Three 40A-240V Relays RS485 Daisy-chainable HAT for Raspberry Pi

Overview

The Three 40A-240V Relays card enables the Raspberry Pi to control heavy-duty industrial loads. Connected to a Raspberry Pi, it is controlled via I2C using the Command Line, Python, or Node-RED drivers. Stand-alone on a DIN Rail, it operates as a MODBUS RTU slave over RS485, controlled by any standard PLC or SCADA system with no Raspberry Pi required.

• Three SPST-NO heavy duty relays, each rated 40A/277VAC inductive, 40A/30VDC
• RS485/MODBUS port with TVS protection and dual connectors
• Up to 8 cards stackable side-by-side on DIN Rail via I2C; up to 32 cards via RS485
• DIN-Rail mounting hardware included
• Uses only I2C port — all GPIO pins remain free
• Works with any Raspberry Pi from Zero to 5

Software Integration

Write your application using the Command Line or Python library, or use Node-RED for visual flow-based programming with no code required.

INTERFACES AND I/O

DESCRIPTION

The Three 40A-240V Relays card provides everything needed to control heavy-duty industrial loads with Raspberry Pi — or stand-alone on a DIN Rail as a MODBUS RTU slave.

Heavy Duty Relays

Three SPST-NO contacts rated 40A/277VAC inductive, 40A/VAC resistive, and 40A/30VDC. Heavy-duty 40A/750V terminal blocks accept large-gauge wire for direct connection to high-current loads.

RS485 / MODBUS

The on-board RS485 transceiver with TVS protection and dual connectors operates in two modes: controlled by the Raspberry Pi over I2C, or as a standalone MODBUS RTU slave where any PLC, SCADA, or HMI can control all relays — no Raspberry Pi required. Up to 32 cards can be chained on a single RS485 bus.

Status LEDs

Each relay has a dedicated LED indicator that lights when the relay is energized, giving immediate visual feedback of relay state. A power LED shows when the card is powered.

Push-Button

A momentary push-button at the edge of the card is routed to GPIO 26 (pin 37). It provides a convenient way to safely shut down the Raspberry Pi without a keyboard or monitor — write a script to monitor the pin and issue the shutdown command when pressed. The button can also be used for any other custom task such as toggling relays.

DIN-Rail Mounting

The card mounts on a standard DIN Rail using the included brackets. Multiple cards connect side-by-side on the DIN Rail via a 4-pin male-female side header, sharing the I2C bus without any external wiring or cables.

Software Self-Test

A single CLI command runs a self-test before installation: each relay cycles on and off at 0.3-second intervals. The relay clack and LED changes confirm that all three relays are functioning correctly.

TECHNICAL DETAILS

CARD LAYOUT

MECHANICAL SPECIFICATIONS

DIP SWITCH AND STACKING

The onboard six-position DIP switch configures the stack address and the RS485 port behavior. Multiple cards connect side-by-side on a DIN Rail via the 4-pin side header, sharing the I2C bus. Up to 8 cards can be addressed independently using the ID bits.

ELECTRICAL SPECIFICATIONS

POWER REQUIREMENTS

PLUGGABLE CONNECTORS

All signal connections use pluggable screw terminal connectors accepting wire gauges from 26 to 16 AWG. Relay output contacts connect to heavy-duty 40A/750V terminal blocks. Connectors can be unplugged from the board for convenient field wiring without disturbing the rest of the installation.

COMMUNICATION INTERFACES

FIRMWARE UPDATE

The onboard microcontroller firmware can be updated in the field. The update tool downloads the latest firmware from Sequent Microsystems' servers and flashes it to the board over I2C.

Running the Update

~$ cd ~/3relind-rpi/update/

~/3relind-rpi/update$ ./update <id>

Replace <id> with the board stack address (0-7). Full instructions including recovery procedures are available at:

https://github.com/SequentMicrosystems/3relind-rpi/tree/main/update/README.md

COMPATIBILITY

DOWNLOADS

COMPLIANCE

RoHS and REACH Compliance Declaration

WHAT'S INCLUDED

When you purchase the card you will receive the following items:

1. Three 40A-240V Relays Card for Raspberry Pi

2. Mounting Hardware
	Four M2.5×18mm male-female brass standoffs Four M2.5×5mm brass screws Four M2.5 brass nuts

3. Connector Plugs
	Pluggable connector plugs for all card connections including RS485 port and power.

4. DIN-Rail Mounting Brackets and Screws
	Two DIN-Rail mounting brackets with screws for installing the card on a standard DIN Rail without a Raspberry Pi.

QUICK START

1. Plug your card on top of your Raspberry Pi and power up the system.
2. Enable I2C communication on Raspberry Pi using raspi-config.
3. Install the software from GitHub:
   ~$ git clone https://github.com/SequentMicrosystems/3relind-rpi.git

   ~$ cd /home/pi/3relind-rpi

   ~/3relind-rpi$ sudo make install

   ~/3relind-rpi$ 3relind

The program will respond with a list of available commands. To read more about the CLI, please refer to the README file. If you would rather use a different platform, you can access the same functionality through dedicated, ready-to-use integrations in the Downloads tab above.