VP-EC-8KO Designing Relay Output Modules

The design criteria for a relay module has to have real world applications: heavy duty relay contacts of at least 10 A Ac at 120 VAC, transient protection, RS485 interfaces, Modbus RTU protocols, etc.

The VP-EC-8KO (Final Version)

The Schematic

The design is based on the PIC18F45K20, and as with all of our PIC based projects, the ICSP (In Circuit Serial Programming Port) is available for programming the device, as well as making each device a development platform for expanding the capabilities or dreaming up new applications.


  • Power: 24 VDC
  • Network: RS485, Jumper selectable 9600 or 19.2K Baud, N81
  • Modbus RTU Protocol (Functions 1, 5 and 15)
  • Modbus ID: Dipswitch Selectable
  • Relay Output: OMRON G5LE-14 24 VDC, 10 A, SPDT (Single Pole Double Throw)
  • Protection: MOV's on contact outputs
  • Relay Test: Jumper to test relay operation, cycles thru relays K1 thru K8 ON
  • 5VDC Switching Power Supply
  • 3.3 VDC Linear Regulator


  • PIC18F45K20
  • c/w ICSP Port
  • Can be used as a development platform for power relay projects


  • Power On: BLUE LED
  • Relay 1 thru 8: RED LED ON
  • Heart Beat: AMBER LED (Flashes to indicate proper operation)
  • RS485: GREEN LED Rx, RED LED Tx.


  • DIN Rail Enclosure Bud Industries Model: DMB-4774

Original Design

The original design was based on a version that would also be compatible with the Raspberry Pi: it had a separate cpu module that has the same GPIO bus structure and the Raspberry Pi, but was a lot cheaper to produce. Also, there was a provision to have a Zigbee module for wireless applications.

Complete original unit with the Raspberry Pi:

Complete original unit with the PIC_PI PCB:

Here is the PIC_PI PCB and schematic:

Finally, the module had to fit into a readily available DIN rail mount enclosure, like the one from BUD Industries.



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