Reid Carlberg

Connected Devices, & Other Adventures

Tag Archives: i2c

Connect I2C Devices Using Cat5 and RJ45 Ports


The other day I shared my approach to reading data off a couple Arduinos with a Raspberry Pi using I2C and Node.js. Works great, but the wiring was ugly and brittle.2arduinos1pi

Mess or not, the wiring did teach me two things: I2C works as long as there is conductivity and yes you can power a couple of Arduinos from the RPi. I2C doesn’t require a particular wiring sequence or whole bunch of cables extending from a central point to a single device, so when I decided to beef it up a bit, I started to imagine a way of connecting devices over a small distance. I was hopeful it would be easy and not require 100% custom wiring. The requirements boiled down to something like this:

raspberry pi arduino i2c

I decided to look for a solution built around Cat5 cable — it’s cheap, has great connectors and I’ve worked with it a fair amount. I found it at Microcenter.  I’m using a couple of el cheapo cable splitters and an RJ45 F-F joiner, for most of the connection, and it works great. I still need to solder 1/2 a Cat5 cable for the Raspberry Pi and another 1/2 for each device, but I can now put the Arduinos (somewhat) arbitrarily far away from the RPi node and add more until my power runs out.


Still not ideal, but like I said it works great.  Although relatively inexpensive, each splitter is about $10, and I could do without stripping the Cat5 wires and soldering them to pins.  Next up I think I’ll add a couple of RJ45 ports to a proto shield, and connect that directly to each Arduino.  I’ll net a little more soldering, but it will be easier soldering so should be faster.

Here’s what the current system actually looks like in my attic.  Note that the Arduino and RPi are far enough away that it’s impractical to photograph them together.


Questions I’ve pondered:

Why am I doing this instead of just getting an Arduino with an Ethernet shield?  I’m trying to solve two problems, communication and power.  if I go with the Ethernet shield, I’m in it for $60 each if I include a power over ethernet (POE) module, which is a lot.  It solves the power problem and the connectivity problem, but I still have to wire ethernet.

Why don’t I just go wireless? Well, I could.  In fact, I have a bunch of XBees sitting around from Dreamforce 13 that I could harness.  However, I’d still have to get power to each one of these devices, so I’ve only solved one problem.

How will I scale this beyond my attic? I’ll probably need to scale this to the other floors of my house to get the data I want.  I suspect the easiest way to scale it will be to add a nearly identical system on each floor.  Otherwise I’ll have to deal with stringing wires, and I hate stringing wires.

Read Data from Multiple Arduinos with a Raspberry Pi B+ using I2C and Node.js


Last year I wrote about controlling an Arduino using a Raspberry Pi and I2C.  I2C being, of course, a 30+ year old protocol that everybody and their brother still supports.  I tried this again, for reasons I’ll state in the very last sentence of this blog, and it failed. Significantly.

Why? Well, turns out I had forgotten the cardinal rule of I2C.

Link your devices, master and slave, sender and receiver, using a shared ground wire.  Otherwise, you’re going to have a bad time.  Really.

Mmm’k, with that out of the way:

1) The Adafruit tutorial is still the best one I’ve found for enabling I2C on your Raspberry Pi. However, raspi-config now helps you do this under advanced options.  I used the advanced options and then doubled checked everything Adafruit told me to do. (Rule #1: do what Adafruit says.)

2) Add your user to the i2c group.  The command is “sudo adduser pi i2c“. This will make your life easier.  Thank you ABC Electronics.

3) Configure your Arduinos as slave senders.  The example code that comes with the Wire library in the Arduino IDE is perfect for this. Wire ground to ground, SDA to SDA and SCL to SCL. Once you add power to the Arduino, install the slave sender example code and boot your pi, you should be able to see your I2C device using the i2cdetect -y 1 command.  Mine with two Arduinos looks like this:


My code looks a little different than the slave sender sample, since I have a photocell and a temperature probe on each one.  There’s a fair amount of boiler plate code to read those values, but the fundamental structure is the same as the sample code: configure an I2C address, wait for a request, send a fixed length string in response.

#include <Wire.h> 
#include <OneWire>

// Define analog pin
int sensorPin = 0;
int lightLevel = 0;

int DS18S20_Pin = 2; 
OneWire ds(DS18S20_Pin);  // on digital pin 2

String deviceId = "D001";
int wireId = 4;

String lightLevelString = "";
String output;
int m;
char c[5];
char d[10];

// Setup
void setup() {
 // Init serial
// Main loop
void loop() {
 // Get temperature
 int sensorValue = analogRead(sensorPin);
 lightLevel = map(sensorValue, 10, 1000, 1, 100);
 m = sprintf(c, "%03d", lightLevel);
  float temperature = getTemp();
  dtostrf(temperature, 6, 2, d);

  output = deviceId;

void requestEvent() {
  char newOut[16];
  output.toCharArray(newOut, 16);

float getTemp(){
  //returns the temperature from one DS18S20 in DEG Celsius

  byte data[12];
  byte addr[8];

  if ( ! {
      //no more sensors on chain, reset search
      return -1000;

  if ( OneWire::crc8( addr, 7) != addr[7]) {
      Serial.println("CRC is not valid!");
      return -1000;

  if ( addr[0] != 0x10 && addr[0] != 0x28) {
      Serial.print("Device is not recognized");
      return -1000;

  ds.write(0x44,1); // start conversion, with parasite power on at the end

  byte present = ds.reset();;    
  ds.write(0xBE); // Read Scratchpad

  for (int i = 0; i < 9; i++) { // we need 9 bytes
    data[i] =;
  byte MSB = data[1];
  byte LSB = data[0];

  float tempRead = ((MSB << 8) | LSB); //using two's compliment
  float TemperatureSum = tempRead / 16;
  return TemperatureSum;

4) Install node.js on your rpi, and then use npm to install the i2c library.  You should now be able to read your Arduino via Node.js.  My code declares two devices, both with addresses matching the settings in my Arduino code, and it looks like this:

var i2c = require('i2c');
var address = 0x18;

var device1 = new i2c(address, {device: '/dev/i2c-1'});

var device2 = new i2c(address, {device: '/dev/i2c-1'});

var devices = [ device1, device2 ];

function handleTimeout() {
	  setTimeout(function() { handleRead(); }, 1000 );

function handleRead() {
  	for (i = 0; i < devices.length; i++) {
		    devices[i].readBytes(null,15, function(err,res) {


The output is pretty simple — it just prints the data it receives, like so:


My final setup consists of two Arduinos (D001 and D002 in the data) connected to one Raspberry Pi B+.  You’ll notice that the RPi is powering both Arduinos.  (Red is power. Back is ground. Blue is SCL. Yellow is SDA.)  Everything is integrated via the breadboard.


Lessons learned (and remembered):

1) Always connect the grounds.  If you’re not seeing your minimally configured, powered and connected slave-sender device, check the ground first.

2) You can easily power a couple of Arduino’s from the RPi.  Nice.  I wonder how many you can power. I’m sure it depends on what you’re doing.

3) The SDA and SCL wires seem to be pretty flexible. You don’t have to chain them in a particular order as far as I can tell. Just make sure there’s some kind of connection and you’re good to go.

4) Arduinos, with built in analog to digital converters, are a lot less painful (and more reliable) to work with when it comes to reading basic sensors and sharing that data with an app running on the RPi.  My original reason for revisiting this use case was dissatisfaction with reading a photocell directly on the RPi using Python (tutorial) and Arduinos are cheap — $10 at MicroCenter.

Control an Arduino with a Raspberry Pi Using Node.js and I2C


You might also find this post helpful: Read Data from Multiple Arduinos with a Raspberry Pi B+ using I2C and Node.js

As a software developer, I usually stay away from lower level communication protocols. My logic has historically been: they’re a lot of work, and my time is better spent higher up the stack.

A couple of weeks ago, after suffering through a variety of failures and uncertainty higher up the stack, I decided the only way out was to go at least a little deeper. So I bit the bullet and connected a Raspberry Pi to an Arduino using the I2C protocol, and then was able to control the Arduino using Node.js. This post covers how I did it.


My use case was fairly simple.  I decided to drive a Wild Thumper using the Salesforce1 mobile app.  The Wild Thumper is controlled by an Arduino compatible board, aptly named the “Wild Thumper Controller“.  You might remember I had a couple of these with me at the Connected Device Lab this Dreamforce.  In order to communicate with Salesforce, it needs to have some kind of a gateway that can talk to the Internet using HTTPS, which is where the Raspberry Pi comes in.  If you have a different flavor or Arduino sitting around this should still work.

There are a lot of really great technical introductions to I2C (including — gasp — one from 2001), but you don’t really need them to get started.  Keep these two basic ideas in mind:

  • You’re going to work in a Master-Slave configuration.
  • You need to connect three pins.  SDA, SCL and Ground.

That’s it.

Step 1: Slave Configuration on the Arduino compatible Wild Thumper Board is pretty easy.  In fact, you can simply use the “Wire > Slave Receiver” sample code included with the IDE to get started (below). Load this in the Arduino IDE as is, send to your device and then start the serial monitor.  Put it aside.

// Wire Slave Receiver
// by Nicholas Zambetti <>

// Demonstrates use of the Wire library
// Receives data as an I2C/TWI slave device
// Refer to the "Wire Master Writer" example for use with this
// Created 29 March 2006
// This example code is in the public domain.
#include <Wire.h>

void setup()
  Wire.begin(4);                // join i2c bus with address #4
  Wire.onReceive(receiveEvent); // register event
  Serial.begin(9600);           // start serial for output

void loop()

// function that executes whenever data is received from master
// this function is registered as an event, see setup()
void receiveEvent(int howMany)
  while(1 < Wire.available()) // loop through all but the last
    char c =; // receive byte as a character
    Serial.print(c);         // print the character
  int x =;    // receive byte as an integer
  Serial.println(x);         // print the integer

Step 2: Setting up the Raspberry Pi is a little more difficult, but it’s well documented.  If you are using a standard Raspbian distro, the I2C port is not yet enabled.  Adafruit has an excellent post on enabling the I2C port.  Follow those instructions, then c’mon back.

Step 3: Wire your Raspberry Pi and your Arduino together and make sure the RPi can see the Arduino.  Pinouts on the RPi are a little irritating.  I use the Pi Cobbler.  It gives you a handy, easy to read guide to which pins are where.  If you are connecting directly to the GPIO pins, be sure to consult a pinout digram.

Before you connect everything, your RPi should produce this when you use the i2cdetect command.


The basic configuration is really simple.  Master SDA pin to Slave SDA, Master SCL pin to Slave SCL, Master GND pin to Slave GND.  On the Wild Thumper Controller, SDA is A4, SCL is A5 and GND is any pin on the outside edge of the board.  And an Arduino Uno R3, there are special pins for I2C, above the AREF pin, clearly labeled on the back side of the board. In my case, the result looks like this (that’s the Wild Thumper Controller on the right, Pi Cobbler on the left):


After connected, your RPi should produce this (note that there is now a device on 4).


Step 4: Install Node.js on to your Raspberry Pi using this simple link.  If you prefer to be a bit more hands on, you can follow these instructions.  If you prefer to be really hands on, you can install it by compiling from the source, but it will probably take > 2 hours and so far in my experience it’s no different.

Step 5: Install something that lets Node.js talk to the Raspberry Pi’s I2C system.  You should use the I2C library.  There are a couple of others out there, but this is easy and popular.  Now you should be able to do something like this:

var i2c = require('i2c');
var device1 = new i2c(0x18, {device: '/dev/i2c-1', debug: false});
device1.writeByte(0x2, function(err) { console.log("error"); console.log(err); });

And see a “2” in the Arduino serial port monitor (like so).


There you have it — the basics of controlling an Arduino using a Raspberry Pi with Node.js and I2C. My original goal was to control the Wild Thumper via the Salesforce1 app.  I was able to accomplish that pretty easily.  Here’s my basic controller for the Thumper, and here’s the Node.js app running on the RPi.  I created a simple UI on the Salesforce1 mobile app (below)  that sends commands to the Thumper using the Streaming API and connects to Salesforce using the excellent nforce.


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