In this tutorial I am going to show you how to program an AVR(ATmega328) and an Arduino UNO using the GPIO on the Raspberry Pi.
Adding an Arduino or an AVR to your projects will give you much greater flexibility.
Hook up the Raspberry Pi to the Arduino UNO or AVR.
The image below shows how to connect a Raspberry Pi 2 and an Arduino UNO. click the image to make it larger