I make beer, which requires yeast, preferably lots of yeast. Normally I buy yeast prepared by commercial labs that cater to homebrewers, but I make 10 gallon batches so I would buy two packs of yeast and even then wonder if I had enough.
After reading this piece by Dr. M.B. Raines (showing that continuous stirring generates about ten times the number of yeast cells as compared to just aeration), I was convinced to build a magnetic stir plate to help grow yeast and save some money.
I used a 120mm computer fan with a LM317 voltage regulator to adjust the speed using this simple circuit:
Details on the circuit can be found here. The component values aren’t critical. I used some capacitors I had around and aimed to keep the ratio of the values of C1 and C2 about the same.
I attached some rare-earth magnets to the fan with a hot melt glue gun and stuck the works in a plastic project box. People say a metal box won’t work, but I’ve used metal-topped stir plates in the past, so I’m not sure.
The potentiometer worked well to control the stirring action over a range of speeds. I don’t have a hemacytometer to count the cells, but it seems to work well, making the starter medium much more cloudy and giving a vigorous fermentation.