I was searching for a tiny and handy remote for hiking with my copters. Normally (e.g. for my videocopter and my 250 race copter) I use a FrSky Taranis.
After a bit of googling I identified the Walkera Devo 7E (from Ebay Germany for about EUR 52) as my solution which can (as well as a few other Walkera TXes) be enhanced with the opensource firmware Deviation TX (http://www.deviationtx.com). Deviation TX allows the use of one or more additional radio modules to use the TX with other transmission protocols. For FrSky support I deceided to buy a CC2500 radio module from Banggood for about EUR 9. To give me more comfort I additionally added 2 3Pos switches (e.g. flight mode selection).
As documentation of the mods in the internet currently (November 2015) seem a bit mixed up caused by different software revisions I briefly want to describe how I did the job.
1. Install Deviation TX
Install latest nightly build from Deviation TX website. FrSky support is already included, no additions required.
I used deviation-devo7e-v4.0.1-583c468 from http://www.deviationtx.com/downloads-new/category/235-releases-nightly-2015-10-25 (which will not remain there forever).
Install guide: http://www.deviationtx.com/user-manual/user-manual-7e-10
2. Install and activate additionally radio module CC2500
The module installation is described here https://bitbucket.org/PhracturedBlue/deviation/wiki/Devo7eModuleInstall and details for the CC2500 module here https://bitbucket.org/PhracturedBlue/deviation/wiki/ModuleList
To clarify: 6 wires need to connect from module to the Devo 7E. Additionally, a connection needs to be soldered each from modules pin 5->10 and 7->9.
To activate the CC2500 module and use them in model configuration the hardware.ini on Devo 7E USB storage need to be updated: https://bitbucket.org/PhracturedBlue/deviation/wiki/Enabling_modules
Remove the ; before the two lines
; enable-cc2500 = A14
; has_pa-cc2500 = 1
If you only added one additional radio module (the CC2500) you will need to change A14 to A13.
Now you should be able to select FrSky as model protocol and activate bind procedure.
3. Install and activate 2x 3Pos switch
Installation is described very detailled here: http://www.rcgroups.com/forums/showthread.php?t=2468549
You can use two holes covered under aluminium foil. Then you have to solder 5 cables to the PCB and connect them with the switches via 4 diodes.
After hardware modification you need to update the hardware.ini as well. Add a line extra-switches=3×2
Devo 7E TX (EUR 52)
CC2500 radio module (EUR 9): http://m.banggood.com/index.php?com=detail&t=index&products_id=922595
Antenna for radio module (EUR 3): http://m.banggood.com/index.php?com=detail&t=index&products_id=68700
2x 3Pos switches (2x EUR 2.50): local dealer
4x normal diode e.g. 1N4001 (4x a few cents)
Some cables about 6-8 different colors