Replacing a standard thermostat (SIEMENS-RAA30 16GR) with a self made, arduino-based, big LCD screen is a fun project.
DISCLAIMER: Mains power is used in both controlling the furnace and for arduino power so please do not proceed if you are not comfortable working with mains power. I assume no responsibility if you hurt yourself or others while making this project.
The main features of the thermostat are:
Code available on github for anyone to download/experiment/fork here: https://github.com/akorbeti/thermostat/tree/master
I used a seeeduino lotus which is arduino compatible but with the added benefit of the grove interface in one board.
The grove interface is a four-wire interface for many components found usually on seeedstudio. I happened to have lots of components lying around using the grove interface so I just went with that.
The project can also be done with normal arduino, nanos etc. It is just a matter of choice. The program does not use any board specific code.
You will also need to create an insulation for the board to be sure that the pins will not touch anything else when we stuff them all in the box