In fine Crooked Bullseye form, our first stab at a DIY Stir Plate (read about it here, this will all make more sense) was, perhaps not a disaster, but certainly not a success. But we learned a bunch of things!
The basic plan, was using an old computer case fan, wired into a 5v DC converter, with 2 rare earth magnets glued to it. Stick it in a box (old cookie tin) that can support a 2l flask full of liquid. Make killer starters.
So, here were the fatal flaws in the past attempt:
No Speed Control
At first, we tried with literally no speed control between the power source (we had 3 to choose from, all just old phone charges with the end cut off). They all spun the fan too fast for the stir-bar (it just barrel-rolls, not spins). The next day we wired in a 10k linear potentiometer (as suggested by the internet), but this was equally useless. It basically just functioned as an on/off, as there was way too much resistance for most of the sweep. As usual, I (Chris) put a request out to the good folks on the GTA Brews Homebrew group on Facebook, and immediately got told what I actually needed was a PWM (Pulse Width Modulator), which you can buy on ebay from China for less than $2 with free shipping, if you can wait 3 weeks for delivery. I can.
The plan had been to use a metal cookie tin from Christmas, but in testing with the new PWM, it was still virtually impossible to get the stir-bar spinning. This was because the stir-bar was too attracted to the lid of the case. So it occurred to me that every metal case either on the market or else made by other DIY’ers (even if those jerk-offs didn’t mention it) is that the metal needs to be non-magnetic. So plan B was a Pilsner Urquell wooden case I got a shipment from a PR firm in.
With those issues fixed, the set-up was dead easy.
Using long bolts and nuts/washers, the distance between the fan and the lid can be varied. Because the bolts have rounded heads and the bottom of the case is rather thin, I decided to use a piece of scrap pine in the case. It also means I could attached the PWM to it, so the dial isn’t right at the bottom of the case. I used the fan to scribe drilling guides on to the wood, and drilled the holes for the bolts.
Slight hiccup was realized at this point: The shaft on the PWM wasn’t long enough to get through the side of the box and still get a secure connection to the dial. So I scribed the width on the inside of the case, and chiseled out 5 of 7 plys.
So a quick test (works), then it was time to piece it all together and stick it all in the box. The good news is, the PWM I bought actually used screw-down contact points, so it didn’t even need to be soldered.
And here it is working:
Okay, so everything is looking good, but I was still a little concerned the whole thing might catch fire while spinning up a starter, so I left it running while making dinner. And the DC converter burned-out (not violently or anything, just stopped working). So now I need to just play around and find a DC converter that does the trick.
— — — Update re: above — — —
So, look very closely at the ac/dc converter you’re chopping the device-end off of. Because, if you like me don’t, you might find that it was actually only an ac/ac step-down converter, which explains why it burned out. A proper ac/dc converter installed, and it’s working perfectly.
Feel free to ask any questions in the comments!
— — — Further Update — — —
So especially with starter and yeast slurry in the jar, it was difficult to get a good fast spin going. The magnets didn’t seem to have enough force to keep the bar spinning, and it would start barrel-rolling, rather than spinning. So I popped the smaller 1/4″ magnets off, and replaced them with 1/2″ ones. Lee Valley Tools (where I buy them) says that the 1/2″ of more than twice the attraction and it seems to be true. Turn the dial up to 11, and you can have a vortex right down to the bar, and still have a good vortex even in the big 4 litre jar I use for step 2 and 3 builds.