• Sensitive for benzene, alcohol, smoke
• Output voltage boosts along with the concentration of the measured gases increases
• Fast response and recovery
• Adjustable sensitivity
• Signal output indicator

• Power: 2.5V ~ 5.0V
• Dimension: 40.0mm * 21.0mm
• Mounting holes size: 2.0mm
• Sensitive gases:
  • ammonia
  • nitrogen oxides
  • alcohols
  • aromatic compounds
  • sulfides
  • fumes

Initial preparation of the module shouldn’t present any undue difficulties if you strictly follows the procedure lined here. Well, first of all power up the module alone with a regulated 5VDC power supply. The heater of the MQ-135 sensor requires 5V and have 33Ω±5% resistance, so your power supply must render a minimum 200mA of current for the sensor part. Remember that at first the MQ-135 gas sensor have to be kept on continuously for its preheating time (over 24 hour) before you can actually play with it. Thereafter, introduce the MQ-135 sensor to the gas you want to detect and slowly adjust the trimpot until DO gets low.

Now every time your sensor gets introduced to that gas at predefined concentration, DO will go low (0V) else will remain high (5V). Note that the green LED (OUT_LED) should also light up when gas is observed. Also note that when the sensor is powered up for gas detection, it needs around 60-120 seconds to settle, because the heater will need that much time to heat the sensor up. Remember, the MQ-135 gas sensor is suitable for detecting (or measuring) of NH3, NOx, alcohol, Benzene, smoke, CO2, etc.

int sensorValue;
int digitalValue;
 
void setup()
{
  Serial.begin(9600); // sets the serial port to 9600
  pinMode(13, OUTPUT);
  pinMode(2, INPUT);
}
 
void loop()
{
  sensorValue = analogRead(0); // read analog input pin 0
  digitalValue = digitalRead(2);
  if (sensorValue > 400)
  {
    digitalWrite(13, HIGH);
  }
  else
    digitalWrite(13, LOW);
  Serial.println(sensorValue, DEC); // prints the value read
  Serial.println(digitalValue, DEC);
  delay(1000); // wait 100ms for next reading
}

• Normal air returns approximately 100-150
• Alcohol returns approximately 700
• Lighter gas returns approximately 750

#include <LiquidCrystal.h>
int sensorValue;
const int rs = 12, en  = 11, d4 = 5, d5 = 4, d6 = 3, d7 = 2;
LiquidCrystal lcd(rs, en, d4, d5, d6, d7);
void  setup(){  lcd.begin(16, 2);
Serial.begin(9600);                            //  sets the serial port to 9600
 }
void loop(){sensorValue = analogRead(0);       //  read analog input pin 0
Serial.print("AirQua=");
Serial.print(sensorValue,  DEC);               // prints the value read
Serial.println(" PPM");
lcd.setCursor(0,0);
lcd.print("ArQ=");
lcd.print(sensorValue,DEC);
lcd.print("  PPM");
lcd.println("       "); 
lcd.print("  ");
delay(100);                                   //  wait 100ms for next reading
}

• https://github.com/maaz-siddiqui/ESP32_MQ-135_SH1106

• MQ-135 Gas Sensor Technical Data