Two-board sensor network

/*
  16 — Two-board sensor network
  Boards: BOTH. One sketch: on the NodeMCU V3 ESP8266 development board it compiles
  as the SENDER (DHT11 -> WiFi), on the ESP32 as the RECEIVER (WiFi -> LCD). Upload it to each board
  with the matching board selected in the IDE. Power both boards at the same time
  from suitable USB power sources; the kit electronics are the two boards, sensors and LCD.
  Libraries: "DHT sensor library" (Adafruit), "LiquidCrystal I2C" (Frank de Brabander).

  Wiring:
    NodeMCU (sender):  DHT11 VCC -> 3V3, GND -> GND, DATA -> D5
    ESP32 (receiver):  LCD VCC -> VIN (5 V), GND -> GND, SDA -> GPIO 21, SCL -> GPIO 22

  Steps: 1) upload to the ESP32 first, read its IP from Serial;
         2) put that IP in RECEIVER_IP below; 3) upload to the NodeMCU.
  Two devices, one job — and what comes with it: a missed message, how often
  to send, what the display shows when the sender goes quiet (here: "no data"
  after 15 s). The first distributed system most people build, and the reason
  the kit ships two boards.
*/

const char* WIFI_SSID   = "YOUR-WIFI-NAME";
const char* WIFI_PASS   = "YOUR-WIFI-PASSWORD";
const char* RECEIVER_IP = "192.168.1.50";        // <- the ESP32's IP from step 1

#if defined(ESP8266)
/* ================= SENDER: NodeMCU + DHT11 ================= */
#include <ESP8266WiFi.h>
#include <ESP8266HTTPClient.h>
#include "DHT.h"

const int DHT_PIN = D5;
DHT dht(DHT_PIN, DHT11);
const unsigned long SEND_EVERY_MS = 5000;

void setup() {
  Serial.begin(115200);
  dht.begin();
  WiFi.mode(WIFI_STA);
  WiFi.begin(WIFI_SSID, WIFI_PASS);
  while (WiFi.status() != WL_CONNECTED) { delay(500); Serial.print("."); }
  Serial.println("\nsender ready");
}

void loop() {
  delay(SEND_EVERY_MS);
  float t = dht.readTemperature(), h = dht.readHumidity();
  if (isnan(t) || isnan(h)) { Serial.println("DHT read failed"); return; }

  WiFiClient client;
  HTTPClient http;
  String url = String("http://") + RECEIVER_IP + "/update?t=" + String(t, 1) + "&h=" + String(h, 0);
  http.begin(client, url);
  int code = http.GET();                      // a missed message just returns an error code
  http.end();
  Serial.print(url); Serial.print(" -> "); Serial.println(code);
}

#elif defined(ESP32)
/* ================= RECEIVER: ESP32 + LCD ================= */
#include <WiFi.h>
#include <WebServer.h>
#include <Wire.h>
#include <LiquidCrystal_I2C.h>

WebServer server(80);
LiquidCrystal_I2C lcd(0x27, 16, 2);
String lastT = "--", lastH = "--";
unsigned long lastSeen = 0;
const unsigned long STALE_MS = 15000;

void setup() {
  Serial.begin(115200);
  lcd.init(); lcd.backlight();
  lcd.print("Joining WiFi...");
  WiFi.mode(WIFI_STA);
  WiFi.begin(WIFI_SSID, WIFI_PASS);
  while (WiFi.status() != WL_CONNECTED) { delay(500); Serial.print("."); }
  Serial.println();
  Serial.print("Receiver IP: "); Serial.println(WiFi.localIP());   // put this in RECEIVER_IP
  lcd.clear(); lcd.print(WiFi.localIP()); lcd.setCursor(0, 1); lcd.print("waiting...");

  server.on("/update", []() {
    lastT = server.arg("t"); lastH = server.arg("h");
    lastSeen = millis();
    server.send(200, "text/plain", "ok");
  });
  server.begin();
}

void loop() {
  server.handleClient();
  static unsigned long lastDraw = 0;
  if (millis() - lastDraw < 500) return;
  lastDraw = millis();

  lcd.setCursor(0, 0);
  lcd.print("T:"); lcd.print(lastT); lcd.print("C H:"); lcd.print(lastH); lcd.print("%   ");
  lcd.setCursor(0, 1);
  if (lastSeen == 0)                       lcd.print("waiting...      ");
  else if (millis() - lastSeen > STALE_MS) lcd.print("no data         ");   // sender went quiet
  else { lcd.print("age "); lcd.print((millis() - lastSeen) / 1000); lcd.print("s        "); }
}

#else
  #error "Select an ESP32 (receiver) or ESP8266 (sender) board"
#endif