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New here, looking for some clarification on a bright/dim LED circuit.

Discussion in 'LEDs and Optoelectronics' started by kyjosh, Dec 15, 2009.

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  1. kyjosh

    kyjosh

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    Dec 15, 2009
    I should know this... but right now I'm sitting at work scratching my head and wishing I could go home and test it. I'm working on ideas for my next simple project, putting LEDs into a 1:18 scale model car. I was thinking about doing turn signals, and using either blinking LEDs or a 555 chip; but would eliminate the ability to have the corner markers or parking lights running steady with the head lights unless I used to two LEDs... and space is at a premium.

    Would it be possible to hook an LED to the power source with two different jumpers and two different resistors and put a switch on one jumper? One jumper could connect to the LED with its proper resistance for normal forward voltage and current; the other could be connected with more resistance to lower voltage/current and thus lower its brightness. The lower resistance jumper (or the bright jumper) could have a switch in it to open its path and send current through the dim jumper. When the switch is closed though the current would travel through the less resistance of the bright jumper.

    That way with the press of a button one LED could be either bright or dim. It just seems way too simple though... and I'm sure there is something I'm missing that makes it impossible; and its driving me crazy trying to figure out what.

    I hope what I said makes sense. I wish I had a way to make a diagram of what I'm talking about. Even if no one can figure out what I'm trying to say, I'm appreciative of you reading!
     
  2. kyjosh

    kyjosh

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    Dec 15, 2009
    I think I put this in the wrong forum, I guess this should have went in the help forum. I'm already off to a bad start. ;-)
     
  3. Resqueline

    Resqueline

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    Jul 31, 2009
    No, it's ok, it's a project so you can post it here, and it actually is as easy as you suggest.
    Two resistors, in series or in parallell, with a switch in parallell or in series with one of them. The switch can be a transistor, and the resistors can be high-side or low-side to suit any transistor controlling scheme.
    The "big" challenge is determining suitable values to get the brightness you want, but a variable resistor & some experimenting should do the trick.
     
  4. kyjosh

    kyjosh

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    Dec 15, 2009
    Thanks. I was worried that maybe I'd need a diode on the higher resistance side to prevent shorting the battery.

    So taking this a step farther, I could hook the lower resistance line up to the LED coming off of a 555 timer with the switch in between right? That way the light would glow dimly when the "head lights" were on, and with the push of a button it would flash.
     
  5. Resqueline

    Resqueline

    2,848
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    Jul 31, 2009
    Yes, the LED anode to +, the cathode to - via a resistor, & the cathode to the 555 via a resistor & switch. Should work.
     
  6. kyjosh

    kyjosh

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    Dec 15, 2009
    Ok. So this won't be as complicated of a circuit as I thought. I've got the parts ordered, and they should be here by the end of the week; so hopefully this weekend I can rough it out.
     
  7. kyjosh

    kyjosh

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    Dec 15, 2009
    Just a couple more questions:

    1) Does anyone know how many LEDs a 555 or a 4017 Decade Counter can drive unassisted? I haven't seen any specs on exactly how much current they can source. I didn't think I would need a transistor just to drive one or two LEDs, but every schematic I've studied has either a transistor or a diode in it.

    2) This is more just out of curiosity, but I'm a bit confused by how the 555 is triggered. Is it triggered by a low when the trigger cap fully charges and opens the circuit, or is it triggered by a high when the cap dishcharges?
     
  8. Resqueline

    Resqueline

    2,848
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    Jul 31, 2009
    1); It depends on the supply voltage, the CMOS type and the LED type.
    Ordinary LED's require 20mA while high-brightness LED's manage with 2mA.
    Unbuffered (UB) CMOS can only deliver one tenth of the current a buffered (B) version can deliver.
    At 5V you might squeeze 2-5mA out of a B version and as much as 20mA at 15V.
    I think I remember that a 555 can deliver as much as 100nmA.

    Regard all data presented here as approximate as I didn't bother to check the datasheets.

    2): I don't remember the answer but it can also be found in the datasheets, that usually has many examples too.
     
  9. kyjosh

    kyjosh

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    Dec 15, 2009
    Ok so I went over the data sheets for both the LM555 and the 4017. I should have known to look over them fromt he start but I'm relatively new to all of this; I'm an engineering technology student going for an AAS. So most of the projects I have done have been in class, where all of the needed info was provided.

    The 555 can source up to 200ma, but I will probably still need a transistor to get proper current at my voltage (9V).

    I have no idea about the 4017 though. Either I missed it in the data sheet, I'm looking for the wrong terminology, or its not in there.

    I guess either way though it is better to be safe than sorry.
     
  10. Resqueline

    Resqueline

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    Jul 31, 2009
    Yes, much class-time would be "wasted" otherwise, but I think schools should focus a little more on the ways of finding necessary information.

    Wikipedia has a nice writeup on the 4000 series btw. but doesn't provide any numbers.

    The 4017 datasheet refers you to the 4000 family data - a datasheet common to the whole series, where the output currents will be listed.
     
  11. kyjosh

    kyjosh

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    Dec 15, 2009
    I would agree with schools pushing more information locating. I mean being given a lab with all of the info is great for the classes where we are learning the theory; but when it comes time to do something with it most of us are just left scratching our heads.

    Though it seems there used to be more classes geared towards that sort of thing, but over time they have been whittled down to make room for more general ed classes.
     
  12. kyjosh

    kyjosh

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    Dec 15, 2009
    Well, I've fabbed up the simple turn signal flasher circuit on a breadboard, and wow this is going to be a bigger circuit that I thought. Just the aStable circuit for one flash frequency is going to take up quite a bit of circuit board.
     
  13. Resqueline

    Resqueline

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    Jul 31, 2009
    Hehe, yes, it doesn't take much circuits to cover quite a bit of "real estate", no wonder surface mounted technology emerged.
     
  14. kyjosh

    kyjosh

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    Dec 15, 2009
    Yeah, I don't think I'm actually going to be able to do everything I wanted; I just won't have enough space to put the board if it gets that big.

    I was originally just going to do head lights, turn signals, and four-ways in a regular model car; but I came across a great deal on 1:18 scale Crown Victoria with mounting points for a light bar, so I've decided to do strobes, wig-wags, and a light bar. I'm going to ditch the turn signals for this one and do them on a regular car.

    So I added a 4017 and have a neat looking double strobe light bar set up now.

    Resqueline, would you recommend using IC holders when I solder my chips to my circuit board?
     
  15. Resqueline

    Resqueline

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    Jul 31, 2009
    I see.
    I don't know if a ready-made blinking LED varies its current draw enough to turn on & off a transistor. If so your circuit could be made smaller.

    Sockets or not is just the flip of a coin. Depends on how you value the IC's (reuse?) versus the work of replacement. I've often used sockets, but the IC's rarely fail. Static discharge and reverse polarity can do them in though.

    You must try to shoot some pic's/ videos and post when you're done.
     
  16. kyjosh

    kyjosh

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    Dec 15, 2009
    I thought about using a blinking LED, but with that I'm stuck with its blink rate.

    I've got some sockets ordered, I think I'm going to use them cause I just don't trust my soldering skills.
     
  17. kyjosh

    kyjosh

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    Dec 15, 2009
    Well, I have my astable circuit with the 555 soldered onto a circuit board. I have the holder for the 4017 in place, so I hope to have it set up and a few long output leads soldered on tomorrow.

    Because my soldering skills are... well they suck.. the circuit is ugly. It sure won't win any beauty pageant, but at least it works.
     
  18. kyjosh

    kyjosh

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    Dec 15, 2009
    I pretty much have to start over with this. Because I was so impatient, I started building the circuit before I actually had the car that I was putting it into. I was worried that there wouldn't be enough room for the board I was using, so I ended up wiring up the transistors "in line" and off the board. My thought was that I could cut the board down the wires would be easier to shove around to make it all fit. It turned out to be the opposite of that. I have plenty of room for a larger PCB, but the tangled mess of wires that it created is too large to fit.

    So I'm going to have to start over on a new board. At least most of the lights are already in the car now, so I just have to solder their leads to the new board.
     
  19. kyjosh

    kyjosh

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    Dec 15, 2009
    This project is on hold for now. I still have to redo my board, and with me being back in class now I just don't have the time. I might try to get some work done on it during spring break or maybe even before, but I know for sure I'll be back on it when summer rolls around.

    I ran into another problem with it... though not really electrical. The lightbar I bought was glued together really really really well, and I ended up screwing it up when I hacked it appart. So if I want to use my original design I have to order a new bar.

    I do have a different light bar that came with the model, but its real life counter part uses rotating lights and not LEDs/strobes, and I want to be as realistic as possible. I know how to simulate rotating lights, I just need 3 LEDs in a triangle or 4 or more in a circle. I could do it with a 4017, but I would really need two of them to make the effect work properly, and I doubt I could fit a second 4017 on my board and still have it fit in the car.

    A PIC would work great, but I don't the first thing about them.
     
  20. Resqueline

    Resqueline

    2,848
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    Jul 31, 2009
    Tough luck.. But I know "breaking new ground" is usually challenging, and "first things first". See you around later then. :)
     
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