Published October 16, 2025 © MIT

Building a Smart D Bistance Measuring System with ESP32

Measure distance with HC-SR04 + ESP32, show live readings on web serial, and send data to MQTT/Firebase for smart automation YaranaIoT Guru

BeginnerFull instructions provided8 hours29

Building a Smart D Bistance Measuring System with ESP32

Things used in this project

Software apps and online services

Arduino IDE

Story

Vikas apne rooftop water tank ka level manually check karta tha. Ek din usne YaranaIoT Guru ka tutorial dekha aur ESP32 + HC-SR04 se distance (level) monitoring system banaya. Ab phone par web page ya MQTT dashboard se wo kab pump on karna hai dekh leta hai. Usne paani bhi bachaya aur time bhi.

“Simple sensor — big impact!” — Vikas

“Simple sensor — big impact!” — Vikas

/* 
  Smart Distance Measuring System with ESP32
  - HC-SR04 ultrasonic sensor
  - Simple web UI: shows current distance and timestamp
  - Optional: MQTT publish
  Author: YaranaIoT Guru
*/

#include <WiFi.h>
#include <WebServer.h>
#include <Ticker.h>

// Optional MQTT
//#include <PubSubClient.h>

#define TRIG_PIN 26
#define ECHO_PIN 25    // Use voltage divider on Echo if sensor runs at 5V

// WiFi - EDIT THESE
const char* ssid = "YourWiFiSSID";
const char* password = "YourWiFiPassword";

// MQTT - optional (uncomment includes above and configure)
//const char* mqtt_server = "YOUR_MQTT_BROKER_IP";
//const char* mqtt_topic  = "home/sensor/distance";

WebServer server(80);
Ticker sensorTicker;

volatile unsigned long lastDuration = 0;
float lastDistance = 0.0;

// Read distance (blocking) - returns distance in cm, or -1 on timeout
float readDistanceCm() {
  // Ensure trigger low
  digitalWrite(TRIG_PIN, LOW);
  delayMicroseconds(2);
  // Send 10us HIGH pulse to trigger
  digitalWrite(TRIG_PIN, HIGH);
  delayMicroseconds(10);
  digitalWrite(TRIG_PIN, LOW);

  // Read echo pulse duration (in microseconds)
  unsigned long duration = pulseIn(ECHO_PIN, HIGH, 30000UL); // timeout 30ms => ~5 meters
  if (duration == 0) {
    return -1.0; // timeout/no echo
  }
  float distance = duration / 58.0; // convert to cm
  // Alternative more exact: distance = (duration / 1e6) * 343.0 / 2.0 * 100.0;
  return distance;
}

// Periodic sensor read (called by Ticker)
void sensorReadTask() {
  float d = readDistanceCm();
  if (d >= 0) {
    lastDistance = d;
  } else {
    lastDistance = -1.0;
  }
  lastDuration = micros();
  // Optionally: publish to MQTT here
  // if (mqttClient.connected()) { ... }
}

String htmlPage() {
  String html = "<!doctype html><html><head><meta charset='utf-8'><meta name='viewport' content='width=device-width,initial-scale=1'>";
  html += "<title>ESP32 Distance Monitor</title></head><body style='font-family:Arial,Helvetica,sans-serif;'>";
  html += "<h2>ESP32 Distance Monitor — YaranaIoT Guru</h2>";
  html += "<p>Distance: <strong>";
  if (lastDistance < 0) html += "No echo / out of range";
  else html += String(lastDistance,2) + " cm";
  html += "</strong></p>";
  html += "<p>Last update: " + String(millis()/1000) + " s since boot</p>";
  html += "<p><a href='/raw'>Raw JSON</a> | <a href='/calibrate'>Calibrate</a></p>";
  html += "<script>setTimeout(()=>location.reload(),3000);</script>";
  html += "</body></html>";
  return html;
}

void handleRoot() {
  server.send(200, "text/html", htmlPage());
}

void handleRaw() {
  String payload = "{";
  payload += "\"distance_cm\": ";
  if (lastDistance < 0) payload += "null";
  else payload += String(lastDistance,2);
  payload += ", \"timestamp_ms\": " + String(millis());
  payload += "}";
  server.send(200, "application/json", payload);
}

void handleCalibrate() {
  // Simple calibration page instructing how to measure offset if needed
  String p = "<h3>Calibration</h3><p>Place a known distance object and note reading. Use offset in code if required.</p><p><a href='/'>Back</a></p>";
  server.send(200, "text/html", p);
}

void setup() {
  Serial.begin(115200);
  delay(100);

  pinMode(TRIG_PIN, OUTPUT);
  pinMode(ECHO_PIN, INPUT);
  digitalWrite(TRIG_PIN, LOW);

  // WiFi
  Serial.printf("Connecting to %s\n", ssid);
  WiFi.begin(ssid, password);
  int attempts = 0;
  while (WiFi.status() != WL_CONNECTED && attempts++ < 30) {
    delay(500);
    Serial.print(".");
  }
  if (WiFi.status() == WL_CONNECTED) {
    Serial.println("\nWiFi connected. IP: " + WiFi.localIP().toString());
  } else {
    Serial.println("\nWiFi connect failed. Starting AP mode...");
    WiFi.softAP("ESP32-Distance-AP");
    Serial.println("AP IP: " + WiFi.softAPIP().toString());
  }

  // Web server routes
  server.on("/", handleRoot);
  server.on("/raw", handleRaw);
  server.on("/calibrate", handleCalibrate);
  server.begin();

  // start periodic sensor reads every 2 seconds
  sensorTicker.attach(2.0, sensorReadTask);

  // Initial read
  sensorReadTask();
}

void loop() {
  server.handleClient();
  // add small yield
  delay(10);
}