I designed and built a pet heater for my girlfriend's hedgehog Gene Parmesan. With a temperature sensor and an Arduino nano, the blanket turns on when temperature drops below 72˚F. Hedgehogs need a warm environment, and San Francisco can get pretty cold in the winter. 

I also designed the enclosure and 3D printed it on my Anet A8.

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The heater is just a resistor pad, and the lights indicate when the pad is being heated (red) and when temperature is above 72˚ (green).

Now Gene is always toasty.