Introduction To Relay

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On August 22, 2025

Relays are a great way to control high-voltage or high-current devices like lamps, motors, or appliances with a low-power microcontroller like an Arduino. In this post, I’ll show you how to get started with a relay, a button, and an Arduino.

Whether you're just starting out or adding a relay to your DIY automation project, this guide is perfect for beginners.

What is a Relay?

A relay is an electrically operated switch. It allows one circuit (like your Arduino) to control another circuit (like a lamp or fan) without direct electrical connection. This is useful when the controlled circuit operates at a different voltage or current level than the Arduino.

What You Need

Component	Description
Arduino (Uno, Nano, etc.)	Your microcontroller brain
Relay Module	1-channel 5V relay
Push Button	To trigger the relay
10kΩ Resistor	Used for button pull-down
Jumper Wires	For connections
Breadboard	for prototyping

Schematic

Wiring summary:

Button: Connected to pin 3, with a 10kΩ pull-down resistor (one leg to GND).
Relay module: IN pin to Arduino pin 2, VCC to 5V, GND to GND.
When button is pressed, the relay toggles its state (on/off).

const int buttonPin = 3;     // Button connected to pin 3
const int relayPin = 2;      // Relay connected to pin 2

bool relayState = false;     // Current state of the relay
bool lastButtonState = HIGH; // Previous button state
unsigned long debounceTime = 50; // Debounce time in milliseconds
unsigned long lastDebounceTime = 0;

void setup() {
  pinMode(buttonPin, INPUT_PULLUP); // Enable internal pull-up resistor
  pinMode(relayPin, OUTPUT);
  digitalWrite(relayPin, LOW);      // Start with relay OFF
}

void loop() {
  bool reading = digitalRead(buttonPin);

  // Check for button state change
  if (reading != lastButtonState) {
    lastDebounceTime = millis(); // Reset debounce timer
  }

  // If stable for debounce time, toggle the relay
  if ((millis() - lastDebounceTime) > debounceTime) {
    if (reading == LOW && lastButtonState == HIGH) {
      relayState = !relayState;
      digitalWrite(relayPin, relayState ? HIGH : LOW);
    }
  }

  lastButtonState = reading;
}

How It Works

The button is connected with a pull-down resistor, so it reads LOW when unpressed and HIGH when pressed.
When the button is pressed, the Arduino toggles the relay state.
Debouncing is used to prevent accidental multiple triggers from a single press.
The relay turns ON or OFF based on the current state.

Note: Some relay modules are active-low, meaning they activate when their input is LOW. If that’s the case, just reverse the digitalWrite logic.

✅ Applications of Relays

Home automation: Lights, fans, heaters
Industrial control: Pumps, valves, motors
IoT systems: Smart energy management

⚠️ Safety First

Relays can control high voltage AC. Always make sure you're handling high voltage wiring with care.
Never touch the relay terminals when it's powered.
If you're new to handling AC, stick to low-voltage testing, or seek help before working with mains electricity.

Tutorials