Lesson 1.14: Relay

A relay lets a 5 V signal control a completely separate circuit: another supply, another voltage, another current. It's the bridge between the Arduino and loads it can't handle by itself.

Parts

How it works

Inside there's a coil and a moving contact. When current flows, the coil becomes an electromagnet and moves the contact from NC (normally closed) to NO (normally open). You hear a click on every change.

The coil of a 5 V relay draws around 70–90 mA, too much for a pin. An NPN transistor acts as the middleman: the pin only supplies the small base current, and the transistor carries the coil current.

When the current is cut, the coil generates a reverse voltage spike that can destroy the transistor. A diode across the coil, cathode towards positive, gives it a safe path.

The relay's contacts are isolated from the control circuit: that's why the GNDs don't need joining here. In this course they're only used with low voltages; never connect mains power (110–230 V) on a breadboard.

Wiring

Component	Connects to	Note
Transistor, base	Pin 7	Through a 1 kΩ resistor
Transistor, emitter	GND
Transistor, collector	Relay, coil (terminal 1)
Relay, coil (terminal 2)	5V
Diode	Across the coil	Cathode (the band) towards 5V
Relay, COM	Power module 5V rail
Relay, NO	Motor, terminal 1
Motor, terminal 2	Power module GND rail

Code

// Switches the relay on for two seconds out of every four.
const int RELAY_PIN = 7;

void setup() {
  pinMode(RELAY_PIN, OUTPUT);
  Serial.begin(9600);
}

void loop() {
  digitalWrite(RELAY_PIN, HIGH); // transistor conducts: coil energised
  Serial.println("Relay on: motor running");
  delay(2000);

  digitalWrite(RELAY_PIN, LOW);
  Serial.println("Relay off: motor stopped");
  delay(2000);
}

What the code does

The program doesn't know there's a relay: it sets a pin HIGH or LOW, just like with an LED. The circuit does all the work.
With HIGH on the base the transistor conducts, and the coil current flows from 5V to GND through it.
The two-second pauses are kind to the relay: its contacts take a few milliseconds to move and it isn't made to switch hundreds of times a second.
If you use a relay module instead of a bare relay, check whether it switches on with HIGH or LOW: many work the other way round.

Step by step

Identify the transistor's legs in its datasheet, and the relay's: two for the coil, COM, NO and NC.
Build the control circuit: pin 7, resistor, transistor, coil and diode.
Build the load circuit separately, with the power module: COM, NO and the motor.
Upload the code and open the serial monitor at 9600 baud.

Result

Every two seconds the relay clicks and the motor starts or stops, even though it runs from a different supply than the Arduino.

If it doesn't work

If you hear the click but the motor doesn't start, the load is on NC instead of NO, or the power module is off.
If there's no click, check the transistor: with its legs in another order it won't conduct.
If the board resets or the transistor heats up when switching, the diode is missing or reversed.