Flashing light circuit

Discussion in 'General Electronics Chat' started by magnet18, Feb 23, 2011.

  1. magnet18

    Thread Starter Senior Member

    Dec 22, 2010
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    Hey guys, I have a project where I need a row of 10 red LED's and a row of 10 green LED's, with all the red LED's on except the one on the high pin, which will have the green LED on. Will the attached circuit work for this?
    I know the 555/4017 portion will work Ive already built it and got it working. I'm unsure of the transistor logic, since I came up with that bit myself.

    Thanks :)
     
  2. nerdegutta

    Moderator

    Dec 15, 2009
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    Hi.

    How did you calculate the value for the resistor to the LEDs?

    Why npn and pnp? Why not use either one? (just curious)

    You might want to ground pin 13 and 15 on the 4017 IC.
     
  3. Wendy

    Moderator

    Mar 24, 2008
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    Your 555 is smoked. Pin 7 and VCC do not mix.

    My Cookbook

    Still working on the drivers, way overcomplicated.

    Why not this?

    [​IMG]

    Just change the resistor to match the power supply.

    ************

    OK, reread your schematic, reread your post. Don't think your schematic on the LEDs work, too many crosslinks (all the bases are common on the second layer).

    Are you wanting the LEDs to be red, then switch to green when it is selected?
     
    Last edited: Feb 24, 2011
  4. magnet18

    Thread Starter Senior Member

    Dec 22, 2010
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    Sorry, that was a bad on my part drawing the schematic.

    I'm not questioning your judgment, but I'm curious why that is bad?
    Too much current?

    Yep.
     
  5. magnet18

    Thread Starter Senior Member

    Dec 22, 2010
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    From what I can tell 330 seems to work most of the time.
    If there is a way to do it with one please let me know :)
    meh. I'll get around to it sometime.
    ;)
     
  6. Wendy

    Moderator

    Mar 24, 2008
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    Each leg needs to process the signal separately. Yes, they are mostly in the same state, except the one that isn't. :D

    I'm guilty of trying to save too many parts, so I recognize the symptoms. For something like this, you need one driver per output, and focus on that.
     
  7. magnet18

    Thread Starter Senior Member

    Dec 22, 2010
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    So would it work if I broke up the ground lines and gave each segment its own 100K and ground?
    How would you do it?
     
  8. Wendy

    Moderator

    Mar 24, 2008
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    Ground is ground the world round.

    I would draw a simple transistor circuit (or two transistor circuit) that does what you want it to do. Green light on when it is high, red light on when it is low.

    Another old engineering statement, KISS

    Keep It Simple, Stupid!

    I could draw something? I remember a similar post many years ago (green light/red light), but it just tickles. I suspect you want to try your hand at it though.
     
  9. magnet18

    Thread Starter Senior Member

    Dec 22, 2010
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    I tried...

    I can give it a shot, if you don't hear back in 20 minuets send in a rescue team ;)
     
  10. magnet18

    Thread Starter Senior Member

    Dec 22, 2010
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    Alright, heres what I gots.

    [​IMG]

    questions?
    comments?
    concerns?
    rants?
     
  11. Wendy

    Moderator

    Mar 24, 2008
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    Clouds of magic blue smoke!!!

    current limiting?

    Remember, a resistor on the emitter to ground increases the input impedance of the transistor.
     
  12. magnet18

    Thread Starter Senior Member

    Dec 22, 2010
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    I'm guessing I did something wrong...

    Remember, no; Learn, yes.

    better?
    [​IMG]
     
  13. Wendy

    Moderator

    Mar 24, 2008
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    OK, here is my take...

    [​IMG]

    The second schematic no current will flow for the green LED. I circuited the smoke issue (though I had it wrong the first time).

    Symmetry is a partial way to look at it, thought there are other solutions. Here's mine...

    [​IMG]
     
  14. magnet18

    Thread Starter Senior Member

    Dec 22, 2010
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    Why would no current flow for the second one, assuming the green led is connected to an output on the 4017?
    What did you have wrong the first time?
    What is blue smoke?
    Would this work?
    [​IMG]
    I'm aiming for minimal parts/construction time, it only needs to work for maybe an hour total, most its going to be on at one time is 8 minuets.
     
  15. nerdegutta

    Moderator

    Dec 15, 2009
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    Nope, sorry. I can't.

    I made a bit of the above circuit. Video. I swapped the 555 with a PIC 16F628 generating the output.
     
    Last edited: Feb 24, 2011
  16. Wendy

    Moderator

    Mar 24, 2008
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    That isn't what I drew, and in simplifying, lost it. Will not work.

    Resistances on an emitter of a transistor mean the base resistance is increased. You switched where the emitters were, removed a resistance (more magic smoke), which left the matching LED with no limiting resistor.

    LED junctions MUST have a resistance. So do transistor BE junctions.

    Both directions of current flow must be covered.

    [​IMG]
     
  17. Wendy

    Moderator

    Mar 24, 2008
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    OK, part of the problem you are fighting is you haven't had the formal course on transistors. It is easy to forget sometimes.

    Take this illustration...

    [​IMG]

    For the moment, ignore the LED, pretend it isn't there. It doesn't exist.

    You have a transistor and a resistor on the emitter. This resistor is 1KΩ.

    Let's say the transistor has a DC gain of 200, gain is beta, β.

    A common collector design, also known as voltage follower, has no gain, but the input impedance is β X Re (emitter resistance).

    So the input resistance of this design is 200KΩ, and the 1KΩ resistor will get whatever voltage is feed on the base delivered to it. In other words, it is a current amplifier.

    A CMOS circuit can not deliver much current, but has the full voltage swing. This circuit will drop 20ma used by the LED to 0.1ma from the CMOS circuit. It also doesn't truly switch on/off, it is analog.

    ******************************

    A common emitter design is totally different. Both designs need a resistor in the path of the Base to ground current somewhere. This is what your missing. With the common collector design I've made the current limiting resistor do double duty, but you can not always get by with it in electronics.
     
    Last edited: Feb 24, 2011
    nerdegutta and magnet18 like this.
  18. magnet18

    Thread Starter Senior Member

    Dec 22, 2010
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    That... Kinda made sense

    I got the general jist but the specifics didn't sink in.. Ill have to look into what gain is.

    I think I'm just going to go with yours, for the sake of my LED's. Blowing Ultrabrights is not on my to-do list.
     
  19. Wendy

    Moderator

    Mar 24, 2008
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    Opps, how much current are you talking about? I was in the 20ma range. 1W LEDs are in the 600ma range.
     
  20. magnet18

    Thread Starter Senior Member

    Dec 22, 2010
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