The basic idea behind these homebrew stirplates is to control the speed of a motor (computer case fans being a cheap and accessible source) by varying the supply voltage. The best way to do it would actually be using pulse width modulation via a microcontroller, but I didn’t have one on hand, and energy efficiency probably isn’t a major concern for most people interested in powering a motor drawing around a couple watts. The simplest way to provide voltage adjustment (though not regulation, which I’ll get to in a second) is simply to put a resistor in series with the motor, which is what a lot of home brewers do. The problem with that (aside from the engineer in me hating the kludginess) is that electronic components are sold on profit margins more frequently associated with Wal-Mart stores, and are therefore made of plastic. If you have one lying around, or pick one up at Radio Shack, it will typically be rated 0.5 W, sometimes less. We’re going to be sinking around 12 V • 150 mA = 1.8 W into it. It will melt.
So, the bare minimum takeaway here is to use a potentiometer rated for at least 2 W continuous power. A better, or at least more elegant, solution is to incorporate some actual voltage regulation. Which brings us to one of the most useful components ever invented, the LM317 adjustable voltage regulator. The LM317 is capable of providing anything from 1.25 to (Vin – 1.7) V with good regulation (±1%), using only two resistors to set the output voltage:
Vout = 1.25(1 + R2/R1)Replace R2 with a potentiometer and you have a nifty little 1.5 A adjustable power supply using only three components. A couple filter capacitors probably aren’t necessary for our purposes, but they’re cheap insurance. The only downside to the LM317 as a regulator is that it’s inefficient; any excess current is dissipated in the regulator as heat. Again, that could be up to 1.8 W in this application. The most common regulator package is a TO-220, which can only safely dissipate around 1.5 W. So a heat sink is definitely a good idea. The LM317 has a built-in thermal shutdown, though, so you might be able to get away without a heat sink if you’re careful not to run the fan at very low speeds. If you do sink it, be aware that the tab on the TO-220 package is tied to Vout – so be careful to avoid shorts. In the photo of the internals, you can see that I sealed the heat sink bolt with heat shrink tubing just to be safe.
Technically, an LM317 can have a minimum load current as high as 10 mA, although most will be much lower. That puts the maximum value of R1 at about 120 Ω (10.4 mA). Note that the datasheet specifies 240 Ω because it’s lifted from the specs for the LM117, which tops out at 5 mA. It’s also worth noting that I’m using a 220 Ω resistor simply because I had a 2.5 kΩ pot and didn’t feel like buying another. Do as I say, not as I do.
Circuitry aside, the rest of the build is pretty easy. I had a plastic kitchen container that happens to be the perfect size for my 1 gallon starter jugs. I bought a pair of neodymium rare earth magnets, which are simply adhered magnetically to the hub of the case fan. At 4 pounds of lift each, they aren’t going anywhere. If the fan was attached directly to the container, the magnets would make contact with its surface, so you’ll need some sort of spacer. I used roughly 5 mm lengths cut from a plastic drinking straw. The magnets and the power supply both came from American Science and Surplus, and the only other thing I needed to buy was a stir bar: $6 on eBay. If you’re going to use your stirplate with a convex-bottomed vessel, it might be worth noting the type of stir bar that works for me. It’s 1″ x 3/8″ with a ring.The complete parts list would be:
- Project box or other plastic container
- 12 VDC power supply (at least 300 mA to allow for power spikes on startup)
- PC case fan
- Mounting hardware for fan
- TO-220 heat sink
- 120 Ω resistor
- 1 kΩ potentiometer
- Knob for potentiometer
- 0.1 μF ceramic disc capacitor
- 1 μF electrolytic capacitor
- 2 rare earth magnets
- Magnetic stir bar
Plus wire, solder, perf board, etc. If you’re a tinkerer you probably already have some of this stuff on hand. Even if you had to buy everything online and pay shipping, it would only cost about $30. Tayda Electronics is a great source for components. Avoid Radio Shack like the plague.
Now that I’ve gotten off my lazy ass and put this thing together, I’ll be updating my aeration experiments with one last trial, to test my assertion that frequent shaking and a stirplate are equivalent.