int mq135pin=3;
int ledpin=12;
void setup() {
  pinMode(12, OUTPUT);
  pinMode(3, INPUT);
  Serial.begin(9600);
  // put your setup code here, to run once:

}

void loop() {
  int reading= analogRead(mq135pin);
  if(reading>=400){
    digitalWrite(12, HIGH);
    delay(20);
  }
  else{
    digitalWrite(12, LOW);
    delay(20);
  }
  
  // put your main code here, to run repeatedly:

}