Voltage Conversion For Traffic Light

Discussion in 'The Projects Forum' started by BMK, Dec 24, 2011.

  1. BMK

    Thread Starter New Member

    Nov 9, 2011
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    I am trying to install traffic lights for my N scale model railroad. I purchased and built a Velleman MK131 Traffic Light Kit (see attached diagram). The kit has a “Common Ground” connection for the three LED’s. However, the Traffic Light units have a “Common +” connection for the LED’s. Is there a voltage conversion circuit I can employ or is it as simple as adding a full wave rectifier to each LED output. Any and all assistance would be greatly appreciated.

    Happy Holidays
    Brian Krupicka
     
  2. crutschow

    Expert

    Mar 14, 2008
    13,045
    3,244
    The output is already DC so a rectifier wouldn't have any effect.

    It would appear that you need to add transistors to invert the output signals from the Velleman kit. Just about any small NPN transistor such as a 2N2222 would work.

    Add a 1k resistor from each transistors base to its respective Velleman output.

    Connect the transistor collector to the Traffic Light output through a resistor of the appropriate size to give the desired LED current [R = (Vs-Vled)/Iled].

    Connect the emitter to ground.

    The LED "Common +" connection goes to the Vs plus supply voltage, of course.
     
    BMK likes this.
  3. spinnaker

    AAC Fanatic!

    Oct 29, 2009
    4,887
    1,016
    Don't attach a doc file. Many forum members may not have a viewer and they could introduce viruses. Print to a PDF and attach that instead.
     
  4. BMK

    Thread Starter New Member

    Nov 9, 2011
    18
    0
    Thank you greatly for the assistance.
    Brian K
    Happy Holidays
     
  5. Wendy

    Moderator

    Mar 24, 2008
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    I recommend a basic .gif format myself, that way it can be displayed as part of the post.

    Are the traffic lights prebuilt (and don't match the circuit)?
     
  6. BMK

    Thread Starter New Member

    Nov 9, 2011
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    Yes, the traffic lights are premade.
    They have four wires: Common + and three leads for the three colors.
    Attached is the PDF of the traffic control circuit.

    Brian K

    Merry Christmas
     
  7. Wendy

    Moderator

    Mar 24, 2008
    20,766
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    Just to show you the difference in graphic standards, I captured the image using the <print screen> key and translated it into a .gif format.
    Once there you can show it in your post, or click the attachment below. Even .pdf offers some risks.

    [​IMG]

    I see what Crutschow was talking about. Let me think on it and I'll redraw the final part. I recognize the 4017,
    even though it is not labeled as such, and the diodes are simple gates.

    In your real circuit LD1 - LD12 are actually flipped around, correct? This is an idealized drawing. I could figure it out if I was
    willing to study it, but which LED is which color? You need to adjust your resistor values to match the colors, even more
    so if you are using transistor drivers.

    The 4017 has a current rating of 16ma, less with a reduced battery voltage. That is all it can provide. Do you want 10ma
    or 20ma per LED?

    And Merry Christmas right back at ya!
     
    Last edited: Dec 25, 2011
  8. BMK

    Thread Starter New Member

    Nov 9, 2011
    18
    0
    Thank you for the lesson in posting

    Thank you for your considerations in helping solve the problem.

    The LED's a 20ma

    Brian K
     
  9. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    There was a reason I asked for the colors of the LEDs. It matters, the values of the resistors to limit the current is somewhat color dependent. I took my best guess.

    [​IMG]

    LEDs, 555s, Flashers, and Light Chasers
     
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