Reversing motor direction, help needed

Discussion in 'The Projects Forum' started by KiwiKid, May 12, 2011.

  1. KiwiKid

    Thread Starter New Member

    May 12, 2011
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    Hi,
    For a school assignment we must make a circuit that can raise and lower a barrier arm using a voltage drop that includes a LDR, a Picax 08M chip, a motor and a transistor (or 2) used as a switch. I know how to set it up so it can raise the barrier arm but I don't know how to reverse the motor without having to rewire it every time or using a switch. I have this idea
    http://s1195.photobucket.com/albums/aa391/pieterthegreat/?action=view&current=circuit2.jpg
    but i'm not sure it will work. If it will not can someone please send in a design of a circuit I can use.
    Thanks
    KiwiKid
     
  2. dpeterson3

    New Member

    Oct 9, 2009
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  3. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Welcome to the Forum.

    That circuit is likely to end up in smoke.
    To reverse motor effectively use a H bridge or Relays.
    Or a chip designed for that purpose.

    It will give u less trouble and easy PCB making
     
  4. CDRIVE

    Senior Member

    Jul 1, 2008
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    Are you saying that your instructor has limited you to two transistors max?
    If so, the suggestion by dpeterson3 would be about the only way you could go. However, you would still need limit switching.
     
  5. John P

    AAC Fanatic!

    Oct 14, 2008
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    Hold on a second. For all we know, the motor is a very low-power type that can run on the outputs of the processor, no transistor needed. Or if it's legal to use a couple of extra resistors, two transistors are enough to build a reversing circuit. It won't be incredibly efficient, but for intermittent action it might be OK. You gotta think outside the box.
     
  6. CDRIVE

    Senior Member

    Jul 1, 2008
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    First of all the 08M is not going to directly drive ANY motor of any type or size. The output pin capabilities of a 08M barely suffices to drive a LED. It's a CMOS uC that requires drivers to do any power work.

    Secondly, I'm sure there are many of us that are waiting with anticipation for you to post a schematic of what you have in mind. If not, I certainly am! :rolleyes:

    Keep in mind that bold statements like this will invariably be challenged. When you make statement like this you should fully expect a "Show me"!
     
  7. John P

    AAC Fanatic!

    Oct 14, 2008
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    O what a world of skeptics we live in.

    I went and looked at the specs for the PicAxe to see if they're different from PIC processors generally, and they're just the same. 20mA capacity source or sink. If you think that's not enough to run a motor, see here:
    http://www.picaxeforum.co.uk/showthread.php?t=6920

    And here's the two transistors plus resistors reversible motor drive. I didn't say it was efficient, but there's no reason why it shouldn't work. Did I not say, you gotta think outside the box?

    [​IMG]
     
  8. CDRIVE

    Senior Member

    Jul 1, 2008
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    John, I read the very surprising motor data in your Picaxe forum link and then I stared at your circuit thinking of something clever to say... How about "I'm Eating Crow"? :eek:

    I confess that I had no idea that little milli-current motors like that existed.... Touché!!
     
  9. KiwiKid

    Thread Starter New Member

    May 12, 2011
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  10. KiwiKid

    Thread Starter New Member

    May 12, 2011
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    we also can't use any other switches.
     
  11. KiwiKid

    Thread Starter New Member

    May 12, 2011
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    we have to use a voltage drop.
     
  12. CDRIVE

    Senior Member

    Jul 1, 2008
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    This statement (as is) is rather vague. Want to elaborate?
     
  13. John P

    AAC Fanatic!

    Oct 14, 2008
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    Cdrive--spoken like a gentleman, sir.

    I'm puzzled by "have to use a voltage drop" too.

    Can't they at least give you some PNP transistors?
     
  14. KiwiKid

    Thread Starter New Member

    May 12, 2011
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    Last edited: May 13, 2011
  15. CDRIVE

    Senior Member

    Jul 1, 2008
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    Your link is bad. We still don't know what voltage drop you're referring to and you didn't specify why you can't use John's circuit.
     
  16. CDRIVE

    Senior Member

    Jul 1, 2008
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    Your link is now working but I can't make sense of your schematics and still don't know what voltage drop you're talking about. Are you a verbal minimalist?
     
  17. KiwiKid

    Thread Starter New Member

    May 12, 2011
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    The voltage drop I am reffering to is the circuit with the LDR in it. I will use that to get infomation on light levels, but that is unimportant as I already know how to do that, what I am concerned about is reversing the motor direction.

    What about my diagram is confusing? When I turn a pin on it will 'open' the two transistors connected to it. The current will then flow through a transistor through the motor and down through the other transistor.

    Will Johns circuit work? I am confused on how it will so? But if you can explain that would be great.
     
  18. CDRIVE

    Senior Member

    Jul 1, 2008
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    Thanks for explanation.

    You have conductors that appear to make connections to air. Since there's no antennas in your project this is not acceptable.

    If you use the same or same spec motor.. yes.
    Since your working with microcontrollers in your class I have to assume that you've completed DC and AC theory. You should know this stuff by now. Think Ohms Law and re-read John's posts. He provided links that discussed a very low current motor that WILL work in his circuit.

    I was going to post a schematic for you but my circuit is more complex than John's. If you can't grasp how his works how are you going to follow mine?
     
  19. strantor

    AAC Fanatic!

    Oct 3, 2010
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    One issue I see is that you have npn transistors upstream of the motor. those are only meant to switch ground.
     
  20. CDRIVE

    Senior Member

    Jul 1, 2008
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    You may want to rephrase that, as it's not accurate as stated.
     
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