Arduino Hands-On Programming and Robotics Course

📘 MODULE 5 – Ultrasonic Distance Meter

Lesson 5.3 – Distance Measurement Code

Learning Objectives

By the end of this lesson, students will be able to:

Understand how Arduino measures distance using an ultrasonic sensor.
Learn the purpose of each line in the distance measurement program.
Generate ultrasonic trigger pulses from Arduino.
Receive and measure echo signals from the sensor.
Calculate distance using Arduino code.
Display measured distance on the Serial Monitor.
Upload and test the distance measurement program successfully.

Main Content

Introduction

In the previous lesson, we connected the HC-SR04 ultrasonic sensor with Arduino UNO. The hardware setup is now ready.

The next step is to write the Arduino program that allows the sensor to measure distance.

The ultrasonic sensor works by:

Sending an ultrasonic sound wave.
Waiting for the reflected wave to return.
Measuring the travel time.
Calculating the distance from the travel time.

Arduino performs all these tasks using a simple program.

Complete Distance Measurement Code

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

long duration;
float distance;

void setup()
{
Serial.begin(9600);

pinMode(trigPin, OUTPUT);
pinMode(echoPin, INPUT);
}

void loop()
{
digitalWrite(trigPin, LOW);
delayMicroseconds(2);

digitalWrite(trigPin, HIGH);
delayMicroseconds(10);

digitalWrite(trigPin, LOW);

duration = pulseIn(echoPin, HIGH);

distance = duration * 0.0343 / 2;

Serial.print("Distance: ");
Serial.print(distance);
Serial.println(" cm");

delay(500);
}

Understanding the Program

Let us understand the code section by section.

Step 1: Define Pin Numbers

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

Explanation

The ultrasonic sensor has two important pins:

Trigger Pin (TRIG)
Echo Pin (ECHO)

We connect:

TRIG → Arduino Pin 9
ECHO → Arduino Pin 10

These lines store pin numbers in variables.

Step 2: Create Variables

long duration;
float distance;

duration

Stores the time taken by the sound wave to travel.

Example:

duration = 1500;

means the sound wave took 1500 microseconds.

distance

Stores the calculated distance.

Example:

distance = 25.5;

means the object is 25.5 cm away.

Step 3: Setup Function

void setup()
{
Serial.begin(9600);

pinMode(trigPin, OUTPUT);
pinMode(echoPin, INPUT);
}

Serial.begin(9600)

Starts communication between Arduino and the computer.

Serial.begin(9600);

9600 is the communication speed.

pinMode()

pinMode(trigPin, OUTPUT);

TRIG sends signals.

Therefore it is OUTPUT.

pinMode(echoPin, INPUT);

ECHO receives signals.

Therefore it is INPUT.

Step 4: Send Trigger Pulse

digitalWrite(trigPin, LOW);
delayMicroseconds(2);

First, make TRIG pin LOW.

This ensures a clean pulse.

digitalWrite(trigPin, HIGH);
delayMicroseconds(10);

TRIG stays HIGH for 10 microseconds.

This tells the sensor:

“Send an ultrasonic sound wave now.”

After that:

digitalWrite(trigPin, LOW);

The pulse is completed.

The sensor immediately transmits ultrasonic waves.

Step 5: Read Echo Signal

duration = pulseIn(echoPin, HIGH);

pulseIn()

This function measures how long a pin stays HIGH.

Arduino waits until:

Echo pin becomes HIGH.
Echo pin becomes LOW again.
Time is measured.

The measured value is stored in microseconds.

Example:

duration = 2000;

means sound traveled for 2000 microseconds.

Step 6: Calculate Distance

distance = duration * 0.0343 / 2;

This is the most important line of the program.

The speed of sound is approximately:

0.0343 cm per microsecond

Distance traveled:

Distance = Time × Speed

However, the sound travels:

From sensor to object
Object back to sensor

So total distance is double.

Therefore:

Actual Distance =
(Time × Speed) ÷ 2

Arduino performs this calculation automatically.

Distance Formula

$d=t×0.03432d=\frac{t\times 0.0343}{2}$

Where:

d = Distance in centimeters
t = Time in microseconds

Step 7: Display Distance

Serial.print("Distance: ");

Prints text.

Output:

Distance:

Serial.print(distance);

Prints the measured value.

Example:

Distance: 23.4

Serial.println(" cm");

Prints unit and moves to next line.

Final output:

Distance: 23.4 cm

Step 8: Delay

delay(500);

Waits 500 milliseconds.

This prevents the Serial Monitor from updating too fast.

Program Flow

The Arduino continuously repeats:

Start

↓
Send Trigger Pulse

↓
Sensor Sends Ultrasonic Wave

↓
Wave Hits Object

↓
Wave Returns

↓
Read Echo Time

↓
Calculate Distance

↓
Display Distance

↓
Repeat

Example Calculation

Suppose:

Duration = 1000 microseconds

Using the formula:

Distance = 1000 × 0.0343 ÷ 2

Distance = 17.15 cm

Arduino automatically performs this calculation.

Expected Serial Monitor Output

If objects are moved in front of the sensor:

Distance: 10 cm
Distance: 15 cm
Distance: 22 cm
Distance: 35 cm
Distance: 50 cm

Values continuously change according to object position.

Common Beginner Mistakes

Wrong Pin Numbers

Incorrect:

const int trigPin = 8;
const int echoPin = 9;

while wiring is connected to Pins 9 and 10.

Always match code with wiring.

Missing Serial.begin()

Without:

Serial.begin(9600);

nothing appears on the Serial Monitor.

Incorrect Trigger Pulse

The trigger pulse must be:

HIGH for 10 microseconds

Otherwise measurements may fail.

Loose Wiring

Loose connections can cause:

Random readings
0 cm readings
No readings

Always verify connections.

Real-World Examples

Automatic Parking System

Measures vehicle distance from walls or barriers.

Water Tank Monitoring

Measures water level inside tanks.

Obstacle Detection Robot

Detects nearby objects and avoids collisions.

Smart Dustbin

Detects hand movement and opens the lid automatically.

Security Systems

Detects movement near protected areas.

Best Practices

Keep Sensor Stable

Do not shake the sensor while measuring.

Use Proper Power Supply

Provide stable 5V power to the sensor.

Avoid Soft Surfaces

Soft materials absorb sound waves and may produce inaccurate readings.

Test Multiple Distances

Verify readings at:

10 cm
20 cm
30 cm
50 cm

to ensure proper operation.

Organize Wiring

Use short and secure jumper wires for reliable results.

Summary

In this lesson, we learned how Arduino measures distance using the HC-SR04 ultrasonic sensor. The Arduino sends a trigger pulse, receives the echo signal, measures travel time, and calculates distance using the speed of sound formula. The measured distance is then displayed on the Serial Monitor. This program forms the foundation for many distance-based Arduino projects such as robots, parking systems, smart bins, and automation systems.

Key Terms

Ultrasonic Sensor
HC-SR04
Trigger Pin
Echo Pin
pulseIn()
Microsecond
Distance Measurement
Serial Monitor
Speed of Sound
Trigger Pulse

Quiz

1. Which Arduino function measures the echo pulse duration?

A. digitalRead()
B. pulseIn()
C. analogRead()
D. delay()

Answer: B

2. Which pin sends ultrasonic waves?

A. Echo Pin
B. GND
C. Trigger Pin
D. VCC

Answer: C

3. Why is the calculated distance divided by 2?

A. To reduce error

B. Sound travels to the object and back

C. Sensor limitation

D. To convert units

Answer: B

4. What is the typical trigger pulse duration?

A. 1 ms

B. 100 ms

C. 10 microseconds

D. 1 second

Answer: C

5. Which function starts serial communication?

A. Serial.print()

B. Serial.begin()

C. Serial.read()

D. Serial.write()

Answer: B

Assignment

Practical Task

Build the ultrasonic sensor circuit and upload the distance measurement code.

Perform the following tests:

Place an object at approximately 10 cm.
Record the displayed distance.
Repeat for 20 cm.
Repeat for 30 cm.
Repeat for 50 cm.

Create a table containing:

Actual Distance	Measured Distance
10 cm	____
20 cm	____
30 cm	____
50 cm	____

Compare the readings and note any differences.