The MQ-135 is a general purpose air quality sensor capable of detecting various toxic and harmful gases in the environment such as ammonia (NH3), nitrogen oxides (NOx), alcohol, benzene, smoke and carbon dioxide (CO2). The module has both a digital (D0) output pin that is triggered at a threshold value set with a potentiometer and an analog (A0) output pin that changes according to the gas density in the environment. Since it works on the principle of a heating element inside, the sensor needs to be warmed up for a while (preheating process) before use in order to get the most accurate measurements. This module, which can be easily integrated with Arduino and ESP series boards, is frequently preferred in projects such as smart home systems, greenhouse automation, office air quality monitoring and fire/smoke alarm.
