Building a DIY PWM Driven Stir Plate for Homebrewing (geek edition)

Disclaimer: Since the tone of this blog is still ill-defined, this isn’t a technical discussion of PWM circuits, nor PCB manufacturing, nor is it strictly a homebrew entry… it’s sort of an awkward in-between.

Introduction/Purpose/Background/Problem Statement (skip this if you don’t care)
This is a two-part blog post. The first blog talks about the concept and theory, and releases DIY build plans for the PWM circuit. The second post talks about the (home) manufacturing of the released files.

As I said before, there’s going to be a lot of nerdy interests that cross-pollinate in my blog, and today, an example of this manifests itself in a DIY (overkill, perhaps) stir plate for homebrewing.

One of the many challenges of homebrewing is having the ability to pitch a healthy yeast culture into your pre-fermented solution (called must [wines] or wort [beer]). An extremely simple summary of the fermentation/brewing process is: Create a solution with sugar in it, and add yeast. The yeast consumes the sugars and releases alcohol as a byproduct. The reality of this simple formula is that if you don’t have a good population of yeast that is added (“pitched”) into your solution, the yeast can become over-worked/stressed, and you may find some “off-flavors” in your final product. The other obvious benefit is that when you have more yeast, your fermentation takes off faster! Thus, many homebrewers [especially those that are brewing high gravity/high-alcohol brews] will make a “yeast-starter.” All a yeast starter does is promote a healthy yeast colony prior to pitching into your solution. This is accomplished by providing an easily fermentable solution in a good environment for yeast. This implies an oxygen/sugar rich environment.

Solution (no pun intended)
Thus enters the “stir plate.” If any of you have taken basic chemistry, you’ve probably worked with one before. You set a beaker or container of liquid on a magnetic platform, and drop a magnetic (generally teflon coated) “stir bar” into the beaker, and the “stir plate” will spin the bar around using magnetic coupling. In a homebrew setting, the stir plate allows for higher oxygen permeation in the solution, which will help “jump start” your yeast starter. Mead, or honey-wine is notorious for slow starts in brewing, largely due to the lack of natural yeast nutrient in the must. I had a mead brewing last year for roughly 9 months, and I still had to stabilize before bottling. I told myself I wouldn’t start another without going through the process with a hearty yeast starter. I’ve got two gallons of honey on my counter, so I figured it’s time to get going…

Design considerations
In a DIY/homebrew setting, it makes little sense to buy a commercial stir plate. A quick Amazon search shows a price range of ~$80-250. The first goal of this project is to beat the commercial price. Many DIY solutions use a computer case fan with a strong magnet glued to it instead of electromagnets, which cuts costs significantly. With this major consideration out of the way, the only real problem left to solve is speed control. There are three (in my opinion) solutions to the problem, all with their respective Pro/Con lists.

  • Solution 1: Simple Potentiometer circuit
    The Potentiometer (sometimes referred to as a “Rheostat”) is simply an adjustable resistor. We abuse this property and essentially create a variable “voltage-divider” circuit <LINK>. At the output of the circuit, this just provides an adjustable voltage output.
  • Solution 2: LM317 (or other variable voltage regulator) circuit.
    The LM317 is designed to be used as a voltage regulator. You can tune the output with a few passive components. Since you can change the output on the fly with a potentiometer, we use the pot to create a variable/controllable voltage regulator to use instead of the potentiometer.
  • Solution 3: Pulse-Width-Modulation (PWM) circuits.
    A PWM circuit basically operates by pulsing full power to the output in varying widths per time period. This is most easily explained with a graphic <LINK>. As you vary the output, the full power is applied to the circuit in varying pulses. At 100% on, the circuit remains powered all the time, at 50%, the circuit remains powered about half the time, and so forth. This is used commonly in controlling LED brightness, and of course, to control the speed of fans.

For a hardware “hack,” any of these solutions is sufficient, depending on your needs. They will all provide some sort of speed control to a fan, and thus, your stir plate. One major consideration in a DIY stir plate design is that lower speed control is generally considered to be pretty desirable. This is to prevent violent torque on the stir bar, which can cause the stir bar to be “thrown” out of its rotation origin. One problem in the inherent design of a case fan is that there is a hard “cutoff” voltage where the fan ceases spinning. The potential applied to the fan motor cannot overcome the resistance/friction to torquing the fan, thus, extremely low speed control is not possible ONLY by lowering the voltage. The first two circuits (potentiometer/LM317) are capable of regulating/outputting low voltages, but if a case fan can’t use them… well, you’re SOL.

The other disadvantage to both the LM317 and potentiometer circuit(s) is that there is considerable power “wasted” due to the design of these two circuits. Much of the power used is translated to heat via resistance and/or powering the regulator. Finally, the biggest disadvantage to the simple potentiometer design is that it has a very weak load driving capability (with a standard Pot). The LM317 will be able to power a much larger load (read as: daisy chained fans/stirplates/etc).

The power and drive capabilities in this context aren’t really that important, but are worth mentioning. To me, the major differentiator of the PWM circuit is that it is capable of driving the fan at a very low rotational speed. This is generally desired in the stir plate setting. While it’s pretty “cool” to get a very tall vortex going in a solution, this isn’t required (nor do you benefit from it very much) when making a yeast starter. A slow swirl is all you really need (and possibly want).

Problems with the PWM circuit
The obvious downsides of the PWM circuit is: complexity/cost(/noise). There are more parts required, and this often means the hack-oriented DIYer shies away from building one. I’ll be providing free schematics, board files, and a straightforward Bill-of-Materials (BOM) at the end of this blog entry. This drives up the cost of the PWM by a few cents (dollars?). The other problem with PWM that people experience is that you might hear a low humming, or “knocking” noise while the circuit is in operation. This is due to the (relatively) sharp waveform edges. You are actually hearing the fan “knock” as it is turned on and off rapidly. The simple solution for this is to put a capacitor in parallel with the output transistor. This smooths out the characteristic of the square-ish waveform.

Summary: If you are looking for cheap and dirty, skip the PWM, the potentiometer will probably work in an “acceptable” fashion for you. I run my stir plate at a fixed speed for the most part anyways, if you can get it down to running in the ~5-7V range, good on ya. You can always gut a larger diameter fan for lower rotational speeds, too.

Since this IS an “overkill” PWM circuit, I will be making a separate blog post about the build process and PCB manufacturing. (Next)

Design consideration note: While I’ll be releasing the PCB, I should note that I personally don’t actually care about on the fly speed control. The Potentiometer on my board (which controls duty cycle) is mounted internal to the case. This implies that I am making a fixed-speed board for myself. You can wire a case mounted Pot if you desire.

DISCLAIMER NOTE: As this isn’t strictly an engineering/homebrew/geeky manufacturing blog, there’s a lot of details missing in the overall theory of operation and/or brewing. Please visit the links in the references section if you are curious about filling in the gaps.

The continuation of this blog post will be talking about the manufacturing of the circuit.

Release notes:

Attached .BRD/.SCH/Extremely sloppy BOM. Note that these files are more for self-reference than anything else. If you are actually looking to build this, feel free to drop me a line, I can clean these files up if they’re of any interest to anyone.


Rough Bill Of Materials:

  1. C1,2,3: .22uF – ~$0.08 ea – QTY 3
  2. B3F Switch: $0.35 – QTY 1 (OPTIONAL –> DEBUG)
  3. DC Power Jack: $1.11 (Choose your favorite, foot print is of common board mount)
  4. 555 timer (SO-8 OR DIP-8) – $0.40-0.55
  5. Potentiometer/Trimmer: 100K OHM –  $1.63
  6. Rectifier Diode(s): 1N5819 – $0.53 ea – QTY 2
  7. Resistor: 10K Ohm – $0.10
  8. Power Mosfet: FD 3055 (or compatible. I just had this lying around) – $0.78
  9. Smoothing Capacitor: 100-470uF – Tantalum SMD or any small pitch cap. – $0.50
  10. Fan header: PC Fan Compatible (salvage or buy) – $0.25
  11. Copper Clad PCB OR Protoboard: ~$3.00
  12. PC Fan: Free – $5.00
  13. Enclosure:Free – $10TOTAL

    : ~25 + shipping (worst case). Mine cost me roughly … $0.00, but I’ve got a lot of spare parts lying around…

Yeast Starter Links:

Pulse Width Modulation – PWM

One thought on “Building a DIY PWM Driven Stir Plate for Homebrewing (geek edition)

  1. Hey there, saw your article on the PWM stir plate controller – really great stuff!
    I’ve been fiddling around with trying to design my own, and most of the analog PWM circuits I’ve worked with before give too high of a duty cycle range for a simple stir plate. Especially for stirring a yeast starter, even ~20% of a PC fan’s max speed is way too fast for a magnetic stir bar, and so I’ve been trying to find one designed for slower duty cycle ranges.

    If you could post / send me a cleaned up version of the project you have outlined here, that would be fantastic! I tied opening the .SCH files you have here, but that schematic seems to be for a more complex circuit than the one you have discussed here…Any help would be greatly appreciated!

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