From Raspberry Pi Tutorials:
One of the many applications of a Raspberry Pi is measuring the temperature. In projects like weather stations you want to know the exact value. Using a 1-Wire Raspberry Pi temperature sensor makes it very easy to read it out and you will only need one GPIO pin.
In this tutorial it is described how to use the DS18B20 Raspberry Pi temperature sensor and what software you will need. In other projects, you could use this sensor even as a trigger for cooling your Raspberry Pi case and turning a fan on.
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