ITP student Bruna Calheiros explains the Magic Crystal Mood Ball she made for a class assignment:

To make this I used 5 RGB LEDs (Digital Output), resistors, wires, a TMP36 temperature sensor (Analog Input), a breadboard, and an Arduino Uno. I built a base for the “crystal ball” and put the circuit with the Arduino inside, and made a little black altar with stars in a box so the LEDs could shine inside.

The mood is based on measuring his temperature and changing the color of the RGB LED to the its respective color. As in a mood ring, I set different colors for different temperatures. The temperatures coincides with mood state that the person is in.

For example, if you are feeling comfortable and calm, the temperature of your skin is going to be normal (about 27 degrees Celsius or 82 degrees Fahrenheit), and the ball is Cyan.

It’s a nice touch that Bruna made a paper guide to what the user’s mood is, and the documentation on her site is extensive, in case you’d like to try it yourself.