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:

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)

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.