This air quality sensor sensor module is made using MQ135, a low cost semiconductor sensor which can detect the presence of Ammonia (NH3), Mono-nitrogen oxides (NOx), Alcohol, Benzene, Smoke, Carbon-dioxide (CO2) etc. It has good sensitivity to harmful gases in a wide range. The sensitive material used for this sensor is SnO2, whose conductivity is lower in clean air. It’s conductivity increases