My wheelchair has a remote control that can turn the wheels off, on, and control the speed of the motor discreetly between a high level and a low level. The remote control is very large (8x2x1 inches). I want to build a smaller version of the remote control for my personal use.
After looking up the FCC code for my remote control I have found that it uses two different wavelength, 2405 MHz and 2465 MHz. I therefore understand that the remote control uses FSK (frequency shift keying). After researching this type of coding I have seen that there are two different ways to demodulate the signal.
The first, FM detector demodulation, and the second filter type demodulation. All the information I have found has been theoretical in base. I have not come across anything practical.
How do you suppose I can capture the RF code? I have been looking at FM receivers in the 2.4 GHz range. I do not wish to build my own receiver with filters. I would much rather purchase an off-the-shelf receiver that I can hookup to the computer and analyze the signal inside the computer.
Any help you can give me is much appreciated.
Due to the large difference between the two frequencies given, it is very unlikely that (if the modulation is FSK) it switches between them. It is more likely that those frequencies correspond to some range of independent channels.
There are many commercial radio chips in the 2.4 GHz band that use FSK encoding, among other possibilities. Here is a link to a data sheet for one such, which gives some information about the channel spacings and frequency shift for the bit encoding. http://www.rfmd.com/CS/Documents/ML2724 … asheet.pdf
However, it appears that there is no standard among 2.4GHz remote control units. So, it may be hard to find out exactly how the data are encoded and to use that information to build your own. See this page for a hobby unit example: http://www.bphobbies.com/view.asp?id=c1 … d=c1736904
I’ll guess that those two frequencies in the FCC filings are two user selectable channels. For either chosen channel, the FM would be a relatively small deviation, say, less than +/- 1MHz, to create the 1’s and 0’s, coded.
I’ll take a guess that the the wheelchair remote uses a chip intended for either model aircraft remote control or one intended for toys and a computer mouse. Rather than a fancy spread spectrum or 802.11 chip as these are gross overkills.
Perhaps there’s a clue if, when you open up the remote, the main RF IC in the remote has a part number that is common, not unique to the chair manufacturer. I’ll guess it’s pretty generic, as the chair company doesn’t make these RF things; they’re mechanical people.
Here’s a couple of links
http://www.rcmodelreviews.com/rxcompatibility.shtml
There’s no standards in these worlds. But maybe R/C model aircraft have some extent of interoperability from standards among a few vendors.
In the world of PC mouses, I doubt there’s any standards at all, except for the few mouses that are based on Bluetooth (IEEE 802.15.3).
My suggestion: Tell the chair vendor that you have an engineering friend who wants to make a smaller / better remote and do so pro bono for any/all that might want such in the chair user world. Then maybe they’ll find and provide info. They probably subcontracted this work.