The on-board ATmega328 receives data over multiple digital interfaces (such as TWI, UART and SPI) and does all the PWM and upkeep of the display so your main controller doesn’t have to. There are even two built-in functions that can be activated with solder jumpers.
Three linear slide potentiometers connected to the Arduino’s analog pins 0 through 2. Red and yellow LEDs connected to digital pins 5 and 6 - PWM pins - so you can easily vary their brightness. Three momentary push buttons connected up to the Arduino’s digital pins 10 through 12. Those lines will go low when the buttons are pressed.
The iron is attached to a six foot cable which is terminated with a European 2-prong (CEE 7/7) plug. There is a nice grippy surface at the end of the handle that does double duty as a nice heat shield.