Blynk IoT Mobile App Control with ESP32: Virtual Pins and Events

// Paste the three lines from your device page in the Blynk console
#define BLYNK_TEMPLATE_ID   "TMPLxxxxxxxx"
#define BLYNK_TEMPLATE_NAME "Room Controller"
#define BLYNK_AUTH_TOKEN    "YourDeviceAuthToken"
#define BLYNK_PRINT Serial          // connection logs on Serial Monitor

#include <WiFi.h>
#include <WiFiClient.h>
#include <BlynkSimpleEsp32.h>
#include <DHT.h>

char ssid[] = "YourWiFi";
char pass[] = "YourWiFiPassword";

const int RELAY_PIN = 26;
const bool RELAY_ACTIVE_LOW = true;  // most cheap relay modules switch ON with LOW
const float TEMP_LIMIT = 35.0;

DHT dht(4, DHT22);
BlynkTimer timer;
bool alerted = false;

void setRelay(bool on) {
  digitalWrite(RELAY_PIN, (on ^ RELAY_ACTIVE_LOW) ? HIGH : LOW);
}

// Runs whenever the app or web dashboard writes to V0
BLYNK_WRITE(V0) {
  int value = param.asInt();         // 0 or 1
  setRelay(value == 1);
  Serial.printf("Relay -> %d", value);
  Serial.println();
}

// Runs every time the board (re)connects to Blynk
BLYNK_CONNECTED() {
  Blynk.syncVirtual(V0);             // ask the server for the last V0 value
}

void sendSensor() {
  float t = dht.readTemperature();
  float h = dht.readHumidity();
  if (isnan(t) || isnan(h)) return;
  Blynk.virtualWrite(V1, t);
  Blynk.virtualWrite(V2, h);

  // fire the event once per crossing, not every 5 seconds
  if (t > TEMP_LIMIT && !alerted) {
    Blynk.logEvent("high_temp", String("Temperature is ") + t + " C");
    alerted = true;
  } else if (t < TEMP_LIMIT - 2) {
    alerted = false;                 // 2 degree hysteresis
  }
}

void setup() {
  Serial.begin(115200);
  pinMode(RELAY_PIN, OUTPUT);
  setRelay(false);                   // safe state until synced
  dht.begin();
  Blynk.begin(BLYNK_AUTH_TOKEN, ssid, pass);  // connects Wi-Fi and Blynk
  timer.setInterval(5000L, sendSensor);
}

void loop() {
  Blynk.run();                       // must run often; no long delay() here
  timer.run();
}