// ── Arduino Uno R3 — External LED Blink ──────────────────────────────────────
// Blinks an LED on pin 9 with a configurable period.
// Modify ON_MS and OFF_MS to change the flash pattern.
// ─────────────────────────────────────────────────────────────────────────────

#define LED_PIN   9       // Digital pin connected to LED anode (via 220Ω)
#define ON_MS    500      // How long the LED stays on  (milliseconds)
#define OFF_MS   500      // How long the LED stays off (milliseconds)

void setup() {
  // Tell the ATmega this pin should drive current (output mode)
  pinMode(LED_PIN, OUTPUT);

  // Optional: open Serial so you can watch the blink count in the monitor
  Serial.begin(9600);
  Serial.println("Blink sketch started");
}

void loop() {
  // HIGH drives the pin to 5V → current flows → LED lights up
  digitalWrite(LED_PIN, HIGH);
  Serial.println("LED ON");
  delay(ON_MS);

  // LOW pulls the pin to 0V → no current → LED off
  digitalWrite(LED_PIN, LOW);
  Serial.println("LED OFF");
  delay(OFF_MS);
}