Build a Stir Plate

One of the most useful tools for making a yeast starter is a product called a stir plate. A stir plate is a device that contains a strong magnet just beneath its surface that spins in a circular motion. A container of liquid sits on top of the stir plate, and placed inside the liquid is a coated magnet called a stir-bar. The spinning magnet of the stir plate causes the stir-bar in the liquid to spin as well, providing a continual stirring of the liquid. When used for building yeast starters, the continual stirring of the stir-bar provides constant agitation and aeration of the yeast. The result is smaller starter sizes and less time needed to build more yeast cells.

With a few tools, a little scavenging, and a touch of creativity, you can make your own stir plate rather inexpensively. Without any of the parts on hand this project costs about $40, but many DIYers will probably have some of the materials already.

The basic concept of this project utilizes a small muffin-type DC fan, like the kind found in computer cases, as the drive motor. Computer case fans work well because they are inexpensive, easy to mount, and easy to attach a magnet to.

To power the fan, you will need an AC/DC wall adapter, like those used to power or charge small electronic devices (sometimes referred to as “wall-warts”). I keep a box of these in my workroom that I have saved from old cordless phones and such.

Once you’ve determined the fan and power supply you’ll be using, it’s time to figure out what size potentiometer you will need to control the speed of the fan. To do this, first calculate the resistance of the fan using Ohm’s law: Resistance = Volts/Amps. For voltage, use the measured value of the power adapter. Calculate the wattage by multiplying the volts by the amps. Now select a linear potentiometer with a max resistance somewhat near the resistance you have calculated and a watt rating higher than the wattage you calculated. For example, the resistance of my fan calculated to be: 16 V/0.07 A = 228 Ohms, and the wattage was 16 V x 0.07 A = 1.12 Watts. I was able to find a 250 Ohm, 2 watt potentiometer online for under $4 that works very well. You will need to buy a control knob to go along with it also.

The remainder of electrical supplies needed includes an on/off switch, a 4-pole terminal block, a few short pieces of electrical wire, and a strain relief bushing for the power cord to enter the enclosure.

When selecting a magnet, there are a couple of things to keep in mind. Rare earth magnets such as neodymium magnets are very strong and work really well. Just make sure that the magnet (or stack of magnets) has its poles opposite each other in the longest dimension. This ensures that when the magnet is attached to the fan, its polarity will alternate as it spins.

Finally, you will need some kind of enclosure to house everything in safely and securely. You could purchase a project box or electrical enclosure from an electronics store, but why not be creative and use something like a wooden cigar box or a plastic storage container? I’ve even seen one made out of a coffee can. You will probably need to use some standoffs or spacers to properly space the fan away from the top surface. For my enclosure I used an old metal housing reclaimed from an industrial control panel that I cleaned up and painted. I made the lid from a scrap sheet of Plexiglas.

Parts and Materials
DC fan
AC/DC wall adapter
On/off switch
Control knob
4-pole terminal block,
Electrical wire
Strain relief bushing
Magnets and spacers
Stir bar

  1. Select your Eqiupment

Any size case fan will do — the one I used in my stir plate is an 80mm fan pulled from the case of a decommissioned computer. Once you’ve located your fan for the job, make a note of the rated volts and amperage printed on the fan for use later on.

An adapter with a rating anywhere between 6 to 12 volts should work fine, but I recommend that you measure the actual DC voltage with a voltmeter, since many times this differs from its rating. The one I’m using is rated at 12V, but it measures closer to 16V. That’s a little on the high side, but it has been working well for me thus far.

For the magnets, make sure the total length is as long as or longer than the stir-bar you will be using. I suggest using a stir-bar that is 1½ inches (3.8 cm) or shorter. Lastly, the magnet will probably be strong enough to prevent the fan from operating properly, so it will be necessary to find a non-metallic spacer to mount in between the two. For my stir plate, I used a stack of three magnets, each 3/8-inches in diameter and a ½-inch long (0.95 cm by 1.25 cm), epoxied onto a stack of two wooden nickels.

  1. Test your Equipment

It’s a good idea to test your electrical components before you get started on the assembly process. Connect one wire of your adapter directly to one wire of the fan. Run the other adapter wire to one tab of the on/off switch, then run a wire from the other tab of the on/off switch to the center tab of the potentiometer. Finally, connect the other fan wire to a side tab of the potentiometer. You may find that you have to reverse the wires of the adapter or use the other side tab of the potentiometer to get the fan to work properly.

  1. Attach the Magnet

Attach your magnet to your spacer and your spacer to your fan. For this job, I used a two-part, 5-minute epoxy. The key here is to try to get your magnet as centered as possible on the fan, which keeps the fan well balanced and also helps reduce the chances of your stirbar being thrown off. I spun my fan by hand several times in the process of adhering the magnet until I was pleased with its position.

  1. Prepare the Enclosure

Whatever you have chosen for your enclosure will need some prep work. Drill holes for the potentiometer, on/off switch, strain relief bushing, and terminal block mounting screws. Also figure out how your fan will mount, and drill the necessary holes for that. Finally, give your enclosure a coat or two of paint if needed.

  1. Install and Wire the Components

Once the enclosure is ready, begin installing your components. It is easiest to solder the wires onto the potentiometer and the on/off switch prior to installing them. Install your fan to either the underside of the top surface, or to the bottom of the enclosure. Just make sure that when everything is closed up, that the magnet is as close as possible to the top surface – without touching it of course! Cut the wires to the appropriate length and strip the ends. Route the wires neatly and out of the way of the fan. Make sure your circuitry is the same as it ended up being in Step 1 to ensure that the fan will run properly. It may be helpful to label your wires or use different color wires like I did.

  1. Take it for a Test Spin

With everything installed and tightened up, place a container of water with a stir-bar inside on top of the stir plate. The stir-bar should align itself with the magnet on the fan. Now plug in the adapter, and with the speed control set to the slowest setting, turn on the switch. The stir-bar should start turning slowly and now you should be able to turn up the speed. You might want to experiment with various speeds and let it run for a while to make sure the stir-bar doesn’t get thrown off. After that, you’re ready to propagate some yeast!