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Transmitter digital pots??

Discussion in 'Electronic Basics' started by steamer, Jul 23, 2004.

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  1. steamer

    steamer Guest

    --Have googled all over the place, but can't find an explanation
    for this. I'm trying to take a "standard" Futaba FM R/C transmitter and
    replace a knob-controlled third channel with an on-off or momentary-on
    switch. Trouble is the knob isn't connected to an analog pot, but instead
    somehow puts out a varying (depnding on position) pulse train (am I
    expressing this right?). So how does one interface with the circuitry and
    with what to add a switch which, in one position puts out one pulse train
    but in the other position puts out a different one?
    --Any help or links appreciated,
     
  2. Dan Fraser

    Dan Fraser Guest

    Do a search for "rotary encoder"
     
  3. Rich Grise

    Rich Grise Guest

    The transmitters use pulse-width modulation. I don't remember the
    exact numbers, but for each channel, there's a steady pulse train,
    and when you move the control one way, it increases the pulse
    width, and when you move it the other, the pulse gets narrower.

    So you'd have to look up or measure those pulses, and to duplicate
    it, make a pulse generator with controllable pulse width. This is
    called Pulse Width Modulation, PWM.

    Good Luck!
    Rich
     
  4. Dbowey

    Dbowey Guest

    Rich posted:
    The transmitters use pulse-width modulation. I don't remember the
    exact numbers, but for each channel, there's a steady pulse train,
    and when you move the control one way, it increases the pulse
    width, and when you move it the other, the pulse gets narrower.

    So you'd have to look up or measure those pulses, and to duplicate
    it, make a pulse generator with controllable pulse width. This is
    called Pulse Width Modulation, PWM.
    You guys may be more up-to-date on Futaba than I am, but I believe the control
    knobs and switches simply vary resistance just as in the "old" days. The
    conversion to PWM is done on the transmitter's circuit board. I have a 6 month
    old systen (non-Futaba) that is as I describe.

    The conversion should be as simple as removing the wires from the pot and
    connecting them to the switch and resistor(s). To help any further, we would
    need to know if the pot has two, or three wires on it, and what is the
    resistance of the pot at the end of the control knob movement at each end.

    Don
     
  5. steamer

    steamer Guest

    : You guys may be more up-to-date on Futaba than I am, but I believe the control
    : knobs and switches simply vary resistance just as in the "old" days. The
    : conversion to PWM is done on the transmitter's circuit board. I have a 6 month
    : old systen (non-Futaba) that is as I describe.

    --Been there, not the case. We put a voltmeter on my xmitter last
    night and there was n variation detectable in what was being put out by
    the pot, regardless of position. Hopefully someone will bring an
    oscilloscope to the next class and we can sort it out a bit further.
    --At this stage plan B is to talk to a pal who works at Parallax,
    as I've heard there may be a Basic Stamp hack that will solve the problem.
     
  6. steamer

    steamer Guest

    --Update: a friend arrived at class last night and he brought his
    oscilloscope and a voltmeter with an audible continuity tester. Turns out
    when he used this and rotated the "pot" on the transmitter he got a series
    of beeps. The "pot" is, in fact, a rotary switch that sends a series of
    on/off signals to one or another location on the circuit, depending on
    which way it's rotated. He says this can be duplicated via PIC chip and a
    simple circuit in a day or so. Anyway he hopes to have a working
    switch-activated remote weapon activator that we can plug into the
    transmitter by class time next Monday. Woohoo!
     
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