Our homes and workplaces are filled with CO2 from our lungs. The only way to get rid of all the excessive CO2 is by regular ventilation. But how do you know if it is working? The answer is simple: a device that can measure the current CO2 contentration in the room. 
Tech: Arduino, Raspberryor ESP based hardware, NDIR sensor, display. Code: C++, Python.
Applications: home, office, school, etc.
Goal: measure current CO2 for improvement of well-being. also learn about CO2 dynamics in closed rooms