How to Use Relay : Examples, Pinouts, and Specs

The SRD-05VDC-SL-C is an electromechanical relay manufactured by Ningbo Songle Relay. It is designed to control high-power devices using low-power signals, making it an essential component in automation, home appliances, and industrial control systems. The relay operates by energizing an electromagnetic coil, which mechanically opens or closes its internal switch contacts.

• Home automation systems (e.g., controlling lights, fans, or appliances)
• Industrial control systems
• Motor control circuits
• Microcontroller-based projects (e.g., Arduino, Raspberry Pi)
• Power supply switching

The SRD-05VDC-SL-C relay has five pins, as described below:

1. Power the Coil: Connect the coil pins (Pin 1 and Pin 2) to a 5 V DC power source. Use a transistor or a microcontroller (e.g., Arduino) to control the coil.
2. Connect the Load:
   • Connect the load to the Common (COM) pin (Pin 3).
   • Use the Normally Open (NO) pin (Pin 4) if you want the load to be powered only when the relay is energized.
   • Use the Normally Closed (NC) pin (Pin 5) if you want the load to be powered when the relay is not energized.
3. Add a Flyback Diode: Place a flyback diode (e.g., 1N4007) across the coil terminals to protect the circuit from voltage spikes when the relay is turned off.
4. Control the Relay: Use a microcontroller or a switch to control the relay coil. Ensure the control circuit can supply sufficient current to energize the coil (approximately 71 mA).

• Isolation: The relay provides electrical isolation between the control circuit and the load circuit, making it safe for high-voltage applications.
• Current Rating: Ensure the load current does not exceed the relay's maximum contact rating (10 A).
• Voltage Rating: Verify that the load voltage is within the relay's maximum switching voltage (250 V AC or 30 V DC).
• Debouncing: When using the relay with a microcontroller, implement software or hardware debouncing to avoid erratic switching.

Below is an example of how to control the SRD-05VDC-SL-C relay using an Arduino UNO:

// Define the pin connected to the relay module
const int relayPin = 7;

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

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

• Connect the relay module's control pin to Arduino pin 7.
• Use an external power supply for the relay module if the Arduino cannot provide sufficient current.

1. Relay Not Switching

   • Cause: Insufficient voltage or current to the coil.
   • Solution: Verify that the control circuit provides 5 V DC and at least 71 mA to the coil.

2. Load Not Powering On

   • Cause: Incorrect wiring of the load to the relay pins.
   • Solution: Double-check the connections to the COM, NO, and NC pins.

3. Relay Buzzing Noise

   • Cause: Insufficient or unstable power supply to the coil.
   • Solution: Ensure the power supply is stable and within the operating voltage range (3.75 V to 5.25 V DC).

4. Microcontroller Resetting

   • Cause: Voltage spikes from the relay coil affecting the microcontroller.
   • Solution: Add a flyback diode across the coil terminals to suppress voltage spikes.

Q1: Can I use the SRD-05VDC-SL-C relay with a 3.3 V microcontroller?
A1: Yes, but you will need a transistor or relay driver circuit to step up the control voltage to 5 V.

Q2: What is the purpose of the flyback diode?
A2: The flyback diode protects the control circuit from voltage spikes generated when the relay coil is de-energized.

Q3: Can this relay switch AC loads?
A3: Yes, the relay can switch AC loads up to 250 V with a maximum current of 10 A.

Q4: Is the relay suitable for continuous operation?
A4: Yes, the relay is designed for continuous operation within its rated specifications. Ensure proper ventilation to avoid overheating.