HC-SR04 Ultrasonic Distance Sensor

Measure echo travel time with an HC-SR04 and print distance to Serial Monitor. Handle a missing echo explicitly instead of reporting a false zero distance.

Open in Circuitera ↗Launch in Simulator →

Ultrasonic Distance Sensor: the matching Arduino Uno circuit rendered in Circuitera’s simulator — Matching Circuitera simulator circuit. Real firmware capture; modeled components are idealized.

Wire it up

Disconnect power before changing connections. Follow terminal names rather than wire colors.

From	To	Why it matters
5V	HC-SR04 VCC	Use this tutorial with a 5 V AVR board
GND	HC-SR04 GND	Common reference
D7	HC-SR04 TRIG	Trigger output
D8	HC-SR04 ECHO	Echo input; HC-SR04 output is 5 V

The complete sketch

ultrasonic-distance.ino

const int triggerPin = 7;
const int echoPin = 8;

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

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

  unsigned long duration = pulseIn(echoPin, HIGH, 30000);
  if (duration == 0) {
    Serial.println("No echo");
  } else {
    float distanceCm = duration * 0.0343 / 2.0;
    Serial.print("distance:");
    Serial.println(distanceCm);
  }
  delay(100);
}

The launch buttons use this same code and a matching saved circuit.

Build it step by step

Connect the module
Unplug USB. Connect VCC to 5 V, GND to GND, TRIG to D7, and ECHO to D8. Do not connect the 5 V echo directly to a 3.3 V-only input.
Verify and upload
Open the project as a new sketch, Verify, grant port access, and Upload.
Read the distance
Open Serial Monitor at 9600 baud. Aim at a flat object in front of the transducers. In simulation, adjust the sensor’s distance property.
Explore the plot
Switch to Serial Plotter for numeric distance samples. No echo is a text diagnostic, not a valid distance measurement.

Understand the sketch

Trigger then listen

A 10 µs HIGH pulse on TRIG requests a measurement. pulseIn measures the HIGH duration of ECHO in microseconds.

Round-trip travel

The sound travels to the object and back. Multiplying by approximately 0.0343 cm/µs converts time to round-trip distance; divide by two for one-way distance. Temperature and geometry affect real readings.

A bounded wait

The 30000 µs timeout prevents waiting indefinitely. A zero result means no complete pulse arrived in time, so the sketch prints No echo. delay(100) spaces measurements.

If something is different

Always No echo

Check TRIG/ECHO wiring and common ground. Use a flat target at a sensible range; soft or angled surfaces can reflect sound away.

Garbled text

Match Serial Monitor to Serial.begin(9600).

Readings jump

Keep the target steady. Reflections, temperature, sensor limits, and surface shape affect measurements; simulation is an idealized model.

Expected output and tested scope

Serial output reports labeled distance samples. A missing pulse prints No echo rather than a false zero measurement.

Version 1 · Last source- and build-matched test: 2026-10-25. Actual shipped AVR-GCC WASM compilation and Uno AVR engine execution; no physical hardware test.

Supported tutorial target: Arduino Uno R3 / ATmega328P. Compile: PASS · Modeled execution: PASS. Other boards require a separate review of pins, voltage, libraries and the exact build profile.

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