Give an LED pin a constant name.

Arduino / C: Use const to name the built-in LED pin and reuse it in output instructions.

const byte ledPin = LED_BUILTIN;

void setup() {
  Serial.begin(9600);
  pinMode(ledPin, OUTPUT);
  digitalWrite(ledPin, HIGH);
  delay(50);
  digitalWrite(ledPin, LOW);
  Serial.print("Used pin ");
  Serial.println(ledPin);
}

void loop() {
}

Expected output EXAMPLE

Used pin 13

Details

Targets Arduino Uno R3 (ATmega328P, 16 MHz) with the Arduino AVR core. Open as a sketch in the Arduino IDE and select Arduino Uno. Serial output uses 9600 baud. Tested with simavr; electrical characteristics and physical wiring are outside this simulation. On the Uno R3, LED_BUILTIN names digital pin 13. const prevents accidental reassignment; byte is suitable for these small pin numbers. This example flashes once after upload or Reset.

✓ Verified

Tested on Ubuntu 24.04.5 LTS · 2026-11-02 18:28:04 UTC · Uno R3 / ATmega328P at 16 MHz; simavr 1.6+dfsg-3build2 · simavr 1.6+dfsg-3build2; arduino-cli Version: 1.5.1 Commit: 01f3d4f2b Date: 2026-06-05T10:22:17Z; Arduino AVR core 1.8.6

Test inputs, results and environment

The exact published code ran in a dedicated Linux test container with prepared inputs. Snippet process uid 65534 with a read-only root filesystem and temporary working files. Networking disabled. The exact sketch was compiled for Uno R3 and simulated at 16 MHz. UART output is shown below; compiler logs and measured GPIO traces are retained in the test evidence. Fixtures and output records are isolated from the application database. Testing covers this example and its stated checks, not every input, operating system, or runtime version. The Linux container shares its host's kernel; this is not a native Windows test.

Architecture / host kernel: x86_64 / 6.18.54.1-microsoft-standard-WSL2
Snippet identity / locale: uid=65534, gid=65534 (unprivileged process) / C.UTF-8
Interpreter invocation: arduino-cli compile --fqbn arduino:avr:uno; uno-sim sketch.ino.elf (capture UART and D13 transitions)
Container image ID: sha256:ec91410a8f6ec3e985830b5e3a0560306d36f8c1c5ccd91a6699abe707123fa3
Code SHA-256: 92b1a2235e95cd1b0fa8d2fd52d4c8b9db14951e51419b55cd697e7dead4482e
Dependencies: simavr 1.6+dfsg-3build2; arduino-cli Version: 1.5.1 Commit: 01f3d4f2b Date: 2026-06-05T10:22:17Z; Arduino AVR core 1.8.6

Test input setup

These are the prepared inputs used by the test harness. See Details above for the prerequisites and how to run this example.

Simulator inputs: {"sim_ms": 500, "serial_input": "", "pin_input": "none"}

Observed standard output

Used pin 13

Exit code: 0. Standard error: empty. Validation: Exact stdout comparison. System-dependent values above are one recorded run.

Additional assertions that passed

Checks recorded by the test harness. Where Python assertions are shown, browser cases use Playwright and simulator cases may inspect pin transitions: result.stdout is captured output, work is the temporary directory, and Path is pathlib.Path. The harness also imports json and csv.

trace = [tuple(map(int, line.split(','))) for line in Path('pin13.csv').read_text().splitlines()]
pulses = [(trace[i+1][0] - cycle) / 16000 for i, (cycle, value) in enumerate(trace[:-1]) if value == 1 and trace[i+1][1] == 0]
assert len(pulses) == 1 and 48 <= pulses[0] <= 52 and trace[-1][1] == 0