Relay Expansion Node Addon

The Onion Relay Node Addon, relay-node-addon is a wrapper around the libonionrelayexp dynmic C library that provides functions to setup and control the relay expansion.

Relay Expansion

The same library is available for use in C and Python programs.


Programming Flow

After each power-cycle, the chip that controls the Relay Expansion must be programmed with an initialization sequence. After the initialization, the relays can be turned on and off at will.

I2C Device Address

The Relay Expansion is the only expansion that has a configurable I2C device address. This was done so that up to eight Relay Expansions can be stacked on a single Omega, giving the user the ability to control 16 relay modules independently.

The base device address is 0x20, the dip switches control the offset added to the base address:

The table below defines the relationship:

Switch 1 Switch 2 Switch 3 I2C Device Address
Off Off Off 0x27
Off Off On 0x26
Off On Off 0x25
Off On On 0x24
On Off Off 0x23
On Off On 0x22
On On Off 0x21
On On On 0x20

All of the functions in this library will require an address argument that specifies the offset to add to the base address of 0x20.

Relay Module Select

Each relay expansion has two channel which can be called using binary values.


#The Node Addon

The relay-exp-addon exposes a series of methods that perform all of the actions specified in the Programming Flow section.

Install the Addon

Install the addon on your Omega:

opkg update
opkg install relay-exp-node

NodeJS will need to be installed for Node programs to actually run:

opkg install nodejs

Importing the addon into your Node Script

To use the addon within your script you have to import it into your node program as you would a module:

var relayAddon = require("/usr/bin/relay-node-addon");

Example Code

Example code that uses the relay-exp-node addon can be found here in the i2c-exp-node-addons Onion GitHub Repo.

Return Values

All of the functions will either return a 0 indicating success or 1 indicating failure.

Calling Methods

Methods are called in the following format.


Replace method with your funcion of interest.

Available Methods

Refer to the table below for a list and brief description of available relay methods.

Methods Inputs Description
init(int addr) 0-7 Initializes the selected relay expansion (0-7), relay states can be set after initialization.
checkInit(int addr) 0-7 Checks initialization state of selected relay expansion.
setChannel(int addr, int channel, int state) 0-7,0-1,0-1 Sets the selected channel on the selected relay to the specified states.
setAllChannels(int addr, int state) 0-7,0-1 Sets all channels on the selected relay expansion to the specified state.


Each of the main functions implemented inthis library are described below.

Initialization Function

This function programs the initialization sequence on the Relay Expansion, after this step is completed, the functions to set the relay states can be used with success:

relayAddon.init(int addr);


The addr argument is described above in the I2C Device Address section.

Examples Initialize a Relay Expansion with all switches set to 0, meaning the I2C device address will be 0x27:


Initialize with switches set to on-off-on (device address: 0x22):


Initialize with switches set to on-on-off (device address: 0x24):


Check for Initialization

This function performs several reads to determine if the Relay Expansion requires the initialization sequence to be programmed before the relay states can be changed.

relayAddon.checkInit(int addr);


The addr argument is described above in the I2C Device Address section.


Check if a Relay Expansion(with all switches set to On) is initialized:


Set Relay State

Finally the fun stuff! Use this function to change the sate of the relay:

relayAddon.setChannel(int addr, int  channel, int state);


The addr argument is described above in the I2C Device Address section.

The channel argument selects the relay channel in question.

The state argument allows the user to select if the relay will be turned on or off:


Let’s turn Relay0 on and Relay1 off (all switches Off)


Set State for both Relays

In the event that both relays need to be turned on or off at the same time:

relayAddon.setAllChannels(int addr, int state);

This is performed with a single register write so both relays should react at the same time.


The addr argument is described above in the I2C Device Address section.

The state argument allows the user to select if the relays will be turned on or off:


All switches are in Off position, turn both relays on, then turn Relay 0 off, the send a command to turn both off: