Ultrasonic distance sensor (HC-SR04) in Arduino - Step-by-Step Execution

Concept Flow - Ultrasonic distance sensor (HC-SR04)

Start

↓

Send Trigger Pulse

↓

Wait for Echo Signal

↓

Measure Echo Duration

↓

Calculate Distance

↓

Output Distance

↓

Repeat or Stop

The sensor sends a sound pulse, waits for the echo, measures the time, then calculates and outputs the distance.

Execution Sample

Arduino

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

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

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(500);
}

This code triggers the sensor, measures echo time, calculates distance in cm, and prints it every half second.

Execution Table

Step	Action	Pin State	Duration (microseconds)	Distance (cm)	Output
1	Set trigPin LOW	trigPin=LOW
2	Wait 2 microseconds	trigPin=LOW
3	Set trigPin HIGH	trigPin=HIGH
4	Wait 10 microseconds	trigPin=HIGH
5	Set trigPin LOW	trigPin=LOW
6	Measure pulse on echoPin	echoPin=HIGH during pulse	600
7	Calculate distance		600	10
8	Print distance			10	10
9	Wait 500 ms
10	Repeat loop

💡 Loop repeats indefinitely to update distance continuously.

Variable Tracker

Variable	Start	After Step 6	After Step 7	After Step 8	Final
duration	0	600	600	600	600
distance	0	0	10	10	10

Key Moments - 3 Insights

Why do we set trigPin LOW before sending the HIGH pulse?

Setting trigPin LOW first ensures a clean pulse start; see execution_table rows 1-3 where LOW then HIGH creates a clear 10 microsecond pulse.

How does pulseIn measure the echo duration?

pulseIn waits for echoPin to go HIGH and measures how long it stays HIGH, shown in execution_table row 6 with duration 600 microseconds.

Why do we divide duration by 2 when calculating distance?

Sound travels to the object and back, so dividing by 2 gives the one-way distance; this is shown in the calculation at step 7.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'duration' after step 6?

A 10.2 cm

B 600 microseconds

C 500 milliseconds

D 0

💡 Hint

Check the 'Duration (microseconds)' column at step 6 in execution_table.

At which step is the trigPin set HIGH to send the ultrasonic pulse?

A Step 2

B Step 5

C Step 3

D Step 6

💡 Hint

Look at the 'Pin State' column in execution_table for when trigPin changes to HIGH.

If the echo duration doubles, how does the distance value change in variable_tracker?

A Distance doubles

B Distance halves

C Distance stays the same

D Distance becomes zero

💡 Hint

Distance is calculated from duration * 0.034 / 2, so distance changes proportionally with duration (see variable_tracker).

Concept Snapshot

HC-SR04 sensor sends a 10μs HIGH pulse on trigPin.
It listens on echoPin for the returning pulse.
pulseIn measures echo duration in microseconds.
Distance (cm) = duration * 0.034 / 2.
Repeat measurement to update distance continuously.

Full Transcript

This visual trace shows how the HC-SR04 ultrasonic sensor works in Arduino code. First, the trigPin is set LOW to prepare, then HIGH for 10 microseconds to send a sound pulse. The echoPin listens for the returning pulse, and pulseIn measures how long the echoPin stays HIGH, which is the echo duration. The code calculates distance by multiplying duration by 0.034 (speed of sound in cm/us) and dividing by 2 because the sound travels to the object and back. The distance is printed and the loop repeats every 500 milliseconds to update the reading continuously. Variables duration and distance change step-by-step as shown. Key points include why trigPin is set LOW before HIGH, how pulseIn works, and why dividing by 2 is necessary. The quizzes test understanding of these steps and variable changes.