How to Use 5V Relay: Examples, Pinouts, and Specs

A 5V relay is an electromechanical switch that uses a low voltage (5V) control signal to open or close a circuit, enabling the control of higher voltage devices. It consists of a coil, an armature, and contacts. When the coil is energized with a 5V signal, it creates a magnetic field that moves the armature, switching the contacts between their normally open (NO) and normally closed (NC) states.

• Home automation systems (e.g., controlling lights, fans, or appliances)
• Industrial control systems
• Motor control circuits
• IoT projects for switching high-power devices
• Safety systems requiring electrical isolation between control and load circuits

• Operating Voltage (Coil): 5V DC
• Trigger Current: ~70-100 mA
• Contact Ratings:
  • Maximum Voltage: 250V AC / 30V DC
  • Maximum Current: 10A
• Relay Type: Single Pole Double Throw (SPDT)
• Isolation: Electrical isolation between control and load circuits
• Switching Time:
  • Operate Time: ~10 ms
  • Release Time: ~5 ms
• Dimensions: Varies by manufacturer, typically compact for PCB mounting

The 5V relay typically has 5 pins. Below is the pin configuration:

1. Power the Relay Coil:
   • Connect the Coil+ pin to a 5V DC control signal (e.g., from a microcontroller or power supply).
   • Connect the Coil- pin to ground.
2. Connect the Load Circuit:
   • Connect the load's power source to the COM pin.
   • Connect the load to either the NO or NC pin, depending on the desired behavior:
     • Use NO if the load should be powered only when the relay is activated.
     • Use NC if the load should be powered by default and disconnected when the relay is activated.
3. Control the Relay:
   • Apply a 5V signal to the Coil+ pin to activate the relay and switch the load circuit.

• Flyback Diode: Always connect a flyback diode (e.g., 1N4007) across the relay coil terminals to protect the control circuit from voltage spikes caused by the collapsing magnetic field when the relay is deactivated.
• Transistor Driver: If the control signal source (e.g., a microcontroller) cannot supply sufficient current to the relay coil, use a transistor (e.g., 2N2222) as a driver.
• Isolation: Ensure proper electrical isolation between the control and load circuits to prevent damage to sensitive components.
• Power Ratings: Do not exceed the relay's voltage and current ratings for the load circuit.

Below is an example of how to control a 5V relay using an Arduino UNO:

• Connect the Coil+ pin of the relay to a digital pin on the Arduino (e.g., pin 7) through a transistor driver.
• Connect the Coil- pin to the Arduino's GND.
• Connect the load circuit to the COM and NO pins of the relay.

// Define the relay pin
const int relayPin = 7;

void setup() {
  // Set the relay pin as an output
  pinMode(relayPin, OUTPUT);
}

void loop() {
  // Turn the relay ON (activate)
  digitalWrite(relayPin, HIGH);
  delay(5000); // Keep the relay ON for 5 seconds

  // Turn the relay OFF (deactivate)
  digitalWrite(relayPin, LOW);
  delay(5000); // Keep the relay OFF for 5 seconds
}

1. Relay Not Activating:

   • Cause: Insufficient current from the control signal.
   • Solution: Use a transistor driver circuit to amplify the control signal.

2. Load Not Switching:

   • Cause: Incorrect wiring of the load circuit.
   • Solution: Verify connections to the COM, NO, and NC pins.

3. Microcontroller Resetting When Relay Activates:

   • Cause: Voltage spikes from the relay coil affecting the control circuit.
   • Solution: Add a flyback diode across the relay coil terminals.

4. Relay Buzzing or Clicking:

   • Cause: Insufficient or unstable power supply to the relay coil.
   • Solution: Ensure a stable 5V DC supply and check for loose connections.

Q1: Can I use a 5V relay to control AC devices?
Yes, a 5V relay can control AC devices as long as the voltage and current ratings of the relay contacts are not exceeded.

Q2: Can I directly connect a 5V relay to an Arduino pin?
While it is possible, it is not recommended unless the Arduino pin can supply sufficient current. Using a transistor driver is a safer approach.

Q3: What is the purpose of the flyback diode?
The flyback diode protects the control circuit from voltage spikes generated when the relay coil is de-energized.

Q4: Can I use a 5V relay with a 3.3V microcontroller?
Yes, but you will need a transistor driver circuit to ensure the relay coil receives the required 5V signal.