This is my home built double stir plate. It is big enough to put 5l mason jars on it. I used a old lego technics box with two computer fans and a built in 12V adapter. It won’t win a beauty contest but it does its job very well.
The magnets were salvaged from an old hard disk. After roughing up and cleaning the surface. I used 2 component glue to stick them to the fans. Don’t mind the extra glue on the sides it was from a previous project.
It is preferable to choose standard size 120mm 12V fans. They are fairly common and you can replace them any time if one should fail.
First i marked the exact position of the mounting holes with a pen. After measuring a second time i bored the 4 holes in the box. The fans were mounted with simple bolts and nuts. Once you have mounted everything, check that the fan can spin freely, if not add some spacers. I had to remove some material from the box because the magnets touched the surface.
I wanted to be able to control fan speed. After some research i chose to build a simple diobus. This is a very cheap and easy solution. You just need some 1N4001 rectifier diodes and a multi step switch. The diodes drop the voltage step by step.
The 0.75V drop per diode was not enough. So instead of just one diode i added 2 diodes in series. If you want, you can even switch the fan on and off with the first step. I chose to add on/off switches because i wanted all of the 6 speed steps available.
You can find a very good how to on building a diobus here:
It is even possible to add LEDs to show you what speed step you are on.
I directly mounted the power supply into the box. It is connected to mains voltage by a standard chassis power supply connector.
Well there you have it, a simple and very useful project. Please comment if you have any questions or remarks.