ESP8266 Remote PC Power Switch (Home Assistant & mDNS) - power relay + resistors

#include <ESP8266WiFi.h>
#include <ESP8266WebServer.h>
#include <ESP8266mDNS.h>

// Project: Remote PC Power Control via Relay & Resistor Voltage Divider State Monitoring
// Hardware components:
// 1. Controller: NodeMCU ESP8266 V3 (CH340)
// 2. Relay: 1-Channel 3.3V Low-Level Trigger Relay Module
// 3. Resistors: 1x 1k Ohm, 1x 2k Ohm (Voltage divider for PLED sensing)
// 4. Power: Micro-USB (5V)
// 5. Connection:
//    - Power Control (Relay Output):
//        Relay VCC -> NodeMCU VIN (5V) or 3V3
//        Relay GND -> NodeMCU GND
//        Relay IN  -> NodeMCU D1 (GPIO5)
//        Relay COM & NO -> PC Motherboard POWER SW pins
//    - Power LED Sense (Resistor Divider Input):
//        PC Motherboard PLED (+) -> 1k Ohm Resistor -> NodeMCU D2 (GPIO4)
//        NodeMCU D2 (GPIO4)      -> 2k Ohm Resistor -> NodeMCU GND & PLED (-)
//
// Schematic Diagram:
//
// [ NodeMCU ]                         [ Relay Module ]                  [ Motherboard F_PANEL ]
//  Pin D1 (GPIO5) -------------------> IN
//  VIN (5V) -------------------------> VCC
//  GND ------------------------------> GND
//                                      COM ---------------------------> Pin 7 (POWER SW +)
//                                      NO ----------------------------> Pin 8 (POWER SW -)
//
// [ NodeMCU ]                         [ Voltage Divider ]               [ Motherboard F_PANEL ]
//  Pin D2 (GPIO4) <----+-------------- [ 1kΩ Resistor ] --------------> Pin 1 (PLED +)
//                      |
//               [ 2kΩ Resistor ]
//                      |
//  GND ----------------+----------------------------------------------> Pin 3 (PLED -) / GND
//
// Features:
// - Web-server for remote control via VPN
// - Real-time power state sensing via resistor divider (5V to 3.3V)
// - mDNS support (access via http://mainpc.local)
// - Static IP configuration (192.168.0.133)
// - Force Shutdown support (8s long press via /force)
// - Physical FLASH button on board triggers the relay
// - English UI with status indicator
//
// Hardware Notes:
// - Relay module IN pin has built-in SMD resistors on PCB.
// - The 1k/2k ohm divider steps motherboard 5V down to 3.3V safe for ESP8266 GPIO.
// - Pin D2 uses standard INPUT logic. HIGH = PC ON, LOW = PC OFF.

// Your network config
const char* ssid     = "WiFI_NAME_REGISTRY_DEPENDANT"; // Change this
const char* password = "WiFI_PASSWORD";               // Change this
const char* hostName = "mainpc";                      // Access in browser with mainpc.local

const int relayPin  = 5; // D1 (GPIO5) -> Relay IN Pin (Control)
const int sensePin  = 4; // D2 (GPIO4) -> Resistor Divider Middle (PLED Sense)
const int buttonPin = 0; // FLASH button (GPIO0)
const int ledPin    = 2; // Onboard LED (D4)

// Static IP Configuration
IPAddress local_IP(192, 168, 0, 133);
IPAddress gateway(192, 168, 0, 1);
IPAddress subnet(255, 255, 255, 0);
IPAddress primaryDNS(192, 168, 0, 1);

// Durations in milliseconds
const int standardPress = 500;
const int forcePress    = 8000; // Force kill duration (8 sec)

ESP8266WebServer server(80);

void pressButton(int durationMs) {
  Serial.print("Action: pressing relay for ");
  Serial.print(durationMs);
  Serial.println("ms");

  digitalWrite(ledPin, LOW);   // Onboard LED ON (active LOW)
  digitalWrite(relayPin, LOW); // Relay ON (Low-Level Trigger)

  delay(durationMs);

  digitalWrite(relayPin, HIGH); // Relay OFF
  digitalWrite(ledPin, HIGH);   // Onboard LED OFF
}

// True if PC is ON (5V from PLED through divider yields HIGH on D2)
bool isPcOn() {
  return digitalRead(sensePin) == HIGH;
}

void handleRoot() {
  String html = "<html><head><meta charset='UTF-8'><meta name='viewport' content='width=device-width, initial-scale=1'>";
  html += "<style>body{text-align:center;font-family:Arial,sans-serif;background:#f4f4f4;padding:20px;}";
  html += "h1{display:flex;align-items:center;justify-content:center;margin-bottom:30px;color:#2c3e50;}";
  html += ".indicator{display:inline-block;width:22px;height:22px;border-radius:50%;margin-left:15px;background:#2ecc71;box-shadow: 0 0 10px #2ecc71;transition: 0.3s;}";
  html += ".btn{width:100%;max-width:300px;height:80px;font-size:20px;font-weight:bold;color:white;border:none;border-radius:15px;margin:15px 0;cursor:pointer;display:block;margin-left:auto;margin-right:auto;transition:0.2s;}";
  html += ".btn:active{transform:scale(0.98);}";
  html += ".on{background:#2ecc71;box-shadow:0 5px #27ae60;} .off{background:#e74c3c;box-shadow:0 5px #c0392b;}";
  html += "p{color:#7f8c8d;margin-top:20px;font-size:14px;}</style>";
  html += "<script>";
  html += "setInterval(function(){ fetch('/status').then(r => r.text()).then(status => {";
  html += "var ind = document.querySelector('.indicator');";
  html += "if(status==='ON'){ind.style.background='#2ecc71'; ind.style.boxShadow='0 0 10px #2ecc71';}";
  html += "else{ind.style.background='#e74c3c'; ind.style.boxShadow='0 0 10px #e74c3c';}";
  html += "});}, 2000);";
  html += "function safeToggle() { fetch('/status').then(r => r.text()).then(status => {";
  html += "if (status === 'ON') { if (confirm('PC is already ON. Power action?')) location.href='/power'; }";
  html += "else { location.href='/power'; }";
  html += "});}";
  html += "</script></head>";
  html += "<body><h1>PC Remote <span class='indicator'></span></h1>";
  html += "<button class='btn on' onclick='safeToggle()'>POWER ON/OFF</button>";
  html += "<button class='btn off' onclick=\"if(confirm('Are you sure you want to FORCE SHUTDOWN?')){location.href='/force';}\">FORCE SHUTDOWN</button>";
  html += "<p>Network name: http://" + String(hostName) + ".local</p></body></html>";

  server.send(200, "text/html", html);
}

void handleStatus() {
  server.send(200, "text/plain", isPcOn() ? "ON" : "OFF");
}

void handlePower() {
  server.send(200, "text/html", "<h1>OK! Standard press sent.</h1><script>setTimeout(function(){window.location.href='/';},2000);</script>");
  pressButton(standardPress);
}

void handleForce() {
  server.send(200, "text/html", "<h1>OK! Force shutdown sent.</h1><script>setTimeout(function(){window.location.href='/';},4000);</script>");
  pressButton(forcePress);
}

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

  pinMode(relayPin, OUTPUT);
  pinMode(ledPin, OUTPUT);
  pinMode(sensePin, INPUT); // Standard input mode for reading divider voltage
  pinMode(buttonPin, INPUT_PULLUP);

  digitalWrite(relayPin, HIGH); // Keep Low-Level Relay OFF initially
  digitalWrite(ledPin, HIGH);  // Onboard LED OFF initially

  if (!WiFi.config(local_IP, gateway, subnet, primaryDNS)) {
    Serial.println("STA Failed to configure static IP!");
  }

  WiFi.mode(WIFI_STA);
  WiFi.begin(ssid, password);
  while (WiFi.status() != WL_CONNECTED) { delay(500); Serial.print("."); }

  Serial.println("\nWiFi Connected!");
  Serial.print("IP Address: ");
  Serial.println(WiFi.localIP());

  if (MDNS.begin(hostName)) {
    Serial.println("mDNS responder started: http://" + String(hostName) + ".local");
  }

  server.on("/", handleRoot);
  server.on("/status", handleStatus);
  server.on("/power", handlePower);
  server.on("/force", handleForce);
  server.begin();

  MDNS.addService("http", "tcp", 80);
}

void loop() {
  MDNS.update();
  server.handleClient();

  if (digitalRead(buttonPin) == LOW) {
    delay(50);
    if (digitalRead(buttonPin) == LOW) {
      pressButton(standardPress);
      while(digitalRead(buttonPin) == LOW) delay(10);
    }
  }
}