How to make a cheap and functional magnetic stirrer

In my post about this years cyser I killed my started using an old discarded magnetic stirrer, because it heated up after i had been running half a day and thereby heated my starter and knocked out the yeast. Buying a magnetic stirrer is ridiculously expensive, so I have been looking for alternatives.

I had been working on my own home made magnetic stirrer, and even though it was not quite finished and fancy looking, it was still working. I got the idea from reading www.instructables.com where I found complete instruction on how to make a magnetic stirrer (HERE). I am no wiz with electronics but this was so easy that even I could do it :o)

Getting an old computer fan is not that difficult these days since nobody wants stationary PC's which are though out all the time. Ask around and somebody will probably have PC just collecting dust and will be happy to let you through their old PC out. Just open the PC, unscrew the fan, and rib the wires out of the motherboard. Clean the fan with wet paper towels, and you now have the main ingredient.

Then you need 2 small power magnets (or neodymium magnets). I purchased mine over the net and it was quite cheap. They were 2 mm wide and 8 mm in diameter. I bought 4 just in case......

These have to be glued on to the fan opposite of each other but also with opposite polarity pointing upwards. Use super glue for the job (cyanoacrylate glue), since this is fast drying and very persistent. But you have to do it right first time.

Finally you need some wires, battery pack, on/off switch (optional), and some sort of connectors.

This is pretty much the same set-up as I found on the web. But there are a few things that are not mentioned there, but are equally important. First of all I was not able to get a cheap potentiometer so I could turn the speed up and down. Instead I just used "number of batteries" to control the speed. I quickly found that:

2 batteries - not enough power to turn the fan
3 batteries - OK speed
4 batteries - Fan is spinning way to fast

..... so I only use 3 batteries (and a clip to make up for the forth in my battery pack)

I used purchased teflon coated magnets as stirrer bars, and the magnet inside them are not as strong and the power magnets. If the stirrer magnet is too long, it cannot keep up with the fan due to water resistance. Hence the magnet size need to be matched with the battery size and speed of the fan.

The two smallest magnets work but the largest does not. Size does matter :o)

One thing I found quite interesting was that electronics and Lego use the same scale. I was surprised to find that Lego matches the screw holes of the fan so I gave my boy Julius an ice-cream in exchange for a "few" building blocks. He wasn't so sure that he came out on top in this deal.

So using 4 antennas as pins I could easily slide the fan down on these, and lock the fan in place with the more Legos. I also put some blocks in the corners so my beaker would not slide off.

And there you go - a cheap and functional magnetic stirrer.