Ultrasonic distance sensor (HC-SR04) in Arduino - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output distance in cm?

Given this Arduino code snippet using the HC-SR04 sensor, what will be printed to the Serial Monitor if the sensor measures a 20 cm distance?

Arduino

const int trigPin = 9;
const int echoPin = 10;
long duration;
int 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 * 0.034 / 2;

  Serial.println(distance);
  delay(1000);
}

A 10

B 20

C 0

D 40

💡 Hint

Remember the speed of sound is approximately 340 m/s, and the sensor measures the time for the sound to travel to the object and back.

The duration is the time for the sound to travel to the object and back. Dividing by 2 gives the one-way time. Multiplying by 0.034 converts microseconds to centimeters. For a 20 cm distance, the code prints 20.

❓ Predict Output

intermediate

What does this code print if no object is detected?

Consider this Arduino code snippet using HC-SR04. What will be printed if there is no object in front of the sensor (no echo received)?

Arduino

const int trigPin = 9;
const int echoPin = 10;
long duration;
int 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, 30000); // 30 ms timeout
  distance = duration * 0.034 / 2;

  Serial.println(distance);
  delay(1000);
}

A 0

B 510

C -1

D 30000

💡 Hint

Check what pulseIn returns if no pulse is detected within the timeout.

pulseIn returns 0 if no pulse is detected within the timeout. So duration is 0, and distance calculates to 0.

🔧 Debug

advanced

Why does this HC-SR04 code always print 0?

This Arduino code using HC-SR04 always prints 0 for distance, even when an object is close. What is the bug?

Arduino

const int trigPin = 9;
const int echoPin = 10;
long duration;
int distance;

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

void loop() {
  digitalWrite(trigPin, LOW);
  delayMicroseconds(2);
  digitalWrite(trigPin, HIGH);
  delayMicroseconds(10);
  digitalWrite(trigPin, LOW);

  duration = pulseIn(echoPin, HIGH);
  distance = duration * 0.034 / 2;

  Serial.println(distance);
  delay(1000);
}

A The trigPin and echoPin modes are reversed; trigPin should be OUTPUT and echoPin INPUT.

B The delayMicroseconds values are too small to trigger the sensor.

C The pulseIn function is used incorrectly; it needs a timeout parameter.

D The Serial.begin baud rate is missing and causes communication failure.

💡 Hint

Check the pinMode setup for trigPin and echoPin.

The trigPin must be set as OUTPUT to send the trigger pulse. The echoPin must be INPUT to receive the echo signal. The code has these reversed, so no pulse is sent or received, resulting in zero duration.

📝 Syntax

advanced

Which option fixes the syntax error in this HC-SR04 code?

Identify the option that fixes the syntax error in this Arduino code snippet:

distance = duration * 0.034 / 2

Assume the rest of the code is correct.

Arduino

const int trigPin = 9;
const int echoPin = 10;
long duration;
int 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 * 0.034 / 2;

  Serial.println(distance);
  delay(1000);
}

A Remove the entire distance assignment line.

B Change the multiplication operator * to a dot .

C Replace the division operator / with a comma ,

D Add a semicolon at the end of the distance assignment line.

💡 Hint

Check the end of the line for missing punctuation.

In Arduino (C++), each statement must end with a semicolon. The missing semicolon causes a syntax error.

🚀 Application

expert

How to improve distance measurement accuracy with HC-SR04?

You want to improve the accuracy of distance measurements using the HC-SR04 sensor in your Arduino project. Which approach below is the best to reduce noise and get stable readings?

A Change the speed of sound constant from 0.034 to 0.1 in the calculation.

B Increase the trigger pulse duration from 10 microseconds to 100 microseconds.

C Take multiple distance readings in a short loop, then calculate and use the median value.

D Use delay(5000) between readings to let the sensor cool down.

💡 Hint

Think about how to handle fluctuating sensor readings.

Taking multiple readings and using the median reduces the effect of outliers and noise, improving accuracy. Increasing pulse duration or delay won't help accuracy, and changing the speed of sound constant arbitrarily causes wrong results.