Principle of operations

Most RC radio accept PPM signal as input on their trainer port.

PPM signal is a Pulse Position Modulation signal. When applied to RC rdaio it simply consist in adding the 8 channels of the servos into a single frame.

Each frame has 20ms length

The channels are simply added one after the other whithin this 20 ms frame.

A pulse (0.5 ms) is generated to start a new channel. The duration from the start of the pulse to the next one is equal to the servo value (1 ms to 2 ms).

A little board has been made to generate the PPM train.

It is a USB HID device and can be connected to any USB host plug. So this board may work connected to a PC but also connected to the USB host of an Android phone.

The board then communicates with the host via a custom HID protocol embedded into the firmware.

The other side (host) of the protocol is handled by the android App : JG Joystick2PPM

Schematics

The heart of the system is a PIC18F4550 microcontroller. 

The schematics is quite simple and implements the basic circuitry of a HID device proposed by Microchip.

PCB

The PCB has been manufactured and all the eagle files are available if you want to reproduce it.

Firmware

The board is programmed in C under MPLAB-IDE.

FIrmware is avalable here including .hex file and all source code

Please note that this code is provided under the Cecill V2 open source licence. Details regarding this licence are available here .