A magnetic stirrer is simply a box with a rotating magnet. Homebrewers use magnetic stirrers to start yeast colonies but the community is generally unwilling to pay the exorbitant prices charged by scientific suppliers for these relatively simple devices. Some homebrewers decided to build their own stirrers by glueing powerful magnets to old PC fans mounted in tupperware containers. For an upcoming field experiment, I'll need two battery-powered stirrers. Unfortunately, Cole Parmer is charging $437 for one of these units. In this guide, I'll build two battery-powered stirrers using techniques developed by the homebrewing community. The goal of this project is to save the lab ~$700. I modified this build from a post on the Red Kettle Brew Blog.
Part list
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You can reduce the cost of this project by using old PC fans. I was able to get two old fans from a scrapped computer. You may have wire available too. To assemble the unit, you'll need a drill, soldering iron, screwdrivers and superglue. I'll be using 12 volt batteries (model: Eagle Picher CF-12V12) as my power source but you could use a 12 volt wall plug adapter if you don't need the portability of a battery.
Assembly
- Drill a hole on the side or back of the box. The diameter should be large enough for the battery hookup wire to fit through.
- Drill two holes on the front of the enclosure then mount the power switch and rheostat.
- Cut the connector off the computer fan. Depending on the fan, you may have blue, green or yellow wires in addition to the red and black wires. We only need the red and black so remove the other wires.
- Connect the red (positive) wire from the fan to one prong of the rheostat. The rheostat can work in different directions depending on how you wire it. Follow the picture above if you want the speed to increase when you turn the control knob clockwise.
- Use a piece of wire to connect the rheostat and power switch.
- Cut a long piece of zip wire (red and black wires bonded in parallel). On one end, attach alligator clips to easily connect the stirrer to the battery. Run the other end into the hole drilled in Step 1. Solder the positive wire to the power switch and the negative wire to the negative wire from the computer fan.
- Glue two magnets to the face of the fan. Make sure you have opposite polarities facing upwards on each magnet.
- Mount the fan in the enclosure. I cut pieces of packing foam to get the fans to the desired height but you could use nylon spacers or similar. The magnets should be as close to the top of the enclosure as possible.
My fan, running at the full 12 volts, was spinning too fast. I could have used a lower voltage battery but I decided to solve the problem with resistors instead. I bought a $5 variety pack of resistors and tried a couple until I found a suitable speed. The resistance required will depend on your particular fan. My fans needed a 100 Ohm resistor to slow them down to an appropriate speed.