RC helicopter transmitter

[paris1]

Hi, I'm planning on building my own transmitter to controll a radio controlled helicopter. Could anyone tell me what kind of messages are transmitted to a helicopter to tell it to move? I have the helicopter and the transmitter so basically i want to build a copy of the transmitter i already have. If anyone has a link to any detailed information on this subject i would very much appreciate it.

Thanks

 

[Nigel Goodwin]

Assuming it's a proper model (and not a toy) they use standard RC systems, which send out suitable signals for the servos.

But it would cost more to build than buy, and really for safety reasons you shouldn't attempt it.

 

[dknguyen]

Google Microstar 2000.

 

[paris1]

Thanks for your reply. It is an ok model. It cost about 140 pounds (280 dollars) but it is very very light and made of plastic so i dont think it could do any damage? I realise they can be bought cheaper but it is more about the challenge and i intend on using it as a college project. Thanks

 

[HarveyH42]

As a really bad R/C helicopter (Blade CP, couple of tiny toys) pilot of a couple of years now... I can't imagine doing this, just for fun. The Blade is hard as hell to keep under control, and kind of dangerous.I haven't had a major crash, but on my 4th set of main rotor blades, second tail. I power down quickly before impact to minimize damage. I'd think you would want to have a couple of backups.

There is a forum RCuniverse, or something like that. Helped me get off the ground. Haven't been there in over a year, but the have most anything RC. Should give it a look, ask there. More likely to get info, but suspect they'll think it a waste of good helicopters...

 

[paris1]

Thanks, Ill tried those and got some information. Im looking for more information on exactly what signals are sent out, not really a guide how to build the transmitter. I want to design a new interface for the controls, this is why I am doing this

 

[dknguyen]

You can use Microstar to not build a new receiver, or you can build a new transmitter and receiver using Zigbee modules.

Search terms are "direct sequence", "frequency hopping", "PCM", and "PPM". THe first two applying to 2.4GHz transmissions and the second two referring to RC air vehicle 72MHz AM transmissions (or was it FM? I don't remember).

 

[paris1]

Thanks they helped my understanding a lot. My next problem is to understand how the transmitter controls several channels on only one frequency. Thanks

 

[dknguyen]

You will find this out by Googling on how to hack a PPM receiver. What it does is it uses a serial-parallel shift register (serial being an input single input processed from the single RF frequency band and each parallel line being one of your channels). What this can do is propogate the single SAME pulse down each parallel output (or channel) sequentially- the pulses are not sent down each channel simulatenously but sequentially. At the beginning of each cycle you produce the pulse that will be propogated down each channel by sending a SYNC pulse into the data-input of the shift register. Then you pulse the clock every time you want the pulse to propogate to the next channel.

So if I have 3 channels and wanted:
Channel A: 1s
Channel B: 2s
Channel C: 3s

At the beginning of each cycle I would sent a SYNC pulse which I would then propogate sequentially through each channel by controlling the clock pulses. After 1s, Channel A would have had a pulse of 1s width which is long enough so I would send clock pulse which would then transfer the pulse to the nex channel- Channel B. Then after Channel B has been high for enough time (2s) I would send another pulse to move the pulse onto the next channel- Channel C. After I decided channel 3's pulse was there for long enough I would then send another clock pulse which would then make the pulse dissapear since it is propogating to a dead-end non-existent channel. Then I would start all over again with a sync pulse.

As you can see, if your serial input pulse is 1:1 time-wise with your channel pulses, you have a limit on how many channels you can control based on your required pulse widths. So a servo that requires a pulse width of 1ms-2ms at a rate of 50Hz, would only allow you to control a maximum of 10 channels since the pulses. To control more channels you would have to somehow reduce the pulse widths so you could cram more of them into the same time frame for more channels or use shift registers in parallel or something.

Of course, with an MCU you could just generate all the channel pulses from the RF input simultaneously which removes this limit depending on the RF's data protocal- it just takes many more pins on the MCU.

 

[VISN]

Have you been to www.rcgroups.com ?
There is a massive wealth there on everything RC related, including an DIY Electronics forum.

 

[paris1]

Thanks Dknguyen, I have been looking everywhere for an explanation. That clears up a lot.
I have been to rcgroups but i might have missed a lot. ill look at it more now thanks