The Glowies use a small microcontroller, but they are really quite simple in both parts and function. The core of the unit is a silicon diode used as a temperature sensor (actually, two of them). These Glowies sense when temperature drops, and it turns blue. If the temperature rises, it turns red. Plus, it’s completely solar powered, so you never have to change the battery. And I used very inexpensive parts, so you can’t get much cheaper!
Temperature-sensitive LED Glowies - [Link]