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

The 5V Single Pole Double Throw (SPDT) relay, manufactured by Relay (Part ID: SPDT), is an electromechanical switch designed to control high-power circuits using low-power signals. It features three terminals: Common (COM), Normally Open (NO), and Normally Closed (NC), allowing it to toggle between two circuits. This relay is widely used in applications requiring electrical isolation, automation, and remote control of devices.

• Home automation systems
• Motor control circuits
• Industrial equipment
• IoT projects
• Switching between power sources
• Controlling high-voltage devices with microcontrollers (e.g., Arduino)

Below are the key technical details of the 5V SPDT relay:

The 5V SPDT relay has five pins, as described in the table below:

1. Power the Relay Coil: Connect the VCC pin to a 5V DC power source and the GND pin to the ground.
2. Control the Relay: Use a microcontroller (e.g., Arduino) or a transistor to control the relay coil. When the coil is energized, the relay switches from NC to NO.
3. Connect the Load:
   • Connect the load to the COM terminal.
   • Use the NO terminal if you want the load to be powered only when the relay is active.
   • Use the NC terminal if you want the load to be powered when the relay is inactive.

• Back-EMF Protection: Always use a flyback diode (e.g., 1N4007) across the relay coil to protect the driving circuit from voltage spikes.
• Isolation: Ensure proper electrical isolation between the control and load circuits to prevent damage.
• Current Rating: Do not exceed the relay's maximum current rating (10A) to avoid overheating or failure.
• Power Supply: Use a stable 5V DC power source to ensure reliable operation.

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

• Connect the relay's VCC pin to the Arduino's 5V pin.
• Connect the relay's GND pin to the Arduino's GND pin.
• Connect the relay's control pin (e.g., IN) to Arduino digital pin 7.
• Use a flyback diode across the relay coil for protection.

// Example code to control a 5V SPDT relay with Arduino UNO

#define RELAY_PIN 7  // Define the pin connected to the relay control

void setup() {
  pinMode(RELAY_PIN, OUTPUT);  // Set the relay pin as an output
  digitalWrite(RELAY_PIN, LOW);  // Ensure the relay is off initially
}

void loop() {
  digitalWrite(RELAY_PIN, HIGH);  // Turn the relay on (activates NO terminal)
  delay(1000);  // Keep the relay on for 1 second
  digitalWrite(RELAY_PIN, LOW);  // Turn the relay off (activates NC terminal)
  delay(1000);  // Keep the relay off for 1 second
}

1. Relay Not Switching:

   • Cause: Insufficient voltage or current to the relay coil.
   • Solution: Ensure the control signal provides 5V and sufficient current (typically ~70mA).

2. Load Not Powering On:

   • Cause: Incorrect wiring of the load to the relay terminals.
   • Solution: Verify the load is connected to the correct terminals (COM and NO/NC).

3. Microcontroller Resetting:

   • Cause: Voltage spikes from the relay coil.
   • Solution: Add a flyback diode across the relay coil to suppress back-EMF.

4. Relay Overheating:

   • Cause: Exceeding the relay's current rating.
   • Solution: Ensure the load current does not exceed 10A.

• Q: Can I use the 5V SPDT relay with a 3.3V microcontroller?
  A: Yes, but you will need a transistor or relay driver circuit to boost the control signal to 5V.

• Q: Is the relay suitable for AC loads?
  A: Yes, the relay can switch AC loads up to 250V, provided the current does not exceed 10A.

• Q: Do I need an external power supply for the relay?
  A: If your microcontroller cannot supply sufficient current, use an external 5V power supply for the relay.

• Q: Can I use the relay for PWM signals?
  A: No, relays are not suitable for high-frequency switching. Use a solid-state relay or MOSFET for PWM applications.