We are not done yet! For now, there are no connections between the Arduino Yún and the Arduino Uno board, so the Yún board won't be able to access the DC motors and the sensors of the robot. To solve this issue, the first step is to connect the power from the Arduino Uno board to the Yún board. This way, when we power the project using the battery, the Yún board will be powered as well.
To do so, simply connect the ground pins together and plug the Vin pin on the Arduino Yún to the 5V rail of the Arduino Uno, as shown in the following image:
To finish connecting the two Arduino boards, we need to connect them so they can speak together when the project is under operation. For this, we are going to use the I2C interface of the Arduino boards so they can send messages to each other. I2C stands for Inter Integrated Circuit and is a simple communication protocol that was developed for communication between circuits, and is widely used in electronics. There are two wires to connect for that purpose: SDA and SCL. To do so, simply connect pin 2 of the Yún board to pin A4 of the Uno board, and pin 3 of the Yún board to pin A5 of the Uno board, as shown in the following image:
The following image summarizes the connection between both boards:
Finally, you can power up the project by inserting the DC jack connector of the battery into the power connector of the Uno board as shown in the following image:
If everything was done correctly in this step, you should see that both boards (the Yún and the Uno) are powered up, with some of their LEDs on. To help you build the robot, we also included two pictures of the sides of the robot that show you the different connections. The following is an image of a side of the robot that shows the power connections to the Yún:
The following image shows the connections from the I2C interface to the Yún:
