This instructables is to show how to build your own magnetic stirrer from parts you may have at home.

This one was built to mix E-cigarette vape juice. You could use it to stir almost any liquids, so it can be used for mixing vape juice, mixing for cooking, or in a chemistry lab.

Time:
About 30 minutes

Tools required:
Multimeter
Screw driver
Pliers
Drill and drill bits
diagonal cutters or strong scissors
[optional] soldering iron and solder
[optional] adjustable crescent wrench

Parts required:
Project box (some sort of container)
12VDC Computer case fan
12VDC power supply
16 gauge wire
SPST switch (optionally illuminated)
Potentiometer and knob
Flea/mixer bar
Superglue (or other strong contact adhesive)
Hard drive magnet
Female blade connectors (as needed)
Wire nut (as needed)
[optional] Rubber feet (so the box doesn't walk)
[optional] Fuse (safety says always use a fuse!)

Possible parts sources:

Almost all of this is out of my junk box.

Box - anything square(ish) and preferably plastic in case something comes loose. I used a 8x6x3" Radio Shack project box, which costs $9.

12VDC computer fan. That is a fan from a donated (junk) computer. You can use a case fan or power supply fan. The power requirements should be marked. The one shown is a Cooler Master PL12S12L, 12VDC 0.13A that spins at 2000 RPM. Similar fans at TigerDirect can cost about $10.

12VDC power supply. It must output more than the rating of the fan. I found this one in my junk box. It's 12VDC 1.2A. If you don't have one, ask around. Any respectable geek has a box full of these. If you can't find one, they can be bought for about $15.

16 gauge wire. You can grab this from anywhere. I prefer to color code so I don't get confused. Worst case, you can use part of the wire from the power supply.
Red = positive
Black = negative
Blue = special (negative for switch light)

SPST switch. It can be almost any switch, as long as it's rated for 12V and at least the amps of the power supply. This one uses a round hole and has a nut, which is simpler to install. You can use a small automotive switch, or a power switch from something else (like a power supply). This switch is rated for 12V @ 30A from RadioShack and would cost $4.49. If you buy a switch, make sure it doesn't say "momentary", unless you want to hold the button the whole time it's running. You can use other switches (SPDP, DPDP, etc). They just have extra poles you won't use

Rheostat/Potentiometer and knob. This is to be able to adjust the speed. You can source this from almost anything with a knob (volume knob, tuner, etc), as long as it can handle the power. The knob is optional, but looks better. This is a 25 ohm, 3watt rheostat from Radio Shack that costs $4.49. For this fan, we need to handle 1.56W Volts * Amps = Watts , so 12V * 0.16A = 1.56W . You can probably find a better potentiometer and use resistors as needed to get other speeds.

Stir Bar / Flea. This is the little white thing that spins. I couldn't find any locally. We got a 7 pack for $8 on Amazon. I recommend a variety of sizes. The size of the flea is significant to how fast the mixture spins. I used a fairly large one for the demonstration videos.

Hard drive magnet (or neodymium magnets bar magnet from the hardware store). To extract it from a hard drive will be a little (fun) work. Hard drives use Phillips, Torx, or Security Torx screws. There's always one (or more) under a sticker Sometimes it's faster/easier/more fun to use a hammer and chisel. You're going to destroy the drive anyways. The two magnets are on the back part of the arm. They are going to be difficult to separate. You can't pull them apart, but you can twist and slide them apart by hand. Don't get your fingers between them.

Connectors, fuse, feet. most people have these in their junk box. If not, you can get them cheap from auto parts stores, home improvement stores, or Radio Shack. Or ask your friends.

If you buy everything new, it will cost you around $50, which is cheaper than a commercial magnetic stirrer would cost. In my case, it was $8 because I had the rest of it laying around.

This instructable assumes you know something about basic electronics. I won't go into detail about how to solder or make connections. For what we're doing, it's very simple. You'll notice I used both connectors and solder. That's up to whoever builds it.

Disclaimer: By reading and/or using these instructions, you take on all liability for anything that may happen.

Step 1: Layout

Think about where you want all the parts when you're done, and make sure things fit. This is a simple step, but people always forget.

You want to put the fan in the middle of the top, so you can center whatever is being stirred.

I chose to put the knob in the middle of the front, and offset the power switch to the right, with the wiring on the right. You can do it any way you want. I recommend both the power switch and knob are both on the front for easy access.

Make sure the chosen fan can spin freely with the magnet on. You can just lay the magnet on the cover and try to spin the fan. If it doesn't, you may need to use washer as spacers. There's usually a bit of a gap, so you should be fine.

Make sure your fan won't hit the back of the potentiometer or switch, and that you can secure the wires safely away.

Step 2: Mechanical Assembly

Now that you've decided where you want everything, it's time to play with power tools!
(Kids, ask your parents to do it. They love playing with power tools!)
You want to be careful doing this step, so everything looks good.

Drill the appropriately sized holes for your potentiometer, power switch, and a hole in the back just large enough to slide the power cord (without the plug) in.
Mount (but do not wire) the potentiometer and power switch
Carefully center the fan on the top, and mark your four mounting holes. Drill 4 holes just large enough for the 4 screws to go in.
Take the hard drive magnet, and try to center it as best as you can on the fan. You'll want the magnet facing away from the fan. Use superglue to attach the metal side of the magnet to the fan. Do not otherwise modify the fan. You do not want to do other modifications to the fan.
Mount the fan to the top with the appropriate screws.
[optional] put rubber feet on the box. This is a good idea so it won't make too much noise, or "walk".

Step 3: Electrical Connections

Reference the provided diagram.

Cut the connector off the power cord. This will save you hours of trying to find the right socket.
Put the power cord through the hole in the back of the box, and tie a knot in it so it won't pull back through.
Verify the positive and negative power supply wires with your multimeter. I like to tie a knot in the positive wire for easy identification. You'll notice that in the previous photo.
[optional] put a fuse at the end of the positive wire. The next step will connect to the other side of the fuse.
Connect the positive wire to the appropriate pole of the switch (bottom on mine)
Connect a wire from the appropriate pole of the switch (center) to the center pole of the potentiometer.
Verify the rotation of the knob with the ohm/resistance setting of your multimeter. When turning the knob to the right, the resistance between the center and one pole should drop.
Connect the appropriate pole of the potentiometer to the positive (yellow if present, or red).
Connect the negative power supply wire to the black wire of the fan.
[optional] If you are using an illuminated switch, you will need to make a connection from the appropriate pole on the switch (top) to the negative power supply wire.
You can test your electrical work now. Plug it in, hit the power switch, and see if the fan spins.

Soldering, wire shrink, blade connectors, and/or wire nuts are all your choice. I chose based on the physical connections.

Step 4: Button It Up

At this point, you should have all of the physical and electrical connections made and tightened. Double check them.

If you are sure you are complete, make the necessary physical attachments.

You can now test it with a flea to see it spin. You don't want to do this for very long, so you don't risk wearing the coating.

Step 5: Try It Out (First Demonstration)

This demonstration mixes oil and water. They don't normally mix. As you can see in the video, it mixes very well.

For this demonstration, I used a fairly large flea, because of the thickness of the oil and the total volume. As you will see in the video, the vortex does not pull all the way to the flea, so it doesn't aerate the mixture. That is usually desirable.

If you are using a thin fluid or a smaller volume, you may want to use a smaller flea.