This is the second version of a my previous project, Arduino CO2 Sensor. The most important changes are: the add of an OLED display, the Carbon Monoxide Sensor (toxic gas that derivates from incomplete combustions), the ventilation system and the possibility to record data. I have printed the case in PLA with my Prusa i3 x. The MQ135 (air-quality sensor) is controlled by an Arduino Mega and it is connected with a HC-06 bluetooth module to interact with an Android smartphone (and see the ppm level). A yellow led will switch on when the sensor detects 1000 ppm and a red led will switch on when it detects 1500 ppm.
I have decided to construct this sensor in particular for schools. Infact, I have noticed that the level of attention decreases with a high CO2 level. In my website you can see the CO2 graph that I have recorded in my classroom.
I hope you can find this sensor useful!

P.s. :
-Arduino CO2 Sensor v.1: "https://www.thingiverse.com/thing:1859623"
-Arduino code under revision
-Here the libraries: "https://drive.google.com/file/d/0B9TC96icExppb2o4RWNVakM1VUE/view"
-Full tutorial at this link: "https://andreacescato.tech.blog/2017/07/06/co-co2-sensor-v-2/"
-Mechatronics Projects: "https://andreacescato.tech.blog/"