#define led_pin  18 // Define 18 as led_pin

void setup() {

  Serial.begin(9600);
  pinMode(led_pin, OUTPUT); //Setting led_pin as output

}

void loop() {
  
  digitalWrite(led_pin, HIGH); // Switch the led_pin on
  delay(1000);                // Wait for a second
  digitalWrite(led_pin, LOW); // Switch the led_pin off
  delay(1000);                // Wait for a second

}