I've made a magnetic stirrer using a computer fan, a lid, and a few neodymium magnets from some weird magnetic toys I found. The power supply I used was from some old battery charging device, it works pretty well.

Homemade stir bar: I connected three or four of the neodymium magnets together to make the stir bar. To make it acid resistant I put them inside a piece of a bic pen that I cut to the appropriate length, and then sealed it to make it air tight.

I glued 4 neodymium magnets on the top of the fan this was necessary because if they are not glued, the fan will begin to spin to fast and fling them off. The four magnets were glued so that two  were on each side of the center part on the fan.  I made sure the two magnets were facing the opposite direction of each other - this allows the magnetic stir bar to align its self with both of the magnets. I then placed a lid that held the vessel that was going to be stirred above the magnets. The lid was also glued to the fan to make it sturdier.

This is great since I made it for next to nothing, and it works well for liquid mycelium cultures and some chemistry reactions. The biggest disadvantage with it is that it only has one speed. Since it can't be adjusted, it cannot stir liquids that have a high viscosity. I've remedied this problem with using different power supplies.


This is a video of it stirring near 2.5 liters of water http://www.youtube.com/watch?v=n7cKbIxnTdA

Here I have attached some Images of the materials used, and it in action.

Any powerful magnets would work. Rare earth magnets, assuming they are not the plasticized type, would work well.

I will post a step by step set up for this device when I get the chance, but if there is anything you are unclear about, please ask me. I'd be happy to explain it in more detail.

All right, I will make the effort to get it up in a day or two. If you don't want to wait there is a lot of information about how to make a magnetic stirrer on Youtube, Instructables, and also Sciencemadness.

Why not just get a real potentiometer/variable resistor for $2 - $3?

Radioshack, amazon.com, mouser.com, any place that sells electronic parts.  Here are a few too..

http://www.google.com/products?rlz=1C1GGLS_enUS347US347&sourceid=chrome&q=potentiometer&um=1&ie=UTF-8&ei=KsY8S5XQO4-HlAeQ58GTBw&sa=X&oi=product_result_group&ct=title&resnum=3&ved=0CD4QrQQwAg

Just be sure the one you get is rated for the voltage and power you'll be pushing through it .  They're generally not meant to go directly in-line with a load, but a computer fan is small enough that it shouldn't be too much of a problem to find one that can handle it.  Also be sure to get one with a linear taper, not audio/logarithmic.

You'll want to use it as a voltage divider to get the effect you expect, with turning all the way in one direction killing all power the the motor, and turning all the way the other way giving full power.
Just connect the two side leads to either pole of the power source and use the middle lead as the input to the fan.

Just use a LM317 variable regulator for speed control.
It is fun to notice that I'm not the only one utilizing those mangnet toys for stirrbars. I wrap mine with teflon thape to make them more oval and then shrink some heatshrink tubing ontop. Also, those hdd neodiym mangnets work well for homemade magstirresrs.