Have you ever used HID devices?

Of course! You are probably using several right now!

HID stands for "Human Interaction Devices", it's the USB protocol used for Keyboards and Mouses and it's probably the easiest way to interact with a computer as it's recognized by any OS system.

Some Arduino boards like Leonardo or Micro aren't using the original ATmega328p AVR chip as UNO. They implement a Atmega32u4. The name doesn't seems very different... but this controller will give you the possibility to communicate with the USB port of a computer through HID. When you play with an Arduino UNO, you need to add a chip to interconnect the USB (like FTDI or 32u2) as HID is incompatible (except V-USB for AVR library, but the implementation is slower).

This HID capability will provide us with a great way to interface between Bluetooth and Raspberry Pi because we can re-use some of the Mouse or Keyboard libraries for an easy interaction and also use a Serial terminal communication for more complex data transfers.

Hardware

So, the hardware is pretty simple and can be re-used in many different projects:

• A Leonardo or Micro Arduino board. I used Micro to save space, but the Leonardo has a 3.3V power supply, which is useful for powering the Bluetooth. If you use a Micro, I recommend you to add an external 3.3V VCC. 
• A NRF24L01+ on the SPI port (Pin 3/10/11/12/13, follow the schematic under). 
• A button. To avoid headache, this safety button can block the HID. Sometimes a wrong loop can make your board send thousands of keyboard and mouse commands!!! Its only function is to say "send nothing if the button is pressed!" 
• I used a tiny breadboard, some male/female headers and some wires to plug in everything together (See the picture above). I will try to optimize it and make a 3D printed box later when I will have finished other more important things :p

Software

All the software is available on this Github. For the Bluetooth HID, I had to manually add the libraries mouse/keyboard/Joystick (I use Arduino 1.6.9). This doesn't really make sense, as they are supposed to be already implemented...

You can try removing the first three includes, then select Arduino Micro or Leonardo board and compile (Be careful, it's not going to work with UNO!). If it says keyboard, mouse or joystick not recognised, you have to download the library yourself "write arduino keyboard.h in google". Then you move them inside the Arduino installation folder (java/library in mac) and restart Arduino software!

The libraries you need depend on what you want. At first I was using a joystick lib, but it seemed to slow the Python code down. So I made another function to send data through Serial as well.

The mouse HID is very useful if you want to follow the movement of the head. So we keep the possibility of controlling our Python program with a normal mouse or with the Micro device.