Most standard RC transmitters are bulky, and fragile. It's always a big deal to carry them around, especially when you have small quads.
Some time ago, I bought a micro quad: the Faze from HorizonHobby. It's basically a clone of the LadyBird or the CX-10. I had a great fun flying it around, but it ended up… faulty (battery can't hold harge, some dead LEDs, and finally a dead motor).
The transmitter is reaaally small ! So I thought I'd try to use it to fly my other quads. Indead of just replacing the PCB, I decided to keep it so I don't have to deal with the trim buttons, pots reading and conversion, … The communication protocol is not standard so I opened the transmitter and got to work.
One of the advantages of this transmitter is that the RF module (a NRF24L01 or a clone, doesn't matter) is on a separate daughter board, instead of being embedded on the PCB. It makes the reserve easier (the pinout is also written on the bard) I just wired a LogicAnalyzer on the pins and sniffed the protocol.
I wanted to get a standard output, so I could connect a JR or a taulink module. PPM is the easiest solution.
Current plan is to embed a small Attiny85 inside the remote, and have a small 4pins headers sticking out of the transmitter. Stay tuned.