LESSON 01 / Year 12 · Digital output

Control an LED.

An output, a current, a timing diagram.

The challenge

Blink an external LED without blocking the loop. Before running the code, draw the state of D8 against time.

Equipment: UNO R4, red LED, 1 kΩ resistor, wires and breadboard. To test without wiring, you can use the built-in LED with LED_BUILTIN, then move to this lesson's external LED.

Wiring diagram: Control an LED — Click to enlarge · SVG · PNG. Connectors are spread out for readability: follow the pin names on your board.

Connect, then check

From	To
D8	1 kΩ resistor then LED anode
LED cathode	GND

The resistor is in series. Its orientation does not matter; the LED has polarity. For a red LED with an assumed 2 V drop, the estimated current is (5 − 2) / 1000 = 0.003 A, or 3 mA. Compare this estimate with the LED data sheet and a measurement.

The complete program

01_led.ino

Complete project ↓

// UNO R4 Minima ou WiFi. LED externe : D8 -> 1 kohm -> anode ; cathode -> GND.
constexpr uint8_t LED = 8;
constexpr unsigned long PERIODE_MS = 500;
unsigned long dernierChangement = 0;
bool allumee = false;

void setup() {
  pinMode(LED, OUTPUT);
  digitalWrite(LED, LOW);
}

void loop() {
  const unsigned long maintenant = millis();
  // La soustraction non signee fonctionne aussi au debordement de millis().
  if (maintenant - dernierChangement >= PERIODE_MS) {
    dernierChangement = maintenant;
    allumee = !allumee;
    digitalWrite(LED, allumee ? HIGH : LOW);
  }
}

Understand the instructions

Instruction	Explanation
`pinMode(LED, OUTPUT)`	Configures D8 as an output.
`maintenant - dernierChangement`	Elapsed time; unsigned subtraction handles counter overflow.
`allumee = !allumee`	Inverts a logic state.
`digitalWrite(...)`	Sets HIGH or LOW; the resistor limits the current.

A change every 500 ms produces a 1 s on + off cycle, or a frequency of 1 Hz. Not using delay() keeps the loop available.

YOUR TURN / YEAR 12

Modify the period to obtain a 2 s cycle. What happens to the frequency?

GOING FURTHER / YEAR 13

Propose a cycle with 100 ms on and 900 ms off. How can you store the state and its duration?

Validate and troubleshoot

Observe ten cycles and compare their duration with the prediction.
LED off: check orientation, GND and D8.
LED always on: check the circuit and uploaded program.
Never remove the resistor to “test”.

Next step: Read a logic state →

SOURCES AND FURTHER READING

Check the documentation.

Original Quiet Bee content. A suggested progression for working on the information chain, prototyping and validation; it does not replace the curriculum or your teacher's instructions.