Arduino Mega · Relay

Arduino Mega Relay Simulator: Wiring, Code and a Live Circuit

This is a single-channel 5 V relay module wired to an Arduino Mega 2560, clicking on and off once a second live in your browser. The arm on the drawing moves between NC and NO as the board switches it.

VCC to 5V, GND to GND, and IN to pin 2. The pin only gives the signal; the module powers the coil.

Run it in the editor Read the code

5 V logic
No library
IN on pin 2
Coil on 5V

Arduino Mega · Relaylive0.000 s 0.00x

Click to open it in the editor

The circuit itself, running on the simulator with the sketch below. Press what can be pressed; click anything else to open it in the editor.

Wiring the Relay to the Arduino Mega

Every connection in the circuit above, with the board’s own pin names.

Relay pin	Arduino Mega pin	Why
VCC	5V	The coil is a 5 V coil; it takes about 70 mA, more than any pin can give.
GND	GND	Common ground.
IN	pin 2	A logic signal only: the module’s transistor switches the coil.
COM, NO, NC	The load	The switched side: COM is the arm, NO is open until the relay pulls in, NC is closed until then.

The Arduino Mega drives IN at 5 V and the module’s own transistor carries the coil current, so the pin only supplies a few milliamps. The simulated relay triggers on HIGH; many blue modules trigger on LOW, set by the part’s trigger property.

Pins to leave alone on the Arduino Mega

Pins 14 to 19 are the three extra serial ports and 20 and 21 are I2C, so keep them free if you will need those. An Uno shield’s I2C on A4 and A5 is not connected to anything on a Mega.

On a real bench

The Mega is long, so put the breadboard beside its double row of pins 22 to 53 and leave the left-hand headers free. The Relay uses only a few of its 54 pins; a Mega earns its size when one sketch drives a keypad, a display and several sensors at once.

The code

The sketch the circuit above runs. Open it in the editor, change a line and press Run: it compiles in your browser.

Relay on Arduino Mega pin 2

const int RELAY_PIN = 2;   // IN, pin 2

void setup() {
  Serial.begin(9600);
  pinMode(RELAY_PIN, OUTPUT);
  digitalWrite(RELAY_PIN, LOW);      // released at start
}

void loop() {
  digitalWrite(RELAY_PIN, HIGH);     // pull in: COM to NO
  Serial.println("Relay on");
  delay(1000);
  digitalWrite(RELAY_PIN, LOW);      // release: COM back to NC
  Serial.println("Relay off");
  delay(1000);
}

digitalWrite(RELAY_PIN, HIGH) turns on the module’s transistor, which energizes the coil and pulls the arm from NC over to NO about 10 ms later. LOW lets it fall back.

The relay is off at the start because setup() writes LOW before anything else. Do the same in a real project, so the load does not flick on while the board boots.

Serial.begin(9600) opens the serial monitor. Serial is the USB port; Serial1 to Serial3 are on pins 18 and 19, 16 and 17, and 14 and 15.

Common mistakes with the Relay on the Arduino Mega

Active low modules

Many cheap relay boards switch when IN goes LOW, so the sketch seems inverted and the relay is on at boot. Check the board, and in the editor set the relay’s trigger property to low to match.

Driving a coil straight from a pin

A bare relay coil wants 70 mA or more and kicks back a voltage spike when it releases. On a module the transistor and the flyback diode are already there; a bare relay needs both, and never a direct pin.

Switching mains without care

The contacts are rated for mains, but the board and the screw terminals are not a safe enclosure. Learn on a 12 V lamp or a motor, and leave mains to a properly rated, enclosed module.

Questions and answers

Can I simulate a relay with an Arduino Mega 2560?

Yes. The circuit at the top of this page is an Arduino Mega 2560 with a relay module, running in your browser on a simulated ATmega2560. The simulated relay has a pull-in and release time, and its contacts really connect COM to NO or NC, so a lamp wired through it switches. Press Run it in the editor to change the wiring or the code; it is free and needs no account.

Can the pin power the relay?

No: a pin gives about 20 mA and the coil wants more. The module’s transistor does the switching; the pin only drives the transistor.

What do NO and NC mean?

Normally open and normally closed. With the coil off, COM is connected to NC; with it on, COM is connected to NO. Wire a load to NO to have it off until the relay switches.

How fast can a relay switch?

A few times a second at most. It takes about 10 ms to pull in and its contacts wear with every click, so for anything faster, or for dimming, use a MOSFET.

Build your own Relay project

Open this circuit in the editor, change the wiring or the code, and keep your version in a free account.

Start building free Open the editor