Relay Module - Controlling High Voltage

What is a Relay?

A relay is an electrically operated switch. It allows a low-voltage circuit (Arduino) to control a high-voltage circuit (like 120V AC lights).

Why Use a Relay?

• Arduino outputs only 5V and 40mA
• Cannot directly control home electrical devices
• Relay acts as a bridge between Arduino and high voltage

Types of Relay Modules

Relay Module Specifications

Common 1-Channel 5V Relay

Pin Configuration

Connecting the Relay Module

Basic Connection

Arduino          Relay Module
5V       ------  VCC
GND      ------  GND
Pin 7    ------  IN
Load (230V bulb):
L (Live)  ---[Bulb]---  COM
                       |
                      NO (when triggered)
                      
N (Neutral) ----------------------

Circuit Diagram

Low Voltage Side (Arduino)     High Voltage Side (230V)
+------------------+          +------------------+
|                  |          |                  |
|   Arduino        |          |   Mains Power    |
|   Pin 7 ---------+---IN    |                  |
|   5V   ---------+---VCC    |   Bulb/Lamp      |
|   GND  ---------+---GND    |        |         |
+------------------+          |        +--COM    |
                              |        |         |
                              |   NO---+         |
                              |                  |
                              +------------------+

Basic Code

Turning Load On/Off

const int relayPin = 7;
void setup() {
  pinMode(relayPin, OUTPUT);
}
void loop() {
  digitalWrite(relayPin, HIGH);  // Turn on (NO closes)
  delay(3000);                    // Wait 3 seconds
  
  digitalWrite(relayPin, LOW);   // Turn off (NO opens)
  delay(3000);                    // Wait 3 seconds
}

With Button Control

const int relayPin = 7;
const int buttonPin = 2;
void setup() {
  pinMode(relayPin, OUTPUT);
  pinMode(buttonPin, INPUT_PULLUP);
}
void loop() {
  if (digitalRead(buttonPin) == LOW) {
    digitalWrite(relayPin, HIGH);  // Turn on
  } else {
    digitalWrite(relayPin, LOW);   // Turn off
  }
}

Project: Temperature-Controlled Fan

Control a fan based on temperature:

const int relayPin = 7;
const int tempPin = A0;
const int fanOnTemp = 30;  // Celsius
void setup() {
  pinMode(relayPin, OUTPUT);
  pinMode(tempPin, INPUT);
}
void loop() {
  int reading = analogRead(tempPin);
  float voltage = reading * 5.0 / 1024.0;
  float tempC = (voltage - 0.5) * 100;
  
  if (tempC > fanOnTemp) {
    digitalWrite(relayPin, HIGH);  // Turn on fan
  } else {
    digitalWrite(relayPin, LOW);   // Turn off fan
  }
  
  delay(1000);
}

Project: Automatic Night Light

Turn on light when it gets dark:

const int relayPin = 7;
const int ldrPin = A0;
const int darkThreshold = 300;
void setup() {
  pinMode(relayPin, OUTPUT);
  pinMode(ldrPin, INPUT);
}
void loop() {
  int lightLevel = analogRead(ldrPin);
  
  if (lightLevel < darkThreshold) {
    digitalWrite(relayPin, HIGH);  // Night - light on
  } else {
    digitalWrite(relayPin, LOW);   // Day - light off
  }
  
  delay(500);
}

Safety Warnings

⚠️ IMPORTANT: High Voltage Danger

Working with mains voltage (120V/230V) can be lethal!

Safety Rules

1. Always disconnect power before wiring 2. Use proper insulation on all connections 3. Never touch bare wires when powered 4. Keep the circuit isolated from Arduino 5. Use proper enclosures for final projects

Recommended Practices

• Use proper electrical box for high-voltage parts
• Label your circuit clearly
• Test with low-voltage load first (like 12V)
• Have someone experienced supervise
• Consider using optocouplers for extra isolation

Multiple Relays

Controlling multiple devices:

const int relay1 = 7;
const int relay2 = 6;
const int relay3 = 5;
const int relay4 = 4;
void setup() {
  pinMode(relay1, OUTPUT);
  pinMode(relay2, OUTPUT);
  pinMode(relay3, OUTPUT);
  pinMode(relay4, OUTPUT);
}
void loop() {
  // Turn on sequentially
  digitalWrite(relay1, HIGH);
  delay(500);
  digitalWrite(relay2, HIGH);
  delay(500);
  digitalWrite(relay3, HIGH);
  delay(500);
  digitalWrite(relay4, HIGH);
  delay(2000);
  
  // Turn off all
  digitalWrite(relay1, LOW);
  digitalWrite(relay2, LOW);
  digitalWrite(relay3, LOW);
  digitalWrite(relay4, LOW);
  delay(2000);
}

Summary

Relay modules bridge Arduino and high-voltage devices:

• Low-level trigger activates with LOW signal
• NO (Normally Open) closes when activated
• NC (Normally Closed) opens when activated
• Always prioritize safety with high voltage
• Can control lights, fans, motors, appliances

Next Lesson

In the final lesson, you'll complete a project combining everything you've learned.