# What is the lowest power LED for longest runtime?



## SciPug (Jun 7, 2005)

I have a kinda crazy idea for a mod (I'm really reaching above my station here), but in order for it to be useful I need to know what type of LED I should use.

It needs to be able to run directly off of a rechargeable 123 (I can throw on a resister), color doesn't matter, and it doesn't need to be any brighter than half as bright as a night light. It just has to use the least amount of juice possible so it can run and run and run without draining the battery down too much.

Is there a particular type of LED (3 mil?--I really don't know enough to even guess), and then of those, is there a particular color that uses the least amount of juice? 

If it blinks, that should cut the battery drain in half, right? If so, is there some very small item I can but between the battery and the LED to make it blink?

It also should be as small as possible.

Sorry for all the newbie questions.

Thanks! If it works, I promise to post it with pictures. But it's going to take some time if it works at all.


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## Lynx_Arc (Jun 7, 2005)

*Re: What is the lowest power LED for longest runti*

If you want to go cheap.. either a 26k LED or the newer nichias and resistor it to run at about 1-5ma. I used a 4 LED array in parallel run at 4ma total for a nightlight and it ran a looooooong time off 4 well used AA batteries. Blinking may save battery or give you a brighter output but kind of sucks for use as a nightlight then.


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## paulr (Jun 7, 2005)

*Re: What is the lowest power LED for longest runti*

The nice thing about LED's is you can run them basically as low-powered as you want and they still give off usable light. They're not like incandescents whose color drops past the yellow and through the red into the (invisible) infrared as the current gets lower. Running the led at 1 mA or so will make more than enough light to find the bathroom at night.

Green leds are generally considered to give off the most usable light for the juice, not because they're more efficient but because the dark-adapted human eye is most sensitive at that wavelength.

It would help if you explained what you're trying to do. Are you trying to make a locator light (something that you can find in the dark), or something to actually see by? There used to be a chip called the LM3909 that would blink a led suitable for use as a locator. With a red led it would run for several months on an AA cell, or years on a D cell, while giving a fairly strong pulse once a second or so. So you could easily find it in the dark, but you couldn't light your way around a room with it.

There is something like that in some Eveready household 2D lights now. They have a blinking red led that runs for years on the internal cells.

Maybe what you really want is a glowring, which is a little glass tube with tritium gas (slightly radioactive) and a phosphor so it glows in the dark. Again, it's just a locator, not an illumination device, but it lasts for many years, no batteries needed. You can't buy them retail in the US (crazy regulations, you can buy glow in the dark wristwatches that use exactly the same stuff in the dial markers) but you can find them on ebay or through some overseas sellers here on CPF.


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## SciPug (Jun 7, 2005)

*Re: What is the lowest power LED for longest runti*

[ QUOTE ]
*paulr said:*
It would help if you explained what you're trying to do. Are you trying to make a locator light (something that you can find in the dark), or something to actually see by? There used to be a chip called the LM3909 that would blink a led suitable for use as a locator. With a red led it would run for several months on an AA cell, or years on a D cell, while giving a fairly strong pulse once a second or so. So you could easily find it in the dark, but you couldn't light your way around a room with it.

There is something like that in some Eveready household 2D lights now. They have a blinking red led that runs for years on the internal cells.



[/ QUOTE ]

Thanks for the info. Can I just say, "Darn it!" I thought I'd come up with a cool, original idea, but it apears Eveready already did it. Oh well. I still want to try it. 

You must be a mind reader. I want to put a blinking locator light on my flashlight. Any advice on how I would go about finding a LM3909 chip? And are they nice and small?

Just to get this clear, any LED will be fine, just run it at 1 mA. Do I do that by using a resister? If so, what resister should I get if I'm using a single 3.7 R123? Like I said, I'm a newb. 

Thanks!


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## andrewwynn (Jun 7, 2005)

*Re: What is the lowest power LED for longest runti*

you can probably just use resistance.. easiest way to get a ball-park figure is use a test resistor say 1000 ohm in series with the LED and the battery.. and measure the Vf of the LED.. now use ohm's law to calculate the voltage you have left.. example: i put 1000ohm -> Led -> bat -> resistor in a loop.. and i measure 1.7V on the LED and 3.0V on the resistor... you will likely measure higher voltage on the resistor with a fresh R123, so calculate your average based on 3.7V battery like this: 

take the 3.7V battery, subtract the 1.7V LED and get 3.0V drop needed on the resistor... now take 3.0V and divide by the desired mA on the LED.. and since you want your answer in ohms and not miliohms you multiply that results by 1000.. so.. 3.0V /1 ma = 3mohm.. x 1000 = 3000ohm. 

this is a neat idea, would blow away a tritium locator.. an R123 has about 600-700mAH storage, so that's nearly a month runtime.. if you charge your light at least once/week you won't 'spend' more than 1/4 your power on the led if it was on continuous.. if you have it blink with a 1/10 duty cycle, it would last for months and months... neat idea keep us posted.. 

you could probably build a simple 'blinker' with a capacitor and and FET, you'd have to ask one of the EE dudes like Mr Al how best to do that.. LM 3909 sounds like lineartech.com


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## WildRice (Jun 7, 2005)

*Re: What is the lowest power LED for longest runti*

3909 have been discontinued IIRC and therefore tricky and expensive to find. That being said, I know I have at least one, and the circuitry is UBER SUPER simple... 1 cap is all you need (and the IC and LED and battery). PM me if interested. 
I have build a simple flasher with a LED and some sort of low V SCR, but that was a long time ago. the LM3909 would have a much lover current draw and you can also change the flash rate by changing the cap.
Also, I made a locator with some 3mm green LEDs I had. they would run 22mA off of 2x alky batts. At low current (20uA, .02mA or .00002A) it was bright enough to be easily seen behind a piece of frosted plastic in a dark room.

Jeff

JUST ADDED... here is some of the page from 'The Engineers Notebook II' (out of print for some time now) showing the LM3909


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## VidPro (Jun 8, 2005)

*Re: What is the lowest power LED for longest runti*

they also have them LEDs with built in flashing things.
they would not save power as well, but one driven really low, might be neet , and visable because of it changing colors.
the ones im my lighters run for a long time on little power.

stuff like this
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=66952

then those jewlery type blinkey leds that run off a few tiny cells. could be driven with some larger cells if you did it correct. they use very small emitters, so they should use less power.
(although i agree that most leds can just be driven lower ANYWAYS, even big arse ones).

http://www.allelectronics.com/cgi-bin/category.cgi?category=340&item=LED-87&type=store
(a rgb blinkey)

http://www.moreleds.com/flashing.htm
obnoxious flashing leds

http://wear-flashing-jewelry.com/
(especially the rgb or red white and blue round ones, which could be put in a light, and look good too)

if this stuff was driven JUST to the point where the tiny blinker curcuit actually started operating, they might take less power than a real timer chip, i donno.
and 
if it was run off, some BIG cells, it might last for some usable time.

most of this stuff would need 3+ volts connected to it, i think the Domed 'blinkeys' underdriven would be fairly capable of surviving a drop or 2.

and all of this stuff is more continuous, not a blink in 20 seconds, which would save the led itself and use the least power. but underdriven , and the very tiny ones, *might* be as efficent as a total curcuit that had to keep doing timing anyways.

and with the Round dome "body light" things, you toss out the magnet, and the battery holder, and you have a SCREW IN part left, solder up a resister and a few wires, and you have both the light, and the mount.


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## LED_ASAP (Jun 9, 2005)

*Re: What is the lowest power LED for longest runti*

The cheapest, easiest, and fairly cool way would be to buy a flashing buttom from a dollar store and solder two wires off the contacts /ubbthreads/images/graemlins/smile.gif


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