I home brew my own beer, and one important tool for brewing is a stir plate. It keeps yeast in suspension and aerates the growth medium for maximum cell count. To purchase one of these stir plates is easily $100 bucks and is probably overkill for my purposes. For around $10 and a few salvaged materials I can have a pretty snazzy stir plate that suits my purposes just fine.
Step 1: Materials
You really don't need much in the way of materials. Once everything is gathered you can knock out the build in less than 30 minutes or so.
Assortment of washers, nuts, and bolts
cell phone charger
hard drive magnet
25 ohm rheostat (Radio Shack)
simple on/off switch (Radio Shack)
Knob (Radio Shack)
Some type of spacer material (washer, plastic, cardboard)
Tools you will need:
Drill and assortment of bits
wire strippers or utility knife
Step 2: Preparing Materials
First, cut the end of your charger off and peel apart the negative and positive wires. You should probably go ahead and strip the ends of all your wires at this point.
Next, open your cigar box and center the computer fan on the inside of the lid. I held mine in place with spring clamps while I drilled the holes for the bolts. Go ahead and drill a small hole in the back for the charger wire and two larger ones in the front for the rheostat and on/off switch.
Once the wires and cigar box are prepped, you should make a spacer that will go on the fan and give you some distance between the magnet and fan motor. I found that if I try to glue the hard drive magnet directly to the fan that the metal in the motor keeps it from spinning at full velocity and even has difficulty getting started. A piece of plastic, wood, or even cardboard should work fine. Anything that will give you some distance between the magnet and the fan motor.
Once you've glued in place your spacer, you can glue down your magnet. Make sure to center the magnet as close as possible. This may be difficult because hard drive magnets are curved, but try your best or else it may throw your stir bar if its to far off.
Step 3: Spacing the Fan
Once you have all your hole drilled, insert 4 bolts through the top of the cigar box lid. You will need a bit of space between the lid and the fan so that the magnet wont rub the lid and get stuck. One or two nuts should be enough. Once you find you spacing, attach the fan and secure with some nuts from the inside.
Step 4: Wiring
Before I continue, let me just make this clear. I am not an electrician. I have had no problems with my set up thus far, and I hope that remains the case. Just use your common sense.
The fan I salvaged was 12V 0.6A, and the charger was 5.6V 0.5A. I'm not exactly sure how this affects the life span of the charger or fan, but my guess would be that the closer they match the better.
Feed the wires from your charger through the hole in the back of the cigar box. Attach the positive to the rheostat on its left tab (left if you are looking directly at the knob). Attach one wire from the on/off switch to the rheostats center tab, then attach the other wire to the positive wire from the fan. Finally attach the fans negative wire to the chargers negative wire, completing the circuit.
I would suggest soldering and heat shrinking as many connections as possible and maybe using alligator clips on the rheostat connections. It keeps wires from touching and short circuiting.
If you find that the fan spins too fast and is throwing your stir bar, you could add a few resistors inline before the rheostat to help slow it down some more.
Step 5: Finish Up
Once all your connections have been made you can pretty everything up. Insert the rheostat and on/off switch through the holes you drilled earlier and then add the knob to the rheostat.
I would suggest buying an actual stir bar for you stir plate. They are not just piece of metal but actual magnets coated in teflon. Also, make sure that the stir bar isn't longer than the magnet you are using or it will be tossed.
I'm sure there are lot of improvements that could be made to my design, but it serves my purposes just fine. Hope you like my instructable and I would love to see pictures if anyone makes one.