Lesson 4: Ultrasonic Distance

Measure distance with sound. The HC-SR04 times an echo, the UNO turns it into centimetres, and the Serial Plotter draws it as a live graph.

One of 7 projects in the Arduino UNO Sensor & Output Starter Kit - LEDs, Buzzer, Servo, Ultras… — the parts below are from that kit.

Before you start

What you build and learn

• How sonar works: sound out, echo back
• Timing a pulse with pulseIn()
• Why the middle (median) of 5 readings is steadier
• Drawing live graphs with the Serial Plotter

New words

Ultrasound

Sound too high for people to hear. This sensor uses 40 kHz.

Echo

Sound that bounces back off an object.

Microsecond (µs)

One millionth of a second.

Median

The middle value after sorting a list. One wild number cannot move it much.

Serial Plotter

Tools → Serial Plotter: draws the numbers the UNO sends as moving lines.

What this project uses

Every item is in the kit — nothing extra to buy.

UNO R3 UNO R3 Development Board ATmega328P Microcontroller with USB Cable, 5V Logic, 14 Digital I/O, 6 Analog Inputs… UNOR3

In the kit

HC-SR04 HC-SR04 Ultrasonic Distance Sensor Module for Arduino, Raspberry Pi & DIY Robotics Projects, 5V Trigger/Echo… DIY-00043

In the kit

breadboard 400 Tie Point Solderless Breadboard - Half Size Prototyping Circuit Board with Power Rails, for Arduino & Ele… DIY-238

In the kit

jumper wires Jumper Wire Assorted Pack 120 Pieces - Male-to-Male, Male-to-Female, Female-to-Female 10cm Ribbon Cables for… DIY-212

In the kit

Build this with

Arduino UNO Sensor & Output Starter Kit - LEDs, Buzzer, Servo, Ultrasonic & LCD

All 10 parts in one box, with this project and 6 more to build from them.

AED 85.00 See the kit Add kit to cart

Need only these parts? Order them loose · from AED 78.30 ▾

Used in this project tick what you still need

UNO R3 UNO R3 Development Board ATmega328P Microcontroller with USB Cable, 5V Logic, 14 Digital I/O, 6 Analog Inputs… UNOR3 · ×1

AED 26.39

In stock

HC-SR04 HC-SR04 Ultrasonic Distance Sensor Module for Arduino, Raspberry Pi & DIY Robotics Projects, 5V Trigger/Echo… DIY-00043 · ×1

AED 15.31

In stock

breadboard 400 Tie Point Solderless Breadboard - Half Size Prototyping Circuit Board with Power Rails, for Arduino & Ele… DIY-238 · ×1

AED 11.58

In stock

jumper wires Jumper Wire Assorted Pack 120 Pieces - Male-to-Male, Male-to-Female, Female-to-Female 10cm Ribbon Cables for… DIY-212 · ×1

AED 25.02

In stock

Selected parts AED 78.30

Add 4 parts to cart

One of each part. Packs (resistor assortment, jumper wires) already hold more than a single build needs. Delivery is AED 20, free on orders AED 100+.

Connections

Unplug USB before you change any wire. Pin names are the labels printed on the board.

Step by step

One thing at a time. Tick nothing off until it matches the diagram.

1. Unplug the USB cable.Watch out: Always unplug the USB cable before you add, move or remove a wire.
2. Push the sensor into the breadboard with its two eyes facing out over the edge.
3. Wire VCC to the + rail, TRIG to UNO pin 8, ECHO to UNO pin 9 and GND to the − rail.Watch out: Read the pin names on the sensor: VCC and GND must not be swapped.
4. Plug in USB and upload the Lesson 4 code.
5. Open Tools → Serial Plotter and set it to 9600 baud.
6. Hold a book flat in front of the sensor. Move it slowly between 5 cm and 1 m and check a few readings with a ruler.

Sketch for this project

Written for the board in the kit. Copy it into the Arduino IDE; change the pin numbers if you wire it differently.

/*
  DIY KIT - Arduino UNO Sensor & Output Starter Kit
  Lesson 4 of 7: Ultrasonic Distance

  WIRING (unplug the USB cable before you wire)
    HC-SR04 VCC  -> 5V
    HC-SR04 TRIG -> UNO pin 8
    HC-SR04 ECHO -> UNO pin 9
    HC-SR04 GND  -> GND
    Parts from Lessons 1-3 can stay where they are.

  WHAT SHOULD HAPPEN
    Open Tools > Serial Plotter (9600 baud). Move a book slowly towards and
    away from the sensor. Two lines follow it: raw_cm (jumpy) and
    median_cm (smoother). 0 means "no echo" - nothing in range.
*/

const int TRIG_PIN = 8;   // we send the "click" from here
const int ECHO_PIN = 9;   // the sensor answers on this pin

const unsigned long ECHO_TIMEOUT_US = 30000;  // give up after 30 ms (about 5 m there and back)
const int MIN_CM = 2;      // closer than this the sensor cannot measure
const int MAX_CM = 400;    // further than this is out of range
const int SAMPLES = 5;     // readings in each median - try 1 or 9
const unsigned long REPORT_MS = 250;  // a new line 4 times a second

// One measurement in centimetres, or 0 if no echo came back.
long readOnceCm() {
  digitalWrite(TRIG_PIN, LOW);
  delayMicroseconds(2);
  digitalWrite(TRIG_PIN, HIGH);   // a 10-microsecond pulse starts the measurement
  delayMicroseconds(10);
  digitalWrite(TRIG_PIN, LOW);

  // pulseIn() times how long ECHO stays HIGH, in microseconds.
  unsigned long us = pulseIn(ECHO_PIN, HIGH, ECHO_TIMEOUT_US);
  if (us == 0) return 0;          // no echo

  // Sound goes there and back: 58 microseconds for every centimetre of distance.
  long cm = us / 58;
  if (cm < MIN_CM || cm > MAX_CM) return 0;
  return cm;
}

// Take several readings, sort them, keep the middle one (the median).
// One wild reading cannot move the middle value much.
long medianCm() {
  long v[SAMPLES];
  for (int i = 0; i < SAMPLES; i++) {
    v[i] = readOnceCm();
    delay(20);  // let the last echo die away
  }
  for (int i = 1; i < SAMPLES; i++) {   // sort, smallest first
    long key = v[i];
    int j = i - 1;
    while (j >= 0 && v[j] > key) {
      v[j + 1] = v[j];
      j--;
    }
    v[j + 1] = key;
  }
  return v[SAMPLES / 2];
}

void setup() {
  pinMode(TRIG_PIN, OUTPUT);
  pinMode(ECHO_PIN, INPUT);
  Serial.begin(9600);
  Serial.println("Lesson 4: Ultrasonic Distance");
}

void loop() {
  long raw = readOnceCm();
  long smooth = medianCm();

  // "name:value,name:value" is the format the Serial Plotter draws.
  Serial.print("raw_cm:");
  Serial.print(raw);
  Serial.print(",median_cm:");
  Serial.println(smooth);

  delay(REPORT_MS);
}

Upload it

1. Click Copy (or Download .ino and open the file).
2. In the Arduino IDE choose File → New Sketch, select everything in it and paste.
3. Check the bottom of the window says Arduino Uno and your port (Start here, step 5).
4. Click Upload (the → arrow) and wait for Done uploading.

How the code works

Change it and see

What you should see or hear

• Two lines in the Serial Plotter, raw_cm and median_cm, that go up as you move the book away.
• On a flat, hard target between 5 cm and 1 m the numbers are close to what the ruler says.
• 0 when nothing is in range, or the target is closer than about 2 cm.

If it doesn't work

Small experiment

Point the sensor at a cushion and then at a book, at the same distance. Which gives steadier readings, and why?

Check what you learned

Next

• Lesson 5 adds the servo on pin 5.

Hi DIY KIT, I'm stuck on the Arduino UNO Sensor & Output Starter Kit.
Lesson: __
Step number: __
What should happen: __
What happens instead: __
Error message (copied exactly): __
I'll attach a photo of my wiring.

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