We made magnetic stirrers which are used for mixing liquids in experiments or dispersing nutrients when growing bacteria or algae in liquid. I personally use this to dissolve sea salt. A magnetic stir bar is placed into a beaker and is spun by a rotating magnetic field produced by the device. In our DIY version we attached magnets to a pc fan and controlled its rotating speed with a potentiometer.
All the information for making the stirrer is uploaded to our repo which includes bill of materials for Japan, laser cut data, the circuit’s schematic, and photos.
Also, here’s some other designs from How to Grow (Almost) Anything. A simple yet sleek version by Take san and Suda san from Fablab Hamamatsu. And if you don’t have a stir bar you can improvise like Tsuda san from YCAM.