This project uses a temperature sensor and five LEDs sewn into a wrist warmer to respond to ambient temperature.
If the temperature drops below 12°C, the lights start fading in and out. If the temperature is 1°C or below, the lights twinkle.
This guide is written for Mac OS X.
(tl;dr: Wrist warmers are practical warming devices, they can also tell you when it’s cold enough to wear them, through the application of pretty twinkling lights.)
alyson fielding 