical connectors will give you plenty of options)
  • Project box (any box will work.  I’ve seen everything from cigar boxes to Tupperware boxes. I used a conduit box from Lowes for about $9)
  • Bolts and nuts to attach the fan
  • Rubber washers (optional to space the fan, leaving room for the magnet, and providing cushion to reduce noise)

I was able to get a free computer fan and power supply, and already had the wires and connectors I needed (I soldered some of the connections as well, but that’s totally optional), so my total out of pocket cost was about $25-35.  A new stir plate retails for $80-100+ at most stores, so this was a HUGE cost savings and it’s really easy to build.


1) Drill holes in your box for your power switch, control knob, and power wire.  I also drilled a couple of extra holes in the back to allow for some air intake for the fan, so it would keep air moving.  Probably not necessary, but I hate to have electrical items in a closed container without any way to get rid of the heat they produce.

2) Install the switch and knob in the front of the box

diy stir plate

3) Cut off the end of the power supply that would normally connect to your phone, and strip the outer coating several inches down, exposing two wires.  Run those wires into the box and strip the ends.

4) Glue the rare earth magnet on top of the fan (make sure it’s the side that spins!) in the center.  The more precisely centered it is, the more even stirring motion it will have, which will result in less shaking.

5) Bolt the fan to the center of the top of the box, using the rubber washers for spacing, and leaving enough room for the magnet to spin above the fan without being wedged against the lid of the box.

6) Wire it all up.  Instead of describing the process, I have provided a simple diagram below.  Obviously, you could vary this if you weren’t using a on/off switch or if you wanted to include an LED light or other indicators.

stir plate wiring diagram7) Close up the box, plug it in, and take it for a test drive.  Place the stir bar in your erlenmeyer flask (or other flat bottom container) you use for your starters, fill it up with water, and see if it provides a good stir.

DIY stir plate
Remember, it doesn’t need to create a vortex that runs all the way to the bottom of the flask.  It just needs to provide enough disruption to keep everything moving.  As long as the yeast stay in suspension, then you’re in good shape.  You will not get better results by slinging the yeast around like the teacups at the state fair!

Obviously there are many variations on this, so let your imagination run wild.  Here are a couple other good basic examples, of which helped me plan out this build:

I’ve seen some sleek cigar box stir plates, double starter stir plates, illuminated transparent box stir plates, and more.  So have fun – it’s an easy upgrade so make it to fit your style!

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