Monday, December 2, 2013

How to build an inexpensive stir plate

Pitching the right amount of yeast is undoubtably one of the more important aspects of brewing good beer, and the most effective way to make a healthy yeast starter is by using a stir plate. You can easily buy a stir plate, but the cheapest units that I've ever seen run about $45. Although $45 is not an enormous amount of money, you can easily build your own for as little as $10 (if not cheaper), and it is a really fun DIY project! Here I'll show you how I built mine so you can build one too!

Let's start with the parts you'll need:

  • A wooden cigar box (you can usually get them for very cheap, if not for free, from you local cigar store)
  • 12v DC computer fan (I used an 90mm one, but use one that fits nicely in your cigar box)
  • Variable computer fan speed controller or potentiometer (I got one this one)
  • Bottle cap
  • 12 AC adapter (I had one lying around from an old router)
  • A neodymium magnet from an old hard drive, also called a "rare earth magnet." If you don't have an old hard drive laying around, ask friends, check with computer recycling centers, and call your local dump to find one.
  • Electrical tape
  • Strong glue or epoxy (I used "Amazing Goop" household glue)
  • A non-magnetic spacer that fits in a bottle cap (I used felt pads that go underneath furniture, but you can use anything really; get creative!)
  • Four #6-32 x 2” machine screws
  • Twelve #6-32 machine screw nuts
  • Twelve #6 metal washers
  • Four 1⁄4” flat neoprene washers
  • A drill

*In order to make a yeast starter, you will need a 2L Erlenmeyer Flask (I purchased this one from my local brew store) and a magnetic stir bar (I got mine from here). I do not cover using these items or how to make a yeast starter in this post, but you can easily find out how in the forums of and at

Alright, now let's start building our stir plate!

1) First, start by removing the magnet from the hard drive. Use this helpful blog tutorial to learn how to do this. Please be careful when handling neodymium magnets, as they are incredibly strong and you can easily pinch a finger between the magnet and a metal surface. After you've fully removed the magnet from its bracket, you'll have a magnet that looks like this:

2) Next, you will want to get your fan ready. First, cut off the connector end of the fan, right at the base of the connector, so you are left with the long black and red wires coming from the fan. If your fan has a yellow wire on it as well, you can cut it off completely. After that, glue your spacer on top of the fan blade. The point of the spacer is to provide distance between the brushless motor inside the fan and the magnet that you will later affix onto it. If you simply place the magnet directly onto the fan without a spacer, the magnet will prevent the fan motor from operating properly.

3) Once your spacer is glued onto the fan, go ahead and glue the bottle cap onto the spacer. Next, position the magnet onto the bottle cap, but do not glue it down yet. Take your AC adaptor, cut off the end of the adapter, and then carefully separate the two wires. You will notice that one of the wires is marked, usually with a white or grey lines - this is the active wire. The other wire is the passive wire. Splice the active wire to the red wire on the fan, and the passive wire to the black wire on the fan. Plug in the AC adaptor to an outlet to ensure that it operates. Now for the tricky part: finding best position for the magnet. Try to get the magnet centered onto the bottle cap as best as you can, then plug in the AC adaptor. You will see how centered your magnet is based on how much the fan wobbles. Keep moving the magnet around on the bottle cap until your fan can spin with as little wobble as possible (a little bit of wobble is okay, but if you have too much, your stir plate will dance across the floor). Then use a permanent marker and trace the outline of the magnet onto the bottle cap and then glue the magnet down. Separate the AC adapter from the fan, and allow ample time for the glue to set.

4) Now we will affix the fan to the inside of the cigar box. Start by centering the fan on top of the cigar box and marking where each mounting hole is; use these to guide where you will drill. Drill your four holes for your #6-32 machine screws into the lid. Once you drill your holes, use the phillips head bit on your drill to create an indentation (also called countersinking) so your screws can lay flush with the top of the box. See the two pictures below for an example.

Screws are flush with the lid

5) Put the neoprene washers flush on the inside of the lid, followed by a metal washer, and then a nut. The result should look like this:

6) Add a nut followed by a washer on each screw, then place the fan on the screws with the magnet pointing towards the inside of the lid. Adjust the spacing of the magnet from the lid using the screws in order to ensure that there is about 3/16" space between the magnet and the inside of the cigar box lid. Once you've done this, add a washer and nut to the end of each screw to secure the fan in place. The result will look something like this:

7) Drill a small hole on one of the sides of the box and feed the cable of the AC adapter through the hole:

8) Separate the potentiometer from the metal plating on the fan speed controller. You will be left with a little knob and some wires. Cut off the connector ends from the fan speed controller (if there is a yellow cable, cut it off completely), and then splice it between the fan and the AC adaptor. The potentiometer will regulate the voltage traveling between the AC adapter and the fan. Make sure that all cables are connected properly, and then wrap the exposed wires in plenty of electrical tape.

9) Drill a hole in the side of your cigar box that is large enough to fit the potentiometer knob through, then glue it in place; allow adequate time for the glue to dry. Use more electrical tape to band the wire bundles together to make everything look nice and neat:

10) Plug in the AC adapter and ensure that everything works properly. The potentiometer should control the fan speed and the box shouldn't wobble while it is on. If you'd like, you can drill a series of small holes along the bottom sides of the box to allow for airflow. If all is as it should be, then revel in your craftsmanship and make a kick-ass yeast starter!