How to Control a USB Relay Module Using Python on Linux and macOS

USB Relay Modules are invaluable tools for automating and controlling electrical devices through a computer interface. By leveraging Free and Open Source Software (FOSS) such as Python and the pyserial library, users can integrate these modules into projects ranging from home automation to industrial control. In this guide, we’ll show you how to control a 2-channel USB Relay Module, explain relay wiring, and provide use cases for broader applications. Additionally, we’ll include a table for controlling up to 8 channels and explain the logic behind their addressing.

FOSS-based solutions not only reduce costs but also foster adaptability and transparency. Whether you are an enthusiast, researcher, or engineer, this approach empowers you to build reliable and customizable automation systems.

Table of Contents

Use Cases for USB Relay Modules

Relay Wiring: NC, NO, and COM Terminals

Each relay channel has three terminals:

1. COM (Common): Connects to the input power or signal.
2. NC (Normally Closed): Maintains a closed circuit when the relay is OFF.
3. NO (Normally Open): Maintains an open circuit when the relay is OFF.

Wiring Example

• Normally Closed (NC): Use for devices that need to remain ON by default and turn OFF when the relay is activated.
• Normally Open (NO): Use for devices that need to remain OFF by default and turn ON when the relay is activated.

This dual functionality makes USB Relay Modules versatile for both safety-critical and trigger-based systems.

Serial Commands and Logic for Addressing

USB Relay Modules communicate using serial commands, with each command consisting of:

• Header: Marks the start (0xA0).
• Channel: Specifies the relay channel (0x01 for relay 1, 0x02 for relay 2, etc.).
• State: Determines whether to turn ON (0x01) or OFF (0x00).
• Checksum: Ensures data integrity. Calculated as Header + Channel + State.

Simplified Python Script for a 2-Channel Relay

Below is an easy-to-follow Python script to control a 2-channel USB relay:

pip3 install pyserial

import serial
import time

# Set up the serial connection
serial_port = "/dev/ttyUSB0"  # Replace with your device path
baud_rate = 9600  # Check your relay's documentation
timeout = 1

# Initialize serial connection
ser = serial.Serial(serial_port, baud_rate, timeout=timeout)

def send_command(command):
    """Send a command to the USB relay."""
    ser.write(command)
    time.sleep(0.1)  # Small delay to ensure the command is sent

try:
    # Turn Relay 1 ON
    send_command(bytes([0xA0, 0x01, 0x01, 0xA2]))
    print("Relay 1 ON")
    
    # Turn Relay 2 ON
    send_command(bytes([0xA0, 0x02, 0x01, 0xA3]))
    print("Relay 2 ON")
    time.sleep(2)
    
    # Turn Relay 1 OFF
    send_command(bytes([0xA0, 0x01, 0x00, 0xA1]))
    print("Relay 1 OFF")
    
    # Turn Relay 2 OFF
    send_command(bytes([0xA0, 0x02, 0x00, 0xA2]))
    print("Relay 2 OFF")

finally:
    # Close the serial connection
    ser.close()

Conclusion

USB Relay Modules are powerful tools for building efficient and reliable automation systems. With FOSS tools like Python, you can create scalable and cost-effective solutions while maintaining the freedom to customize your setup. By understanding the relay wiring and serial command logic, you can confidently control up to 8 channels for diverse applications such as home automation, lab testing, and IoT integration.

Adopting open-source tools ensures not only adaptability but also the opportunity to contribute to a larger community of innovators. This guide provides a strong foundation for your automation projects, and we encourage you to experiment further.