STM32 HC-SR04 Ultrasonic Sensor with LED Indicator

English Description

This project demonstrates how to use the HC-SR04 ultrasonic distance sensor with an STM32 NUCLEO-F103RB board.
When the measured distance is less than 10 cm, the onboard LED (PA5) turns ON; otherwise it remains OFF.
Distance values are sent over USART2 (115200 baud) and can be monitored with PuTTY.

Pin Map

• PA9 (D8) → HC-SR04 Trig
• PA8 (D7) → HC-SR04 Echo (via voltage divider: 1 kΩ (top), 2 kΩ (bottom))
• 5V → HC-SR04 VCC
• GND → HC-SR04 GND
• PA5 (LD2) → Onboard LED

Circuit Connection

• Trig is driven directly by PA9 with a 10–15 µs HIGH pulse.
• Echo is 5V level; use a 1 kΩ / 2 kΩ divider to bring it to ~3.33 V at PA8.
• Sensor VCC to 5V, GND to common GND (board/sensor/divider bottom all tied).

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한국어 설명

이 프로젝트는 STM32 NUCLEO-F103RB와 HC-SR04 초음파 센서를 이용해 거리를 측정하는 예제입니다.
측정 거리가 10 cm 미만이면 보드 LED(PA5)가 켜지고, 아니면 꺼집니다.
거리값은 USART2 (115200 baud) 로 전송되며 PuTTY 등 터미널로 확인할 수 있습니다.

핀맵 설정

• PA9 (D8) → HC-SR04 Trig
• PA8 (D7) → HC-SR04 Echo (전압 분배기: 1 kΩ 상단, 2 kΩ 하단)
• 5V → HC-SR04 VCC
• GND → HC-SR04 GND
• PA5 (LD2) → 보드 LED

회로 연결

• Trig는 PA9에서 10–15 µs HIGH 펄스를 직접 출력합니다.
• Echo는 5V이므로 1 kΩ / 2 kΩ 분압기를 통해 약 3.33 V로 낮춰 PA8에 입력합니다.
• 센서의 VCC는 5V, GND는 공통 GND로 연결합니다.

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Demo Video

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Core Code

// 1µs timer (TIM2 prescaler set for 1 MHz)
static inline uint32_t micros(void) { return __HAL_TIM_GET_COUNTER(&htim2); }

// Trigger pulse (10–15 µs HIGH)
static void HCSR04_Trigger(void) {
  HAL_GPIO_WritePin(GPIOA, GPIO_PIN_9, GPIO_PIN_SET);
  uint32_t t0 = micros();
  while ((uint32_t)(micros() - t0) < 12);
  HAL_GPIO_WritePin(GPIOA, GPIO_PIN_9, GPIO_PIN_RESET);
}

// EXTI callback for PA8 (Echo): rising = start, falling = stop
void HAL_GPIO_EXTI_Callback(uint16_t GPIO_Pin) {
  if (GPIO_Pin == GPIO_PIN_8) {
    if (HAL_GPIO_ReadPin(GPIOA, GPIO_PIN_8) == GPIO_PIN_SET) {
      echo_rise_us = micros();   // rising edge
      echo_done = 0;
    } else {
      echo_fall_us = micros();   // falling edge
      echo_done = 1;
    }
  }
}

// Main loop snippet
__HAL_TIM_SET_COUNTER(&htim2, 0);
HCSR04_Trigger();
uint32_t wait_start = micros();
while (!echo_done && (uint32_t)(micros() - wait_start) < 40000) { /* wait up to 40 ms */ }

float d = -1.0f;
if (echo_done) {
  uint32_t pulse = (uint32_t)(echo_fall_us - echo_rise_us); // µs
  if (pulse > 250 && pulse < 35000) d = (pulse * 0.0343f) * 0.5f; // cm
  else d = -2.0f;
} else d = -1.0f;

// UART print & LED (10 cm threshold)
int n = snprintf(msg, sizeof(msg),
                 (d >= 0.0f) ? "Distance: %.2f cm\r\n" : "Distance: out of range (%.0f)\r\n", d);
HAL_UART_Transmit(&huart2, (uint8_t*)msg, n, 50);
HAL_GPIO_WritePin(GPIOA, GPIO_PIN_5, (d > 0 && d < 10.0f) ? GPIO_PIN_SET : GPIO_PIN_RESET);