Magnetic Stirrer

A magnetic stirrer uses a rotating magnetic field to mix fluid samples, such as buffers and media for growing bacteria. Since only a small magnet bar has to be put inside the sample/fluid, the risk of contamination is minimized. For protein purification, overnight sample dialysis steps often utilize magnetic stirrers.

Below are beta release DIY instructions for a magnetic stirrer this summer, courtesy of Malcolm Watts and Massey University in New Zealand. The stirrer runs of off a battery and has multiple speed settings.
You can also download the pdf: magnetic_stirrer_betarelease_V01

Magnetic Stirrer Beta Release v01

QUESTIONS RECEIVED  OVER EMAIL (Clarifications and corrections are added on a rolling basis – a new more dynamic platform to make this easier is under development and will be live in the not too distant future!):

Q: I would very much like to build one of your battery-operated stir bases but you left three critical components out of your instructions. There appear to be two tantalum capacitors and one electrolytic capacitor in the circuit and no specifications are given for any of them. Also, my supplier (Jameco) does not carry a BC557 transistor, I presume a 2N2219 would work as well.


- The large electrolytic capacitor isn’t really required. It’s there to keep the system stable when the battery starts to drain. Any 1.8 nanofarad capacitor that’s close will work. Searching for “CAP 1800PF through hole” or on digikey will give you some good results. I would give you a specific capacitor from Jameco, but I’m not really effective at doing searches in Jameco.

- Also, note that the schematic is more accurate than the wiring diagram which needs to be updated.  Follow that first.

- We can also send you a Circuit Board, the one in the document, that will make assembly easier as long as you have a box to put it in. (Note to others: we have a limited supply of these available: email us if you want one sent!)


Q: I gather the two smaller capacitors are 1800 pf or 0.01 µf. I suppose the larger capacitor is not critical, perhaps a 340 µf or 470 µf?

A: Digikey seems to favor pf so it’s hard to search for 1.8nF.  You do want to get as close to 1800 pf as possible. The capacitors are used
as part of an oscillator. Changing the capacitance and resistance values will change the frequency of the square wave controlling the motor.If you want to order from Jameco instead: is the closest I can find in Jameco’s stock. It’s 1500 pF instead of 1800 pF you can also take that one and put it in parallel with this one:

Q:  There is no breakdown voltage specified for the zener diode.

A: The zener diode is acting as a flyback diode. The breakdown voltage shouldn’t matter much as long as it’s above 1.5 otherwise, the LED will always be on. The forward voltage is more important it will depend on your motor, again I wouldn’t be too worried about it as long as it’s below the acceptable voltage for the transistors you are using.

Q:  I am assuming that the 5k6 designation for one resistor is a typo. Since a 5K resistor would be a special item, should that read 5.6K or 56K which are standard? Note also that it does not appear on the wiring diagram. The BC557 transistor is very strangely biased.

A:  It reads as 5.6K Ohm. That notation is intended to emphasize the decimal point so that it’s harder to miss and doesn’t disappear when photocopied or printed.
Q: Is it really important what the frequency of the multivibrator is? Isn’t what we’re interested in what might be called the duty cycle? What is important is the amount of time the current to the motor is on and the amount of time it is off. Also, it has been my experience that, unless you are buying expensive, high-tolerance components, capacitors can vary greatly from the stamped value. Ceramic disc capacitors may vary as much a 20%. That being said, I would think a nominal 15,000 pf or 20,000 pf capacitor would probably work. Of course the frequency would be different but does it matter as long as it is in the ball park?
A: The frequency probably doesn’t matter so much, but I’d imagine that you’d lose resolution and maybe use more power, but in the grander scheme of things I don’t think that’s a big issue.