Driving large number LEDs from AA batteries?

Discussion in 'General Electronics Chat' started by Detonator, Dec 2, 2011.

  1. Detonator

    Thread Starter Member

    Mar 6, 2010
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    Hi guys

    This is going to sound like a dumb ass question no doubt, but here goes anyway...

    I off camping for summer soon and wanted some better lighting this year, was looking at some of these "ring" LED camping lights coming out of China etc such as this one :
    http://img.dxcdn.com/productimages/sku_2516_2.jpg

    Now i made a few LED projects so far and love playing with LEDs so figured "hmm simple, make my own version of this". But one thing i just don't get is what LEDs they could be using to run 24 (36 in some models) of them from 4 AA batteries? Surely 4 AA is only 6v of power, and i trying to see how they run 24 or 36 LEDs from just 6v of battery, and for 30hrs they reckon?

    Someone point me in right direction please?
    [​IMG]
     
    Last edited by a moderator: Dec 2, 2011
  2. Wendy

    Moderator

    Mar 24, 2008
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    LEDs do not have to be run at full current to be pretty bright. There is a fly in the ointment though, 6V would only power 1 LED per chain, which means there would have to be 1 resistor per LED. It would be interesting to try to improve that with an electronic drive though.

    Since you are a beginner I suspect you want it as basic as possible, correct?

    LEDs, 555s, Flashers, and Light Chasers
     
  3. Audioguru

    New Member

    Dec 20, 2007
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    White LEDs operate at about 3.5V. The manufacturer of the light tests and sorts LEDs into groups with the same voltage, connects them all in parallel then adds one series resistor to limit the current.

    AA alkaline cells have a capacity of about 2500mAh when their current is 100mA. Then they last 25 hours. At the end of 25 hours the 6V battery will be only 3.2V and the LEDs will be extremely dim. 24 LEDs with a total current of 100mA is only 4.17mA each.
     
  4. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
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    I have some super efficient red LEDs that are very bright with only a few mA through them, but most typical LEDs would not be. Choose the LEDs for high efficiency.

    There is a sw mode boost converter called the LM2731 that is good for driving LEDs. When I released that product, I built a sign using high eff LEDs spelling out that number driven by a single 2731 chip from a couple of AA batteries.
     
  5. Detonator

    Thread Starter Member

    Mar 6, 2010
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    Well i am all ears for any solution, i learn fast from advanced things, i probably more of an intermediate these days. Starting a degree in Electronics in February (at 39yo !!) so may as well get deep in Bill
     
  6. Detonator

    Thread Starter Member

    Mar 6, 2010
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    Thanks given me some ideas, Think i might use a 9v cell now anyhow, figure for a little camping light a 9v cell weighs a lot less than 4x AA, but how does Ah compare for life?
     
  7. Audioguru

    New Member

    Dec 20, 2007
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    A 9V alkaline battery has a very small amount of capacity. With a 100mA load you will see the LEDs dimming until they are not seen anymore in about half an hour or 45 mimutes.
     
  8. Adjuster

    Well-Known Member

    Dec 26, 2010
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    Small 9V batteries have a pretty limited capacities, unless you want to get the expensive lithium ones. http://en.wikipedia.org/wiki/Nine-volt_battery

    On the whole, AA cells give a far better cost/capacity compromise. they are also very readily available, which may be a consideration if you are travelling. AAA cells are less good in my experience, often the same price as AA for less capacity.
     
  9. KJ6EAD

    Senior Member

    Apr 30, 2011
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    For light weight and plenty of capacity, use the lithium AA cells.
     
  10. Detonator

    Thread Starter Member

    Mar 6, 2010
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    Understood, i stick with the AAs then, thankyou
     
  11. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    6,357
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    I have a light exactly like the one you have in the photo above, just LEDs, resistors, and cheap switch, nothing fancy.

    The on off switch goes through a small value resistor (one clockwise, one counter-clockwise) and all of the LEDs are in two parallel on a circular PC board around the outside edge. The inside can be open (for a donut shape) or closed for a disc shape by re-arranging how the batteries sit.

    It actually works very well and lasts a long time on AA batteries, I use it in one of my "junk box" cabinets of parts that I have extra from various projects, and it is bright enough to read the resistor codes correctly from 4 feet away. Lifetime is probably 2 days of continual on, but it goes to about 50% brightness after 18 continual hours on. I know this from forgetting to shut it off.
     
  12. Wendy

    Moderator

    Mar 24, 2008
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    Those suckers are expensive, with experience it would be possible to make a buck booster that would increase efficiency. Don't know how practical it would be, but I suspect it would work well.

    There is also the CR123, a 3.3V cell. They have been used with good results.

    Ultimately I would use alkaline AA batteries. If you have room C or D is even better, they would last for days.

    An auto turn off feature would be a piece of cake, say 30 minutes? It could even be switch selectable.
     
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