Thinking of cheap 4-channel transmitter mod..

[jonhelge]

Hi! I'm thinking of ripping out all the electronics on a cheap 4-channel transmitter except the RF-module, and then build a new "brain" with a Atmel AVR uC. Just using ADC for the pots and then generate the PPM-output on a ordinary output pin. (I can add some extra channels here..) I can't see any problems with this as long as I keep my hands of the RF-module? Or am I totally wrong?

-Jon Helge

[Chippie]

If you dont tamper with the RF section of the tx I dont see a problem....best if you check with the type approval regs for your country.......good luck with coding the avr chip...I admire anyone that can write code, takes me all my time to write text.... ;)

[Mr.RC-CAM]

In the USA, changes to any part of a Part 15 or Part 95 transmitter, that are not supported in the mfg's technical submission, are generally not allowed.

There are some minor exceptions in the USA Part 15 and Part 95 regs for kit/home built equipment. For USA use, I suggest a home builder read all the regs very carefully to ensure your efforts are within the rulings.

Your country will have its own laws regarding RF equipment. Frankly, if you are concerned about regulatory issues, a ham license and a ham based R/C Tx (if allowed in your area) may be the best way to go.

RC-CAM

[OmegaDot]

Suppose that you were to take the original PPM signal from the trainer jack, modify it externally (probably with the microcontroller) and then feed it back into the trainer port. What does the FCC say about that?

[Mr.RC-CAM]

The jack provide a user connection that is meant for interfacing to compatible R/C equipment. Essentially, the mfg's instructions are expected to provide the indications for use. On a practical level, as long as you do not use the trainer interface for disallowed use (i.e., sending telemetry data, voice, etc) no sane agency would care.

BTW, the FCC rarely says anything useful, even when asked. Chaulk it up to years of budget cuts I suppose. However, recently they did seem to have offered some advice to Talk Show host Howard Stern though. ;)

RC-CAM

[Mr DIY]

Hi! I'm thinking of ripping out all the electronics on a cheap 4-channel transmitter except the RF-module, and then build a new "brain" with a Atmel AVR uC.

I have toyed with idea for some time, but never got round to it. The closest point I have got to the finished product was to build up the micro board. I purchased some nice biggish graphic displays for this project as well, but must find the time to complete it. In the mean time, I bought a PCM 9X radio, resulting in the project hitting the back shelf. :)

As for regulations, the main concern for any country would be about what is being radiated. That would be the RF signal plus any EMC from your digital circuitry. Our controllers are quite slow anway, (less than the 30MHz) so the EMC should not be a problem. The signal that modulates the RF module however, should be the right waveform (a perfect square wave is not good) in order to reduce the RF bandwidth to within specified limits. This is your main concern. The RF module may wave waveform shaping built in .. or it may not. But dont let this stop you. The wave shapping is easy to do anyway.

Brian

[imeins]

There is a thread in the "radio" section where I started to do this to an old Futaba gold series tx. I got to the stage of removing all it's own guts and building the Mega16 controller but then got busy with other things so it went onto the long waiting list.. All I kept of the original electronics was the RF modules. So, we all started but never quite finished it !!

Ingmar

[vintage1]

The only thing apart from RFI from teh digits you need to worry about is teh depth and sharpness of teh FM modulation - go too far or too fats swinging teh frequency and you will generate sidebands on adjacent channels.

If you can scope out the waveform of the existing modulator and make sure yiurs looks similar, I'd say go for it. Access to a frequency counter and if possible an RF spectrum analyser should tell you if you are within spec. Expensive beasts to hire, and extremely expensive to buy tho.

[Bruce Abbott]

Your cheap 4-channel transmitter probably already uses a microcontroller to generate the PPM signal. You could remove this chip and replace it with a daughter board with the same pinouts, but which has an AVR processor onboard. You could then add connections to other peripherals (more channel inputs, switches, LCD display etc.).

Provided that the PPM output from your board duplicates the voltage levels and pulse widths of the original MCU, RF output purity should not be compromised. Chances are that the radio already has low pass filtering and level shifting circuitry to limit the transmitted bandwidth, which you will want to keep in place.