void setup() {
  pinMode(18, OUTPUT);
  pinMode(19, OUTPUT);
  pinMode(21, OUTPUT);  // Adding pin 21 as an output
}

void loop() {
  digitalWrite(18, LOW);  // 000
  digitalWrite(19, LOW);
  digitalWrite(21, LOW);
  delay(1000);

  digitalWrite(18, HIGH);  // 001
  digitalWrite(19, LOW);
  digitalWrite(21, LOW);
  delay(1000);

  digitalWrite(18, LOW);   // 010
  digitalWrite(19, HIGH);
  digitalWrite(21, LOW);
  delay(1000);

  digitalWrite(18, HIGH);  // 011
  digitalWrite(19, HIGH);
  digitalWrite(21, LOW);
  delay(1000);

  digitalWrite(18, LOW);   // 100
  digitalWrite(19, LOW);
  digitalWrite(21, HIGH);
  delay(1000);

  digitalWrite(18, HIGH);  // 101
  digitalWrite(19, LOW);
  digitalWrite(21, HIGH);
  delay(1000);

  digitalWrite(18, LOW);   // 110
  digitalWrite(19, HIGH);
  digitalWrite(21, HIGH);
  delay(1000);

  digitalWrite(18, HIGH);  // 111
  digitalWrite(19, HIGH);
  digitalWrite(21, HIGH);
  delay(1000);
}