Arduino Uno Relay Simulator: Wiring, Code and a Live Circuit
This is a single-channel 5 V relay module wired to an Arduino Uno, 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.
- 5 V logic
- No library
- IN on pin 2
- Coil on 5V
Wiring the Relay to the Arduino Uno
Every connection in the circuit above, with the board’s own pin names.
| Relay pin | Arduino Uno 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 Uno 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 Uno
Pins 0 and 1 are the USB serial port on a real Uno, so anything wired there fights the upload and the serial monitor. Pin 13 carries the onboard LED.
On a real bench
On a real bench the Uno stands beside the breadboard: the Relay goes into the breadboard and male-to-male jumpers run from the Uno’s female headers to its rows. Power the Uno from USB while you build, and unplug it before you move a wire.
The code
The sketch the circuit above runs. Open it in the editor, change a line and press Run: it compiles in your browser.
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 goes over the USB cable, and the serial monitor in the editor reads it at whatever baud rate the sketch sets.
Common mistakes with the Relay on the Arduino Uno
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 Uno?
Yes. The circuit at the top of this page is an Arduino Uno with a relay module, running in your browser on a simulated ATmega328P. 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.