HC-SR04 Ultrasonic Sensor with ESP8266

Introduction

The HC-SR04 Ultrasonic Sensor is widely used to measure distance by emitting ultrasonic waves and measuring the time it takes for the echo to return. It works like sonar:

• The TRIG pin sends a short ultrasonic pulse.
• The ECHO pin receives the reflected signal from an obstacle.
• Using the time taken for the pulse to return, we calculate the distance.

The ESP8266 reads this timing and converts it into centimeters (cm) or inches (in). Ultrasonic sensors are commonly used in robotics, automation, parking sensors, and water level monitoring.

Components Required

• ESP8266 (NodeMCU / Wemos D1 Mini)
• HC-SR04 Ultrasonic Sensor
• Jumper Wires
• Breadboard
• USB Cable

Circuit Diagram

⚠️ Important: The HC-SR04 ECHO pin outputs 5V, but ESP8266 works at 3.3V. Use a voltage divider (2 resistors, e.g., 1k + 2k) or a logic level shifter to protect the ESP8266.

• HC-SR04 VCC → 5V
• HC-SR04 GND → GND
• HC-SR04 TRIG → D2
• HC-SR04 ECHO → D3

Arduino Code

// HC-SR04 Ultrasonic Sensor with ESP8266

#define TRIG_PIN D2 // GPIO5
#define ECHO_PIN D3  // GPIO4

long duration;
int distance;

void setup() {
  Serial.begin(115200);
  pinMode(TRIG_PIN, OUTPUT);
  pinMode(ECHO_PIN, INPUT);
}

void loop() {
  // Step 1: Clear TRIG pin
  digitalWrite(TRIG_PIN, LOW);
  delayMicroseconds(2);

  // Step 2: Send 10us pulse
  digitalWrite(TRIG_PIN, HIGH);
  delayMicroseconds(10);
  digitalWrite(TRIG_PIN, LOW);

  // Step 3: Measure echo time
  duration = pulseIn(ECHO_PIN, HIGH);

  // Step 4: Convert to distance
  distance = duration * 0.034 / 2;  // Speed of sound: 343 m/s

  // Step 5: Print distance
  Serial.print("Distance: ");
  Serial.print(distance);
  Serial.println(" cm");

  delay(1000);
}

Step-by-Step Code Explanation

Define TRIG and ECHO pins

#define TRIG_PIN D2
#define ECHO_PIN D3

TRIG sends the pulse, ECHO receives it.

Send 10µs Trigger Pulse

digitalWrite(TRIG_PIN, HIGH);
delayMicroseconds(10);
digitalWrite(TRIG_PIN, LOW);

This starts the ultrasonic wave.

Measure Echo Duration

duration = pulseIn(ECHO_PIN, HIGH);

Returns the time in microseconds that the echo pin stayed HIGH.

Convert into Distance

distance = duration * 0.034 / 2;

• Speed of sound = 0.034 cm/µs
• Divide by 2 (round trip).

Output

Place an object 10 cm away → Serial Monitor prints:

Distance: 10 cm

Move object further → Distance increases.

Real-Life Applications

• Obstacle Avoidance Robots – Detects distance to objects.
• Parking Assistance – Measures distance to walls/objects.
• Smart Trash Bin – Opens lid automatically when hand is near.
• Water Level Monitoring – Measures tank water depth.
• Home Automation – Presence detection for lights/alarms.

Troubleshooting

Issue	Cause	Solution
Always reads 0	Wrong wiring	Check TRIG/ECHO pins and voltage divider
Values fluctuate	Noise / unstable supply	Use capacitor across VCC & GND
ESP8266 resets	ECHO pin gives 5V	Use voltage divider (e.g., 1k + 2k resistors)
No output in Serial Monitor	Wrong baud rate	Set Serial Monitor to 115200 baud