Often times I like to make lagers and pilsners in the waning months of winter here in Utah. Unfortunately for me, I have about 20 gallons of lagers planned, and don't have the patience to shake up 4 or more 2L jars every time I walk by. This coupled with the fact that I am planning on using washed yeast that is very old, effectively ruled out normal starter making procedures.
I did some reading and decided that a stir plate was in order. Apparently in some cases you can double your yeast cell count beyond a typical stagnant-wort starter. We shall see!
I have a few boxes of old computer parts and pieces laying around my office from the last 15 years of assembling and disassembling computers. The only part that I purchased for this was a $4.00 potentiometer from RadioShack, because I was desperate to find a way to tune the speed down of the stir bar.
- Thermaltake 92mm fan: 12v
- Potentiometer: I don't even remember the part number, other than that it was marked as being able to handle a "maximum 3 watts".
- cardboard strips and duct tape
- 120v mini 1/4" toggle switch
- Random yellow LED
- magnet sourced from defunct 40GB Western Digital hard drive
- clear portion of CD jewel case
- 1.5" octagonal stir bar
While testing out the parts at my disposal, I quickly learned that all the fans which I had were way too fast and threw the stir bar off center. I used an ATX computer power supply on a test bench to provide 12v and 5v power. 12v was always too much for the fans and would result in the stir bars being thrown very quickly. Connecting the 5V rail of the power supply made the fans spin slower, but the stir bar would still get thrown off relatively quickly.
I tried a half dozen wall warts from 19v to 4v, and all were still too quick. Fixing a resistor of unknown capability would either slow down the fans too much and keep them from starting, or still keep the fans going too fast for a stable stir bar.
Ultimately, 5V seemed to be the sweet spot. I used a leftover USB phone charger, and connected it to my 12V fan. The fan was the slowest spinning (and quietest) of all of my fans, even on 12V. While running at 5V wide-open, it still would throw the bar. The potentiometer worked perfectly at this point. I can turn the fan down so it barely starts up, all the way to the point where it throws off the stir bar.
The whole thing is wired into the toggle switch and LED, onto the USB cable.
The magnet is merely taped onto the fan where I think the center is. The CD jewel case allows me to easily position my starter vessel over the middle of the fan, then adjust once it is started.
The trick seems to be to find a slow/quiet fan to begin with, and use a lower than normal voltage, in addition to a potentiometer or some kind of adjustment mechanism.