Blinking an [LED](https://www.onzuu.com/category/led-emitters-infrared-uv-visible) is the “hello world” of [microcontrollers](https://www.ampheo.com/c/microcontrollers): configure a GPIO pin as an output, then toggle it HIGH/LOW with a delay.


**Wiring (don’t skip this)**

* MCU GPIO pin → 220–1kΩ [resistor](https://www.onzuu.com/category/resistors) → LED anode (long leg)
* LED cathode (short leg/flat side) → GND

(Or wire LED to VCC and have the pin sink current—either works.)

**Generic pseudocode**

init_gpio_output(LED_PIN);

while (1) {
  set_pin_high(LED_PIN);
  delay_ms(500);
  set_pin_low(LED_PIN);
  delay_ms(500);
}

**[Arduino](https://www.ampheo.com/c/development-board-arduino) (Uno/Nano)**

const int LED_PIN = 13;

void setup() {
  pinMode(LED_PIN, OUTPUT);
}

void loop() {
  digitalWrite(LED_PIN, HIGH);
  delay(500);
  digitalWrite(LED_PIN, LOW);
  delay(500);
}

**[STM32](https://www.ampheo.com/search/STM32) (HAL / CubeIDE) example**

Assume your LED is on PA5 (common on [Nucleo boards](https://www.onzuu.com/search/Nucleo)):

// after MX_GPIO_Init() has set PA5 as Output Push-Pull
while (1)
{
  HAL_GPIO_TogglePin(GPIOA, GPIO_PIN_5);
  HAL_Delay(500);
}

In CubeMX, set PA5 → GPIO_Output and generate code.

**Bare-metal AVR example ([ATmega328P](https://www.ampheo.com/search/ATmega328P))**

Blink on PB5 ([Arduino Uno](https://www.ampheo.com/product/a000046-25542493) LED):

#include <avr/io.h>
#include <util/delay.h>

int main(void) {
  DDRB |= (1 << DDB5);        // PB5 output

  while (1) {
    PORTB ^= (1 << PORTB5);   // toggle
    _delay_ms(500);
  }
}

**Common mistakes**

* No resistor → LED can burn out or overload the pin.
* Wrong polarity (LED reversed).
* Pin not actually configured as output.
* Using the wrong pin name/port for your board.