[electron_plumber] made an LED that can be blown out like a candle, and amazingly it requires no added sensors. The project uses an Arduino ... LED itself as a temperature sensor.

The LM35 temperature sensor and red led are powered with 5V from the Arduino board. As you can see in the sketch on the first line I included the LiquidCrystal library (header) that includes useful ...

He mounted a small temperature sensor on his graphics card near the GPU, wiring it to an Arduino. The Arduino monitors his video card, lighting an RGB LED blue if conditions are alright.