Dimming circuitry puzzlement

Hi all!

You good people should know that YOU are responsible for my downfall. All I did was a WWW search on flashlights and this forum looked interesting. That was a week ago and now I'm hooked. I've got 18 high power LED's on order, two from the Shack burning on NiCad's just to look at, a Sapphire in my pocket and a Brinkman came home last night. This LED thing has all the markings of a major obsession. Thank you!

This is my first post here and I know it is a bit wordy, so please forgive me this time around.

I intend to build a custom array of 9 Blue-Green 20,800 mcd LED's and mount them in a retrofited lantern case.

Some of you here can probably help me make a couple of decisions based on your experiences. I'd appreciate your input.

I want to use this lantern as both a bright area light (hiking at night, camp setup in the dark, etc.) and as a security light at low level (by the latrine so folks don't fall in). Being bright shouldn't be a problem with the blue-green LED's. Dimming them efficiently, however, is a different story.

If I use a 12 volt power supply (8 AA batteries), and wire each set of 3 LEDs in series so each LED gets 4 volts, I should be able to use one of the dimming circuits OddOne posted on his website at: OddOne White LED Information.

The thing is, I REALLY WANT to keep this thing compact and just use 3 AA's. That suggests I have to put the LED's in parallel and stick the 4.5 volts across them (yep, one resistor for each LED). But, I'm clueless on how to do the circuitry for a dimmer! I'm simply not smart enough to figure out if OddOne's circuitry will work at the lower voltage or what changes would need to be made. I've relentlessly searched the net for an answer to no avail. Maybe you can help.

Since the batteries really only need to make it through weekend campouts, I'm hoping the 3 AA's would have the capacity for about 8 hours of bright use and another 16 to 20 of dimmed use.

I invite you to shoot holes in any of this and will welcome all constructive criticism or suggestions. I'm not doing this for commercial gain, but to provide a specific lighting solution for some Boy Scouts.

This newbie thanks you for taking the time to wade through all this.

Most especially, I want to thank you for kindling this nascent insanity for things LED that's overtaken me ... lordy, I dreamed about the blasted things last night!

Random

<FONT COLOR="#000000" SIZE="1" FACE="Verdana, Arial">This message has been edited by Random on 01-20-2001 at 08:50 PM</font>

Well you've taken the first step and admitted that you have a problem. Now if your like the rest of us, you'll reward yourself for this admission and go out and get another light.
There is more in this world to worry about than whether you have too many flashlights....

Welcome, Random - you're in good company here.

It's getting late, so I'm not going to devote much time to this last post of the night - but I'd recommend a FET based chopper - I (personally) *HATE* using resistors in LED lights - ~25% of your energy is wasted in the resistor, depending on the design. FET pulse width choppers have two main advantages - Vds of the FET is near zero, so the power lost in the FET is insignificant (wanna verifiy this? check out the dimming tailcap for the streamlight SL20 and SL35) - second - pulsing the LEDs can give you the illusion of a brighter light.

Just a random thought. No pun intended.

Do you need to have the light dimming variable or would a simple high low choice work for you. I was thinking, on high don't run any resistor and on low, run thru one. I really don't belive you need any resistor on high with only 4.5 volts (with blue-green LED`s). I wonder if just a small rheostat type thing from the shack would work. Might waste a lot of power as heat though. Well I better let our more knowledgeable members handle this one. Welcome aboard!

<FONT COLOR="#000000" SIZE="1" FACE="Verdana, Arial">This message has been edited by KenB on 01-21-2001 at 06:43 AM</font>

K.Horn - Amen!

hmmwv - the FET based chopper sounds exactly like what I want. I "think" that's what OddOne's dimmer circuit is since his is based on a IRF 51 MOSFET. His circuit appears valid over 9-18 VDC and I don't have the knowledge to judge if it would operate efficiently across the 3-4.5 VDC range. My first post above has the link to his site with the schematic. Guess I'll just have to kludge the parts together and give it a try. Unfortunately, much of what you wrote is way over my pea brain's ability to comprehend (my random attempts at learning electronics have mostly fizzled). BTW, great pun! Loved it.

KenB - I'd prefer an infinitely variable dimmer. You are dead-on about the resistors and I should have thought of that. Thanks for pointing out they aren't needed at 4.5 volts. I'd rather not dim the LED's using a potentiometer to cut voltage ... energy cost is just too high. During week-long backpacking summer treks we'll want to stretch the battery life as much as we can so we don't carry any more extra weight in the form of replacement batteries than we have to. Every ounce counts over 60 some miles on foot!

Thanks for the input so far. You've already been very helpful.

<FONT COLOR="#000000" SIZE="1" FACE="Verdana, Arial">This message has been edited by Random on 01-21-2001 at 07:16 AM</font>

While I'm no electronics guru, I have worked extensively with 555 timer circuits (the basis for the oscillator in OddOne's schematics) and they are spec'ed to operate from 3-16 volts. Operating them on a pair of AA's will start out fine, but as the combined voltage of the batts start to fall below 3V, which they will due normally under load after some time, the operation of the 555 oscillator will become unstable. It absolutely needs 3V at minimum. I would feel much more comfortable using 3 AAs as this threshold of 3V would only occur when the batteries are at or very close to death. MOSFETS, on the other hand will have no problem across this range, and they should operate without failure. The only variable in OddOne's circuit, would be the current limiting resistor, which, at 4.5V should be close to zero ohms. I hope this helps.

Regards,

Ed

Edwood - IMHO, you are indeed of guru status ... your Sunblaster is the proof!

OddOne says his FET circuit is designed for a 12v rechargeable wet-cell battery. Do you think it would be okay to eliminate the 4.5 VDC current limiting resistor since I won't be using a charger in this scenario? There wouldn't be any threat of exceeding 4.5 volts across the LEDs. You mention the current limiting resistor as the "only variable' ... how about just doing away with it altogether?

I'm not confident I can build his 555 timer circuit, even though it seems the better of the two.

I'll heed your advice on the 3 AA's.

Thanks for your input!

<FONT COLOR="#000000" SIZE="1" FACE="Verdana, Arial">This message has been edited by Random on 01-21-2001 at 10:18 AM</font>

Heres my thought, but it isn't variable. I have actually done this and it works. Get 2 LED's on 3 AA cells, then put a tiny DPDT switch to put the LED's in series. They do light but very dim, like 2ma if I recall correctly. It works for a marker light of sorts and would last almost forever at that draw. But again it isn't variable, but very simple. You could do the same thing with any even number LED's. I wish I knew enought about electronics to build in a FET chopper. But alas, I break them not build them

Brock

One other thought, is it possible to change the LEDs in a Eternal light to these high output blue-green ? It would not be the same as completely building it yourself but the batt. setup and variable brightness would already be there...? How much do you think it will cost to build this other circuit ? You could use the rest of the LEDs to build a blue-green Edwood Sunblaster. That would be the champ of LEDs, at least for now.

<FONT COLOR="#000000" SIZE="1" FACE="Verdana, Arial">This message has been edited by KenB on 01-21-2001 at 12:05 PM</font>

I spent a little time in the Shack today looking at prices. I'm guessing the dimmer circuit will end up costing in the $12-15 range. I want this lantern conversion to cost less than $30.

Good idea about changing out an Eternalight! I don't know if it's feasible ... I've yet to order one & don't know how complex that might be.

The extra blue-greens I ordered will go onto a circuit board that just happens to fit inside my 2-cell Mag. I'm sticking 9 on to start and will scale up from there. At 20,800 mcd for each LED x 9 = 187,200 mcd which is equivalent to 33 whites (if my math and assumptions are right). I wonder how many it will take to blind myself?

Now ... if I can just figure out how to fit the dimmer circuit inside the Mag so the LEDs are pulse modulated and can be dimmed ... power the array with 12 volts from 8 AA's (they'll fit in a battery holder inside the 2D Mag with space left over) and retain the Mag's water resistant integrity, I may have a flashlight to be reckoned with.

Yep, that would be REAL bright!!!

Random, FWIW, I took this photo of a blue green 20800 mcd LED (Nichia??) obtained from BG Micro for 2.95 ea. I shot this at a paper target from 12" away. The squares on the target are 1" X 1".
<img src=http://members.bellatlantic.net/bg20800.jpg>

Random, FWIW, I took this photo of a blue green 20800 mcd LED (Nichia??) obtained from BG Micro for 2.95 ea. I shot this at a paper target from 12" away. The squares on the target are 1" X 1".
<img src=http://members.bellatlantic.net/~edraucik/bg20800.jpg>

Edwood:

INHO, that is one funky pattern! Thanks for making the effort to take the shot and post it. You're right that those are the LED's I'll be using.

After seeing your image, I hope that by having the LEDs in an array, light spilling from neighboring LEDs will help 'fill' the pattern. I'd better do a little geometry and figure angles to insure some overlap before I place those buggers on a board.

Muchas gracias!

Random

Yes - the tourquoise LEDs have exceptional brightness, but except for one vendor, they have that hourglass pattern. The one who fixed the problem pattented their technique - so most will still have the hourglass.

I ran across some devices in my parts bin - they are the chips used in the infinity to get 3.0+ v from a 1.5 (down to 0.8) supply. Best part is that the "advanced" version of the chip has a control pin to regulate the voltage - presto - chopper, dimmer, step up voltage source all in one. If you stack 5 of these chips on top of each other, they would take the same space as a t 1-3/4 led.

Unfortunately, this project is behind my new nimh charger for the surefire 9N and sanyo 3000 mah nimh cells - but when I get around to it, it would be a single part solution for dimming and boost.

hmmwv:

What an elegant solution! The chip must be tiny! At my advanced age, I'm not sure I could SEE the pin outs, much less do anything with them.

But, I will file your idea away for the day when I've had an Infinity awhile and feel the need to do some reverse engineering or destructive threshold testing

<FONT COLOR="#000000" SIZE="1" FACE="Verdana, Arial">This message has been edited by Random on 01-22-2001 at 01:55 PM</font>