How many LEDs can be put together with 12v dc supply

Discussion in 'General Electronics Chat' started by reno12469, Jan 17, 2009.

  1. reno12469

    reno12469 Thread Starter Active Member

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    How many LEDs can i put together with a 12v dc supply. The LEDs are 5MM ultra bright white. They operate on 3.0-3.5Vdc, 20mA. They have a 15-25 degree viewing angle. I also want to split the number of LEDs in half and have them flash back and forth without using PIC.
  2. mik3

    mik3 Senior Member

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    You can put 3 leds in series with a limiting resistor. The total current through the leds and the resistor will be 20mA. You can use as many branches of 3 leds as the power supply can power. What is the maximum current the supply can give you?
  3. Bernard

    Bernard Senior Member

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    Use a 555 chip, circuits abound in AAC, source about 6 strings and sink another 6 strings[ of 3 ea. ]. 3X 3V ea.= 9V leaving 3V to drop 3/.o2=150Ω, .o6W
  4. reno12469

    reno12469 Thread Starter Active Member

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    its a 12 v battery
  5. thedt

    thedt Member

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    like car battery or a lil baby battery?
  6. Bill_Marsden

    Bill_Marsden Moderator Staff Member

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    A straight 555 will have some trouble doing this, with 12V its positive output is 10.6VDC, due to the Darlington transistor drivers. There is just not enough voltage left over, so you'd need to reduce the number of LEDs on that side to 2, though it would drive 3 LEDs to ground. There are ways around this, are you interested?

    [​IMG]

    The OP will have to give us a flash rate for me to finish determining the values of the resistors and capacitors.

    Attached Files:

  7. reno12469

    reno12469 Thread Starter Active Member

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    Its a car battery and i would like each one to flash once in one second.
    Last edited: Jan 18, 2009
  8. mik3

    mik3 Senior Member

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    You can wire up to 1000 LEDS or more as I told you on post #2 on a small car battery.
  9. reno12469

    reno12469 Thread Starter Active Member

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    Ok. But i want something like this that they flash back and fourth in one sec and using as many of the 5MM LEDs as possible in each one hooked up to the 12 v battery.

    [​IMG]
    LED Strobe.JPG
  10. Bill_Marsden

    Bill_Marsden Moderator Staff Member

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    You have to upload the picture somewhere to display it in a post. AAC has good picture forums, or you can do it locally on an attachment. My blog has several tricks and tips for this, this one is what you want.

    Here is a schematic that will do what you want. If you choose to add more LEDs just parallel new chains with separate resistors.

    [​IMG]

    You can increase efficiency a bit by making C1 1µF, R1 10KΩ, and R2 750KΩ. It will drop the current draw from 10ma to 1ma during the alternate cycle.

    Attached Files:

  11. reno12469

    reno12469 Thread Starter Active Member

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    Thank you so much.
  12. Bernard

    Bernard Senior Member

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    Is the battery in a vehicle? , if so might wish to use 14V rather than 12, only effects resistor values a little. The 2N 2222 shoul be able to drive several strings.
  13. reno12469

    reno12469 Thread Starter Active Member

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    So what are you saying i should do. Cause it will be in a car.
  14. Bill_Marsden

    Bill_Marsden Moderator Staff Member

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    Are you interested in using my design? It will work, but to work properly it will need some slight redesign.

    As is with 12V the LEDs will get around 50ma (opps), with the engine running and battery at full charge the circuit will be around 13.7, and the LEDs will get 107ma (double opps).

    [​IMG]

    Change R3 and R4 to 150Ω (gee, seen that before) and with the engine off they get 10ma, with the engine on they get 21ma.

    If you want a tweak to make them the same current (same brightness) with engine on / off let me know, figure one transistor and a couple of diodes added.

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

    Heck with it, here it is...

    [​IMG]

    Attached Files:

    Last edited: Jan 20, 2009
  15. reno12469

    reno12469 Thread Starter Active Member

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    Im planning on making LED rope lights for my car. I was planning on using this design to make them. There is going to be 48 Blue LEDs on a strip.



    [​IMG][​IMG][​IMG]
  16. Bill_Marsden

    Bill_Marsden Moderator Staff Member

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    Kind of a significant detail to leave out. I calculate those LEDs will use 36ma per leg. If you have 16 legs that works out to under 0.6A (assuming 20ma X 16 = .3A). That leaves the 555 in the dust, 0.2A is its max. Want me to redesign with drivers to handle the correct current?
  17. reno12469

    reno12469 Thread Starter Active Member

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  18. Bill_Marsden

    Bill_Marsden Moderator Staff Member

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    OK, but to be able to help you you need to nail the specs down, the first post didn't say how many LEDs, so we went with 3, the max number on a chain (that's what assumptions do). Now it is 48 blue LEDs. Is that total or per side?

    The engine running does nail the voltage down a bit, we can assume 13.7V. That can still vary a bit, but not nearly so much as with the engine off.

    I assume the current target is still 20ma.

    Nice price on the LEDs BTW.

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

    OK, 91Ω gives you a current of 25.3ma, which is good enough.

    You do realize blue LEDs (especially alternating) is illegal on road vehicles? I think red lights are too. Think emergancy vehicle.

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

    OK, I've drawn up some schematics for you to look at. I haven't built any of them. I think you're going to get a ticket, but if you're old enough to drive and build one of these it is your lookout.

    [​IMG]

    The transistor drivers of Circuit #2 and #3 can handle 200ma OK, but if you up the current you might think about increasing the transistors to something that can handle higher ratings.

    Attached Files:

    Last edited: Jan 21, 2009
  19. reno12469

    reno12469 Thread Starter Active Member

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    Ok here is what i want. I am makeing a rope light that is 8 feet long and has one blue LED every 2 inches hence 48 LEDs. I dont want them to flash or anything, just to be on solid. The diagram that i showed you before was for 60 blue LEDs at 20ma with a 12V power supply. The new one i want to do is with the 48 3.8VDC blue LEDs with a target goal of 20ma on a power supply of around 13.7V while the vehicle is running. I know that blue lights are illegel in vehicles but sometimes you have to have some fun. These Lights will be on the INSIDE of my 2002 Chevy Suburban. I will have two of these rope lights on the inside, one on each side.

    Another question is does anyone know what kind of LEDs are using in police lighting?
    Last edited: Jan 21, 2009
  20. Bill_Marsden

    Bill_Marsden Moderator Staff Member

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    As to the type used in cop cars, can't help you. I know they are the high quality high brightness types similar to the Rebel series.

    Basically you're back to basics. Figure each resistor is 110Ω and 3 LEDs per chain for 20ma. The quantity is off, but you get the idea. Betcha still get a ticket, but I could be wrong.

    [​IMG]
    Last edited: Jan 21, 2009
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