HC-SR04 Ultrasonic
Distance Sensor Guide

// HC-SR04 Ultrasonic Distance Sensor
// Tishvi Docs · Wiring: VCC→5V | GND→GND | Trig→9 | Echo→10

const int trigPin = 9;
const int echoPin = 10;

long  duration;    // echo pulse width in microseconds
float distance;    // calculated distance in centimetres

void setup() {
  pinMode(trigPin, OUTPUT);
  pinMode(echoPin, INPUT);
  Serial.begin(9600);
}

void loop() {
  // 1. Send a 10µs trigger pulse
  digitalWrite(trigPin, LOW);
  delayMicroseconds(2);

  digitalWrite(trigPin, HIGH);
  delayMicroseconds(10);   // 10µs pulse
  digitalWrite(trigPin, LOW);

  // 2. Measure how long the Echo pin stays HIGH
  duration = pulseIn(echoPin, HIGH);

  // 3. Convert to centimetres
  // Speed of sound = 343 m/s = 0.0343 cm/µs
  // Distance = (duration / 2) * 0.0343  →  simplifies to duration / 58.2
  distance = duration / 58.2;

  // 4. Print to Serial Monitor
  Serial.print("Distance: ");
  Serial.print(distance, 1);   // 1 decimal place
  Serial.println(" cm");

  delay(200);  // read 5× per second
}

// HC-SR04 Proximity Alert
// LED on pin 13 (built-in) lights when object < 20cm

const int trigPin     = 9;
const int echoPin     = 10;
const int ledPin      = 13;    // built-in LED
const int alertRange  = 20;   // alert if closer than 20cm

void setup() {
  pinMode(trigPin, OUTPUT);
  pinMode(echoPin, INPUT);
  pinMode(ledPin,  OUTPUT);
  Serial.begin(9600);
}

void loop() {
  float distance = measureDistance();

  Serial.print("Distance: ");
  Serial.print(distance, 1);
  Serial.print(" cm  →  ");

  if (distance < alertRange && distance > 2) {
    digitalWrite(ledPin, HIGH);  // LED ON
    Serial.println("⚠ TOO CLOSE!");
  } else {
    digitalWrite(ledPin, LOW);   // LED off
    Serial.println("OK");
  }

  delay(100);  // 10× per second
}

float measureDistance() {
  digitalWrite(trigPin, LOW);
  delayMicroseconds(2);
  digitalWrite(trigPin, HIGH);
  delayMicroseconds(10);
  digitalWrite(trigPin, LOW);
  long dur = pulseIn(echoPin, HIGH);
  return dur / 58.2;
}