const int mqxPin = 2;
int redLed = A5;
int sensorThres = 500;

void setup()
{
  pinMode(mqxPin, INPUT);
  pinMode(redLed, OUTPUT);
  Serial.begin(9600);
}

 void loop()
 {
   int analogSensor = analogRead(mqxPin);
   Serial.print("Output MQ-2 : ");
   Serial.println(analogSensor);
   if (analogSensor > sensorThres)
   {
     digitalWrite(redLed, HIGH);
   }
   else
   {
     digitalWrite(redLed, LOW);
   }
   delay(500);
 }