I have an experiment that I want to do that involves growing bacteria in relatively large volumes (around 10 L) in a cold room for several weeks. Typically in this sort of experiment you stir the cultures with a magnetic stir bar. This keeps the oxygen level up and prevents the bacteria from falling out on the bottom of the vessel or adhering to the sides. Unfortunately this is an unfunded side project and it is prohibitively expensive to purchase the nine commercial stir plates I would need to do this (at $300 a piece). I decided to try to build a stir plate alternative.
Electric motors and magnets are cheap, so I thought that I could epoxy a couple of circular rare Earth magnets to an electric motor and be done with it. This creates a spinning magnet well enough, but the speed of the motor is way too fast for a stir bar in a culture vessel to keep up. The speed can be reduced by dropping the voltage, but there are practical limitations to this if you want the motor to spin at 60-100 rpm (something like 0.6 % of what the motor was designed for, which would necessitate a ridiculously small voltage).
It turns out that there is an easier way to control the speed of the motor, using something called pulse-width modulation. A pulse-width modulator outputs “pulses” of the input voltage. The longer the pulse, the higher the average voltage received by the motor, and the faster the motor spins. Here’s an image from the robotics site micromouse that illustrates the process.
I searched high and low for an off-the-shelf pulse width modulator and couldn’t find one. Fortunately there are two relatively straightforward ways to build one. The elegant way uses a microcontroller and requires programming a simple piece of firmware (necessitating some knowledge of C). This seemed a little advanced for a first attempt, but good instructions can be found here (I think the LED light example would transfer to this application without modification). I opted for the uglier analogue circuit method using a kit made by Velleman (DC to Pulse Width Modulator K8004), available online from Radioshack (but not in stores).
I have very little experience with soldering, but after a couple of false starts and some guidance from YouTube things went pretty quick. Total assembly time on the kit was around 4 hours. I could probably do it again in less than half the time. Here’s what the assembled board looks like wired into a power source (I used a 19V drill battery) and my electric motor with magnet (I won’t show you the backside with my ugly soldering effort). I had very little hope that it would actually work as described, but was pleasantly surprised!
Here’s the whole prototype, spinning a beaker of water at low speed. The speed controls are pretty twitchy, but manageable. What I’ll try to do next is connect a series of nine electric motors (with magnets) to the single PWM, built into a wood frame sturdy enough to support the incubation vessels. Should be pretty straightforward and I think the increased resistance on the circuit will make speed control even easier.
Final cost for the project? Kit = $30 (x 1), Motor = $3 (x 9), Magnet = $3 (x 18), all together = $111. That’s a significant savings over the $3K the commercial version would have cost…
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