Datasheet : https://www.olimex.com/Products/Components/Sensors/SNS-MQ135/resources/SNS-MQ135.pdf http://www.robotshop.com/media/files/pdf/MQ-135-Gas-Sensor-UserManual.pdf FEATURES -Wide detecting scope -Fast response and High sensitivity -Stable and long life -Simple drive circuit APPLICATION They are used in air quality control equipm ents for buildings/offices, are suitable for detecting of NH3,NOx, alcohol, Benzene, smoke,CO2 ,etc.