Interfacing HC-SR04 Ultrasonic Sensor with Arduino

What You'll Build

A pocket-sized digital distance meter: point the sensor at a wall, your hand, or a box, and the Arduino Uno shows the distance in centimetres on an I2C 16x2 (1602) LCD — updated several times a second, with a live bar graph on the second row.

Thanks to the I2C backpack on the display, the LCD needs only four wires (power, ground, SDA, SCL) instead of the twelve a plain 1602 requires. That leaves the rest of the Uno's pins free for future upgrades.

No computer is needed once the sketch is uploaded. Power it from a USB power bank and it becomes a portable ruler for 2 cm to about 4 m.

What you will learn:

• How the HC-SR04 turns a burst of ultrasound into a distance
• How to connect an I2C LCD to an Arduino Uno and find its I2C address
• How to write a stable measurement routine (timeouts, median filtering, temperature compensation)
• How to format numbers cleanly on a 16-character display
• How to debug the most common wiring, address and contrast problems

Difficulty: Beginner | Build time: about 25 minutes | Board: Arduino Uno R3

What Is the HC-SR04?

The HC-SR04 is a low-cost ultrasonic ranging module. It has two metal "eyes": one is a transmitter that emits a short burst of 40 kHz sound (far above human hearing), the other is a receiver that listens for the echo. Bats and submarines use the same principle.

The module has four pins: VCC, TRIG, ECHO and GND. Because the Uno's I/O is 5 V, the sensor connects directly — no level shifting is needed on this board.

How the Measurement Works

Every reading is a short conversation between the Arduino and the sensor:

1. The Arduino pulls TRIG HIGH for 10 µs.
2. The sensor emits eight 40 kHz pulses of ultrasound.
3. The sound travels to the nearest object, bounces back, and reaches the receiver.
4. The sensor raises ECHO HIGH from the moment the burst leaves until the echo returns.
5. The Arduino measures how long ECHO stayed HIGH (using pulseIn()) and converts that time to a distance.

The Distance Formula

Sound travels at roughly 343 m/s at 20 °C, which is 0.0343 cm per microsecond. The measured time covers the trip to the object and back, so the distance is half of that:

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

That is where the well-known shortcut comes from: distance ≈ time ÷ 58.

Why Temperature Matters

The speed of sound changes with air temperature, approximately:

speed (m/s) = 331.3 + 0.606 × temperature (°C)

A 10 °C swing shifts the speed by about 1.8 %, which is roughly 1.8 cm of error at 100 cm. The sketch below includes a TEMP_C constant so you can correct for your room temperature.

Understanding the I2C 1602 LCD

A "1602" LCD shows 16 characters on each of 2 rows. The display itself uses the Hitachi HD44780 controller, which normally needs about six signal wires plus power, contrast and backlight connections.

Most modules now come with a small I2C backpack soldered on the back. It contains a PCF8574 port-expander chip that talks to the LCD for you, so the Arduino only has to use the two-wire I2C bus:

On the Uno, the hardware I2C pins are fixed: SDA is A4 and SCL is A5 (they are also broken out on the two extra header pins near AREF on the R3 board). The backpack already includes the pull-up resistors the bus needs, so you do not have to add any.

The I2C address

Every device on an I2C bus has an address. Backpacks usually ship as 0x27 (PCF8574) or 0x3F (PCF8574A). Using the wrong address is the number one reason an I2C LCD stays blank, so this guide includes a short scanner sketch that tells you the right one.

The two small controls on the backpack

• Blue trimmer potentiometer — sets the display contrast. Turn it with a small screwdriver.
• Backlight jumper — a small cap across two pins. Leave it on for the backlight to work; removing it turns the backlight off.

Wiring the Circuit

HC-SR04 to Arduino Uno

I2C LCD to Arduino Uno

Build order that avoids mistakes

1. Run the Uno's 5V and GND to the breadboard power rails first.
2. Connect the four LCD backpack wires (GND, VCC, SDA, SCL).
3. Wire the HC-SR04 last, placing it at the board edge so it has a clear view forward.
4. Check that no wire crosses the sensor's two "eyes" — anything in front of them can cause false echoes.

Install the LCD Library

The sketches below use the LiquidCrystal I2C library.

1. In the Arduino IDE open Sketch → Include Library → Manage Libraries… (or the Library Manager icon).
2. Search for LiquidCrystal I2C.
3. Install the one by Frank de Brabander.

Wire.h, the library that handles I2C itself, is already built into the Arduino IDE, so nothing else needs installing.

Step 1 — Find the I2C Address

Upload this scanner first. It asks every possible address "is anyone there?" and prints the ones that answer. Open Tools → Serial Monitor at 9600 baud to read the result. This is the only sketch in the project that uses the Serial Monitor, and it exists purely to avoid guessing the address.

Step 2 — Test the LCD on Its Own

Note the address from the scanner (most likely 0x27 or 0x3F). Now prove that the display works before involving the sensor. If this small sketch works, any later problem must be in the sensor wiring or code — not the LCD. This is the most useful debugging habit in electronics: test one subsystem at a time.

If your address is not 0x27, change the value in LCD_ADDR.

Step 3 — The Distance Meter Sketch

This is the complete sketch. It reads the sensor five times, takes the median (so a single bad echo cannot spoil the result), converts the time to centimetres using a temperature-corrected speed of sound, and prints the value with a bar graph on the second row.

Set LCD_ADDR to your scanned address and TEMP_C to your room temperature for best accuracy.

How the Code Works

The LCD object

LiquidCrystal_I2C lcd(LCD_ADDR, 16, 2); tells the library which I2C address to talk to and the display size. After that, lcd.init() starts the display and lcd.backlight() switches the backlight on. All the usual commands (setCursor, print, write, clear) work exactly like on a normal LCD.

Temperature-corrected constant

SOUND_CM_PER_US is calculated once at compile time from TEMP_C. Using a constant means the maths inside the loop is a single multiplication.

readEchoMicros()

This sends the 10 µs trigger pulse and calls pulseIn(ECHO_PIN, HIGH, ECHO_TIMEOUT_US). The third argument is critical: without it, pulseIn() waits up to one full second when nothing is in range, freezing your sketch. With a 25 ms timeout, a missed echo costs almost nothing.

readDistanceCm()

Ultrasonic readings occasionally spike — a stray reflection, a soft surface, electrical noise. Instead of averaging (which a single wild value drags off target), the sketch sorts five readings and takes the median, which simply ignores outliers. A reading of 0 (timeout) is discarded before sorting.

The 60 ms gap between pings follows the datasheet recommendation and prevents the sensor from hearing the echo of its previous burst. Five samples at 60 ms give a fresh result about every 0.3 seconds.

showDistance()

• Every string is padded to exactly 16 characters, so a new value fully overwrites the old one and no leftover digits remain. This is more reliable than calling lcd.clear() every loop, which causes visible flicker and slows the I2C bus.
• dtostrf() formats the float with a fixed width, so the decimal point stays in the same column as the number changes from 5.2 to 123.4.
• The F() macro keeps text in flash memory, saving precious Uno SRAM (only 2 KB).
• Character code 255 is the solid block in the standard HD44780 character set, which makes the bar graph without any custom characters.

Accuracy and Limitations

The HC-SR04 is excellent value, but knowing its limits will save you from chasing phantom bugs:

• Narrow cone, not a laser. The sensor "sees" in a cone of about 15°. At 2 m that cone is roughly 50 cm wide, so it reports the nearest object anywhere inside it.
• Angled surfaces fool it. If the target is tilted more than about 45° away from the sensor, the sound bounces off to the side instead of back, and the reading is wrong or missing.
• Soft materials absorb sound. Fabric, foam, curtains and clothing return a weak echo, so reliable range drops sharply. Hard flat surfaces such as walls, boxes and books work best.
• Very small objects are hard to detect. Thin poles and wires may not reflect enough sound.
• Minimum distance is about 2 cm. Closer than that, the receiver is still "ringing" from the transmitted burst.
• Air temperature and humidity affect results. That is why TEMP_C exists.
• Realistic range: the specification says 400 cm, but for consistent results on typical targets expect about 2–3 m.

Troubleshooting

Ideas to Extend the Project

• Show inches too. Divide the centimetre value by 2.54 and print it on the second row instead of the bar graph.
• Add a hold button. Wire a push button to a free pin with INPUT_PULLUP and freeze the reading while it is pressed.
• Min / max tracking. Store the smallest and largest values seen since power-up and show them on demand.
• Tank level meter. Mount the sensor facing down over a water tank and convert distance to percentage full.
• Add a buzzer for proximity alerts — see the full parking sensor project linked below.
• Smaller changes, big effect. Increase SAMPLES to 7 for even smoother numbers, or reduce it to 3 for faster updates.

Frequently Asked Questions

My I2C address is not 0x27. What do I do? Run the scanner sketch and use the address it prints in LCD_ADDR. 0x3F is the other common one.

Can I use other pins for SDA and SCL? On the Arduino Uno the hardware I2C pins are fixed: SDA is A4 and SCL is A5. TRIG and ECHO, on the other hand, can be any digital pins — just change TRIG_PIN and ECHO_PIN.

Why is it better to use I2C for the LCD? It needs only four wires instead of about twelve, which makes the circuit neater, faster to build, and leaves almost every Uno pin free for other parts.

Why use the median instead of the average? An average is pulled toward a bad reading. If four readings are 30 cm and one is 250 cm, the average is wrong by 44 cm, while the median stays at 30 cm.

Can I power the sensor and LCD from the Uno's 5V pin? Yes. The HC-SR04 draws only about 15 mA and the LCD backpack a little more, which the Uno's USB supply handles comfortably.

Why does the sketch wait 60 ms between pings? Faster than that, the sensor can receive the leftover echo of the previous pulse and report a false, short distance.

What is the actual maximum range? The specification says 400 cm. In practice, with a large flat target, 2–3 m is dependable.

Do I need the temperature constant? No — the meter works fine with the default. It only improves accuracy if your room is much hotter or colder than 25 °C.

My LCD has no backpack and 16 separate pins. Can I follow this guide? That is a parallel LCD, which needs more wiring and the standard LiquidCrystal library. The sensor logic is identical; only the LCD wiring and initialisation change.

Related on SolderHub

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• Arduino Mega 2560 vs Uno: Which Board Should You Choose?

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