Picture of I2C between Arduino's
Ever wanted to connect more Arduino's to transfer data or commands between them?
What purpose? Maybe you don't have enough I/O Pins or Analog In's. Maybe you want to do separate tasks on the same project but still want to communicate between the Arduino's. Or maybe you just need to do a simple thing more complicated. 

There are many ways to send a command to another Microcontroller:

UART -  Requires pins 0 and 1. The problem with UART is that only one device can send data to another. And also it's already in use by the USB port of the Arduino to connect to the computer.

-  Basically put a pin HIGH and read it on the other Arduino. But with this you have a
limited number of commands and devices.

I2C - I2C Requires Analog Pins 4 and 5 and two pull-up resistors. You can connect more than 100 Arduino's on the same 2 pins. It's simple, reliable and easy-to-use. 

Remove these adsRemove these ads by Signing Up

Step 1: I2C Basics

Picture of I2C Basics
I2C is a bit complex in theory. The Arduino environment makes it all simple but anyway, here is a small part of the theory you should know. 

 I2C (pronounced I-squared-C) created by Philips Semiconductors and commonly written as "I2C" stands for Inter-Integrated Circuit and allows communication of data between I2C devices over two wires. It sends information serially using one line for data (SDA) and one for clock (SCL).

Master and Slave

The I2C protocol defines the concept of master and slave devices. A master device is the device that is in charge of the bus. This device controls the clock and generates the START and STOP signals. Slave devices listen to the commands sent by the Master and respond to them.

Basic details:

Transfer rate
: 10 Kb/s (low speed) - 100Kb/s 
SDA - Serial DAta line
SCL - Serial CLock line
128 possible addresses 
16 reserved addresses
112 devices max

Devices have to share both 5V (Power) and GND (Ground)

1-40 of 43Next »
CarnefixN10 hours ago

Hi. I want to use this System with multiple Arduino Mega 2560.

From what i have understood, I don't need the Voltage connection as long as the devices get the same reference Voltage. So I would only need the CLOCK, DATA and GROUND connection.

Is that correct?

cornelam (author)  CarnefixN8 hours ago
Hi, that is correct.
However, if you have some spare time, test it with and without the resistors and give me some feedback on it.

Thank you!

Best regards,
dan300820 days ago

you say "Devices have to share both 5V (Power) and GND (Ground)"

is there a reason for this? couldnt I just have both devices powered at the same voltage?

They have to get the same voltage reference.

akospenda6 days ago

Hey, I'm using this as comms between 2 arduinos. I got it to respond to a photoresistor via the slave but when I tried to add ultrasonic sensor they interact and neither of them work, any idea why?

ðkital9 days ago

hi i just wanna ask if i can make an arduino as master at the same time as slave on both arduino?

or can i send data from master to slave? and slave to master?

kinda confused.,

cornelam (author)  ðkital9 days ago

The Arduino only has one I2C Bus, so you can only be either a Master or a Slave at any particular time.
A slave cannot write to a Master but a Master can request data from a slave. Here is an example:

Best regards,
Cornel Amariei

I just came across this instructable today and while the basics work fine, I found out the hard way that you don't need pull-up resistors anymore. In fact it doesn't work if you use them. The Arduinos I tried were Uno, Due and Leonardo and neither of them requires resistors between 5V and SDA or SCL. It's also prudent to look up the SDA and SCL pin mapping for the different types of Arduino (ie: Leonardo: 2 and 3, Uno: 4 and 5, Due, 20 and 21) in the documentation of the wire library:


You might be right about the no need for doing it, but Im using it with Uno and it works fine for me.

Hi. What is max distance between two arduinos allowable ? thank you

ImeshU3 months ago

Hi... thank you very much for sharing mach valuable information... but i'm having trouble using pin4 and pin5 as i am using a gsm shield.. it consumes pin 4 and pin there any possibility to use pin 9 and pin 10 or any other pin couple ??? i am new to arduino...thank you very much again...

Find out the address of the GSM shield comms and use any different address. Just solder the header or wires to A4 and A5 and connect to another Arduino.

frivolas5 months ago

Hi All!

Thanks cornelam for this instructable! Clear and easy to follow. I finally get a grasp on WTH is I2C.

Just a quick question, is there any particular convention to assign addresses to the slave devices? Can I just name any device as I want (1,2,3...116), or should I name them in a particular way?


Jayw2 frivolas1 month ago

You can use any address and it does not have to be in any particular order (although some are reserved; look it up online), although I tend to name them consequetely for the ease of programming :)

casonye2 months ago


pkgdelion2 months ago

still need help!!!

cornelam (author)  pkgdelion2 months ago
How can I help? :-)

ibenkos3 months ago
Smart idea! Thanks for shearig :)

I'm having trouble with flickering in my common cathode 4-digit 7-segment display as my code gets more complex. I've decided to drive it with a dedicated AVR chip that I will interface with my Arduino. That is a perfect example of how "I2C between Arduinos" is extremely handy. The I2C slave doesn't have to be a full Arduino. There are tons of examples of how to program ATTiny and ATMega microprocessors with the Arduino IDE. Making your own I2C peripheral is now pretty easy. Thanks for the info!

thnx xD

thanks, if you only need one or a few nodes 1k5 works great, but the more nodes i have, i prefer higher values to lower the current. I made a I2C developer pcb with a atmega328 and i2c bus to make a BMS for lifepo4 batteries with attiyby 85 i2c nodes on each battery

Rifath9 months ago

finally i know about I2C by you, very very thank you

thaear10 months ago


schel11 months ago

PERFECT!!! EXACTLY what I was lookin' for!!

daryllukas1 year ago

Do the devices have to share the same power source? is it possible to use two different power source e.g. one Arduino is taking its power from USB, and the other
from a battery pack?

cornelam (author)  daryllukas1 year ago

Yes it is, only the Ground has to be common

Cool. I actually figured it out before seeing you reply. But thanks :)
tekkentux4 years ago
Is that pin 3 and 4 (as in the text), or pin 4 and 5 (as in the picture)? Thank you!
seems to be 4 & 5 for the uno.
binibp2 years ago
Can anyone attach de code for master and slave in dis two .pde attachments are not readable.....plz help....plz plz vry urgent....
odalcet binibp2 years ago
// ============= master1.pde


#define LED_PIN 13
byte x = 0;

// ======================================================

void setup()
Wire.begin(); // Start I2C Bus as Master
digitalWrite(LED_PIN, LOW);


// ======================================================

void loop()

Wire.beginTransmission(9); // transmit to device #9
Wire.send(x); // sends x
Wire.endTransmission(); // stop transmitting
if (x > 5) x=0;
// ==================== end of file ====================

// ============= slave1.pde


#define LED_PIN 13
#define LED_1 12
#define LED_2 11

int x;

// ======================================================

void setup()
Wire.begin(9); // Start I2C Bus as a Slave (Device Number 9)
Wire.onReceive(receiveEvent); // register event

pinMode(LED_1, OUTPUT);
pinMode(LED_2, OUTPUT);

digitalWrite(LED_PIN, LOW);
digitalWrite(LED_1, LOW);
digitalWrite(LED_2, LOW);

x = 0;

// ======================================================

void loop()
//If value received is 0 blink LED 1
if (x == 0)
digitalWrite(LED_1, HIGH);
digitalWrite(LED_1, LOW);
//If value received is 1 blink LED 2
if (x == 1)
digitalWrite(LED_2, HIGH);
digitalWrite(LED_2, LOW);

// ======================================================

void receiveEvent(int howMany)
x = Wire.receive(); // receive byte as an integer
// ==================== end of file ====================
varuninnz2 years ago
hey man i have a quick question, I was using the same analogy 2 connect arduino UNO and mega 1280. I want the UNO to be my master because UNO works properly with processing while my mega 1280 does not work with processing.

I did as described in this tutorial Basically I connected
SDA pin 4 of UNO to Arduino Mega SDA digital pin 20
SCL pin 5 of UNO and SCL is 21 of mega
Ground UNO to ground MEGA (I dont think it matter which ground on the board you connect to)
5V of UNO to Vin of Mega.

Uploaded the master program to UNO and slave to mega as decribed on the website.
Now when I run the serial monitors I get a lot of gibberish popping up on my serial monitor. Please help as I dont understand whats going on. I dont believe the gibberish Im receiving is right?
I have a few 1k resistors lying around. Should I use your pull-up resistor theory?
act casual2 years ago
how do you enable the pull ups
my wire seen it not do it, i have 0022

hje3 years ago
In the reference design it says nothing about common ground and Vcc. You can do without :D as it is a 2-wire bus. Otherwise great introduction to the subject.
chris911ny3 years ago
I have read in some articles that the wire library will enable internal pull-ups, is that the case or we really need to add pull-ups?
jeremyvnc4 years ago
Pins 4 & 5 are SDA and SCLK for I2C
WyoJustin4 years ago
Thanks for taking the time to post this! It is just what I was looking for.

easydaman4 years ago
Nice! Thanks man!
A little sidenote for when working with an Arduino Mega: Comm-Pins are 20 and 21. But well... figuring that out takes 5 minutes. On the other hand: you just saved them ;-)
amando964 years ago
 sweet, making an i2c controlled motor driver to learn some of this...
cornelam (author) 5 years ago
 Thank You!
The fact is that you don't need an extra wire per slave.  :-)
That's the beauty of I2C. You simply put on each slave unique address and put the master to talk with that specific address...and it works. Simple and reliable. 
1-40 of 43Next »