This sample code is for smoke detector using MQ2 sensor on Maker NANO

smoke_detector

#define MQ2             A0
#define GREEN_LED       4
#define YELLOW_LED      5
#define RED_LED         6
#define PIEZO           8
int sensorValue = 0;

void setup()
{
  Serial.begin(9600);   // Initialize serial communications with the PC
  pinMode(RED_LED, OUTPUT);
  pinMode(GREEN_LED, OUTPUT);
  pinMode(YELLOW_LED, OUTPUT);
  pinMode(PIEZO, OUTPUT);
  pinMode(MQ2, INPUT);
  digitalWrite(GREEN_LED, HIGH);
}

void loop()
{
  sensorValue = analogRead(MQ2);
  Serial.println(sensorValue);

  if (sensorValue > 300)
  {
    digitalWrite(GREEN_LED, LOW);
    digitalWrite(RED_LED, HIGH);
    tone(PIEZO, 1000, 300);
    delay(700);
  }

  else
  { 
    noTone;
    digitalWrite(GREEN_LED, HIGH);
    digitalWrite(RED_LED, LOW);
  }
}