Arduino CO2 Sensor v.1

I have printed the case in PLA with my Prusa i3 x. The MQ135 (air-quality sensor) is controlled by an Arduino Nano and connected with a HC-06 bluetooth module to interact with an Android smartphone (and see the ppm level). The magnets hold the face to the case. 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.
I hope you can find this sensor useful!

P.s. :
-Arduino CO2 Sensor v.2: "https://www.thingiverse.com/thing:2422144"
-Arduino code under revision