//Declare Variables to hold Pin Numbers
int RED = 18;
int YELLOW = 19;
int GREEN = 21;

void setup() 
{
  // put your setup code here, to run once:
  Serial.begin(115200);
  Serial.println("Hello, Lights!");

  //Setup Pin D18,19,21 as an Output (Actuator)
  pinMode(RED, OUTPUT);  //RED
  pinMode(YELLOW, OUTPUT);  //YELLOW
  pinMode(GREEN, OUTPUT);  //GREEN
}

void loop() 
{
  digitalWrite(RED, HIGH);
  delay(1000);
  digitalWrite(RED, LOW);
  delay(1000);
  digitalWrite(YELLOW, HIGH);
  delay(1000);
  digitalWrite(YELLOW, LOW);
  delay(1000);
  digitalWrite(GREEN, HIGH);
  delay(1000);
  digitalWrite(GREEN, LOW);
  delay(1000);
}