int a_pin = A0;
int d_pin = 2;

int sensor_value;

void setup() {
  Serial.begin(9600);
  pinMode(LED_BUILTIN, OUTPUT);
  pinMode(d_pin, INPUT);
}

void loop() {
  // readAnalog();
  readDigital();
  Serial.println(sensor_value);

  if (sensor_value == 0) {
    Serial.println("Smoke is detected");
    digitalWrite(LED_BUILTIN, HIGH);
  }
  else {
    Serial.println("No Smoke");
    digitalWrite(LED_BUILTIN, LOW);
  }
}

void readAnalog() {
  sensor_value = analogRead(a_pin);
}

void readDigital() {
  sensor_value = digitalRead(d_pin);
}