An MQ135 air quality sensor is a type of MQ gas sensor that detects, measures, and monitors a wide range of gases present in air like ammonia, alcohol, benzene, smoke, carbon dioxide, etc. It operates at a 5V supply with 150mA consumption. Preheating of 20 seconds is required before the operation to obtain the accurate output. The MQ-135 is sensitive to multiple gases including Benzene, Acetone, Alcohol, and Ammonium. 
 
Specifications and Features:
The MQ135 air quality sensor specifications and features are listed below.
It has a wide detection scope.
High sensitivity and faster response.
Long life and stability.
The operating voltage: +5V.
Measures and detects NH3, alcohol, NOx, Benzene, CO2, smoke etc.
Range of analog output voltage: 0V-5V.
Range of digital output voltage: 0V-5V (TTL logic).
Duration of preheating: 20 seconds.
Used as an analog or digital sensor.
The potentiometer is used to vary the sensitivity of the digital pin.
Heating Voltage: 5V±0.1.
Load resistance is adjustable.
Heater resistance: 33ohms±5%.
Heating consumption:<800mW.
Operating temperature: -10°C to -45°C.
Storage temperature: -20°C to -70°C.
Related humidity: <95%Rh.
Oxygen concentration: 21% (affects the sensitivity).
Sensing resistance: 30kiloohms to 200kiloohms.
Concentration slope rate: ≤0.65.
Preheat time: over 24 hrs.
Simple drive circuit.