void setup() {
  pinMode(8, OUTPUT);
  pinMode(7, OUTPUT);
}

void loop() {
  digitalWrite(8, HIGH); //  left LED on
  digitalWrite(7, LOW);  // right LED off
  delay(100);           
  
  digitalWrite(8, LOW);  //  left LED off
  digitalWrite(7, HIGH); //  right LED on
  delay(100);           
  
  digitalWrite(8, LOW);  // off
  digitalWrite(7, LOW);  // off
  delay(100);           
}