#define MQSEN A5
#define LED 3

void setup() {
pinMode(LED, OUTPUT);
pinMode(MQSEN, INPUT);
Serial.begin(9600);
}
void loop() {
int MQVALUE=analogRead(A5);
if(MQVALUE>500)
{
  digitalWrite(LED,HIGH);

}
else
{
  digitalWrite(LED, LOW);
}
Serial.println(MQVALUE);
}