Customs: LEDs to a pair of AA's - Resistor and Battery Life Q'

Discussion in 'Creative General Discussion' started by blaynescott, Mar 16, 2012.

  1. blaynescott

    blaynescott Well-Known Member

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    Hi everyone,

    I'll be tackling an LED lightup in a static custom of mine that has a faux-rock base (to hold the power source). I've watched every LED toy custom video I could find, read up here at TFW and Instructables, but I still have 2 questions:

    What is the longevity of a single LED connected to a pair of AA batteries? How long (varying by battery type, of course) should an LED hold out before running out of power?

    I've run a single LED to an AA set before, but don't have it handy to test (it's in a store in Toronto currently).

    Also, does adding a resistor increase the battery life, or just the brightness of the LED?

    Thanks!
    -Blayne
     
  2. Shwiggie

    Shwiggie Likeable dryskinned biped

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    It depends on a few factors: the rated amp-hours of the battery (mAh), the size of the resistor, and the draw of the diode.

    First, the resistor is there to limit the amount of current going to the diode. LEDs are current driven; you give it a voltage source of a certain amount (say 3V, in this case...hope they're not white LEDs BTW) and it draws whatever current available. You put a resistor in there to limit that current to a level that's safe for the LED (keeping it from burning out) as well as maximize battery life.

    The LED packaging lists the max current; divide the voltage by that (Ohm's Law), and you have your ball-park minimum resistor value. A resistor the next size up from that will get you the safe side of the max brightness. Or go with a bigger resistor to reduce current draw at the expense of brightness. You could put a potentiometer in series in the circuit to vary it.

    When you figure what current will drive your circuit, then you can divide the sum of the battery mAh values by that current and you will calculate an estimate of what battery life you can expect. Just remember that electronics are not an exact science, and getting a value of, say, 1.42 hrs may be off quite a bit due to slop in element tolerances, chemical fluctuations, and temperature.
     
  3. blaynescott

    blaynescott Well-Known Member

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    I've yet to buy the LEDs or resistors - I only have the switch on order from an eBay seller at this point.

    I'm aiming to have either 2 or 4 AAs powering a single LED - with the goal of having a battery life that lasts can last for weeks+. Is that something that is doable without it overheating the LED?

    I can think of consumer electronic devices whose 'little red light' is on for years at a time without overloading - I'm hoping I can achieve something similar (on a smaller time scale) with this project.

    Any suggestions on good specs to look for when buying resistor/LED to achieve long battery life while running off a pair of Energizer rechargable AAs? :) 

    Thanks!

     
  4. David Hingtgen

    David Hingtgen Chromaticon

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    What color is the LED? 2 AA's in series will put out 3 volts, which may be acceptable for some colors like red and green---this is why so many small toys, pens, etc can run with no resistor at all. White and Blue LED's can often demand 5 volts or so, thus 2 AA's might not be enough. 4 would over-volt most any LED out there. (and 2 in parallel would probably only light a red one if that) 3 in series would give 4.5 volts, should work for most blue/white ones. Could always probably fit 3 AAA's in the space of 2 AA's if need be---same voltage as 3 AA's, only the milliamp-hours would differ.

    Before you figure resistance/life, you've gotta know what the specs of the LED is first.
     
  5. Shwiggie

    Shwiggie Likeable dryskinned biped

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    Great point, David. I should have gone into that more than I did:
    If this thing is static and tied to a base, and you're concerned about battery life, just install a jack for an AC adapter (such as this one) and stick a voltage regulator in the circuit. Here's an example circuit...just use a 9VDC pack, for example. If you're using a single LED, get a 3.3V regulator; for blue/white LEDs, use a 5V. The caps in there are just for signal clean-up; if you're just running LEDs then you needn't bother with them.

    But, as David said, it depends on the LEDs you're using.
     
  6. blaynescott

    blaynescott Well-Known Member

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    I'll be needing a Blue LED, but I'm considering if it would be easier to go with a blue 'gel' (or tinted blue plastic) in front of the light shining from the statues eyes. I would prefer to go with a blue LED, then write the power supply around it's requirements. Are there particular stats on a Blue LED I should look for when buying one that would't be overwhelmed by x3 AA's?

    That was my first reaction, but the fellow this statue will be for lives in Brazil - with a different plug/voltage system than I have here in Canada. To make sure it's something I can test-power up before doing the statue, I'll have to go with the AA's option.

    I'm seeking the ideal setup to power a single blue LED, such as one of these that has a heat sink attached to it, min. 3.6V - 4.2V.

    There are also just basic blue LEDs, but they seem to have a lower min. voltage input range, 3.2V-3.4V.

    What would be the ideal setup for a long term battery circut using AA's with either of these? I'm still learning about electronics work, but know once I have a firm grasp on a setup that works without exploding, I'll be fine. :) 
     
  7. Shwiggie

    Shwiggie Likeable dryskinned biped

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    A jack should be compatible with an AC adapter anywhere: he'd just have to go get one himself, but it would work. But I see where you're going.

    As for using a battery...well, remember that circuit I linked with a 9V and a 5V voltage regulator? I'd go that route for the following reasons: a) that'd be simplest from a user standpoint, since he'd have only one battery to change out, b) it'd take less battery space, c) many blue LEDs use 5V input, and the regulator provides that; and d) you already have a schematic ready-made for it :p 

    But, if you are stuck on AAs, keep in mind that you'll need 4.5V at least for a blue LED. But, if it requires more voltage, you're looking at 6V with 4 AAs...so you'll need to calculate the needed resistance differently (my method is more simplistic and assumes more or less equivalent voltage), in order to drop more of the voltage across it and spare the LED so it doesn't blow.

    This page tells you how to figure everything...basically, subtract the 6V (or whatever it turns out to be) by the forward voltage of your LED and divide by the current at which you want it to run.

    Also, I suggest you use a small bread board to prototype the circuit & make adjustments before you start soldering. I said earlier, there's a lot of slop in electronics, and your figures won't be perfect...the best example being resistance values. If you were to take four AAs (6V) with a 5V LED with 15mA max current, the resistor value would be 66.66666666666666Ω. You'll actually need to use something more like 68Ω, as they don't make a 66.66666666666666Ω resistor. But given that the battery voltage might be more like 6.6V (fresh AAs can have up to 1.65V), it'd make more sense to go with a 110Ω resistor (the calculated value is 106.66666666Ω; next most common size being 110Ω), since you don't want current to exceed 15mA. Sure, it'll be dimmer, but you won't over-current your LED, and the battery will last longer due to reduce current draw as it discharges. And you can experiment to see just how much dimmer you can go with a higher resistance because of the accordingly lower current.
     

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