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Microcontroller Based RC Receiver Project

Discussion in 'General Electronics' started by Daniel Rubin, Aug 29, 2003.

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  1. Daniel Rubin

    Daniel Rubin Guest

    Size my friend... I don't need the resource required to control the
    servos... I am planning on using PWM output from the PIC to vary the
    speed of 4 small motors directly. I am shooting for something under 2
    grams with 1000 ft. range.
     
  2. TexMex

    TexMex Guest

    Outside of the bit of pique, what the Natural Philosopher said is
    correct. You have to amplify the radio-frequency signal at the
    antennae, filter out the signals you aren't interested in, and
    demodulate the resulting FM signal before you can do much digital
    processing. What you have to do to get a useable signal to your PIC
    is three-fold…. : )

    The first part, the amplifer/filter, is called the "front end" of the
    receiver. You need a very low noise transistor(s) to do the
    amplification, since all semiconductors (resistors too) generate some
    background noise all by themselves. Once the signal is boosted to a
    higher level, the filtering can be done in a lot of ways- passive and
    active filters, and various combinations of both. The demodulation
    process can be done many ways too- phase locked loops, digital
    filters, etc. Each has it's advantages. Regardless, you're pretty
    much locked into having to use a few passive components.

    For your project I would think one of National Semiconductor's FM IC's
    would fit the bill. Cheap, small, and light. Get their IC application
    book and take a look at some of their sample schematics. Just keep in
    mind that RF receiver/amplifier design is a complex project, and
    things like temperature drift, noise, and shielding come into play-
    things that aren't always in the digital designers toolbox.

    One idea to initially circumvent some of these difficulties would be
    to get hold of an RC receiver and schematic and using a scope, find
    the decoded signal point. Wire from there to your PIC. Then you can
    program your digital processing with a known good signal. Once you've
    got your code working, you can delve into the design of your front
    end. It's tough to de-bug both components of a prototype analog and
    digital system at the same time, as you'll have a lot of
    interdependent variables to sort out.

    I applaud your curiosity, and even if the result isn't as good as a
    multi-million dollar corporation's offerings, you'll have gained a
    wealth of practical knowledge, and the personal satisfaction of having
    done something not many of us can do!

    Good Luck,
    John
     

  3. I think a DSP is fast enough to do the detection from 455khz tho. AND
    maybe do digital filtering as well. So the last half of the IF strip is
    digitally possible IYSWIM
     
  4. Johnboy

    Johnboy Guest

    What I expect the processor RX to be is 1or 2 dual gate mosfet rf amps
    a few passives, and maybe one coil. Then the processor takes over
    with the use of its single master crystal. User selects any 72 or
    75mhz
    frequencies via dip switch, then the processor provides phase lock
    loop,
    IF strip, narrow banding, FM/PCM demod, and up to 24 channel output.
    Lets go way beyond the antique National IC design. As we all know the
    decoder is super simple and not worth even an additional ten cent IC.
    Also go to a high number of channels. Granted that the plane and car
    users don't need all the channels, but the Scale Boat builders can use
    them. A trace cut on the board can be used to lock out the unwanted
    band. This concept is exactly what the major radio suppliers do not
    want
    to see, since they make most of their profit selling 19 cent crystals
    for
    $15.00 each, and selling 20 different receivers. One receiver will
    fill the
    bill for ALL possible RC activities except the 27 and 53 bands. These
    bands are too far from the 72-75 range to use the same front end. A
    few
    passives and coil change would be needed for the lower bands. I may
    be way off base expecting this much performance from a processor
    based RX but is there anyone out there wanting to give it a try?
    Johnboy
     
  5. TexMex

    TexMex Guest

    Yes, depending on the S/N. Programing a DSP to handle low S/N in a
    mission critical situation is not for the faint of heart, or
    hobby-level funding. At a minimum, going to need an In Circuit
    Emulator, high-dollar recording digital scope, a digital signal
    generator/digital noise source, and the usual lab equip. A circuit
    sim. program is nice to learn on, but I haven't seen one for under
    $10K that will handle mixed mode analog/digital processing in a
    non-idealized environment (Spice on a PC ain't gonna get it done.)
    Cell phone grade dsp is a REALLY bad idea when a 25lb, 150 mph Extra
    swinging a 15" meat cleaver is involved. And yeah, the National rf
    IC's are old, but you can set 'em up with a 50 MHz analog scope and a
    Radio Shack multimeter.

    I suspect that most of this discussion is moot, though, as there is
    none so blind as will not see. The premise of the thread smacks of a
    pianist, who having mastered Eine Kline Nachtmusic on his Hammond,
    demands to be shown how to play the Brandenberg, III. At Charte'.
     
  6. Tom Burke

    Tom Burke Guest

    -----BEGIN PGP SIGNED MESSAGE-----
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    Older remote controls used either PWM or PPM (Pulse position Mod)...
    Both effectively amplitude shift keying (ASK)... We designed a
    project once that used a single (8 pin?) motorolla IC that was
    intended to demod FM signals with some minor support circuitry. We
    used all SMT caps & coils (they all looked like SMT resistors), and a
    PIC to extract the signal. It worked well enough, but there was a
    real problem with multipathing in our environment. We operated in
    the 50MHz regime.



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  7. J

    J Guest

    Well because you may want it to look like a real airplane, in which case
    ARTF is a no go and a lot of the kits are not much better, or you may want
    it to serve a certain purpose, e.g. a load carrier, in which case most kits
    are no good. With a RX though, you could even buy a micro 7 channel model,
    for about $30 which will still do everything a homebrew could, for less
    money, less wasted time and it will be much more reliable ;)

    Justin.
     
  8. J

    J Guest

    AH ha, building a roswell flyer are we?
     
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