LED help

[SwamperGene]

This is probably simple, but it's driving me nuts...

I built a light bar for a photocell-based timer setup for a slot car track. I used 4 white LED's from some pen-type flashlights, however after close to a year of very reliable use now one is getting too dim to drive it's photocell. Cheapo that I am, I found some new one's at a dollar store and proceeded to duplicate the setup with 4 new LED's. Problem is, I have the same problem.

The new pen-lights had 4 AG3 batteries and measured 6.1 volts, my meter showed about 30mA with the LED in the pen-light. As usual there is no resistor or circuitry in the lights. I'm hooking up the LED's in series, the 9 volt junk box wall wart is putting out 14.1 volts, and there is no resistor. No matter what order I put them in, the one closest to the negative leg is noticably dimmer than the other three. Figuring that maybe I was under-powering them, I tried a (measured) 21 volt source with 60 ohm resistor thrown in, they were all brighter but still the last one is dimmer than the other three, even if I remove the resistor. Individually, at 4.5 volts straight up they all appear about the same, but the leads start warmimg up. Any thoughts?

[JPKNHTP]

-JPKNHTP

[positronicle]

--Edited by Positronicle--

[JPKNHTP]

-JPKNHTP

[dyarker]

You need to use a resistor in series with LEDs.

AG3 batteries are very small, internal battery resistance is probably sufficient to limit current in the pen-type flashlights.

If one LED position is always dimmer, even with swapping LEDs around, then something is in parallel with the connections at that position. Maybe a glob of solder flux. Maybe a dust bunny in high humidity. Maybe one strand of stranded wire is touching the other LED terminal, and is corroded enough to make a high resistance short. But something!

<small>[ March 23, 2006, 10:45 PM: Message edited by: Dale Y ]</small>

[cato]

If you put a resistor in parallel with each led, the resulting chain of resistors will form a voltage divider that will force the same voltage drop across each led.

Of course, the resistor value has to be low enough to that each LED can pull current through the resistor if its hungrier than the one ahead of it in the chain....

As mentioned above, you should use a series resistor to make sure your not over currenting the leds. You might want to consider using 2 parallel legs of 2 leds each to get you more voltage to play with ...

also...check the specs on the photocell...it might be more sensitive in the infrared or in the ultra violet....using white leds if probably very inefficient as far as generating signal from the cell is concerned....consider using infrared or ultra violet leds or at least red or blue ones...

.photo diodes or photo transistors are likely to be more peaky in their frequency response and also much faster in switching than the photo cell...so...if you use an led that produces light in the matching wavelength...you will need even less led power....and you can accurately time the cars down to the micro second :-p

[dyarker]

Agree on the photo transistors and IR LEDs.

Disagree on paralleling each LED with a resistor. That may equalize the voltage across each LED, but will probably make differences in brightness worse. Brightness is dependent mostly upon current, not voltage. With a series only path, the current through each LED is identical.

In LEDs, the voltage drop varies very little over large changes in current. There is a fixed voltage needed to force electrons out of orbit, plus a fairly small voltage drop that varies with current due to low internal resistance. Light is created when electrons drop to a lower orbit. More current, more light; while voltage stays nearly constant.

Brightness of incandescent lamps is more dependent on applied voltage. The voltage causes current to flow through the resistance of lamp, causing heat, the lamp glows. Lamps are more self regulating than LEDs. As the lamp gets hotter, the resistance of the filament increases. So the current does not increase as much as it would for a fixed resistance with the same voltage change.

cheers,

[SwamperGene]

Wow...thanks for all the quick replies! Lots to think about. For a better idea of what I'm working with, below is a pic of the setup. This was taken shortly after the original build, as I said it worked flawlessly for quite some time. The CDS photoresistors (I should have made that clear at first) are pulls from...don't laugh...automobile rear view mirrors from cars with auto dimming lights and self adjusting mirrors (two in each mirror). I originally set up a testing rig and these are very consistant. They are hooked directly to the parallel port, and must be held below 1K ohms. At the distance shown, those white LED's hold them at about 750, which is perfect. A 75 watt floodlight put above the track works well, but as people reach to fix deslotted cars a shadow can trigger a lap, hence the need for a dedicated light bridge.

I like the idea of the divider circuit, I may try that today. Thanks again for the input guys!

<img src="http://home.comcast.net/~swampergene/im ... ocells.jpg" alt=" - " />

[JPKNHTP]

-JPKNHTP

[SwamperGene]

Thanks for the kind words!

I tried that LED wizard, though I've used a few other online calcs I do like that one as it gives more info. If you consider that all the LED's are from the same brand/model dollar store penlight, I guess you could say they should all be the same. Of course, the reality is that it's anyone's guess.

On a side note, here's what alerted me to the original problem, and it invokes another question....as I was running some cars one day on the white lane, I started seeing stray laps on yellow, two lanes over. This only occured if I ran cars on other lanes, without running cars it can sit all day with no problem. Further testing along with the visual observation indicated the yellow lane LED was low, just barely keeping the photocell below the threshold. The question this brings up is what, relative to a car running on the track, could cause this? Are CDS cells subject to EMF (they are mounted next to, but not touching, the common rail)? Could interference be a factor, causing ripple in the LED's wall-wart - even when plugged into a different line?

[SwamperGene]

Well, I went a little different route and built a small filter/regulator using an LM317. I adjusted my 21V wall wart down to about 16 and threw in a 10 ohm resistor for good measure, all the little lights seem to be happy. Colors were visibly a little varied but they all were bright, using a test photoresistor there was only a 20 ohm difference between the 4 LED's. I let them sit lit up for about 2 hours and the colors seemed to even out a little, though the resistance in the photocell increased by about 10 ohms, so I guess they are being overdriven a little bit. I'm sure the penlight mfg doesn't expect they will be lit up for hours at a time. Today they will be put back to work on the track and maybe I can adjust the voltage down a little further, hopefully the filter will take care of any ripple that may have been the cause of the original problem.

Thanks again for all the input!

[JPKNHTP]

-JPKNHTP

[SwamperGene]

I'm taking a guess based on the fact that I know they will have to be driven pretty high, which is why I tried these as they came from 6V (6.1 measured) penlights (even though I know there is some current limiting in the button cells, I figure the Vf has to be on the higher end). I have a temp gun handy that I use on the cars, there is no temp increase in the wires at all?

Here's the odd part, when I first started using these, like you I came up with the 150 ohm value, they fried in about 10 seconds? The second time around I put in a trimpot, if I went above 50 ohms I'd get heat buildup and the lifespan on the LED's dropped dramatically. As I said, with these cheapo lights I think it's a crap shoot. I'm certainly not an engineer, just basing this stuff on what I'm seeing.

I don't know if these numbers help but before I dismantled one of the penlights I took a couple measurements.

Battery voltage: 6.1
Current in circuit: 30-35 mA (50 at power on)
Voltage drop: 2.2 (would this mean 3.9v Vf?)

[JPKNHTP]

-JPKNHTP

[Chris Smith]

Rule number one for LEDs...

LEDS have a 11 year life span, 365 days a year, 24 hours a day, when properly powered up.

Never run other leds parallel on any circuit with other leds [before the resistor],..... always power each one with its own resistor,..... and keep your voltage supply constant,..... and you too can have your LEDs last 11 years straight, bright, and constant.

<small>[ March 25, 2006, 08:25 PM: Message edited by: Chris Smith ]</small>