LED flashlight failure report

So Jim, what are the electronics for? I sorta figured the new LEDs used (only) appropriate dropping resistors like older conventional LEDs.
Bob Swinney

Likely it's a dc to dc converter of some sort, much more efficient than wasting power in a dropping resistor. I'm amazed at the kind of efficiencies I see claimed in chip manufacturer's ads these days.
Jim just confirmed what I've been saying for the last ten years or so, repair of consumer electronics has become more of a mechanical job than an electronic troubleshooting one. The parts themselves hardly ever fail these days, it's mainly "loose disconnections" that keep the parts from doing their intended tasks.
Jeff

Resistors waste power. The best way to get a lot of light out of LEDs is to pulse them at a relatively high rate putting lots of current through them. I've designed a flashing red LED beacon for a client (it's used in an alarm system) that puts 250 mA through two strings of LEDs (0.5 A total) . They are pulsed at 35 kHz using a small, very cheap microcontroller (25c in quantity) with the power stored in an inductor being switched across them with a MOSFET. Current taken from the supply is < 10 mA. The whole thing is on a tiny 30 mm by 45 mm PCB using mostly surface mount parts.
Leon

The electronics are two transistors, a couple of resistors, and a flyback inductor, to boost the voltage so that a white LED that takes four volts to turn on, can be run off of two AAA batteries that only make three volts.
Now if I only could have found the loose connection that made my daughter's cell phone stop working - not that I didn't inspect every bit of the board under a microscope for an hour or so!
Jim

Thanx, Jeff! I shoulda known.
Bob Swinney

Deja vu all over again Jim. My youngest (17 year old) son sat on or whacked the stuubby little antenna on his two month old Motorola camera phone (which I'd just bought him to replace the previous phone which he lost somewhere.) and it twisted over in the plastic threads of the housing so that it's inside end scrubbed several tiny surface mount parts right off the board.
Because our cellphone carrier (Cingular) is no longer doing any service work on phones at their stores here in Taxachusetts, sick phones have to be mailed in for repair, so I figured there was no chance I could FTF sweet talk a local repair guy into doing me a favor and fixing it on the sly. The mail-in service operation wanted $150 to exchange a "damaged" phone. (Yeah, I know, I should have sprung for the "all risk" insurance Cingular wanted to sell me, but I must have been projecting my cautious experience and not thinking about the kid's, so I declined it.)
I couldn't Google up anything in the way of a print for the phone, and I figured I'd go nuts trying to reverse engineer it and then obtain the components it needed, so I just pulled the SIM card out of it, chucked the phone in the trash, and bought the kid a rugged, non-flip, internal antenna ex-Cingular Nokia complete with wall and car chargers for $30 on eBay.
Stuck the SIM card into it, made a quick phone call to Conversent so they could plug the phone's ID numbers into their computer, and he was on the air again. He can bloody well live with a plebian phone until he's on his own - Ha!
(It's times like this when I think children are G-d's punishment for having sex.)
Jeff

Yep! Like Jeff said, some sort of a DC to DC thingie. Comments regarding mechanical repair to modern electronics reminds me: RCA Colortrack TV receiver - mid '80s model. Modularized; a problem tracked down to the tuner control module - opened up tuner control module and found a note under the shield that the "new" set had already been serviced before it was sold to me. The pecker tracks therein must have been the first fix for an "intermittent" in the module because it came clear (again) when I reinstalled it.
Some time later, when the problem reappeared I was able to fix it with a popsicle stick propped under the rear of the tuner control module. Now this was "really cool" - complex electronic problem fixed with a popsicle stick!
Over the years, the module would exhibit the same problem which was handily repaired via the popsicle stick fix. Ultimately, a stack of popsicle sticks failed to clear the problem - like it was still intermittent, over a cycle of months, or sometimes years.
Optovisor (finally) to the rescue revealed a tiny PCB crack around an edge card connector on the front of the module. This was 4 or 5 years ago. After soldering over the crack, the old RCA has given trouble-free service as a bedroom set ever since.
Bob Swinney

You didn't say you got it working again, but I gather you did. Nice work, Jim. When it comes to electronics, I'm lost. Only good fortune in my shop yields a repair once the magic smoke gets out. I don't have a clue how to trouble shoot the vast majority of devices these days.
Harold

I seem to remember reading about pulsing LEDs to achieve higher percieved brightness. But the LEDs would have a much shortened life compared to running them at their rated current even though the average current was the within spec. Wish that article was handy now. Anybody know? ERS

You had some doubts??????
Gunner
"To be civilized is to restrain the ability to commit mayhem. To be incapable of committing mayhem is not the mark of the civilized, merely the domesticated." - Trefor Thomas

Yep, it was working again when I was done. It was actually a pretty easy diagnosis, as there were *two* parts floating around free inside the housing, and one of them (the heatsink do-hicky) was a brittle fracture off its base, so it could only go back on one way!
I felt kinda bad because I didn't take the time to reverse engineer the schematic for it, but my free morning time was up at that point.
Jim

It doesn't shorten their life, they are specified for high current pulses at a low duty cycle.
Leon

The apparent brightness is the same if the average current is the same. Pulsing them does not make them look brighter.
This assertion is based on an experiment I did today.
I ran two identical red LED's (automotive brake-turn parts if it matters, LiteOn LTL 912-VRKSA), one with DC current and the other getting pulses of 10X that current with a 10% dutycycle so the average current was the same.
I ran this at a variety of currents and frequencies. In every case, the LED's looked identical and indistinguishable unless the frequency was low enough so there was noticable flicker. At 40 Hz, where there was noticable flicker, the pulsed LED didn't look any brighter but it was certainly more irritating.

The voice of reason! Thanks, Don.
Bob Swinney

Try varying the duty cycle, at a fixed current/frequency (say 1000 hz, something up out of noticable by a good ways). IIRC, running one this way the apparent brightness hit a plateau (visually indistinguishable to an LED running the same current 100%) somewhere between 30-50% duty cycle.
Take that duty cycle and ramp the current up, and you might get a more noticable difference in output. In specific, if the rated current of the LED is 20mA, and you find no noticable brightness increase between 50% and 100% on time, you should see a difference when you run 40mA at 50%.
It has been 16 years or so since I ran that experiment, but I recall it reasonably well.

Particulary if you're an epileptic....Blinking lights, even LEDs, at the "wrong" rates can trigger epileptic seizures. That's why you sometimes see warnings in theatre programs when they are using flashing strobe lights in the show. See for example:
Quite a while ago I heard reports of helicopters crashing in Viet Nam because a few pilots pilots had previously unrevealed predispositions to epilepsy which caused them to go into siezures when the rotor blades chopped sunlight reflecting off something in the cockpit. But, that may just be an urban legend, it's tough for me to believe that the folks in charge of such things wouldn't have checked prospective student pilots out well enough to spot that.
Jeff

More than likely it was a FET in a constant Current mode driver. So the 'resistance' of the FET constantly changes as the battery depletes giving the same light output until there is not enough control or battery.
One concern with the 5 minute - it might be to hot and hurt semiconductor.
Martin

Jim - know the direction of hit - then look at the massive object - how the gravity does stuff. Might be a break in the pcb due to a pull... Mechanical fit that isn't mated directly...
Martin
jim rozen wrote:

Ah, it's a moot point now.
Last night I mentioned to my wife that I had fixed the flashlight.
"Oh, what went wrong with it?"
"Some of the parts had come off the circuit board, and I was eminently clever and managed to fix it. Damn I'm smart."
Or some other kind of preening to that effect.
I proceeded to unscrew the lens to show her what a great microelectronics handyman I really am.
The lens flipped out of my fingers as it screwed off the barrel of the light, and flipped across the room. Because I tracked it closely as it spun across the floor, I found it right away under the kitchen table.
The circuit board, with LED attached, had come loose and flew off in some other direction.
Correction: some other *dimension*. It's gone now, I can say that for sure after seaching for an hour. After looking high and low, I finally decided that it must have dropped down into the hole in the kitchen floor where there had been an old heating pipe at one time.
So I went down the basement and peered up onto the top of the beam that was right under the hole. Yep, there was something there. With the aid of an inspection mirror and a (different) flashlight I extracted the object.
Dumbfounded, I held an adapter that I had made years ago, to fit PR-style flashlight bulbs into antique navy battle lanterns, which take DC bayonette lamps. I'm sure I must have looked for *that* thing for a few hours, ten years ago.
Maybe in ten years I'll be looking for something else and find that LED on its board. In the meantime I think I'll plug that damn hole up.
Jim