Introduction: How to Make a Cheap, Portable Magnetic Stirrer

By alexngai.wordpress.com/
About: My name is Alex Ngai. I am currently an electrical engineering student at Cornell University. I’m interested in bio-mimetic walking, running, swimming, and flying robots, automation, and 3D bioprinting. Cybo…

I needed a magnetic stirrer to keep some cells agitated for cell culture, but didn't want to spend the thirty or so dollars needed to buy one. This magnetic stirrer uses an old computer fan and some cheap neodymium magnets with a moldable plastic stir bar.

The parts are:
- computer fan
- small magnets: http://www.dealextreme.com/p/super-strong-rare-earth-re-magnets-8mm-20-pack-4248 (free shipping)
- moldable plastic (polymorph, instamorph, shapelock)
- lexan
-batteries
-superglue (cyanoacrylate)

Step 1: Glue the Magnets to the Fan

Prepare and clean the fan for use. Your fan may have two wires (red for positive and black for negative). If it has three wires, the white wire is a tachometer and is not needed. Glue the magnets on opposite sides of the fan head. The magnets in the fan's motor may push the magnets around a bit, so arrange accordingly. I arranged the magnets to have opposite polarities facing up.

Step 2: Add Spacers

The magnets will increase the profile of the fan, so you will need spacers before putting a flat surface on top. The spacers can be anything, I used coins initially. Eventually I snapped off a few pieces of spare plexiglass I had lying around and glued them to the edges of the fan. Either way, make sure the spacers are higher than the profile of the fan with the magnets glued on.

Step 3: Add the Main Platform

I cut a small square of plexiglass over the fan as a platform for any containers I will need to stir. Glue or attach any thin, waterproof material to the spacers.

Step 4: Make the Stir Bar

The stir bar is made of two magnets embedded in the moldable plastic. Simply place the plastic in a container of hot water until it turns clear. Place two magnets (polarities opposite facing) onto the plastic and surround with more of the moldable plastic until the magnets are covered. Then round off the edges and mold until the bar is roughly cylindrical with the embedded disk magnets still inside.

Step 5: Powering Your Magnetic Stirrer

The magnetic stirrer can be powered by any voltage source the fan is rated to. However, make sure your magnets are strong enough to couple together from the fan to the stir bar with the rated speed. If the speed is too high, the stir bar will not spin correctly. I used a 9V battery at first, but used this set of 3 AAA batteries I had lying around. I didn't need torrential currents for mine, so I used a lower voltage. This is a magnetic stirrer, not a blender. The lower voltage will result in a slower fan speed, and the stir bar will better be able to better keep up with the fan speed.

Step 6: Use It

Place any container large enough to allow the magnetic stir bar to fit and start agitating/stirring! Since this unit is so small, you can place it on your desk and use it to stir your cold drinks. I wouldn't recommend hot drinks though, heat will cause the polymorph stir bar to soften and lose shape. If you want an improvement, you can try embedding magnets in ceramic and glazing the stir bar.