First of all, english isn't my mother languaje, so you may find some grammatical mistakes in the explanations. I will be grateful if you help me to correct my Instructable. That being said, let's start.

A magnetic stirrer is a laboratory equipment, used for, well, stirring, to enhance or promote some chemical reactions. It's used for stirring low viscosity liquids, using a magnetic bar immersed in the liquid. This kind of stirring has lots of benefits, like being able to stir in a closed container, the use of less mechanical parts than other kind of stirrers (or shakers), being quieter than mechanical stirrers, etc.

A magnetic stirrer is a very useful tool for me: I am a home brewer, and I love growing and culturing my own yeast, first because it's funny, and then, because it helps me to save lots of money, as I don't have to buy yeast. In this article, we can see the benefits of constantly stirring our growth media for yeast, in comparison with the hand shaking of the media, the method which I've been using until today.

But buying a magnetic stirrer is a nonsense for me: they are very expensive, and have a lots of features that surpass my needs, so I decided to build mine, a very simple magnetic stirrer, which will be enough to ensure the best yeast culturing.

Materials Needed

- Cooler, fan (It will be the motor of our stirrer. Mine is a 5v fan from a laptop base)

- Power supply for our fan (I used a 5v charger for an old phone)

- Two small neodymium magnets

- Balsa wood - High Impact Polystyrene (HIPS)

- Some cables and solder

- Male and female power supply jacks

- Glue. I used wood glue, superglue and epoxy bi-component adhesive.

- Plywood

Step 1: Preparing Your Fan and Power Supply

First of all, we got to prepare our motor to be connected to the power supply. So, we have to solder a female power supply jack to our fan, and the male power supply jack to the phone charger. For this project, the polarity isn't that significant, so you can't solder wrong your cables, as you just have two options and both are correct.

In a fan is important a correct polarity on the connections to ensure a proper airflow, but here, we are interested just in spinning, and not in blowing air.

Step 2: Fan and Magnets.

In this step, we are going to make this piece. It will hold the magnets and attach them to the fan. The wood piece is a balsa stick, and both magnets are glued to it with super glue. In this step, there are some variables to take them at count:

-Orientation of the magnets: This magnets have their poles, each one in every face. So we got to glue them one with the north magnetic pole looking upwards, and the other, with the south magnetic pole looking upwards.

-Distance between the magnets: You have to make sure the separation between your magnets is a bit longer than the length of your stir bar

-The piece needs to be glued aligned to the fan axis.

At this this step, I suggest you test your fan, if it still working despite the magnetic influence of your magnets, if the spinning is balanced or not, and if the piece with the magnets interacts fine with our stirring bar

Step 3: The Case

Now, the magnetic mechanism works fine, you need an enclosure to put your fan in. You could use any box big enough to contain or fan, the cable and the magnets spinning, but I preferred to make my box from scratch, to make it as small as possible.

First we need four strips of balsa wood. They need to be longer than our fan sides, and wider than our fan plus magnet plus the piece of wood that holds them. They will be the sides of our prism.

We need to make a hole in one of them, to put the power supply female connector through it. I have made the hole with my drill, and then I modified it with a chisel, as the shape of the connector is like the union of a square and a circle. I made the hole as fit as possible, this will help to keep the jack in place.

To assemble the sides of the box, I've held together the four pieces of wood with tape, while I put wood glue on them. Then I've arrayed the pieces in a squared based prism, and secured them with a rubber band.

After some curing time, I've glued my wood prism to its base, a piece of high impact polystyrene. At this step, I've used some epoxy bi-component adhesive.

Step 4: Assembling

In order to glue the fan in its place, first we need to make some separator to put between the fan and the HIPS, to leave room beneath the cooler for the cable. I've made them with more wood, glued with super glue. Also used superglue to attach the fan in the case, and the connector in the sides of the box.

Now you should check if your magnets aren't beneath the level of the sides of your case.

Step 5: Improvising

The magnets were well located, they weren't higher than they should be. But, when the magnets interacts with our stir bar, the spinning part of the fan raises up, elevating the magnets position, and therefore, putting them in contact with the superior part of the case.

I hope the raise of the fan is noticeable in the video above.

To fix this, I've made a simple plywood separator wich elevates the place of the top of the case, or the stand for the jars.

Then I've tested the stirrer to check if the separator accomplish its purpose.

As you can see in the videos, this build is near its end

Step 6: Finishing Touches

After gluing the separator, I've started sanding the sides of the box. First with sandpaper grit 80, then 150, and 600 for a smooth finishing. I've used some liquid wax as finishing of the wood sides.

Also, I've built a removable plate to put between the flash or the jar, and the separator, to prevent spills to go directly to the plywood.

And that is my first stirred starter, before inoculating on it a wild mixed culture I've harvested from some fruit. You may notice that the color of the starter media was modified in an editing software for the cover of this instructable.

Step 7: Possible Variations.

You can tweak this project at your desire. I made the most simple version of a DIY stirrer, as it satisfies my need. But you could try using a 25 ohm potenciometer to regulate the fan spinning velocity. Also you could use a bigger fan or a dc motor instead of a fan, put an on/off switch, an on/off led, or even use a heated plate to keep your liquid warm. You could use a box you already have laying around in your home/workshop, instead of building one.

Hope you enjoyed my first instructable, and my first tutorial written on English. Any suggestion you have about the project or the instructable, will be well received.

Step 8: Update - About the Magnetic Bar (Stirring Bar)

Some members of the community asked in the comments section about the stirrer, or how to make one, if mine is bought or DIY.

I have two magnetic bars at the moment. They are the ones showed in the photos above. They were bought on Aliexpress some time ago. I decided to bought mine for just one reason: The bough ones are made of PTFE. Let me explain. PTFE (polytetrafluoroethylene), better known as Teflon, is a polymer with very cool characteristics for lab appliances. It is very resistant to chemical corrosion, which allows you to stir lot of liquids with it, without fearing to your bar to be destroyed with the liquid you are stirring. Lets imagine a stir bar made of Polystyrene mixing some nail polish remover, it will be destroyed in a few seconds. That wont occur to a PTFE bar. So, if you have to sanitize it, which is something you always have to do in yeast propagation, sanitize your equipment, you can do it with any chemical, and your magnetic bar will be fine with it. But there are other things about this material. This polymer is a thermostable polymer, which means it can't be melted again after being molded for the first time. This allows everything you built in this material to withstand high temperatures without receiving any deformation, so you could hot sterilize it by putting it on your media while you boil it, which is the method used to kill any microorganism in the grow media used for yeast starters. Also, PTFE has a very low friction coefficient. That is an advantage, because the stir bar has less resistance to its spinning that if it were made of any other material.

But there exists other shapes of magnetic stirrer "bars". I made a model on blender to show you different shapes, without using any internet photo. You may notice that every kind of stirrer has a pivot point, where the bar makes contact with the vessel in which is contained with the liquid to stir. This pivot point is always located beneath the gravity center of the stir bar. These shapes of bars has different uses, related to the kind of stirring you need to get.