08_example_2.ino

// Arduino pin assignment
#define PIN_LED   9
#define PIN_TRIG 12   // sonar sensor TRIGGER
#define PIN_ECHO 13   // sonar sensor ECHO

// configurable parameters
#define SND_VEL 346.0     // sound velocity at 24 celsius degree (unit: m/sec)
#define INTERVAL 25       // sampling interval (unit: msec)
#define PULSE_DURATION 10 // ultra-sound Pulse Duration (unit: usec)
#define _DIST_MIN 100.0   // minimum distance to be measured (unit: mm)
#define _DIST_MAX 300.0   // maximum distance to be measured (unit: mm)

#define TIMEOUT ((INTERVAL / 2) * 1000.0) // maximum echo waiting time (unit: usec)
#define SCALE (0.001 * 0.5 * SND_VEL)     // coefficent to convert duration to distance

unsigned long last_sampling_time;   // unit: msec

void setup() {
  // initialize GPIO pins
  pinMode(PIN_LED, OUTPUT);
  pinMode(PIN_TRIG, OUTPUT);  // sonar TRIGGER
  pinMode(PIN_ECHO, INPUT);   // sonar ECHO
  digitalWrite(PIN_TRIG, LOW);  // turn-off Sonar 
  
  // initialize serial port
  Serial.begin(57600);
}

void loop() { 
  float distance;

  // wait until next sampling time
  if (millis() < (last_sampling_time + INTERVAL))
    return;
  last_sampling_time = millis();   // update time

  // measure distance
  distance = USS_measure(PIN_TRIG, PIN_ECHO);

  int duty; // LED brightness (0 = max bright, 255 = off)

  if ((distance == 0.0) || (distance > _DIST_MAX)) {
      distance = _DIST_MAX + 10.0; 
      duty = 255;  // LED OFF
  } else if (distance < _DIST_MIN) {
      distance = _DIST_MIN - 10.0;
      duty = 255;  // LED OFF
  } else {
      // Brightness control
      if (distance <= 100 || distance >= 300) {
          duty = 255; // minimum brightness (off)
      } else if (distance == 200) {
          duty = 0;   // maximum brightness
      } else if (distance == 150 || distance == 250) {
          duty = 128; // 50% brightness
      } else {
          // Smooth brightness control using map()
          if (distance < 200) {
              duty = map(distance, 100, 200, 255, 0); // brighter as distance → 200
          } else {
              duty = map(distance, 200, 300, 0, 255); // dimmer as distance → 300
          }
      }
  }

  // apply brightness to LED
  analogWrite(PIN_LED, duty);

  // output distance + duty cycle for debugging
  Serial.print("Min:");        Serial.print(_DIST_MIN);
  Serial.print(", distance:"); Serial.print(distance);
  Serial.print(", Max:");      Serial.print(_DIST_MAX);
  Serial.print(", duty:");     Serial.println(duty);
}

/* =====================================================
   Function to measure distance using Ultrasonic Sensor
   ===================================================== */
float USS_measure(int pin_trig, int pin_echo) {
  unsigned long duration;

  // Generate trigger pulse
  digitalWrite(pin_trig, HIGH);
  delayMicroseconds(PULSE_DURATION);
  digitalWrite(pin_trig, LOW);

  // Wait for echo pulse
  duration = pulseIn(pin_echo, HIGH, TIMEOUT);

  // convert duration (usec) to distance (mm)
  float distance = duration * SCALE;
  return distance;
}