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.
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Signing UpStep 1: The Parts
These are the required parts:
- Thermoelectric Cooler ($14.75)
- Power Supply ($4.75)
- Motor ($1.25)
- Magnets x2 ($0.66 total)
- Potentiometer, 5K ($1.00)
- Switches x2 ($2.00 total)
- LED ($0.10)
- Enclosure ($1.95)
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Thank you
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 $2 depending on parts on hand)
I'm not trying to ruin your instructable, but rather I want to help with ideas so you don't destroy the parts you've spent good money to buy, in addition to the disappointment that comes with the circuit failing. If you'd like a schematic to try out and to add to your 'ible, I'd be happy to draw one up for you. Just ask.
That's the beauty of community participation - helping others succeed.
Peace!
Please post a schematic of the modifications you proposed.
Why use a TEC instead of just a heating coil?
You have forgotten to mention that your power supply will need >10 amps, which isn't exactly inexpensive (more than 30$) and you also need to lable your amperage amounts on your potentiometers, etc...