• 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.

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