MQ-135 Sensor Features
Wide detecting scope
Fast response and High sensitivity
Stable and long life
Operating Voltage is +5V
Detect/Measure NH3, NOx, alcohol, Benzene, smoke, CO2, etc.
Analog output voltage: 0V to 5V
Digital output voltage: 0V or 5V (TTL Logic)
Preheat duration 20 seconds
Can be used as a Digital or analog sensor
The Sensitivity of Digital pin can be varied using the potentiometer Where to use MQ-135 Gas sensor The MQ-135 Gas sensors are used in air quality control equipments and are suitable for detecting or measuring of NH3, NOx, Alcohol, Benzene, Smoke, CO2. The MQ-135 sensor module comes with a Digital Pin which makes this sensor operate even without a microcontroller that comes in handy when you are only trying to detect one particular gas.  If you need to measure the gases in PPM the analog pin need to be used. The analog pin is TTL driven and works on 5V and so can be used with most common microcontrollers.
 
Resources: 
1. Measuring CO2 Concentration in Air using Arduino and MQ-135 Sensor
2. Interfacing of MQ135 Gas Sensor with Arduino