HC-SR04 Ultrasonic Distance Sensor

1. Introduction

The HC-SR04 is one of the most popular distance sensors for hobbyists and makers. It uses ultrasonic sound waves (40kHz) to measure distances from 2cm to 400cm without physical contact.

The sensor works like a bat's echolocation: it emits an ultrasonic pulse, waits for the echo, and calculates distance based on the time it took for the sound to return.

2. Technical Specifications

How It Works

1. Send a 10µs HIGH pulse to the TRIG pin
2. Sensor emits 8 ultrasonic bursts at 40kHz
3. ECHO pin goes HIGH and stays HIGH until echo returns
4. Measure ECHO pulse width to calculate distance

3. Wiring Guide

*ESP32 is 3.3V - use a voltage divider on ECHO pin or use 3.3V-compatible ultrasonic sensor

Figure 1: HC-SR04 wiring with Arduino Uno

4. Project: Distance Meter

Complete Code

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

void setup() {
    Serial.begin(115200);
    pinMode(trigPin, OUTPUT);
    pinMode(echoPin, INPUT);
    Serial.println("HC-SR04 Distance Meter");
}

void loop() {
    // Clear trigger pin
    digitalWrite(trigPin, LOW);
    delayMicroseconds(2);

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

    // Measure echo pulse duration
    long duration = pulseIn(echoPin, HIGH);

    // Calculate distance
    float distanceCm = duration * 0.0343 / 2;

    // Validate reading
    if (duration == 0 || distanceCm > 400) {
        Serial.println("Out of range!");
    } else {
        Serial.print("Distance: ");
        Serial.print(distanceCm, 1);
        Serial.println(" cm");
    }

    delay(100);
}

Median Filter for Stable Readings

float getMedianDistance(int samples) {
    float readings[samples];
    
    for (int i = 0; i < samples; i++) {
        digitalWrite(trigPin, LOW);
        delayMicroseconds(2);
        digitalWrite(trigPin, HIGH);
        delayMicroseconds(10);
        digitalWrite(trigPin, LOW);
        
        long duration = pulseIn(echoPin, HIGH);
        readings[i] = duration * 0.0343 / 2;
        delay(10);
    }
    
    // Simple bubble sort
    for (int i = 0; i < samples-1; i++) {
        for (int j = 0; j < samples-i-1; j++) {
            if (readings[j] > readings[j+1]) {
                float temp = readings[j];
                readings[j] = readings[j+1];
                readings[j+1] = temp;
            }
        }
    }
    
    return readings[samples/2];  // Return median
}

5. Applications

• Obstacle avoidance: Robots and autonomous vehicles
• Parking sensors: Reverse distance warning
• Liquid level: Non-contact tank level monitoring
• Proximity detection: Automatic doors, hand sanitizers
• Mapping: Simple LIDAR-like scanning with servo

6. Troubleshooting

Always reads 0 or timeout

• Check VCC is connected to 5V (not 3.3V)
• Verify TRIG and ECHO pin connections

Inconsistent readings

• Use median filter (see code above)
• Avoid soft or angled surfaces
• Add delay between measurements

Maximum range not achieved

• Target may be too small or angled
• Ensure clear line of sight

7. Summary

• Ultrasonic distance sensor (2-400cm range)
• Requires 5V power supply
• Distance = (pulse duration × 0.0343) / 2
• Use median filter for stable readings
• Not suitable for sound-absorbing materials