A magnetic stirrer is a desktop helper to mix any liquid solution. This is probably one of the simplest way to make one, as it involves a reduced number of parts that can be reclaimed ones. With brand new parts, this build will cost you under 5€.
1x enclosure (printed or DIY)
1x Power Switch 6mm 3P
1x USB-A inlet
1x 60mm fan (12v)
1x 5mm led (optional)
1x 180 ohms resistor (optional)
5x 6x8mm neodymium magnets : 2x for the rotor, 3x for the stir bar (flea)
Or 2x 6x8 neodymium magnets for the rotor + 1x stir bar (20>30mm or +-1")
Build instructions : https://youtu.be/6bDGU4-4F88
prusa i3, CoreXY, Tarantula
Pack the stir bar into packing tape to avoid corrosion.
The simpler the better
Most of the DIY magnetic stirrers uses a 12v fan powered by a 12v PSU. But they uses a potentiometer to step down the voltage. To be able to vary the voltage can be usefull, but usualy you will power the fan with 3 to 6v. 3v is for a very gentle stirring and 6v is very fast. So what if you power the stirrer with +5v USB power ? Well it's the good idea : except if you do fine chemicals, it works with 80% of the maker's needs and you probably already have USB ports available anywhere. So all you really need is a power switch and a fan.