Smart Home Relay Controller with ESP32 and MQTT: End-to-End Build

#include <WiFi.h>
#include <PubSubClient.h>
#include <Preferences.h>

const char* WIFI_SSID = "YourWiFi";
const char* WIFI_PASS = "YourWiFiPassword";
const char* MQTT_HOST = "192.168.1.50";
const char* MQTT_USER = "hall";
const char* MQTT_PASS = "secret";
const char* BASE = "home/hall";

const int N = 4;
const int RELAY_PIN[N]  = {16, 17, 18, 19};
const int BUTTON_PIN[N] = {32, 33, 25, 27};
const bool ACTIVE_LOW = true;          // typical relay modules

bool state[N];
bool lastButton[N];
uint32_t lastChange[N];
uint32_t lastMqttTry = 0;

WiFiClient net;
PubSubClient mqtt(net);
Preferences prefs;

String topic(int i, const char* leaf) {   // home/hall/relay/<i+1>/<leaf>
  return String(BASE) + "/relay/" + String(i + 1) + "/" + leaf;
}

void publishState(int i) {
  if (mqtt.connected())
    mqtt.publish(topic(i, "state").c_str(), state[i] ? "ON" : "OFF", true);
}

void setRelay(int i, bool on) {
  state[i] = on;
  digitalWrite(RELAY_PIN[i], (on ^ ACTIVE_LOW) ? HIGH : LOW);
  prefs.putBool(String("r" + String(i)).c_str(), on);  // survive power cuts
  publishState(i);
}

void onMessage(char* t, byte* payload, unsigned int len) {
  String msg;
  for (unsigned int k = 0; k < len; k++) msg += (char)payload[k];
  msg.trim();
  msg.toUpperCase();
  for (int i = 0; i < N; i++) {
    if (topic(i, "set") == t) {
      if (msg == "ON")  setRelay(i, true);    // idempotent commands
      if (msg == "OFF") setRelay(i, false);
    }
  }
}

void tryMqtt() {
  String will = String(BASE) + "/status";
  String id = "relay-" + String((uint32_t)ESP.getEfuseMac(), HEX);
  if (!mqtt.connect(id.c_str(), MQTT_USER, MQTT_PASS, will.c_str(), 1, true, "offline")) return;
  mqtt.publish(will.c_str(), "online", true);
  for (int i = 0; i < N; i++) {
    mqtt.subscribe(topic(i, "set").c_str(), 1);
    publishState(i);                     // report the real state
  }
}

void setup() {
  Serial.begin(115200);
  prefs.begin("relays", false);
  for (int i = 0; i < N; i++) {
    pinMode(RELAY_PIN[i], OUTPUT);
    pinMode(BUTTON_PIN[i], INPUT_PULLUP);
    lastButton[i] = HIGH;
    lastChange[i] = 0;
    // restore the state saved before the power cut (default OFF)
    bool saved = prefs.getBool(String("r" + String(i)).c_str(), false);
    state[i] = saved;
    digitalWrite(RELAY_PIN[i], (saved ^ ACTIVE_LOW) ? HIGH : LOW);
  }
  WiFi.mode(WIFI_STA);
  WiFi.setAutoReconnect(true);
  WiFi.begin(WIFI_SSID, WIFI_PASS);      // no waiting: buttons work at once
  mqtt.setServer(MQTT_HOST, 1883);
  mqtt.setCallback(onMessage);
}

void loop() {
  // 1. Local buttons: toggle on press, 50 ms debounce
  for (int i = 0; i < N; i++) {
    bool b = digitalRead(BUTTON_PIN[i]);
    if (b != lastButton[i] && millis() - lastChange[i] > 50) {
      lastChange[i] = millis();
      lastButton[i] = b;
      if (b == LOW) setRelay(i, !state[i]);
    }
  }

  // 2. Network, never blocking the buttons
  if (WiFi.status() == WL_CONNECTED) {
    if (!mqtt.connected() && millis() - lastMqttTry > 5000) {
      lastMqttTry = millis();
      tryMqtt();
    }
    mqtt.loop();
  }
}