Magnetic Stirrer W/ Hotplate for <$30





Introduction: Magnetic Stirrer W/ Hotplate for <$30

A magnetic stirrer with a hotplate allows you to mix and heat solutions with ease. These are commonly used in chemistry classrooms and cost upwards of $160! But I'll show you how to build one for less than $30.

Step 1: The Parts

These are the required parts:
And that's all you need! Total cost: 26.46. Not bad for something worth more than $160!!!

Step 2: The Stirrer

To create the stirrer, epoxy the magnet set onto the shaft of the motor. Make sure it is well centered.
Next, complete a circuit with the power supply, switch, and the potentiometer. See the schematic if you have questions.
To mount the setup into the enclosure, use hot glue to secure the motor in the center of the enclosure. Drill holes to mount the switch and the potentiometer.

Place the second magnet set in the liquid to remotely stir it.

Step 3: The Hotplate

Adding the hotplate is very easy! Just add an extra switch and LED to the thermoelectronic cooler to the power supply. Check the schematics if you have questions.

To mount the heater/cooler, identify the heating side and place it face up on the metal. Secure the device with a dot of hot glue. Make sure this is centered above the motor you mounted in the last step.

Step 4: The Combination

Combine both setups either at the power supply or the stirrer switch. Check out the schematics for more detail.



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    35 Discussions

    The idea overall is a fair one but there are a few points which need to be mentioned, some already made by others.

    1. You need to connect the middle tag of the potentiometer (to either side) otherwise it will have no effect at all.

    2. Ideally, you might want to make (or buy, they are dirt cheap - £2 or so) a little motor controller from an auction site. The reason is that the motor will then have full torque even at very low revs. A potentiometer for this is not really ideal especially if the medium being stirred is perhaps a bit "thicker" than water - so requiring lower speed and full torque.

    3. Don't wire the LED in series with anything other than perhaps a 560 Ohm - to 1K resistor. Wired as shown will result in the LED becoming a DED (Dark emitting Diode).

    4. Don't use hot melt unless you will never be running more than about 50C - it softens quite easily. Araldite is not bad - and silicone is also resilient to high temp.

    5. For even a fairly modest peltier unit (that is at least big enough to be potentially useful), you are going to want at LEAST 5A at 9V. Personally I would suggest a motor, controller and power supply for 10A at 12V (fairly cheap on an auction site though certainly not $5)

    6. You need to heatsink the bottom of the peltier junction - otherwise within the space of a few seconds, the entire module will just keep getting hotter as it has no ability to "pump the heat away". A nice big heatsink is required - which then means a new layout is needed with regards tot he stirrer system.

    One possibility is to use four peltier junctions, each heatsinked and then have a small gap between them, such that the motor shaft can come up through the middle of them all. Either way, without a heatsink, this will not work for more than a few seconds on "cooling mode". Heating mode will work ok though as the entire assembly will just keep getting hotter.

    In short, a good idea but not one to construct yourself unless you can understand each of the points above. If you just construct "as shown" you will see a very brief flash from the LED and a stalled motor - or one barely running at all equivalent to slowest possible speed setting).

    So FAB idea for an instructible, but does need a little polishing to make it work.

    Wow...I didn't realize how easy it would be to make one of these...I just spent like $150 for one of these on Amazon yesterday...Oh well, live and learn I suppose lol...

    Cool instructable, will prove helpful for those interested in chemistry but on a difficult budget.

    If I may point out a couple of things that readers may find helpful -

    - The potentiometer in both schematics show the wiper disconnected, there will be no change in the current of your circuit unless the wiper is connected to one terminal or the other on the pot.

    - Your LED will become totally crisp if you hook it up in series with the TEC like you show it. The TEC draws AMPS not milliamps. Hook the LED up across the power connections of the TEC with a resistor in series with it (for 9v try 470ohm)

    - Your 9v wall-wart adapter will not supply enough current to the TEC unless you use a fairly beefy one (2-3amps) so a switcher version would help depending on temp desired.

    - By reversing the polarity to the input of the TEC, you can swap hot/cold sides. Makes a great closed-loop temp regulation system if more circuitry is added.

    - The potentiometer you show is not capable of the current levels required of it. The 5k value will make any control you DO have be confined to the extreme end of travel (turn it a tiny bit and it goes from stall to full speed) and may burn out very quickly depending on the load and current draw of the motor.

    - Motors hate to be controlled by current, the first thing to go is their torque so they become useless quickly. The BETTER way is to use PWM (Pulse Width Modulation). This is VERY simple to do - just use a 555 timer with a transistor (plus two or three resistors, two capacitors and a pair of diodes) and then you can use that pot you have. (extra cost is about $2 depending on parts on hand)

    I'm not trying to ruin your instructable, but rather I want to help with ideas so you don't destroy the parts you've spent good money to buy, in addition to the disappointment that comes with the circuit failing. If you'd like a schematic to try out and to add to your 'ible, I'd be happy to draw one up for you. Just ask.

    That's the beauty of community participation - helping others succeed.


    2 replies

    What would be an appropriate potentiometer for this application? Would it be the same for a 12v 92 peltier device?


    Please post a schematic of the modifications you proposed.

    I made one of these but the 'thermoelectric cooler' i used a peltier unit which it gets cold on one side and hot on the other with a 12v wall supply and it got quite hot

    1 reply

    What do you think he was using......If you've seen a peltier cooler you would know that is what he had from his pictures.

    Can you tell me if the magnet holders in this project are readily available and where you get them?
    Thank you

    where did you buy your stuff, cause I only paid like $1 or $2 or the thermoelectric cooler and i already had most of the stuff.

     What wattage was your thermoelectric cooler?

    Why use a TEC instead of just a heating coil?

    1 reply

    If you use a TEC in addition to converting all the electricity it gets into heat some of the energy is used to pull heat from the other side of the TEC to the hot side. IE if you have a TEC and put 100w of electricity in it might give you 110 w of heat (100w from the electricity 10w drawn from the cold side. It's just more efficient.

    I wonder why no one asked, why the R doesn't change the motor speed LOL

    So the hot glue will not melt because the otherside is kept cool correct?

    2 replies

    Last comment:
    You have forgotten to mention that your power supply will need >10 amps, which isn't exactly inexpensive (more than 30$) and you also need to lable your amperage amounts on your potentiometers, etc...

    He also didn't predict that such current running through the LED or a potentiometer would quickly damage it. A thyristor-based (or other kind of power dimming device) regulation is needed.

     yes, put it in parallel with an appropriate resistor.

    You can stack TECs to make a very large temperature difference IE h>c>h>c>h>c so one side cools (or heats) the next one in line.  But you need a lot of amperage and some decent cooling or a constant liquid filled container on it.  the bottom cool side would probably get down to freezing or thereabout

    what kind of magnets are they

    yed but use nut and bolts because hot melt glue melts......!!!!