I know there are lots of sites showing how to make a DIY stirplate but I thought this might be worth posting because I have not seen one quite like the one I came up with totally by accident. Before I get to it, note that this is not a variable speed stirplate. I don’t really think you need to vary speeds for the purposes of yeast stirring but that’s just my opinion.
As I said above, this turned out to be a totally by accident stirplate because I already had the fan that ended up fitting the Amazon purchased project box. This stirplate should take you all of 10 minutes to put together.
120mm computer fan – You should be able to find one for under $10 if you don’t have one already. A smaller fan will defeat the purpose of this DIY because it will not fit the project box out of the box.
120mm Project box– I think this is the key to this project because the 120mm fan fits this project box PERFECTLY. Absolutely no need to drill holes, line stuff up or anything. The fan fits the box like a glove. You just need the right diameter and length screws to sandwich the fan in between the project box and project box top. $7
Magnetic stir bar – I went with a 1″ stir bar which worked well with the 3/8″ diameter magnets I ended up using. Bigger diameter magnets will probably need a longer stir bar. $6
Rare earth magnets – I bought some rare earth magnets from ebay for about $8. 1/8″ thick magnets is about the max that will fit under the project box without spacers. I tried 2 sets of magnets, first some 1/8″ in diameter x 1/16″ in thickness which proved too weak to keep the bar centered for an extended period of time. I then tried some 3/8″ diameter X 1/8″ thick magnets that worked perfectly. Most diy’s I have seen use computer hardrive magnets but I did not try those so cannot comment on them.
Power supply – If you’re like me, then you probably have a pile of these lying around so I didn’t add this to the project cost. I had a 7.5v power supply lying around that worked well for my .5L starters. Powered the fan just fine and did not get very warm when left for extended periods.
Screws – You will need screws that are skinny enough to fit through the fan holes and be long enough to reach the project box holes. I think about 1 1/4″ or so screws will work. Take your project box to home depot or whatever to make sure you have the right screws. I used regular old wood screws with flat heads for mine.
Building this should be super easy and will probably take you less than 10 minutes. You will need super glue or hot glue gun (hot glue gun is better), a screw driver, a drill and either electrical tape or sauter to connect the wires.
First step is to glue the magnets to the fan center. Easiest way to do this is to put the magnets on the stir bar to get the proper magnet polarity. Then mark the bottoms with a black sharpie so you know which side of the magnet to glue down. Note that placement is important for the stir bar not to spin off center if you are using small round magnets. It seems that the closer they are to the center of the stir bar, the faster the stir bar spins off. My first effort worked fine for a few hours but the bar would eventually spin off center. After some tinkering, I found that the magnets are best placed at the far ends of the stir bar when using round magnets like I did. Finally, either super glue or hot glue the magnets to the fan center. Hot glue is best as the magnets can easily be removed and re-positioned if you end up with the stir bar spinning off.
Once you have that glued on, cut whatever end is on your power supply and connect it to your fan. Again, this one is not using a variable speed controller. Make sure you have the polarity correct as getting it wrong could mess up your fan. I had mine wrong and the fan got really warm and started to smell of burnt electronics. Some power supplies will only work in the correct polarity, other will make the fan spin backwards so a multimeter may come in handy. Also, make sure you put a knot on the power supply on the inside of the project box so that you don’t accidentally pull the cord and disconnect the wires. I sautered mine and used shrink tubing to protect the connections. Of course you will need to drill a hole on the project box big enough to fit the wire.
Now it’s just a matter of sandwiching the fan in between the project box and the project box top. Don’t screw the top on until after you have tested it as you may have to re-position the magnets as I mentioned above. A good test is to leave the bar spinning overnight. You’re good if it’s still spinning in the morning. My first attempt stirred fine for a few hours but would not last overnight.
My power supply is not particularly strong so I don’t get a huge vortex when the flask is filled to capacity but it stirs the yeast more than enough. You don’t need a huge vortex, you just need the yeast to stir and this does a great job. Feel free to add a speed controller and larger power supply if you need it. I only make 1L starters for my small batches so this is perfect for me.