ESP8266 NodeMCU Relay Module – Control AC Appliances

The rise of home automation has transformed how we interact with everyday appliances. Using an ESP8266 NodeMCU and a relay module, it’s now possible to control AC appliances remotely via a simple web server. This tutorial shows you exactly how to make it happen, even if you’re just getting started with IoT projects.

What Is ESP8266 NodeMCU?

The ESP8266 NodeMCU is a low-cost Wi-Fi-enabled microcontroller perfect for building smart automation systems. It offers:

Built-in Wi-Fi capabilities
Multiple GPIO pins for controlling external devices
Compatibility with the Arduino IDE

Compared to traditional Arduino boards, the ESP8266 shines with its wireless communication capabilities, making it a go-to choice for connected applications.

If you’re unfamiliar with pin mapping, check this ESP8266 GPIO reference for details on which pins are safe and recommended for use.

What is a Relay Module and How Does It Work?

A relay module acts as an electrically operated switch, allowing low-voltage digital signals to control high-voltage AC devices safely. Key features include:

Multiple channels (1, 2, 4, 8, or 16 relays)
Built-in optocouplers for isolation between control and power circuits
COM (Common), NO (Normally Open), and NC (Normally Closed) terminals

To learn more about the internal workings, explore how relays function in circuits.

Safety First: Working with AC Appliances

Handling mains voltage is dangerous if not done properly. Before wiring an AC appliance:

Always disconnect power before touching connections
Use heat-shrink tubing or electrical insulation
Avoid working directly with high voltage unless you’re experienced
Start by using low-voltage devices (like 12V lights) for testing

Components Required

Before starting the project, gather the following:

ESP8266 NodeMCU board
Relay module (1-channel or more)
An AC appliance (lamp, fan, etc.)
Jumper wires, breadboard, USB cable
Arduino IDE installed on your computer
Install ESP8266 board in Arduino IDE

Connecting the Relay Module to ESP8266

Here’s how to wire the relay module to the ESP8266:

ESP8266 NodeMCU Relay Module – Control AC Appliances (Web Server)

Connect VCC of the relay to 3.3V on ESP8266 (or external 5V with optocoupler)
GND to GND
IN pin to a GPIO pin (e.g., D1 or GPIO5)
AC appliance connected to COM and NO (Normally Open) terminals

Note: Use a proper power source if driving more than one relay.

Writing the Arduino Sketch

With the hardware wired up, the next step is coding.

Use Arduino IDE to program the ESP8266
Include necessary libraries: ESP8266WiFi.h, ESP8266WebServer.h
Define your GPIO pin used for the relay
Set up Wi-Fi connection
Create HTTP routes to handle ON/OFF requests

Here’s a basic snippet:

digitalWrite(relayPin, HIGH); // Turns ON
digitalWrite(relayPin, LOW);  // Turns OFF

This code allows you to toggle the appliance from your web interface.

Building a Web Server Interface

With the ESP8266 hosting a simple web server, you can interact via a browser.

Create a basic HTML page with two buttons: ON and OFF
Link the buttons to ESP8266 endpoints (/on, /off)
Use ESP8266WebServer or ESPAsyncWebServer for efficient handling of requests

Your interface can look like this:

#include "ESP8266WiFi.h"
#include "ESPAsyncWebServer.h"

// Set to true to define Relay as Normally Open (NO)
#define RELAY_NO    true

// Set number of relays
#define NUM_RELAYS  5

// Assign each GPIO to a relay
int relayGPIOs[NUM_RELAYS] = {5, 4, 14, 12, 13};

// Replace with your network credentials
const char* ssid = "REPLACE_WITH_YOUR_SSID";
const char* password = "REPLACE_WITH_YOUR_PASSWORD";

const char* PARAM_INPUT_1 = "relay";  
const char* PARAM_INPUT_2 = "state";

// Create AsyncWebServer object on port 80
AsyncWebServer server(80);

const char index_html[] PROGMEM = R"rawliteral(
<!DOCTYPE HTML><html>
<head>
  <meta name="viewport" content="width=device-width, initial-scale=1">
  <style>
    html {font-family: Arial; display: inline-block; text-align: center;}
    h2 {font-size: 3.0rem;}
    p {font-size: 3.0rem;}
    body {max-width: 600px; margin:0px auto; padding-bottom: 25px;}
    .switch {position: relative; display: inline-block; width: 120px; height: 68px} 
    .switch input {display: none}
    .slider {position: absolute; top: 0; left: 0; right: 0; bottom: 0; background-color: #ccc; border-radius: 34px}
    .slider:before {position: absolute; content: ""; height: 52px; width: 52px; left: 8px; bottom: 8px; background-color: #fff; -webkit-transition: .4s; transition: .4s; border-radius: 68px}
    input:checked+.slider {background-color: #2196F3}
    input:checked+.slider:before {-webkit-transform: translateX(52px); -ms-transform: translateX(52px); transform: translateX(52px)}
  </style>
</head>
<body>
  <h2>ESP Web Server</h2>
  %BUTTONPLACEHOLDER%
<script>function toggleCheckbox(element) {
  var xhr = new XMLHttpRequest();
  if(element.checked){ xhr.open("GET", "/update?relay="+element.id+"&state=1", true); }
  else { xhr.open("GET", "/update?relay="+element.id+"&state=0", true); }
  xhr.send();
}</script>
</body>
</html>
)rawliteral";

// Replaces placeholder with button section in your web page
String processor(const String& var){
  //Serial.println(var);
  if(var == "BUTTONPLACEHOLDER"){
    String buttons ="";
    for(int i=1; i<=NUM_RELAYS; i++){
      String relayStateValue = relayState(i);
      buttons+= "<h4>Relay #" + String(i) + " - GPIO " + relayGPIOs[i-1] + "</h4><label class=\"switch\"><input type=\"checkbox\" onchange=\"toggleCheckbox(this)\" id=\"" + String(i) + "\" "+ relayStateValue +"><span class=\"slider\"></span></label>";
    }
    return buttons;
  }
  return String();
}

String relayState(int numRelay){
  if(RELAY_NO){
    if(digitalRead(relayGPIOs[numRelay-1])){
      return "";
    }
    else {
      return "checked";
    }
  }
  else {
    if(digitalRead(relayGPIOs[numRelay-1])){
      return "checked";
    }
    else {
      return "";
    }
  }
  return "";
}

void setup(){
  // Serial port for debugging purposes
  Serial.begin(115200);

  // Set all relays to off when the program starts - if set to Normally Open (NO), the relay is off when you set the relay to HIGH
  for(int i=1; i<=NUM_RELAYS; i++){
    pinMode(relayGPIOs[i-1], OUTPUT);
    if(RELAY_NO){
      digitalWrite(relayGPIOs[i-1], HIGH);
    }
    else{
      digitalWrite(relayGPIOs[i-1], LOW);
    }
  }
  
  // Connect to Wi-Fi
  WiFi.begin(ssid, password);
  while (WiFi.status() != WL_CONNECTED) {
    delay(1000);
    Serial.println("Connecting to WiFi..");
  }

  // Print ESP8266 Local IP Address
  Serial.println(WiFi.localIP());

  // Route for root / web page
  server.on("/", HTTP_GET, [](AsyncWebServerRequest *request){
    request->send(200, "text/html", index_html, processor);
  });

  // Send a GET request to <ESP_IP>/update?relay=<inputMessage>&state=<inputMessage2>
  server.on("/update", HTTP_GET, [] (AsyncWebServerRequest *request) {
    String inputMessage;
    String inputParam;
    String inputMessage2;
    String inputParam2;
    // GET input1 value on <ESP_IP>/update?relay=<inputMessage>
    if (request->hasParam(PARAM_INPUT_1) & request->hasParam(PARAM_INPUT_2)) {
      inputMessage = request->getParam(PARAM_INPUT_1)->value();
      inputParam = PARAM_INPUT_1;
      inputMessage2 = request->getParam(PARAM_INPUT_2)->value();
      inputParam2 = PARAM_INPUT_2;
      if(RELAY_NO){
        Serial.print("NO ");
        digitalWrite(relayGPIOs[inputMessage.toInt()-1], !inputMessage2.toInt());
      }
      else{
        Serial.print("NC ");
        digitalWrite(relayGPIOs[inputMessage.toInt()-1], inputMessage2.toInt());
      }
    }
    else {
      inputMessage = "No message sent";
      inputParam = "none";
    }
    Serial.println(inputMessage + inputMessage2);
    request->send(200, "text/plain", "OK");
  });
  // Start server
  server.begin();
}
  
void loop() {

}

Expanding to Multiple Relays

Want to control multiple appliances?

Connect additional relays to unused GPIOs
Update the Arduino sketch to include multiple endpoints
Modify the web interface to show buttons for each device

For instance, using D1, D2, and D3 to control three relays individually.

Adding Security to the Web Server

To avoid unauthorized access:

Add basic HTTP authentication to your web server
Use strong passwords and change default network credentials
Avoid exposing the device directly to the internet

Bonus Tip: Use a captive portal or local network IP restrictions for added protection.

Troubleshooting Common Issues

Here’s how to solve typical problems:

Relay not clicking? Check GPIO logic level; some modules are LOW triggered
Web server unresponsive? Check Wi-Fi credentials and re-upload code
Appliance not switching? Confirm relay connection to NO/COM terminals

Monitor serial output via Arduino IDE to debug responses.

Real-World Applications

The project opens up many real-world automation ideas:

Control room lighting and ceiling fans
Remote start coffee machines or air conditioners
Automate outdoor lighting with timers
Integrate with smart home assistants via MQTT or IFTTT

FAQs

How do I control AC appliances with ESP8266?
Use a relay module to safely switch the AC appliance on or off using ESP8266’s digital GPIO signals, controlled through a web interface.

Is it safe to use ESP8266 with AC appliances?
Yes, if proper isolation is used through optocouplers and if safety precautions are followed strictly.

Can I control multiple appliances with one ESP8266?
Yes, by using a multi-channel relay module and connecting each to a different GPIO pin.

What is the difference between NO and NC terminals?

NO (Normally Open): Circuit is OFF by default, closes when relay is activated
NC (Normally Closed): Circuit is ON by default, opens when relay is activated

How can I secure my ESP8266 web server?
Implement basic authentication, use secure passwords, and restrict access to local networks only.

Conclusion

By combining the ESP8266 NodeMCU with a relay module, you can easily control AC appliances via a browser, creating powerful and flexible home automation solutions. With simple code and a minimal setup, you’re well on your way to building a smarter home or lab project.

Stay safe, start simple, and continue exploring the endless possibilities of IoT and web-based automation.