Power Relay Module

🧩 Project Introduction

This project is a Power Relay Module (Rev 1.0) designed for safely switching high-current or high-voltage loads using low-voltage logic signals. It features full electrical isolation between control and load circuits, making it ideal for automation, smart control systems, and embedded applications.

⚙️ Project Function

The Power Relay Module allows microcontrollers like Arduino, ESP32, or STM32 to control high-power loads through a relay, without a direct electrical connection. The opto-isolated input and power isolation architecture provide robust protection for control-side electronics. A logic-level jumper allows compatibility with both active-HIGH and active-LOW trigger logic.

📊 Project Parameters

• Input Voltage: 12V DC

• Relay Type: SLC-12VDC-SL-C

• Optocoupler: EL814S – ensures electrical isolation of input signal

• Power Isolation: Total separation between logic-side and relay power circuits

• Transistor Driver: BC847B for reliable switching

• Flyback Protection: M7 diode across the relay coil

• Status Indication: LED for real-time relay ON/OFF status

• Logic Select Jumper: Configurable HIGH or LOW trigger

• Connector Pins: VCC, GND, Din (control) + relay output terminals

🔍 Principle Analysis (Hardware Description)

This project consists of the following parts:

1. Input Isolation Stage:
   An EL814S optocoupler isolates the control input from the relay circuit, protecting your microcontroller from potential voltage spikes.

2. Relay Drive Circuit:
   A BC847B NPN transistor drives the relay coil, controlled by the optocoupler. Passive components ensure stable operation.

3. Flyback Protection:
   An M7 diode across the relay coil suppresses back EMF during switching, protecting the transistor.

4. Logic Trigger Select:
   A jumper allows the module to be triggered by either a HIGH or LOW logic level.

5. Power Isolation:
   Logic-side and relay-side power are isolated to ensure full protection of the microcontroller.

6. Status LED:
   Provides visual feedback when the relay is active.

💻 Software Code

🛠️ Assembling Process

1. Solder resistors, capacitors, and diode

2. Place and solder the transistor and optocoupler

3. Add the relay and headers

4. Set the trigger logic jumper (HIGH or LOW)

5. Test using a safe DC load before deployment