What Is a Relay Module? Control Lamps, Sockets and Pumps with Arduino

A relay is an electrically operated switch that uses a small electrical signal to turn a much bigger load on and off. An Arduino pin can only supply 5 V and a few tens of milliamps; that's enough for an LED, but not for a lamp, a fan, a water pump or a device running on 230 V mains. A relay module bridges the gap: the Arduino sends a small signal, and the relay closes its internal contacts so the large current flows through the load. It's the core part of smart home, automatic watering and timer projects.

How does a relay work?

Inside a relay there's a coil (an electromagnet), a movable metal arm and a set of contacts. When current flows through the coil, the magnetic field pulls the arm and closes the contacts; the "click" you hear is that movement. When the current stops, a spring pushes the arm back and the contacts open.

The key point is that there's no electrical connection between the coil side (the control circuit) and the contact side (the load circuit). They're linked only magnetically. That keeps your 5 V Arduino circuit isolated from the mains line.

What's on a relay module?

You can't connect a bare relay straight to an Arduino pin: the coil draws more current than the pin can supply, and when it switches off it produces a voltage spike that can destroy the pin. A module board comes with the parts that solve these problems:

• Driver transistor: switches the 70–80 mA the coil needs using the small signal from the pin.
• Flyback diode: absorbs the reverse voltage produced when the coil switches off, protecting the transistor and the Arduino.
• Optocoupler (on some models): passes the control signal across as light, fully separating the Arduino from the relay circuit.
• Status LED: lights up when the relay is on; very handy for checking that your code works.
• Screw terminals: let you connect the load wires securely without soldering.

What do NO, NC and COM mean?

The load side of a relay has three terminals. Which ones you use decides whether your device is on or off when the relay is not energised:

Specifications (common 5 V module)

Connecting a relay module to Arduino

The control side takes three wires: connect the module's VCC to 5V, GND to GND and IN to digital pin 7. On the load side, you cut one of the wires of the device you want to control and connect its two ends to the COM and NO terminals. The relay simply takes the place of an ordinary wall switch.

1const int RELAY_PIN = 7;
2
3void setup() {
4  pinMode(RELAY_PIN, OUTPUT);
5  digitalWrite(RELAY_PIN, HIGH);  // aktif LOW modülde kapalı başla / start OFF on active-LOW modules
6}
7
8void loop() {
9  digitalWrite(RELAY_PIN, LOW);   // röle çeker, cihaz açılır / relay on
10  delay(3000);
11  digitalWrite(RELAY_PIN, HIGH);  // röle bırakır, cihaz kapanır / relay off
12  delay(3000);
13}

Controlling a relay with a button: on-off switch

In real projects you usually control the relay with a button, a sensor or a timer. The code below toggles the relay each time you press the button, so you can switch a mains lamp with a tiny push button. For button wiring and bounce, see How to Connect a Push Button to Arduino.

1const int RELAY_PIN = 7;
2const int BUTTON_PIN = 2;      // butonun diğer ucu GND'de / other leg to GND
3
4bool relayOn = false;
5bool lastButton = HIGH;
6
7void setup() {
8  pinMode(RELAY_PIN, OUTPUT);
9  pinMode(BUTTON_PIN, INPUT_PULLUP);
10  digitalWrite(RELAY_PIN, HIGH); // başta kapalı / start OFF (active LOW)
11}
12
13void loop() {
14  bool button = digitalRead(BUTTON_PIN);
15  if (lastButton == HIGH && button == LOW) {   // yeni basış / new press
16    relayOn = !relayOn;
17    digitalWrite(RELAY_PIN, relayOn ? LOW : HIGH);
18    delay(50);                                  // sıçrama önleme / debounce
19  }
20  lastButton = button;
21}

JD-VCC and the optocoupler: full isolation

On optocoupler modules, next to the VCC, GND and IN pins there's a pin called JD-VCC with a jumper. With the jumper fitted, the relay coil is also powered from the Arduino's 5 V. If you remove the jumper and connect a separate 5 V adapter to JD-VCC and its own GND, the coil current comes entirely from that adapter, and the Arduino side only lights the optocoupler's LED. With noisy loads such as motors and pumps, this greatly reduces Arduino freezes and resets.

Relays with the ESP32 and Raspberry Pi Pico

ESP32 and Pico pins output 3.3 V. Most 5 V relay modules will trigger from this signal, but on an active-LOW module a 3.3 V HIGH may not always release the relay fully; if your relay stays on all the time, this is why. The fix is to buy a 3.3 V-trigger module, use an optocoupler board with a JD-VCC split, or add a small NPN transistor. The module itself should still be powered from 5 V. Controlling a relay from a Pico in MicroPython is as simple as this:

1from machine import Pin
2import time
3
4relay = Pin(15, Pin.OUT, value=1)   # GP15, aktif LOW: 1 = kapalı / active LOW: 1 = off
5
6while True:
7    relay.value(0)    # röle çeker / relay on
8    time.sleep(3)
9    relay.value(1)    # röle bırakır / relay off
10    time.sleep(3)

Safety rules for working with mains voltage

• Respect the current rating: never exceed the value printed on the relay (for example 10A 250VAC). With high-power devices such as heaters, air conditioners and kettles, stay well below that limit.
• Switch the live wire: when controlling a lamp or socket, route the live wire through the relay, not the neutral, so no voltage remains on the device when it's off.
• Use proper cable: breadboard jumper wires aren't made for mains; on the load side use insulated cable of a suitable cross-section.
• Insulate the underside of the board: the module's solder side carries mains voltage; don't leave it touching metal surfaces or bare hands.
• Add a fuse: a suitably rated fuse in the load line prevents a fire in case of a short circuit.

Relay, transistor or solid-state relay?

In short: if you'll switch a device on and off now and then, a mechanical relay is the cheapest and easiest solution. To adjust a motor's speed or an LED strip's brightness, choose a transistor or MOSFET, and for AC loads switched many times a second, a solid-state relay. To choose a suitable transistor, have a look at the Transistors tool.

Relay project ideas

• Smart watering: run a water pump when the soil moisture sensor says the soil is dry.
• Motion-activated light: turn a lamp on when a PIR sensor detects someone and off again after a while.
• Humidity-controlled fan: start the bathroom extractor when humidity rises; for measuring, see DHT11 Temperature and Humidity Sensor.
• Remote-controlled socket: switch a socket with an IR receiver and a TV remote.
• Timer: turn an aquarium light on and off at the same time every day.
• Phone control: manage a lamp from a web page using the ESP32's Wi-Fi.

Common mistakes and how to fix them

• The relay works backwards: the module is active LOW; swap HIGH and LOW in the code.
• The relay keeps clicking: the power supply may be too weak; use an external 5 V adapter, especially with several relays.
• The Arduino resets when the motor starts: noise and voltage drop; power the coil separately via JD-VCC and add a capacitor across the motor.
• The LED is on but the load doesn't run: the load wires may be on NC, or a terminal screw may be loose.
• The relay never releases with an ESP32: the 3.3 V HIGH can't switch off an active-LOW module; use a 3.3 V-compatible module or a transistor.
• Connecting a bare relay straight to a pin: the coil current and reverse voltage destroy the pin; always use a module, or a transistor plus a diode.

Once you pair a relay with a sensor, you start building automations that are genuinely useful. To see which sensor does what, take a look at the Arduino Sensors Guide; in the BlueGrays app you'll find relay lamp, watering and lighting projects step by step with circuit diagrams.