Interfacing HC-SR04 Ultrasonic Sensor with Arduino

What is the HC-SR04?

The HC-SR04 is a low-cost ultrasonic distance sensor capable of measuring distances from 2 cm to 400 cm with an accuracy of about 3 mm. It works by sending out a burst of ultrasonic sound (40 kHz) and measuring how long it takes for the echo to return — the same principle bats use to navigate in the dark.

It runs on 5V, works directly with Arduino, and costs under ₹50 / $1. That makes it one of the most popular sensors in the maker community.

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How It Works

1. Arduino sends a 10µs HIGH pulse to the TRIG pin
2. The sensor fires 8 ultrasonic pulses at 40 kHz
3. Those pulses bounce off the nearest object
4. The ECHO pin goes HIGH for the duration it took the echo to return
5. Arduino measures that duration and converts it to distance

Distance Formula

Distance (cm) = Duration (µs) / 58
Distance (inches) = Duration (µs) / 148

This comes from the speed of sound (~343 m/s at room temperature) divided by 2 (round trip).

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Components Required

Component	Quantity
Arduino Uno	1
HC-SR04 Ultrasonic Sensor	1
Jumper wires	4
Breadboard	1
16x2 LCD (optional)	1
10kΩ potentiometer (for LCD)	1

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HC-SR04 Pin Diagram

The sensor has 4 pins:

Pin	Label	Description
1	VCC	Power — connect to Arduino 5V
2	TRIG	Trigger input — connect to any digital pin
3	ECHO	Echo output — connect to any digital pin
4	GND	Ground — connect to Arduino GND

Note: The ECHO pin outputs 5V logic. On 3.3V boards like ESP32 or Raspberry Pi, use a voltage divider (1kΩ + 2kΩ resistors) on the ECHO line to avoid damaging the GPIO.

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Wiring — Arduino Uno

HC-SR04 Pin	Arduino Pin
VCC	5V
GND	GND
TRIG	Digital Pin 9
ECHO	Digital Pin 10

Connect VCC to the Arduino's 5V rail and GND to GND. TRIG goes to D9 and ECHO to D10. That's it — no resistors needed for Arduino Uno.

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Basic Code — Serial Monitor Output

This sketch measures distance every 500ms and prints it to the Serial Monitor.

// HC-SR04 Ultrasonic Distance Sensor
// Prints distance to Serial Monitor

const int TRIG_PIN = 9;
const int ECHO_PIN = 10;

void setup() {
  Serial.begin(9600);
  pinMode(TRIG_PIN, OUTPUT);
  pinMode(ECHO_PIN, INPUT);
  Serial.println("HC-SR04 Distance Meter");
}

void loop() {
  long duration;
  float distanceCm;
  float distanceInch;

  // Clear the TRIG pin
  digitalWrite(TRIG_PIN, LOW);
  delayMicroseconds(2);

  // Send 10µs HIGH pulse to TRIG
  digitalWrite(TRIG_PIN, HIGH);
  delayMicroseconds(10);
  digitalWrite(TRIG_PIN, LOW);

  // Read ECHO pulse duration (in microseconds)
  duration = pulseIn(ECHO_PIN, HIGH);

  // Convert to distance
  distanceCm   = duration / 58.0;
  distanceInch = duration / 148.0;

  // Print results
  Serial.print("Distance: ");
  Serial.print(distanceCm, 1);
  Serial.print(" cm  /  ");
  Serial.print(distanceInch, 1);
  Serial.println(" inches");

  delay(500);
}

Upload this sketch, open the Serial Monitor at 9600 baud, and bring your hand close to the sensor. You'll see the distance update in real time.

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Code with 16x2 LCD Display

Display the distance directly on an LCD — no computer needed.

Additional Wiring for LCD

LCD Pin	Arduino Pin
VSS	GND
VDD	5V
V0	Potentiometer wiper
RS	Digital Pin 12
RW	GND
E	Digital Pin 11
D4	Digital Pin 5
D5	Digital Pin 4
D6	Digital Pin 3
D7	Digital Pin 2
A (backlight +)	5V
K (backlight −)	GND

Code

// HC-SR04 + 16x2 LCD Distance Meter

#include <LiquidCrystal.h>

// LCD pins: RS, E, D4, D5, D6, D7
LiquidCrystal lcd(12, 11, 5, 4, 3, 2);

const int TRIG_PIN = 9;
const int ECHO_PIN = 10;

void setup() {
  lcd.begin(16, 2);
  pinMode(TRIG_PIN, OUTPUT);
  pinMode(ECHO_PIN, INPUT);

  lcd.setCursor(0, 0);
  lcd.print("  SolderHub.com ");
  lcd.setCursor(0, 1);
  lcd.print(" Distance Meter ");
  delay(2000);
  lcd.clear();
}

void loop() {
  long duration;
  float distanceCm;

  // Trigger pulse
  digitalWrite(TRIG_PIN, LOW);
  delayMicroseconds(2);
  digitalWrite(TRIG_PIN, HIGH);
  delayMicroseconds(10);
  digitalWrite(TRIG_PIN, LOW);

  // Measure echo
  duration = pulseIn(ECHO_PIN, HIGH);
  distanceCm = duration / 58.0;

  // Display
  lcd.setCursor(0, 0);
  lcd.print("Distance:       ");

  lcd.setCursor(0, 1);
  if (distanceCm > 400 || distanceCm <= 0) {
lcd.print("Out of range    ");
  } else {
lcd.print(distanceCm, 1);
lcd.print(" cm          ");
  }

  delay(300);
}

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Adding a Buzzer Alert (Parking Sensor Style)

Add an active buzzer that beeps faster as an object gets closer — exactly like a car parking sensor.

Extra Component

Component	Quantity
Active buzzer	1

Connect buzzer + to Digital Pin 8, − to GND.

#include <LiquidCrystal.h>

LiquidCrystal lcd(12, 11, 5, 4, 3, 2);

const int TRIG_PIN   = 9;
const int ECHO_PIN   = 10;
const int BUZZER_PIN = 8;

void setup() {
  lcd.begin(16, 2);
  pinMode(TRIG_PIN,   OUTPUT);
  pinMode(ECHO_PIN,   INPUT);
  pinMode(BUZZER_PIN, OUTPUT);
}

void loop() {
  // Trigger
  digitalWrite(TRIG_PIN, LOW);
  delayMicroseconds(2);
  digitalWrite(TRIG_PIN, HIGH);
  delayMicroseconds(10);
  digitalWrite(TRIG_PIN, LOW);

  long duration = pulseIn(ECHO_PIN, HIGH);
  float dist = duration / 58.0;

  // LCD display
  lcd.setCursor(0, 0);
  lcd.print("Distance:       ");
  lcd.setCursor(0, 1);

  if (dist > 400 || dist <= 0) {
lcd.print("Out of range    ");
noTone(BUZZER_PIN);
  } else {
lcd.print(dist, 1);
lcd.print(" cm          ");

// Buzzer: beep interval shrinks as object gets closer
if (dist < 10) {
// Continuous beep — very close
tone(BUZZER_PIN, 1000);
delay(100);
} else if (dist < 20) {
tone(BUZZER_PIN, 1000, 80);
delay(150);
} else if (dist < 50) {
tone(BUZZER_PIN, 800, 60);
delay(300);
} else {
noTone(BUZZER_PIN);
delay(100);
}
  }
}

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Understanding pulseIn()

pulseIn(pin, HIGH) waits for the ECHO pin to go HIGH, starts a timer, then stops when it goes LOW — returning the duration in microseconds.

Default timeout is 1 second. If no echo returns within 1 second, pulseIn() returns 0. You can set a custom timeout:

// Timeout after 30ms (good for max 400cm range)
duration = pulseIn(ECHO_PIN, HIGH, 30000);

This prevents the code from hanging when no object is in range.

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Improving Accuracy with Averaging

A single reading can be noisy. Take multiple readings and average them:

float getDistance() {
  long total = 0;
  for (int i = 0; i < 5; i++) {
digitalWrite(TRIG_PIN, LOW);
delayMicroseconds(2);
digitalWrite(TRIG_PIN, HIGH);
delayMicroseconds(10);
digitalWrite(TRIG_PIN, LOW);
total += pulseIn(ECHO_PIN, HIGH, 30000);
delay(10);
  }
  return (total / 5.0) / 58.0;
}

Call getDistance() instead of doing the pulseIn inline — smoother, more stable readings.

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Troubleshooting

Problem	Likely Cause	Fix
Always reads 0	TRIG/ECHO pins swapped	Double-check wiring
Reads out of range constantly	Object too close or too far	Stay between 2–400 cm
Very noisy readings	Power supply noise	Add 100µF capacitor across VCC and GND
Works but jumps around	No averaging	Use the 5-reading average above
ESP32 shows garbage	5V ECHO to 3.3V pin	Add voltage divider on ECHO
pulseIn freezes sketch	No timeout set	Add 30000 timeout parameter

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Extensions to Try

• OLED display — use a 0.96" I2C OLED instead of LCD for a cleaner look
• Multiple sensors — point two HC-SR04s left and right for wider detection
• Water level monitor — mount above a tank pointing down
• Blind spot detector — mount on a vehicle bumper with a buzzer
• Gesture control — detect hand distance to control LEDs or servo position

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Related Projects on SolderHub

• HC-SR04 Parking Alert with Arduino, LCD & Buzzer
• Arduino Mega 2560 Pinout — Complete Reference
• ESP32 Web Server — Control LED from Any Device

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