Hi,
i was wondering how to interface arduino with an rc copter that use 40mhz radio controll, any idea on how can i start ? id like not to use the original rc controll, but just arduino + rc transmitter !
Pull the transmitter out of the controller. then remove the 2 joysticks and figure out how they work and what range of resistance they have. Assuming they are similar to spring loaded pots. The connection points the joysticks had on the controller's PCB is where the Arduino will connect too. The hard part will be finding the right ranges to get the copter to take flight and maintain a steady hover.
Unless your trying to use the arduino to take the place of the controller's I2C that translates the position of the joysticks to signals that are transmitted to the copter.
i dont want to use the original rc remote, i wanna build one brand new with arduino and radio transmitter. otherwise i would just use 2 digipot to replace the 2 joystick. i wanna read the signal from the remote, get the algorithm with they encode the 2 pad position into a rf signal, and reproduce it with my arduino. i already did it with an ir remote elicopter, now i wanna try with an rf@40mhz and i was wondering what do i need to read signals and to reproduce em at the right frequency
Check the transmitter and receiver and find the data sheets on them. You may be able to get enough information from them to do what your looking for.
On the transmitter, are there any identifiable ICs, or is it just a bunch (or one) epoxy blob? Do you see any RF components (chokes, coils, etc)?
Trace the pots back to the IC, and if there are any such RF components connected to the IC, that junction is where I would probe. Since it is unknown what the signal really is at this point, you might want to use an oscilloscope set to 5V peak-to-peak - you probably don't need anything fancy bandwidth-wise; even one of the 1 MHz seeed studio kits would likely work OK for this. If you don't have a scope or access to one, then you could try to hook up to an input on the Arduino, and run a program to rapidly sample the input and spit the data out the serial port; provided it isn't running at too high a speed, you might be able to figure out what it is.
Likely it is a square wave, and the RF converts in in some manner to an AM or FM signal for the receiver. However, if there isn't any RF circuitry (ie, the only thing coming from the IC or blob is an antenna) you probably won't be able to figure the signal out, and if you did (lets say you had access to a 100 MHz storage scope, for instance, so you could sample the RF), you wouldn't be able to reproduce the stream with the Arduino (it doesn't run fast enough).
Hopefully the manufacturer cheaped out and used common components and a piecemeal RF section that is separate; but then again, they likely could've cheaped out and stuck everything under a blob of epoxy...
im half lucky, no blob epoxy for me, but the only ic ive got its in a dip16 without anything written on it ... (same thing that happend with the ir remote), this suck me a bit ... i can see a crystal on the board, a coil, a brunch of transistors and the common spread of condensator and resistance...
and i'm a poor guy, no oscilloscope for me 
i just got the n00b arduinoscope