Starship control panel on an Arduino Uno

// Starship control panel: a green "ready" lamp, and two red lamps that flash
// in turn while the launch button is held.
//
//   pin 2   the button: one leg to 5V, the other to pin 2 and through a 10k
//           resistor to GND (a pull-down)
//   pin 3   the green LED, through 220 ohm to GND
//   pin 4   the first red LED, through 220 ohm to GND
//   pin 5   the second red LED, through 220 ohm to GND
//
// The 10k resistor holds pin 2 at 0 V while the button is up, so it reads LOW
// rather than floating. Press the button and it joins pin 2 to 5 V: HIGH.
// This is the opposite of INPUT_PULLUP, where pressed reads LOW.

const int BUTTON = 2;
const int GREEN = 3;
const int RED_A = 4;
const int RED_B = 5;

int lastButton = LOW;

void setup() {
  pinMode(BUTTON, INPUT);
  pinMode(GREEN, OUTPUT);
  pinMode(RED_A, OUTPUT);
  pinMode(RED_B, OUTPUT);
  Serial.begin(9600);
  Serial.println("Mokxi Uno: starship control panel");
}

void loop() {
  int button = digitalRead(BUTTON);
  if (button != lastButton) {
    lastButton = button;
    Serial.println(button == HIGH ? "launch!" : "ready");
  }

  if (button == LOW) {
    // Waiting: green on, both reds off.
    digitalWrite(GREEN, HIGH);
    digitalWrite(RED_A, LOW);
    digitalWrite(RED_B, LOW);
  } else {
    // Held: green off, the reds take turns every quarter second.
    digitalWrite(GREEN, LOW);
    digitalWrite(RED_A, HIGH);
    digitalWrite(RED_B, LOW);
    delay(250);
    digitalWrite(RED_A, LOW);
    digitalWrite(RED_B, HIGH);
    delay(250);
  }
}