Reading Sensors - Distance Sensor

Understanding Sensors

Sensors are devices that convert physical quantities into electrical signals. Arduino can read these signals and use them in your code.

HC-SR04 Ultrasonic Distance Sensor

The HC-SR04 is a popular, inexpensive distance sensor that uses ultrasonic sound waves to measure distance.

How It Works

1. Send a 10μs pulse on Trigger pin 2. Sensor emits 8 cycles of ultrasonic burst 3. Echo pin goes HIGH for time sound travels 4. Calculate distance: distance = (time × speed of sound) / 2

Specifications

Connecting the Sensor

Pin Connections

Circuit Diagram

Arduino          HC-SR04
5V      ------  VCC
Pin 9   ------  Trig
Pin 10  ------  Echo
GND     ------  GND

Reading Distance

Basic Code

const int trigPin = 9;
const int echoPin = 10;
void setup() {
  pinMode(trigPin, OUTPUT);
  pinMode(echoPin, INPUT);
  Serial.begin(9600);
}
void loop() {
  // Send ultrasonic pulse
  digitalWrite(trigPin, LOW);
  delayMicroseconds(2);
  digitalWrite(trigPin, HIGH);
  delayMicroseconds(10);
  digitalWrite(trigPin, LOW);
  
  // Measure echo time
  long duration = pulseIn(echoPin, HIGH);
  
  // Calculate distance (in cm)
  int distance = duration / 29 / 2;
  
  Serial.print("Distance: ");
  Serial.print(distance);
  Serial.println(" cm");
  
  delay(500);
}

Using pulseIn()

pulseIn measures the duration of a pulse:

long duration = pulseIn(echoPin, HIGH);

This returns the time in microseconds.

Creating a Function

For cleaner code, create a function:

const int trigPin = 9;
const int echoPin = 10;
int getDistance() {
  digitalWrite(trigPin, LOW);
  delayMicroseconds(2);
  digitalWrite(trigPin, HIGH);
  delayMicroseconds(10);
  digitalWrite(trigPin, LOW);
  
  long duration = pulseIn(echoPin, HIGH);
  return duration / 29 / 2;
}
void setup() {
  pinMode(trigPin, OUTPUT);
  pinMode(echoPin, INPUT);
  Serial.begin(9600);
}
void loop() {
  int distance = getDistance();
  Serial.print("Distance: ");
  Serial.print(distance);
  Serial.println(" cm");
  delay(500);
}

Practical Project: Distance Indicator

Create an LED that changes based on distance:

const int trigPin = 9;
const int echoPin = 10;
const int greenLed = 7;
const int yellowLed = 6;
const int redLed = 5;
void setup() {
  pinMode(trigPin, OUTPUT);
  pinMode(echoPin, INPUT);
  pinMode(greenLed, OUTPUT);
  pinMode(yellowLed, OUTPUT);
  pinMode(redLed, OUTPUT);
}
int getDistance() {
  digitalWrite(trigPin, LOW);
  delayMicroseconds(2);
  digitalWrite(trigPin, HIGH);
  delayMicroseconds(10);
  digitalWrite(trigPin, LOW);
  return pulseIn(echoPin, HIGH) / 29 / 2;
}
void loop() {
  int distance = getDistance();
  
  // Turn off all LEDs first
  digitalWrite(greenLed, LOW);
  digitalWrite(yellowLed, LOW);
  digitalWrite(redLed, LOW);
  
  // Show distance with LEDs
  if (distance < 10) {
    digitalWrite(redLed, HIGH);  // Very close
  } else if (distance < 30) {
    digitalWrite(yellowLed, HIGH);  // Medium
  } else {
    digitalWrite(greenLed, HIGH);  // Far
  }
  
  delay(200);
}

Other Popular Sensors

Temperature Sensor (TMP36)

int sensorPin = A0;
void setup() {
  Serial.begin(9600);
}
void loop() {
  int reading = analogRead(sensorPin);
  float voltage = reading * 5.0 / 1024.0;
  float temperatureC = (voltage - 0.5) * 100;
  
  Serial.print("Temperature: ");
  Serial.print(temperatureC);
  Serial.println(" C");
  delay(1000);
}

Light Sensor (LDR)

int ldrPin = A0;
void setup() {
  Serial.begin(9600);
}
void loop() {
  int lightLevel = analogRead(ldrPin);
  Serial.print("Light: ");
  Serial.println(lightLevel);
  delay(500);
}

Motion Sensor (PIR)

int pirPin = 2;
int ledPin = 13;
void setup() {
  pinMode(pirPin, INPUT);
  pinMode(ledPin, OUTPUT);
}
void loop() {
  if (digitalRead(pirPin) == HIGH) {
    digitalWrite(ledPin, HIGH);
  } else {
    digitalWrite(ledPin, LOW);
  }
}

Summary

You've learned to read sensors with Arduino:

• HC-SR04 measures distance using ultrasound
• pulseIn measures pulse duration
• analogRead reads analog sensors
• digitalRead reads digital sensors

Next Lesson

In the next lesson, you'll learn to control servo motors.