Help with model train Reverse Loop

Discussion in 'General Electronics Chat' started by greenleader, May 15, 2009.

  1. greenleader

    Thread Starter New Member

    May 15, 2009
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    I have a DCC (digital command control) model train layout. On is a reverse loop. This means that the train enters a loop and the tracks end up connecting to themselves again. DCC trains use AC 19V. There are always 19v on the tracks and a special decoder is installed inside the train. By means of a controller, signals is send through the track to the train telling it at what speed to move, the direction, etc.

    I have attached a picture of what I want to do. The train is coming from C and proceed through A around the loop to exit it again at B. As there are always 19V on the tracks, you have to isolate the inside of the loop from the main line. Now I use a DPDT ( Double pole double throw) switch to supply current to the inside. I don't know if I can speak about negative and positive with AC, but as you can see the current going in has to be the same as inside. DCC trains are not dependant on polarity. The decoder keep it in the right direction.

    So before the train exit at B, I throw the DPDT switch to the other side thus changing the polarity so that it match the outside again. If you do not do this, it will short circuit.

    Now all of the above is working fine. What I want to do, is to install a bi-color LED Light ( Red/Green) at point A an B. So when the train enters A, the LED should be Green and the one at B should be Red. As soo as I throw the DPDT switch over, the two lights must change from red to green alternatively. Because of this circiut been AC, if you connect the LEDs, both red and green are on. On a DC system it worked fine.

    How do I connect this two LEDs to work like I have explained.

    Thanks for listening to my story and for the help
     
  2. StayatHomeElectronics

    Well-Known Member

    Sep 25, 2008
    864
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    How about adding another pole to the switch and power Red(A) and Green(B) from one side and Green(A) and Red(B) from the other side? Your power line can go to the pole of the switch and the LEDs with series resistance can go to ground.

    Rectifying the voltage with a diode and capacitor is probably needed so you do not reverse bias the diodes and burn them out that way. It can be done before the voltage goes to the switch.

    http://en.wikipedia.org/wiki/Rectifier shows an RC filter rectifier
     
    Last edited: May 15, 2009
  3. greenleader

    Thread Starter New Member

    May 15, 2009
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    0
    Thanks

    This sound logic to me. But since I am by no means electronic literate, is it possible to make me a drawing of the switch, more or less like the one I have drawn. I will appreciate it. In my case it is " Monkey sees, monkey do". I have learned quite a lot so far during the building of my layout.

    Regards
     
  4. StayatHomeElectronics

    Well-Known Member

    Sep 25, 2008
    864
    40
    Here is a quick drawing of the idea I proposed:

    The diode will have to be at least greater than the peak to peak voltage of your source for reverse voltage and have a current rating for starting to charge the capacitor. The capacitor may be around 100 uF and at least greater than the peak voltage of the input voltage, leave a little room at the top. The resistors will need to be sized for the LED brightness that you want.

    I drew the switch according to your format. If you can get a 3PDT switch and replace the DPDT switch, everything would happen at the same time.

    Edit: jpeg is attached
     
    Last edited: May 15, 2009
  5. greenleader

    Thread Starter New Member

    May 15, 2009
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    Thanks very much. :)
    The jpeg attachment seems very dark :confused:. One can barely make out the picture. Is it possible to change it to a "lighter" version.

    Regards
     
  6. StayatHomeElectronics

    Well-Known Member

    Sep 25, 2008
    864
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    How's this?
     
  7. greenleader

    Thread Starter New Member

    May 15, 2009
    4
    0
    Looks good now.
    Thanks. I will let you know. I will not be home until the 30th May and will give it a try then.

    Regards
     
  8. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
    2,040
    287
    You should be able to buy off-the-shelf points that are made just for doing that. After all, no self-respecting model railroader can do without a few balloon loops!

    Eric
     
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