Need to make model airplane retract gear controller

Discussion in 'The Projects Forum' started by Jack_K, Dec 26, 2017.

  1. Jack_K

    Thread Starter Senior Member

    May 13, 2009
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    I have a radio control airplane that has electric retractable landing gear. The controller for the retracts has bit the dust. New ones are no longer available so I have to make one. Here's how it works:

    It is controlled by a channel on the RC receiver. The channel uses PWM with the pulses varying from 1000 usec to 2000 usec. The pulse width is controlled by a two position toggle switch on the transmitter. In position 0 the pulse width is 1000 us. In position 1 the pulse width is 2000 us.

    The wire from the receiver to the controller supplies +5 volts, ground, and the PWM signal. When the pulse width is 1000 us the controller outputs +5.0 to the retracts. When the pulse width is 2000 us the controller outputs -5.0 volts to the retracts.

    I suppose a servo IC might perform that function, but I don't know, nor do I have any, or even know what part number it might be.

    So, can anyone come up with a circuit that do what I need?

    Thanks,
    Jack
     
  2. JohnInTX

    Moderator

    Jun 26, 2012
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    I'm guessing that the -5.0V is a reversal through a switch rather than needing a +/-5V supply. If that's the case, a PIC microcontroller with H-bridge control outputs would be a natural, I would think. It could easily discriminate between the two times and output the correct switching signals. What kind of current are we talking about? Any info on the retract motor? Does the retract motor have it's own limit switches?

    I'm sure someone will have a hardware solution if micros aren't your thing,
     
  3. spinnaker

    AAC Fanatic!

    Oct 29, 2009
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    Why not repair what you have? If it is that old, chance are it uses off the shelf parts and there is a chance the schematic is available online.
     
  4. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Could you just use a standard servo to operate a DPDT reversing switch?
     
    JohnInTX likes this.
  5. spinnaker

    AAC Fanatic!

    Oct 29, 2009
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    The way I read it is that it is not the servo that is damaged. It is the circuit that interprets the signal at the receiver that is the issue. Though the OP is not very clear IMHO.
     
  6. LesJones

    Well-Known Member

    Jan 8, 2017
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    A variation on Alec's suggestion in post #4 (Providing that the landing gear motor had limit switches.) would be to stop the new servo in about the mid position then disconnect the wires that went to the motor in the servo and connect them to the landing gear motor. A check would first have to be made that the current taken by the landing gear motor was less than the current for the motor in the new servo.

    Les.
     
  7. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    The specified signal (1ms-2mS wide pulse) is a standard servo signal; hence a standard servo will interpret it to move the servo to one extreme or the other. Shouldn't be difficlt to hang a DPDT (or 2 x SPDT) microswitch(es) on the servo.
     
  8. crutschow

    Expert

    Mar 14, 2008
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    You could use a missing pulse detector circuit (such as one using a 555 timer) to discriminate between the two pulse widths and use that to control a bridge circuit (or relay) to generate either the plus or minus polarity signals to the landing gear motor.
     
  9. Jack_K

    Thread Starter Senior Member

    May 13, 2009
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  10. Jack_K

    Thread Starter Senior Member

    May 13, 2009
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    I'll try to elaborate some. The plane has three gears, nose gear and two main gears. They are servo-less retracts. It appears there may be worm drive gears in the mechanisms, but can't really see.

    The controller is a small PCB with three IC's on it. It has three outputs, one for each gear. Two of the the three outputs work, but the third one doesn't. I have tried all three retracts in the two good outputs and all three work correctly in either of the two, but none work in the bad output. With one or two retracts plugged into good outputs, if I plug a retract into the non-functioning output, none of the other two outputs work.

    I have temporarily connected one retract to a good output and used a 'Y' cable to plug the other two retracts into the other good output. That worked, but I have no idea what the current capability of the outputs are. So I might burn up another (or both) good output.

    I haven't been able to find a schematic, especially since it's a Chinese product. I'm posting a picture.
     
  11. LesJones

    Well-Known Member

    Jan 8, 2017
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    The picture is not good enough to read the part numbers on the three chips. If the data sheet for the chips can be found it would give us an idea of how it works.

    Les.
     
  12. Jack_K

    Thread Starter Senior Member

    May 13, 2009
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    I copied that picture from one of the hobby shops that has none in stock. My actual controller is covered with something.
     
  13. Jack_K

    Thread Starter Senior Member

    May 13, 2009
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    I was able to uncover the circuit board. I found 3 each 9926A ICs, 3 each 4953 ICs, and an unbranded 16-pin SOIC.
    It looks like the MOFETs are the drivers and the 16-pin is the brains.

    The attached PDF file has as much info as I can trace.
     
  14. LesJones

    Well-Known Member

    Jan 8, 2017
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    The three dual P channel and three dual N channel mosfets make sense. They probably form three H bridge circuits. It is likely the ones that make up the faulty channel have failed. Test with you DVM which two devices have a drain pin connected to the output wires of the faulty cahnnel. When you identify them I suggest you change both devices for that channel. I also suggest that you check if the sevos have limit switches and if so are they working on the faulty channel servo as well as the good channel servos.

    Les.
     
  15. Jack_K

    Thread Starter Senior Member

    May 13, 2009
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    Would a bad MOSFET cause the other two channels to not work when a retract is plugged into that channel?
    The retract controller stops the retracts when it detects the end of travel. Perhaps it detects stall current?
     
  16. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Depends on the controller. If it's smart it may sense a fault condition and go into a self-protect mode which disables all channels.
    Quite possibly: much like car window control circuits.
     
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