One of the limitations of the small 8 bit microcontrollers like the Atmel ATMega family adopted by the Arduino board, is the reduced number of I/O ports.

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 ...

These TFT LCD screens can be picked up on eBay for a few dollars. But they’re more suited for 16-bit microcontrollers which operate at 3.3V levels.