Ultrasonic distance sensors

Learn

What you’ll learn

• How ultrasonic sensors measure distance
• How to wire and code the HC-SR04
• Its limits, and how to work around them

Echoes

The HC-SR04 sends a burst of ultrasound (40 kHz, too high for people to hear) and listens for the echo, like a bat. Sound travels at about 343 m/s (0.0343 cm per microsecond), so the time for the echo to come back gives the distance:

distance (cm) = echo time (µs) × 0.0343 ÷ 2

(÷ 2 because the sound goes there and back.) A handy shortcut: cm ≈ µs ÷ 58.

Wiring

HC-SR04 pin	Arduino
VCC	5V
GND	GND
TRIG	any digital pin (e.g. 3)
ECHO	any digital pin (e.g. 2)

To measure: set TRIG HIGH for 10 µs, then time how long ECHO stays HIGH with pulseIn().

Limits

• Range roughly 2 cm to 4 m.
• Soft things (curtains, clothes) absorb sound; angled surfaces bounce it away, so readings can jump.
• Two sensors firing at once can hear each other’s echoes: trigger them one after the other.
• Readings are noisy: take a few and use the median.

Examples

Example: a distance function

float readCm(int trig, int echo) {
  digitalWrite(trig, LOW);  delayMicroseconds(2);
  digitalWrite(trig, HIGH); delayMicroseconds(10);
  digitalWrite(trig, LOW);
  long us = pulseIn(echo, HIGH, 30000);    // time out after 30 ms
  if (us == 0) return 400;                 // no echo: treat as far away
  return us * 0.0343 / 2;
}

Example: median of three

float a = readCm(3, 2), b = readCm(3, 2), c = readCm(3, 2);
float median = max(min(a, b), min(max(a, b), c));

One bad reading can’t fool the median.

Example: a parking sensor

Beep faster as you get closer: delay(map(constrain(cm, 5, 100), 5, 100, 50, 600)); between beeps.

Practice

Try it in the virtual Arduino

Press Start simulator, replace diagram.json, paste the sketch, press ▶. Click the sensor and drag its distance slider: the Serial Monitor should follow it.

diagram.json

{
  "version": 1,
  "author": "RoboLearn",
  "editor": "wokwi",
  "parts": [
    {
      "type": "wokwi-arduino-uno",
      "id": "uno",
      "top": 0,
      "left": 0,
      "attrs": {}
    },
    {
      "type": "wokwi-hc-sr04",
      "id": "sonar",
      "top": -110,
      "left": 90,
      "attrs": {
        "distance": "100"
      }
    }
  ],
  "connections": [
    [
      "sonar:VCC",
      "uno:5V",
      "red",
      []
    ],
    [
      "sonar:GND",
      "uno:GND.1",
      "black",
      []
    ],
    [
      "sonar:TRIG",
      "uno:3",
      "blue",
      []
    ],
    [
      "sonar:ECHO",
      "uno:2",
      "green",
      []
    ]
  ],
  "dependencies": {}
}

sketch.ino

const int TRIG = 3;
const int ECHO = 2;

void setup() {
  pinMode(TRIG, OUTPUT);
  pinMode(ECHO, INPUT);
  Serial.begin(9600);
}

void loop() {
  digitalWrite(TRIG, LOW);
  delayMicroseconds(2);
  digitalWrite(TRIG, HIGH);       // a 10 µs pulse starts a measurement
  delayMicroseconds(10);
  digitalWrite(TRIG, LOW);

  long us = pulseIn(ECHO, HIGH, 30000);   // echo time in µs (0 = nothing within ~5 m)
  float cm = us * 0.0343 / 2;             // sound: 0.0343 cm/µs, there and back
  Serial.print(cm);
  Serial.println(" cm");
  delay(100);
}

Ultrasonic distance sensorOpen full screen ↗

Start simulator

Runs on Wokwi, free, no account needed.

Try it on a robot

The robot simulator’s distance() works like an ultrasonic sensor pointing forwards.

Obstacle avoider

▶ Run↺ Reset

Your code (JavaScript)Load starter code

Robot commands

• motors(left, right): wheel power, −100 to 100
• lineLeft(), lineRight(): true over the black line
• distance(): cm to the nearest thing ahead (max 200)
• distanceLeft(), distanceRight(): cm to the sides
• linePosition(): where the line is, −1 (left) to 1 (right), or null
• millis(): milliseconds since Run
• print(value): show a message below
• setup() runs once; loop() runs 50 times a second

Check the mathsEasy

An echo takes 1,160 µs. How far away is the object? 1160 × 0.0343 ÷ 2 ≈ 19.9 cm (or 1160 ÷ 58 = 20 cm).

Too close warningMedium

In the virtual Arduino, turn the built-in LED on when something is closer than 20 cm. After calculating cm, add digitalWrite(LED_BUILTIN, cm < 20 ? HIGH : LOW); and pinMode(LED_BUILTIN, OUTPUT); in setup().

Smart turningHard

In the robot simulator, when blocked, turn towards whichever side has more space.

Show an answer

function loop() {
  if (distance() < 25) {
    if (distanceLeft() > distanceRight()) motors(-40, 40);
    else motors(40, -40);
  } else motors(60, 60);
}

Quick check

1. Why divide by 2 in the distance formula?

The sensor has two eyesThe sound travels there and backTo convert to metresTo round the number

2. How long must TRIG be HIGH to start a measurement?

10 µs10 ms1 s58 µs

3. Which surface gives the least reliable readings?

A flat wall facing the sensorA curtainA wooden boxA door

Check my answers