I used an LCD shield to display the current temperature and speed of the fan, but you can use the circuit without the LCD display. You also need to select the transistor by the type of fan that you ...

Now you can build your own room temperature monitor using an Arduino board. The circuit is a combination of an Arduino UNO board, LM35 temperature sensor, and a 16×2 Hitachi compatible LCD. It will ...

To address this issue, he put together a simple but very elegant temperature controlled fan to vent the enclosure when the internal conditions go above the optimal temperature.

Alternatively, if the outside temperature is < desired temp, just blow air in regardless of the inside temp. All you need then is a temperature controlled fan.