How to Make a Cheap, Portable Magnetic Stirrer

Picture of How to Make a Cheap, Portable Magnetic Stirrer
I needed a magnetic stirrer to keep some cells agitated for cell culture, but didn't want to spend the thirty or so dollars needed to buy one. This magnetic stirrer uses an old computer fan and some cheap neodymium magnets with a moldable plastic stir bar.

The parts are:
- computer fan
- small magnets: (free shipping)
- moldable plastic (polymorph, instamorph, shapelock)
- lexan
-superglue (cyanoacrylate)
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Step 1: Glue the Magnets to the Fan

Picture of Glue the Magnets to the Fan
Prepare and clean the fan for use. Your fan may have two wires (red for positive and black for negative). If it has three wires, the white wire is a tachometer and is not needed. Glue the magnets on opposite sides of the fan head. The magnets in the fan's motor may push the magnets around a bit, so arrange accordingly. I arranged the magnets to have opposite polarities facing up.

Step 2: Add Spacers

Picture of Add Spacers
The magnets will increase the profile of the fan, so you will need spacers before putting a flat surface on top. The spacers can be anything, I used coins initially. Eventually I snapped off a few pieces of spare plexiglass I had lying around and glued them to the edges of the fan. Either way, make sure the spacers are higher than the profile of the fan with the magnets glued on.

Step 3: Add the Main Platform

Picture of Add the Main Platform
I cut a small square of plexiglass over the fan as a platform for any containers I will need to stir. Glue or attach any thin, waterproof material to the spacers.

Step 4: Make the Stir Bar

Picture of Make the Stir Bar
The stir bar is made of two magnets embedded in the moldable plastic. Simply place the plastic in a container of hot water until it turns clear. Place two magnets (polarities opposite facing) onto the plastic and surround with more of the moldable plastic until the magnets are covered. Then round off the edges and mold until the bar is roughly cylindrical with the embedded disk magnets still inside.
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I have been creating a similar design, but I am looking to take my magnetic fan stirrer into the field. We do experiments in streams, and I am trying to figure out the most effective way to waterproof this design. Any ideas or advice would be appreciated! Right now, the best solution I can see is to seal plexiglass to the top and bottom, but this has proven more difficult than I initially thought...

If you raise the fain up off the surface it'll run more efficiently by allowing the air to flow through.
It turns out this is NOT true. Fans run easiest in a vacuum. If that doesn't make sense, envision the opposite case whereby you try to push heavier air, which would cause the fan to work harder. Having the fan inlet sit flush on the table is perfectly acceptable.
McAldo IronYeti1 month ago

I am not too sure of that.

Putting the fan close to a surface will not make air less dense.
On the contrary, putting a surface in close proximity to the fan will create a situation in which the fan will have to push the air against a surface, or suck it up from an enclosed space underneath. The latter might create a vacuum to an extent, but I am not convinced that would work the way you suggest, bit like sucking through a straw.
If the fan was to operate in a sealed enclosure with most of the air removed, I agree it would spin faster.

aazzaki9 months ago

how its works ?

Mark Le bois9 months ago

Thank you for
these great pictures! I found some more on this website:
AndyPipkin1 year ago
Can't you just take the fan blade off, and drive the stir-bar from the electromagnets that would have made the fan turn?
Austringer2 years ago
Normally, you don't want to use a stir bar with cells since the action of the stir bar and the bottom of the beaker will lyse them. Roller bottles are the common way this sort of thing is done at bench scale.
then that will be my next instructable xD
a cheap multi tier cell roller would be awesome!!!
tnmann101 year ago
rish1131131 year ago
Where will you put the stir bar?? Please reply .
Awesome build, I was able to make my own with great success. I found a 4 pin fan and a library for Arduino to control the speed with a potentiometer, works well. Want to try again sometime with a fan with LED lighting for effect.
limpport2 years ago
Are the fan blades necessary to keep attached? It would spin faster without them, unless it makes it spin too fast....

(Then you'll need a resistor or potentiometer!)
7_Volts2 years ago
didnt understnd yet wht was the use of stir bar and where did u put it ????

EhBmicrobio2 years ago
Great! Very useful to me.

If you change the batteries by a solar panel, you can enter it into the Green Tech Contest.
well you could get get into the green tech contest anyway becouse it is used with water and rare earth magnets
Krayzi992 years ago
Would you be able to use hot glue for the stir bar instead?
i don't see why you cannot use hot glue instead of polymorph, or you could always make one of each.
drdan1522 years ago
Great idea !!!! Very cultured haha ..... I'm totally gonna make one of these
i don't see why you cannot use hot glue instead of polymorph, or you could always make one of each.
lol i replied to the wrong person, i meant to reply to pblanscet.
foxworrior2 years ago
Hello, i have made this but i have made an "improvement", i connected the ground and the positive wires of the fan to a 3.5" mono socket and the positive and ground of a USB cable to the 3.5" mono plugs, and i did the same with a 9v battery clip so i can use either a 9v battery or power from a usb port if i am near a computer.

That is just something to think about if you want to remake it.
ultrabob2 years ago
Looks like a great idea for yeast starters for homebrewing! Nice instructable!
bulwynkl2 years ago
the stirrer does not have to be a magnet ofcourse - just magnetic - a piece of steel ( magnetic stainless?) would do the trick. Magnetic would couple better but it's not absolutely needed.

alternatively you can buy Heamatite bars that have been magnetised - these would make an excellent stirrer - Ceramic clean, hard, food safe

random google search shows for example...

(WRT stainless - someone is bound to point out that stainless steel is not magnetic - well that depends on how much Cr versus Ni is in it. Ferrite (body centered cubic crystal structure) is magnetic. Austenite (face centered cubic) is not magnetic. In stainless steels, Ni is a ferrite promoter and Cr is a austenite promoter. High Ni stainless steels are ferritic - this is what stainless steel knives and razors are made of)

Oops - I got that the wrong way around - Cr is the ferrite promoter, Ni is the Austenite promoter - ferritic CRES has very low Ni.
The only issue i have with ceramics is that some to be properly fired get into the heat range that would effect the strength of the magnets in the stirrer. So i would more be shopping for something that is a food safe polymer resin for general use.. If you're using this for crazy science then be sure it will stand up to what you're doing or consider forming glass. As another aside hacking the fan blades off would help as you are totally restricting air flow and any extra drag is power not going to stirring.. all in all a near concept.
kirkb1502 years ago
Make sure that thermo-plastic is food grade if you plan to use it where consumables are involved. (like for home brew "yeast" starter). Just sayin'.
imuhachev2 years ago
Video, please!)
powerfool2 years ago
I didn't get how you place the magnets. To form one bigger magnet? So, S-N-space-S-N?
Let magnets decide. ;)

Yes, S-N-S-N and So oN.
Spokehedz2 years ago
Very nice! Couple of cheap (or free) additions would make this even better:

A PWM circuit so that you could control the rate of stir. These are cheap online at various auction sites that rhyme with fleabay.

Removal of the blades so that the rate of spin would be increased dramatically and also give more control when the PWM is installed.

Otherwise, great job!
If the fan sits on a table, air flow is stopped and the fan speeds up considerably. Will not gain that much by breaking the wings.

If the fan is not that much big, a potentiometer will do a fine job of a speed controller.
Yes, removal of the blades is easy you can usually just break them off one by one, the plastic is kinda brittle. these are brushless motors, so they may not work well with all pwm speed controls, but there are plenty of fan speed controls sold at your local PC repair shop.
polerix2 years ago
Sculpey / Fimo works well, onced baked, they are just plastic.
sculpy/fimo are porous and even water absorbs to a point in them. real stirrer magnets are going to be more durable, and not lose particles in your solution, and are cheap compared to the stirrer that the fan+magnets are replacing.
It has been a while since I used it, but I'm pretty sure Sculpey say it is not food safe. I thought it was due to the plasticizers in it, which would mean Fimo is probably not food safe either.
Baking most magnets (ferrite, neodymium) would render them non-magnetic (I think SmCo have higher Curie temperatures).
There are all kinds of "plastic". Not all plastic is as tough or inert as the teflon based stuff used in lab magnetic stirrers.
kthrun2 years ago
Made out of PURE AWESOME! Been looking for something to use to stir yeast cultures for homebrewed beer. Thanks!
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