The power of internet...
It all started when a friend of mine asking me to setup his Dji Naza-M on his 450 frame quad copter. Needing a 3 position switch, he opted to buy a new Flysky TH9x because its 9ch and its cheap.
Problem: Mixing a function in stock TH9x is a nightmare. The need to create a failsafe/return to home switch faced a failure due to stock firmware bugs in mixing.
Action: Flash its firmware to custom ER9x which is a godsend for diy rcers. The new firmware has make it a totally new tx inside but still the same on the outside.
Mod: Firmware flashed using arduino uno. Backlight ER9x controlled. Speaker and haptic vibration feedback.