What They Are and How They Work
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!
One of the diodes sticks out a little from the Glowie and serves as the main temperature sensor. The other diode is lightly insulated from the ambient temperature. This makes it slower to “catch up” to temperature changes. Temperature changes are detected by comparing the main sensor to the insulated one.
As a result, the Glowie doesn’t actually know what temperature it is – at least, not in degrees. But it is quite good at detecting temperature changes. A temperature change upwards lights the red LED. A change downward lights the blue LED. Since the Glowie measures and reacts many times per second, borderline changes in temperature result in a flickering of the LEDs. Strong changes have the LED lit solid.