Relay Module for Arduino: A Complete Guide

A relay module allows an Arduino to control high-voltage devices like lights, fans, motors, or home appliances. Since Arduino operates at low voltage (3.3V/5V) and cannot directly switch high-power loads, a relay acts as an electrically isolated switch.

🔹 1. How a Relay Module Works

A relay module consists of:

1. Electromagnetic Relay – A switch that opens or closes when powered.
2. Optocoupler (for isolation) – Prevents high-voltage backflow to Arduino.
3. Transistor (Driver Circuit) – Controls the relay using a small current.
4. Diode (Flyback Protection) – Prevents voltage spikes when turning off the coil.
5. Indicator LED – Shows relay status.

📌 Function:

• When the Arduino sends a signal (HIGH or LOW), the relay activates or deactivates.
• The relay can switch AC (110V/220V) or DC (12V/24V) loads.

🔹 2. Relay Module Types

🔹 3. Relay Module Pinout

A typical relay module has 5 pins:

📌 “Normally Open” (NO) → Device is OFF by default, turns ON when relay is activated.
📌 “Normally Closed” (NC) → Device is ON by default, turns OFF when relay is activated.

🔹 4. Connecting a Relay Module to Arduino

🛠 Required Components

• 1x Arduino Board (Uno, Mega, Nano, etc.)
• 1x Relay Module (5V)
• 1x 220V Lamp (or 12V DC Motor)
• Jumper Wires
• External Power Source (if needed)

🛠 Wiring Diagram

High-Voltage Load Connection (AC Example):

• COM → Live Wire (from Power Source)
• NO → Load (Lamp, Fan, Motor)
• Neutral Wire → Directly to Load

🔹 5. Arduino Code for Relay Control

This basic example turns a relay ON for 5 seconds, then turns it OFF for 5 seconds.

#define RELAY_PIN 7  // Relay connected to Digital Pin 7

void setup() {
    pinMode(RELAY_PIN, OUTPUT);
}

void loop() {
    digitalWrite(RELAY_PIN, HIGH); // Turn ON Relay
    delay(5000);                   // Wait 5 seconds
    digitalWrite(RELAY_PIN, LOW);  // Turn OFF Relay
    delay(5000);                   // Wait 5 seconds
}

📌 For Active-Low Relays (some modules work in reverse):

• Use LOW to turn ON and HIGH to turn OFF.

🔹 6. Controlling a Relay with a Sensor

Example: Turn on a light when it’s dark (using LDR sensor).

#define RELAY_PIN 7
#define LDR_PIN A0

void setup() {
    pinMode(RELAY_PIN, OUTPUT);
    pinMode(LDR_PIN, INPUT);
}

void loop() {
    int lightValue = analogRead(LDR_PIN);  // Read LDR sensor
    if (lightValue < 500) {  // If it's dark
        digitalWrite(RELAY_PIN, HIGH);  // Turn ON relay
    } else {
        digitalWrite(RELAY_PIN, LOW);  // Turn OFF relay
    }
}

📌 Other Relay Applications:
✅ Temperature-Based Control (DHT11 + Relay for Fan).
✅ Motion Detection (PIR Sensor + Relay for Security Light).
✅ Wi-Fi Home Automation (ESP8266 + Relay).

🔹 7. Safety Precautions When Using Relays

⚠️ Avoid Directly Touching High-Voltage Wires.
⚠️ Use Relays with Optocouplers for Electrical Isolation.
⚠️ Do Not Exceed the Relay’s Rated Voltage/Current.
⚠️ For AC Loads, Use Proper Insulation & Fuse Protection.

🎯 Conclusion

• A relay module allows Arduino to control high-voltage appliances.
• It works by switching AC or DC loads using a small control signal.
• Can be used for home automation, security, and industrial applications.
• Always follow safety guidelines when working with high voltage.