LED brightness and the number of LEDs

Friends,

I'm doing a new tap light mod using Doug Owen's newest circuit here:
http://www.worldtorch.com/boost-kit-instructions.php

I noticed that at 3.0 volts in I get about 28 mA. Now, my question is this... which of the following scenerios would seem / be brighter and why? Which would you do?

1) A single 5mm Nichia LED at 28 mA
2) Two 5mm Nichia LED at 14 mA each.
3) Three 5mm Nichia LEDs at 9.3 mA

These little guys have dimminishing returns right (increasing current increases light output at a decreasing rate)?

If space isn't an issue how does one determine the most efficient usage?

Thanks in advance!

Option 3 should produce the most amount of light.

evan,

Thanks for the post. My next question then is how does one determine the best scenerio of 3, 4, 5, 6,.... LEDs? I mean, 100 LEDs wouldn't be good would it?

So, one isn't the answer, and it isn't 100, so how does a person determine the "sweet spot"?

Thanks again!

At 28 milliamps, 3 ought to do it.
I would even say plop in 10 or so leds.
With this many leds, you could probably draw 28+ ma from just 2.7 - 2.8 volts worth of 2xAA batteries direct driven with no boost circuit.
Very simple and efficient.

Its the current flowing through LED's that determines the brightness of LED's

LED's connected in series ... 6 - 21 ... can give best results... @ 25 - 30 mA under normal circumstances.

However, if you put more LED's together, you have to think of heat & its cascading effects.

I have done one mod which contains about 19 Nichia LED's and its sure brighter than a Luxeon 1 Watter (but has very very less throw) compared ... will compare it with the upcoming Nuwai Q III

Thanks & Regards,
ViReN

Great stuff indeed!

So, there isn't a magic formula for this? No nichia pie are squared divided by the n sign of a logs rythem? No way to find the optimum number of LEDs between 1 and 1,000?

3rd_shift - What your saying is if I used 10 LEDs, we're talking 2.7 mA each, that would be a good way to go?


Footnote: I've been using Doug's circuit to boost up really dead batteries and use them in the wife's tap light. It has been working great!

Yes, it's a good way to go with rechargeable batteries.
I only have rechargeable batteries now.

Otherwise, 3 or so ought to do it with a boost circuit for getting your money's worth out of regular batteries.
It ought to seem to run forever like that. /ubbthreads/images/graemlins/rolleye11.gif
Most leds are slightly more efficient and cooler running when individually underdriven. /ubbthreads/images/graemlins/smile.gif

Basically, here's how it works. The less current through an LED, the lower the voltage drop, and more efficient it will be. Halve the current, and you still get more than 1/2 the output, and it reqires less than 1/2 the total power (as Vf drops a little, and current is cut in half per the example)

So, you must balance how much underdrive (and efficiency) you want vs. the cost of LEDs, and the amount of space you work with. Also, consider that at lower currents (probably in the low single digit mA range) you may experience a color shift towards green (I don't have a lot of experience with 5mm LEDs, so I don't know to what extent this effect occurrs), so that may limit how far you decide to underdrive, and thus, how many LEDs you use.

For most white LEDs the efficency increases as the current decreases down to about 3 or 4 mA. Below that it generally stays about the same or even increases. Therefore, there's not much point to driving your LEDs below about 4 mA. 7 LEDs at 4 mA each will give you about 35% to 50% more light than 1 LED at 28 mA. Since the forward voltage will be less, you'll be consuming less power besides.

In case you're looking for maximum possible efficiency, Nichia should by now have production quantities of its promised 40 lm/W 5mm LEDs. The better ones are claimed to get about 50 lm/W. Underdriving these top-ranked LEDs at 4 mA should give you roughly 40% better efficiency, or in the 70 lm/W range. This is about what the better CFLs get. /ubbthreads/images/graemlins/happy14.gif

Where is the 3 volts for the tap light coming from? My tap lights that I am moding take 4AA batteries in series.

I'd like to see a circuit with an auto shut off feature. My tap light over my wine rack has a single LED in it. But when I forget to turn it off it might be 1-3 days before I discover it. Any ideas?

electronics4life - I've got about 5 different styles of tap lights from dollar stores, garage sales, etc.

I have two "mini" tap lights that are only 3.5 inches in diameter. One version has space for 3 x AA and the other is 2 x AA. I really like the 3 x AA version (2 for a dollar at Dollar Tree) because I've been able to mod it a few different ways. For example:

I used Doug's new boost circuit and re-wired the battery case to use two of the three battery spots in parallel and left the middle spot open. This runs his new circuit FOREVER on two new AA batteries and a real long time on two dead batteries.

In fact, it was taking far too long to kill my dead batteries (thing lasts forever!) so I opened it up and put the two batteries in series. Now the light is a bit brighter and will hopefully suck the life out of these guys quicker so I can move onto the next set of batteries from my "dead battery box".

RonM - That's a great idea! Doug actually made an auto shut off for a light he made to help him walk down a dim hall at night. It would shut off in about 20 seconds (when he got to the end of the hall). I think you can just change the capacitor and get a longer "on time" for the light. Great idea! I'll add it to my ultimate tap light which includes:

1) Conversion to LED
2) Super dim blinking red LED fo very faint to find it at night, doesn't use hardly any power. (like the one at the bottom of the page here: http://www.nifty-stuff.com/mini-tap-light.php )
3) Auto shut-off

<Okay, that was all a bit off topic, but still fun stuff!>

Oh... I forgot to add to my list for the "ultimate tap light"

4) An innexpensive and efficient current regulator (like the one Doug made for the first Milky Candle: http://www.worldtorch.com )

I never saw this circuit, is there some schematics?

And an idea about the maximum mA it can safely deliver to a single white led ?

I'm looking for alternatives to custom build a single led flashlight head with the fewest parts possible, smallest footprint and powered by a single 1.5v cell.

Pablo

This is a new circuit he built, and he's still getting all the pages / text together, but you can find it here:

http://www.worldtorch.com/boost-kit-instructions.php

I'm sure he'll end up posting the schematics... no secrets with regard to the WT and his other stuff. He's really in it to help people out and get feedback for the main goals of the WorldTorch Project.

From what I understand (and he can correct if wrong) his goals for this boost circuit were (in no particular order):

1) Efficiency
2) Inexpensive
3) Small footprint
4) Easily accessible / few parts

The design as it stands isn't very bright at 1.5 volts, but I think he has some mods that can increase the mA.

reviewum, are you using perf board for your circuits? If so, where did you get yours from? I started my tap light mod last weekend and putting the LM334 circuit together without a perf board is a pain in the rear.

[ QUOTE ]
reviewum said:
I'm sure he'll end up posting the schematics... no secrets with regard to the WT and his other stuff.

[/ QUOTE ]

Yup, just did. "Read it and weep". Or rather, 'build it and comment'.....

Doug Owen

[ QUOTE ]
electronics4life said:
reviewum, are you using perf board for your circuits? If so, where did you get yours from? I started my tap light mod last weekend and putting the LM334 circuit together without a perf board is a pain in the rear.

[/ QUOTE ]

Sure is....

You can buy perf board from Radio Shack. You can also order the kit from the web page and get all the parts, including the perf board, for four bucks.

Doug Owen

Yup, I used the perf board that was in Doug's kit.

I'm not very good at soldering or at putting circuits together, but this was pretty easy (actually, I had a hard time doing it from schematics, but when I had the image of the real circuit in front of me it made it much easier!).

Here are a few of Doug's circuits I made:

Left to right: Boost circuit on perf board, boost circuit on PCB, WorldTorch on perf board.




Same guys from the back:


I've been wanting to ask Doug if he used a program to lay these out, or just his super brain. The way everything fits so well together in such a small space without having to cross wires / connections all over is amazing!