20 LED project (1.9 - 2.4 in) - is my math wrong

Discussion in 'The Projects Forum' started by aerogal, Jan 24, 2011.

  1. aerogal

    Thread Starter New Member

    Jan 24, 2011
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    My daughter wants to make a duck in grass using colored LEDs for her 5th grade electrical project. We have 6 green, 4 red, 10 yellow. These are all the same (intensity 50-80 mcd; wavelength 590nm; FW current 20 mA; FW supply 1.9V typical 2.4V max). We have a spst micromini toggle switch "rated 3A at 125VAC" and some 22 gauge wire. I've done these projects in the past (eventually having success, the switch being the most frustrating piece of all since I don't know which is +/-) but the LED is new territory. I've read a LOT of posts. I know how to do series, parallel, the difference, etc. (not much more...) The project dimensions will be about 12" x 12". I have several questions.

    1. Can I mount this whole thing on a gift box lid. Kid wants to paint background, duck, etc. We were planning to poke the LEDs through the lid and wire / tape on the back.
    2. If we connect this in *series* I'll need 38-48V, right? That is five (5) 9V batteries. That sounds like a lot. Then I'd need an 860 ohm resistor? Clearly this is new territory.
    3. Or do I do this whole thing in parallel. But I read in another post that I should put a resistor in front of each LED. That is TWENTY resistors. A lot for a tiny 5th grade project.

    Bottom line question - what is the easiest, fastest way to accomplish wiring these 20 different LEDs (so it looks like a duck in grass) without burning them all out? Also, with this switch, is the "longer" lead the positive? I read another post about someone with 32 LEDs only needing a single 9V battery so clearly I'm missing something.

    I really have searched and searched. I'd appreciate some EASY (i.e., "eHOW") help. Had a basic EE course in college but was a long time ago. Thanks.
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    You can break them up into short chains and have a resistor for each. A 9V will power 4 LEDs per chain, (4 LEDs at 1.9V X 4 = 7.6V). A 9 V drops its voltage quickly, so I would use 3 per chat at 5.7V. This means you need a resistor around 180Ω in each chain, maybe 150Ω.

    LEDs, 555s, Flashers, and Light Chasers

    This will explain how it is done.
     
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  3. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    Figure the combined FW V of the LED to a number just less than your battery. (9volt)
    FW V = 1.9-2.4, so 2.2 avg.
    4 x 2.2 = 8.8 volts

    Use four LED's in series with a 9 volt battery and then add a small resistance, determined by experiment quite easily. Something less than 100 ohms. More resistance means battery lasts longer. Also means LED's will be dimmer.

    Switches have no polarity, or +, - ends. They are either 'open', or 'closed'. Unless the switch has illumination built in.

    Type in some keywords in google and you will be assaulted by more 'articles' than you have time to read.
     
  4. wayneh

    Expert

    Sep 9, 2010
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    Be a little careful about the specs on the LEDs. It's a bit unlikely they're truly all the same. I think the red LEDs, for instance, will likely have a lower forward voltage drop than the green ones. So 4 green LEDs in series with the 9V battery might be fine even without a resistor to limit the current, whereas the same arrangement might risk destroying the red ones. For simplicity you might want to keep each string a single color, so you can tweak the current thru that string.

    Do you have a meter that can read current in the 10mA range? You can get one very cheap, less than$5 or even free, at Harbor Freight or elsewhere. They're really handy. I'd aim to start at 5-10mA thru any LED and only increase that if you need more brightness. The LEDs will last forever at 5mA and will be iffy at 20mA.
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    A scientific wild guess tells me that you bought the 20-piece LED assortment, Catalog #: 276-1622, at a Radio Shack store, because the specifications you are giving are vague for the various colors, which is consistent with the manner that Radio Shack poorly labels their components nowadays.

    From comments on the Radio Shack page:

    Link to product page: http://www.radioshack.com/product/index.jsp?productId=2062588

    This is more of what I was expecting to see. Note that the green have a much higher Vf than was specified on the package, even with only 10mA current. From my prior experience with Radio Shack LED assortments, I suggest that you limit the maximum current to 15mA, as some (or many) of the LEDs in the package may be quite old. Back in the 70s', their absolute maximum current specified was 15mA. Besides, the lower your current is through the LEDs, the longer the batteries will last.
     
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  6. Wendy

    Moderator

    Mar 24, 2008
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    8.8 is way to close bro. It is the same problem I have with LED calculators, you need at least a volt for the resistor to do its job.

    Given that 9V batteries very quickly drop to 7V and the problem is compounded. It is why I recommended 3 LEDs per chain.
     
  7. aerogal

    Thread Starter New Member

    Jan 24, 2011
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    Ouch. I'm realizing that maybe having an LED ducky project was not such a good idea.

    I am amazed at all the responses and help I am receiving. But what I'm realizing is that I'm still in the dark of - quite literally - how to do this.

    I know you cannot draw a diagram for me, but I think (perhaps?) some of you may have under-estimated the years between now and the time I took college physics & my (only) EE course.

    Back to the basics... I have a battery holder. Initially, I was going to put 4 AA batteries in it (total 6V). With a 100 resistor, that would be good for 3 LEDs in a series. I get this. But taking the next step up to 20.. What do I do, make parallel circuit with several LED series? Will I have a problem with too low voltage (I'd rather have these lights be dim than fry them). If I go with a parallel arrangement, does a parallel of 4 series with 5 LEDs reasonable? Or should I make 5 series with 4 LEDs? Do I really have to limit this to 3 LEDs (i.e., a parallel circuit with 4 series of 3 LEDs to make a total of 12 lights -- not 20). And if THAT is the case, then I need 4.5V in, right? Or can someone help me with the series/parallel situation, since in theory I got it right but in practice I burned up a bunch of stuff.

    Thank you for your patience.
     
  8. aerogal

    Thread Starter New Member

    Jan 24, 2011
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    I have clicked on your link and am reading through the info. I'm still pretty sure I'm in over my head and we will either end up with a painted duck with holes in it or a beautiful lit up duck. I'm beginning to think that the latter will be the case. If worse comes to worse, we can always use the big bulbs and paint them, which is what my husband suggested.
     
  9. aerogal

    Thread Starter New Member

    Jan 24, 2011
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    Bill - thanks for your diagrams. Looks like I'll have to do LEDs in series in a parallel after all. I'll have to figure out the volt & resistance requirements and keep my fingers crossed.
    Kermit - You say I can do up to 4... that sounds promising, but I just hope I can get the voltage and resistance right. Bill was saying 3, so we may just draw out the duck and see if we can get by with 3 per chain. Then bring down the total number so we're not wiring a monstrosity.

    Regarding the switch - touche. O.k. so I DO remember that from my EE course I took... 25 years ago. And now that you mentioned it, when I turned it around, the light went on.
     
  10. Wendy

    Moderator

    Mar 24, 2008
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    Actually we can help somewhat. Resistors are likely where you will have the most challenge. What is the deadline?

    I would take a 1KΩ resistor and solder it to a battery clip. Measure the LED drop for each diode, and go from there.

    If you do have problem finding the right resistors I've put this up in the new Flea Market Forum.

    Resistor offer
     
  11. aerogal

    Thread Starter New Member

    Jan 24, 2011
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    Bill - deadline is around valentines day. But we probably need to get going on this since we will only be working on it on weekends and burning through a few LEDs in the process. While I have a solder kit, I've never used it (been meaning to self-teach myself - have some guys at GSFC that might teach me some basics). But this is meant to be a basic project. So I'm thinking we might start with 3 in a series and figure that out. If we can get those to work, we will slowly wire the rest together. We have it diagrammed. I'm still unconvinced though. I appreciate everyone's help, I just don't have the equipment to measure these things and am unlikely to go out and get it (lack of time, overall motivation, etc.) However if it doesn't work, not sure what we will end up doing other than using regular radio shack "lamp" bulbs which are so darn ugly.
     
  12. laurich

    New Member

    Jan 22, 2011
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    I know this is very basic but I, like you, have been out of this for a long long time, so I just wanted to point out when begin assembling your project that you realized these LEDs have a positive (anode) and negative (cathode) side and if you apply power in reverse they will not conduct.
     
  13. Wendy

    Moderator

    Mar 24, 2008
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    That is not a problem actually, they have a flat side, that is the negative or cathode side. I figure you will need 150Ω or so.
     
  14. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    If you do not know the cathode, take a look at the side of the led:

    [​IMG]

    You see a cup, where the chip is mounted, that is the cathode, the other lead is the anode.

    This picture comes from this page, with some more info on the leds:

    http://www.merg.org.uk/led/index.htm

    Bertus
     
  15. aerogal

    Thread Starter New Member

    Jan 24, 2011
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    To everyone - THANKS! I mocked up a diagram of a parallel circuit of 6 series (3 LEDs and a 150 ohm resistor per series). I took it in to work (we support NASA GSFC) with all the stuff I bought and asked our engineering functional lead. By the way, I bought the LEDs in packages of 2 each, they are rectangular LEDs and each spec is precisely the same on the back of each package.

    My friend was concerned about voltage drop over the system. He calculated each series current requirement and what it would be provided. It turns out that the 150 ohm resistor is absolutely PERFECT. The calculations work out 100% - literally. This is using a 9V power supply. So I'm going forward with it. My LEDs do have a long end and short end, but to make sure which is the cathode/anode ends, I'll have to use a magnifying glass. The inside is quite tiny to the eye. Thanks for all your help. I'm so glad I drew it out and was able to have the end result verified!
     
  16. Wendy

    Moderator

    Mar 24, 2008
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    Glad to help. If you want something more advanced don't forget the link I showed in post #2, I start simple and go on to other visually interesting projects. It is a hobby of mine, as I like LEDs and visual projects.
     
  17. SgtWookie

    Expert

    Jul 17, 2007
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    Before you trim the LED's leads, the short lead is the cathode.

    Note that not all 9v "transistor" batteries are equal. Some "industrial" 9v batteries actually have 7 cells inside of them, and measure around 10v when new. A "standard" 9v alkaline battery will measure 9v with no load, and may drop to 8.6v with only 25mA load. By the time it gets to 7v, it will be completely discharged.

    If you operate, say, 7 parallel strings at 10mA each, that's a 70mA load. Keep in mind that these 9v batteries have mAh ratings; they are rated for discharge at a specified current over a 20 hour period. Therefore, a 500mAh alkaline 9v battery is rated at a 25mA load current. 9v batteries aren't cheap.

    Radio Shack also has battery holders that will handle eight AA batteries, giving 12v out. AA batteries have a much higher mAh rating than 9v "transistor" batteries do. Starting out with a higher initial voltage means that you can have more LEDs per series string, and that means fewer strings in parallel, so you reduce your total current requirement. Make sense?
     
  18. wayneh

    Expert

    Sep 9, 2010
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    It does to me. I think the odds of success and satisfaction are much higher with the AA strategy. The 9v will start dimming almost immediately and won't last nearly as long. Maybe if you use rechargables and keep several handy.
     
  19. Wendy

    Moderator

    Mar 24, 2008
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    Depends on size constraints. 12V will work, but requires 8 batteries, while 6V requires 4 batteries, but will only work for 2 LEDs per chain. 6 batteries will be 9V, but use just over double the space. This is a judgment call by the OP. If they go either route we'll help them with the redesign. It may seem simple to us (it is) but they may have other ideas.

    It may be the 9V will last long enough for their application. I did mention that 9V quickly drops to 7V, and made sure the LEDs would stay lit, if dimmer.
     
    Last edited: Jan 25, 2011
  20. wayneh

    Expert

    Sep 9, 2010
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    Agreed. The OP said the project is ~1 square foot, so 6 AAs shouldn't be a problem. I'm more concerned about holding them securely to prevent losing connection. 9v batteries are great on that.
     
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