HC-SR04 ultrasonic sensor pinout: VCC, TRIG, ECHO and GND explained

#What it is

The HC-SR04 measures distance with sound, like a bat. One round "eye" is a tiny speaker (marked T), the other a microphone (R). They work at 40 kHz — too high for people to hear. It measures from 2 cm to 4 m and needs just four pins.

This guide is about the sensor itself: its pins and its timing. For the full project with the maths step by step, see measure distance with the HC-SR04. See it in 3D.

#Pins

Looking at the front, with the pins at the bottom, from left to right:

TRIG is an input to the sensor (an OUTPUT on the Arduino). ECHO is an output from the sensor (an INPUT on the Arduino). Both are plain digital signals: just HIGH or LOW.

#How it works: one measurement, step by step

Here is what happens on the wires, with the same timing our 3D lab simulates:

1. TRIG goes HIGH for at least 10 µs, then LOW. A shorter pulse is ignored.
2. The sensor sends eight bursts of 40 kHz sound — that takes a moment (about 250 µs in our lab).
3. ECHO goes HIGH.
4. The sound hits an object and bounces back. When the microphone hears it, ECHO goes LOW.
5. The time ECHO was HIGH is the round trip: 58 µs for every centimetre of distance.

Why 58? Sound travels about 0.0343 cm per microsecond. There and back is two trips, so 1 cm costs 2 ÷ 0.0343 ≈ 58 µs.

#How to wire it to an Arduino

No breadboard needed. Four jumper wires go straight from the sensor's pins to the UNO:

1. VCC → 5V
2. TRIG → pin 9
3. ECHO → pin 10
4. GND → GND

Play in 3DRun the code · press the parts · turn it around

#Example code

The short version from the component page:

void setup() {
  pinMode(9, OUTPUT);
  pinMode(10, INPUT);
  Serial.begin(9600);
}

void loop() {
  digitalWrite(9, HIGH); delayMicroseconds(10); digitalWrite(9, LOW);
  Serial.println(pulseIn(10, HIGH) / 58);
  delay(200);
}

• pinMode(9, OUTPUT) for TRIG, pinMode(10, INPUT) for ECHO — the sensor drives ECHO, the Arduino only listens.
• digitalWrite(9, HIGH); delayMicroseconds(10); digitalWrite(9, LOW); is the 10 µs trigger pulse.
• pulseIn(10, HIGH) waits for ECHO to go HIGH, times how long it stays HIGH, and returns microseconds.
• / 58 turns microseconds into centimetres.
• delay(200) leaves time for old echoes to fade.

#Common mistakes

• Always 0. TRIG and ECHO are swapped, or ECHO is not connected: pulseIn() gives up after one second and returns 0.
• Huge numbers, about 655. Nothing was in range — 38 ms ÷ 58 is about 655. Point it at something within 4 m.
• Jumpy readings. Soft things (cloth, foam) swallow sound, and slanted surfaces bounce it away. Aim at something flat and hard.
• Very slow loop. If no echo ever starts, each pulseIn() waits a full second. Pass a timeout: pulseIn(10, HIGH, 30000).

#Check yourself

#Questions people ask

#What does the TRIG pin do on the HC-SR04?

It starts a measurement. Hold it HIGH for 10 µs, and the sensor sends its burst of ultrasound and then reports the echo on the ECHO pin.

#Can I use one pin for both TRIG and ECHO?

With some libraries, yes — the pin switches between output and input. Using two pins, as here, is simpler and easier to understand.

#Why does my HC-SR04 read 0 cm?

Usually ECHO is not wired, TRIG and ECHO are swapped, or the sensor has no power. Check the four wires against the pin table.

#Is the HC-SR04 waterproof?

No. For outdoor or wet places, use a sealed sensor such as the JSN-SR04T, which works in a very similar way.

#Where it is used

It is the star of measure distance and the eyes of the parking sensor, where a traffic light module shows how close you are.

Last updated 2026-10-29