4xRelay-USB is an old product that stops selling for few years. However, an old customer looking for it a few weeks ago. But we don't have the plastic case anymore, so we reimplement the design again. We using a 3D printed case and new MCU. We shrinking the size of the PCB, to make the case size below 100x100mm. That size should able to print on most type of 3D printer.

old to new


Powered by an External DC adapter, all relays will remain its state even PC is turn off.

Relay state stored at EE, it will resume to the last state when power on.

Simple ASCII command, easy for programming.

USB device work as CDC Virtual Serial Port. (Using usbser.sys at Windows. Natively supported by MacOS and most Linux distributions).

USB Micro-B Receptacle. Same as most Android phone. Easy to find a cable at a local store.

3D Printed case, you can modify it to fit your own usage.



Number of Relays: 4
Connection to PC:

USB Micro-B

CDC Virtual COM port

ASCII Command

Power: 9-12V DC (center positive)
Relay Contact:
(AC on resistive load)
Max. Voltage 240V, Max. Current 5A. Max. Power 120W.
Relay Contact:
(DC on resistive load)
Max. Voltage 30V, Max. Current 4A. Max. Power 120W.


About powered by USB:
This unit can be powered by USB, but a poor quality of the USB cable or a low voltage output of the USB port (notebook computer) may cause the unit unable to turn on its four relays simultaneously. So powered by USB is not guaranteed.

Please consult with a qualified electrical engineer if you are going to controlling AC devices.


Image Gallery


ASCII Command

Action Command Response
Get model name GI\n 4xRelay-USB\n
Get firmware version GV\n V3.00\n
Get status of relay n


e.g.: G1\n




Turn on relay n


e.g.: S1=1\n

Turn off relay n


e.g.: S1=0\n


Get unit's name

 Default is "SENSOR", can set by user.


Set Unit Name

 Up to 8 characters, stored at EEPROM.

N=name\n OK\n


PC program

4xRelayV3 qt



Download 3D files: STL / X_T / STEP

Download PC program: win32 installer

Browse source code: GitHub