Simple Stir Plate –

If you ask a seasoned brewer for advice on improving your beer, one of the key things he/she is most likely to say is to make a yeast starter. An easy way to improve your yeast starter is to use a magnetic stir plate and stir bar during the fermentation of your starter. A constantly stirred yeast starter will yield a higher cell count than an unstirred starter.


First, note that home brewers often use magnetic stir plates and stir bars to help make a yeast starter. But magnetic stirrs are also widely used in chemistry and biology research, especially in medicine. A stir plate mechanism consists of an electric motor and a permanent magnet underneath. The stir bar is a magnet (Ceramic or Alnico or Neodymium) that is encased in a material that will not react with the fluid it is immersed in. The stir bar magnet is placed within a container and made to spin with the external magnetic field produced by the stir-plate. The main advantage of this setup is that you can easily stir the fluids without having something stick into the container or require an opening in the container. The container can be completely sealed.

A few weeks ago, my neighbor got interested on portable magnetic stirrers. All commercially available decent stir magneticrers are very high-priced, and sadly not within his reach. So, I decided to build a simple version for him using common components laying around. With a little skill and some patience, you too can build one yourself for much less!

Basic Theme

The build idea is a borrowed theme which is popular among home brewers who prepare their own yeast for beer. In this post, you can see my random thoughts on the building of that simple homemade stir plate mechanism made with the help of a common computer cooling fan and a couple of other quite inexpensive components. Feel free to use this post as a starting point for creating your own design.

The simple stir plate described in this project uses a 12V/80mm PC cooling fan which is ideal for 1-L conical flasks. Such cooling fans are common in computer and power supply devices. You can even get it free from defunct electronics you may have laying around. In addition to this key component (cooling fan) two more crucial components are required to complete the project – a couple of strong magnets and a fan speed regulator circuitry. Of course, a compact 12VDC(>500mA) power supply unit is also called for, and a small switch mode power supply (SMPS) adapter is preferred.

Ramping Up

First, glue two Neodymium disc magnets (10mm) on the opposite sides of the fan’s hub (opposite polarities facing up). You can apply a small amount of multi-surface adhesive to the fan hub. Next, hold the fan in your hand, and give it a good spin with your finger to check how well the magnets are balanced. If you feel some wobble, carefully reposition the magnets on the hub.

It may take a little work to balance the magnets to spin correctly. If it is off-center it might throw the stir bar. Finally, remember to allow the glue to dry as per the glue maker’s instructions before using it in your project.

In my ‘open-frame’ stir plate build, I also inserted four 40mm machine screws through the holes available in the fans’ frame, and secured those screws with hex nuts (yes, it will depend on the final setup you choose). Look below for a closer view of the mounting!

The magnetic stir bar can be made by embedding two magnets in moldable plastic. You can buy polymorph plastic pellets (see below) to prepare a cylindrical stir bar with embedded disc magnets (opposite facing polarities), or a low-profile bar magnet inside. The recommended size of the magnetic stir bar is 25x10mm. However, this is not a crucial dimension.

If polymorph plastic pellet is not within easy reach, you can use PTFE (aka Teflon) tape instead to rig up your stir bar. Go through this “low budget stirrer” link to get a clever how-to idea I did not test it though.

I was rushing to get a quick-and-dirty prototype, I made my magnetic stir bar with a small Neodymium bar magnet (20x5mm) by tightly wrapping PTFE tape on it. It is crude, but it worked!

As mentioned before, magnetic stir bars are coated magnets used to stir liquids in a sealed container. Neodymium magnets are stronger than ceramic and Alnico magnets. They have the highest coercivity of all permanent magnet types. The actual ‘strength’ you will see here depends on several interrelated factors, which are hard to quantify in a single number. The key factors include the size and strength of the stir bar magnet (bigger is stronger), strength of the magnets used in the stir-plate, distance between the stir bar magnet and magnets underneath (stir plate magnets), viscosity of the fluid, and the rotation speed. While I cannot tell you exactly which magnets will work better in your prototype, begin your experiments by opting the types as used by me here. A bit of keen experimentation should yield an effective stirrer.

As you can see, I mounted an old compact disc (120mm or so) over the fan as a platform (base) for the container I will need to stir. I then secured the base with four nuts.

Speed ​​Controller

A 12V-rated PC cooling fan will run at lower speeds with lower voltages, so we can control how fast the stir plate turns by controlling the fan driving voltage. For example, the 12V cooling fan used in my prototype starts from 3.3V and attains its full speed at 12V with a maximum current draw around 200mA.

This is the breadboard prototype of my linear dc motor/fan speed controller tailored especially for the stir plate project presented here.

The speed controller is centered around the popular 3-terminal fixed voltage regulator 7805 (IC1). The output voltage of IC1 is referenced to its common terminal which is usually grounded. In principle, the output voltage of IC1 can be raised above the designed value by simply biasing the common terminal with a suitable voltage making it easy to obtain odd-ball output voltages.

Here, the biasing voltage is obtained by passing quiescent current of IC1 to ground via a 1KΩ potentiometer (P1). The additional 1KΩ resistor (R1) is included intentionally to minimize the effect of potential output voltage drift with changes in quiescent current. Nw the P1 bias voltage is determined by the sum of the quiescent current and the bias current set by R1. The given configuration will output a drive voltage in 5V-10V scale, adequate for most 12V PC cooling fans. Keep in mind, IC1 requires a small heatsink, and it is essential in this application!

Power Supply & Enclosure

The stir plate can be powered by any voltage source the fan is rated to (12V). Make sure the magnets are strong enough to couple together from the stir plate to the stir bar with the rated speed. If the speed is too high, the stir bar will not spin aright – a lower drive voltage will result in a slower speed, and the stir bar will be able to better keep up with the speed of the stir plate.

The build options for the enclosure are limited only by your imagination. This is a picture to give you a dose of inspiration!

Following is the closeup of my homemade magnetic stir bar – made with a Neodymium bar magnet and Teflon tape.

In conclusion

Here is a casual snap of my homemade stir plate in action. I shot a test video too, but sadly it did not turn out.

I am making the second version of my own stir plate. There were some small heater elements left over, and I think adding a hot plate to the basic stir plate will make it more utilitarian (another big improvement for a lab-quality stir plate). I will document the build and share the project details soon!

Credits & References

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