🛠 Allumer une LED externe en respectant les broches 3,3 V et 9 mA de la Due

Objectives

Move away from all-in-one setups: wire an external LED to a Due pin by correctly calculating the resistor. On a 5 V board, you can take "220 Ω with your eyes closed"; on the Due, the logic is at 3.3 V AND the current per pin is much lower than on AVR chips: 9 mA for group 1, 3 mA for group 2, and 130 mA total across all I/O pins. This tutorial establishes good electrical practices.

Hardware

• 1 Arduino Due
• 1 red LED (Vf ≈ 2 V) or green/blue LED (Vf ≈ 2.1–3.2 V)
• 1 resistor from 100 Ω to 220 Ω
• 2 male-to-male wires, micro-USB cable

Complete code

// LED externe sur la broche 9 (groupe 1 : 9 mA max par broche)
const uint8_t LEDX = 9;

void setup {
  pinMode(LEDX, OUTPUT);
}

void loop {
  digitalWrite(LEDX, HIGH);   // 3,3 V sur la broche -> LED allumée
  delay(300);
  digitalWrite(LEDX, LOW);    // 0 V -> LED éteinte
  delay(300);
}

Resistor calculation: R = (Vpin − Vf) / I. For a red LED at 3.3 V: (3.3 − 2.0) / 0.010 A ≈ 130 Ω → use 150 Ω, the LED will see ~9 mA and the pin will stay within its 9 mA limit. With 220 Ω it is even more conservative and perfectly visible.

Expected verification

The LED blinks every 300 ms. If you disconnect the resistor, the LED shines brighter, but you are injecting uncontrolled current: never leave it in this state. The program runs autonomously on 7–12 V jack power, with USB disconnected.

Common errors

• Forgetting the resistor: on a 3.3 V output, the current quickly exceeds the 9 mA (group 1) or 3 mA (group 2) limits from the official datasheet; unlike an AVR pin rated for 20–40 mA, the Due suffers damage faster, and the total budget of 130 mA across all pins limits the number of LEDs.
• Connecting a pin to a 5 V circuit (a sensor or shield designed for Uno): risk of destruction — the Due is NOT 5 V tolerant, you need a level shifter.
• LED inserted backwards (anode/cathode reversed): nothing lights up, no damage; swap the legs.
• Relying on the 3V3 pin to power a power-hungry module: it is designed for logic, not for actuators; beyond a few tens of mA, use an external power supply.
• Driving a relay or motor directly: you need a transistor/MOSFET with a separate power supply and a flyback diode.