Hard Drive Stir Plate - Homebrew Hack

If you brew beer at home, you’ll find that a stir plate is a very handy thing to have.  Stir plates are the trick to making big, healthy yeast starters, which improves smaller beers and makes large beers possible.

Homebrewers came up with a cheap hack to make a stir plate at home: harvest the rare earth magnet out of an old computer hard drive, glue it to an old PC fan, and put it in a box.  Stir plate.

I started to do exactly that, but got distracted after the first step: there was a perfectly good HDD spindle staring me in the face after I extracted its magnets.  My stir plate shouldn’t be made from hard drive parts… it should be a hard drive.

Some reading and experimentation lead to the discovery that I can’t reasonably control the spindle myself.  It’s a brushless DC motor that requires a fairly intelligent motor driver to control its spin speed.  But there is a motor driver IC already on the HDD board, so I decided I could kinda-sorta make something.  By pulse-width modulating power to the whole controller board.

I built this little circuit.  It’s a 555 timer that generates a ~1Hz square wave.  The square wave opens and closes a DPDT relay, which is hooked to the 12V power (input to the board) and the 5V power (generated on the board with a linear regulator).  The 5V and 12V outputs of the relay go to the hard drive controller board.

The next step is modifying the hard drive itself.  I removed the heads and magnets and two of the platters.  The bottom most platter stays on, and provides both a place for weights and load for the motor (it doesn’t like spinning without any weight).  If you were only using one hard drive, you could use anything as weights.  I had a second hard drive that I gutted, and used two of the four magnets as weights.  They clamp around the spindle and provide a very even weight distribution.

A third magnet (with tall edges dremeled off) sits on top of the spindle.  All of the magnets are epoxied down.  My counter-balance magnets on the platter are held down by spacers that were already in the hard drive, and screwed down pretty tightly with the spindle’s cap.  The magnet on top is only held on by epoxy.

Since the magnet on top sticks up above the edge of the drive, something must be done for a lid.  I bought a 5”x6” rectangle of felt (used for sliding furniture) and a similar sized piece of thick rubber (used for making custom gaskets).  I trimmed them to size and cut holes in them for the magnet to stick through.

Finally, I replaced all of the screws that held the hard drive’s top on with much longer versions (with the same threading).  I poked screw holes in the felt and rubber, and screwed the original hard drive lid down.

Operation is quiet and stable.  However, it is faster than I expected.  I have no way to measure how fast… it may even be spinning full speed for all I know.  If you have any problems or need to slow it down, you can adjust the R1 and R2 resistors in the 555 timer circuit.  I would recommend changing the duty cycle (increase R1, decrease R2) to shorten the length of time the drive is ‘on’ for in each period.  R2 could also be a potentiometer with a knob to adjust the speed.

The last step for me was adding a barrel plug and a HDD molex connector.  I used a 12V/2A wall wort.  My hdd pulls quite a bit of power: my bench supply read 1.5A while spinning.  This puppy certainly isn’t power-efficient.

UPDATE WITH EXPERIENCE AND MODIFICATIONS:

The above design was too fast.  The stir bar flings off to the corner of my (admittedly large) 2L flask.  Maybe it would work on a smaller container.

I got a 2cm x 2cm rare earth magnet that is much, much stronger than the hard drive magnets.  I ended up putting 3 hard drive magnets and the super-strong rare earth magnet around the spindle, and removed the magnet from the top.  

The result is still an over-powerful spin that throws the bar, but the strong magnet is strong enough to suck the bar back into the middle.  Instead of getting a nice, rotational cyclone, I get a loud, chattering, hopping stir bar that goes nuts.

However, it works.  It stirs the yeast effectively, and I had a hell of a strong fermentation after using it.  Just wish I hadn’t accidentally pitched the stir bar into the fermenter… (which, by the way, the stir plate is strong enough to stir.)