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 for isolating high-voltage 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
• Lifetime:
  • Mechanical: ~10 million operations
  • Electrical: ~100,000 operations (at rated load)

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 external power source).
   • Connect the Coil- pin to ground.

2. Connect the Load Circuit:

   • Connect the load's power source to the COM pin.
   • For a normally open configuration, connect the load to the NO pin. The load will be powered only when the relay is energized.
   • For a normally closed configuration, connect the load to the NC pin. The load will be powered when the relay is not energized.

3. Control the Relay:

   • Use a microcontroller (e.g., Arduino UNO) or a transistor circuit to provide the required 5V signal to the relay coil.

• Use a Flyback Diode: Always connect a flyback diode across the relay coil to protect the control circuit from voltage spikes caused by the collapsing magnetic field when the relay is de-energized.
• Avoid Overloading: Ensure the load does not exceed the relay's maximum voltage and current ratings.
• Isolation: Use optocouplers or transistors to isolate the microcontroller from the relay for added safety.
• Power Supply: Ensure the power supply can provide sufficient current to energize the relay coil.

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

// Example: Controlling a 5V relay with Arduino UNO
// Pin 7 is used to control the relay
const int relayPin = 7;

void setup() {
  pinMode(relayPin, OUTPUT); // Set relay pin as output
  digitalWrite(relayPin, LOW); // Ensure relay is off at startup
}

void loop() {
  digitalWrite(relayPin, HIGH); // Turn relay on
  delay(1000); // Keep relay on for 1 second
  digitalWrite(relayPin, LOW); // Turn relay off
  delay(1000); // Keep relay off for 1 second
}

Note: Connect the relay's Coil+ pin to Arduino pin 7 (via a transistor if needed) and Coil- to ground. Use a flyback diode across the relay coil.

1. Relay Not Switching:

   • Cause: Insufficient current to energize the coil.
   • Solution: Ensure the control circuit can supply at least 70-100 mA. Use a transistor or relay driver module if necessary.

2. Load Not Powering On:

   • Cause: Incorrect wiring of the load circuit.
   • Solution: Verify connections to the COM, NO, and NC pins. Ensure the load is within the relay's rated voltage and current.

3. Microcontroller Resets When Relay Activates:

   • Cause: Voltage spikes from the relay coil.
   • Solution: Add a flyback diode across the relay coil and ensure proper decoupling capacitors are used in the circuit.

4. Relay Stuck in One State:

   • Cause: Mechanical failure or overloading.
   • Solution: Replace the relay and ensure the load does not exceed the rated specifications.

Q1: Can I use a 5V relay to control AC devices?
Yes, as long as the device's voltage and current are within the relay's rated specifications (e.g., 250V AC, 10A).

Q2: Do I need a separate power supply for the relay?
Not necessarily. If your microcontroller or power source can provide sufficient current (70-100 mA), you can use the same supply. Otherwise, use an external power source.

Q3: Can I control multiple relays with one microcontroller?
Yes, but ensure the microcontroller can handle the combined current requirements. Use transistors or a relay driver module for better control.

Q4: 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. It is essential for circuit safety and longevity.