Are white LEDs amenable to use as a strobe?

I wonder if the conversion phosphors change the pulse shape; that well might slow it down. Gotta try that some time.

John

What do they use for white LEDs... yag powder or something. Phosphors are interesting; tau's go from under 1 ns to hours.

John

LEDs are faster than bulbs, so I cannot think of a good reason why they wouldn't make a decent timing light. I'd be surprised if someone hasn't already capitalized on the idea!

Jon

I built a strobe out of white LEDs a couple of months ago, and had fun 'stopping' an AC fan I had in the window. My kids thought it was cool, particularly when I started using different rates, showing more fan positions.

It worked quite well, even with cheap LEDs; I built it out of a PIC chip and a big current mirror, with 10 or so LEDs. The brightness was adjustable using a pot. I used a fixed 1% duty cycle (adjustable using a constant in the software), and a variable frequency based on the A/D converter. Simple software, took about 15 minutes to write.

I'll post the software/schematic if anybody is interested.

I'd throw an eyeball over that...

Good observation I'd forgotten to consider. I use such phosphors (not these, but 'such') regularly. Their response times are an exponential decay with a tau in the roughly 1ms-1.5ms range at typical temperatures (it varies from say 5-8ms at liquid nitrogen temps to tens or hundreds of microseconds at about 400 C.)

Jon

Here it is:

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It's a big waste of transistors. Replace each transistor on each LED with a short. The 10 ohm resistors will drop 1 volt at 100 mA, and the LED forward V drop will be about 4V at that current, so the total will be about 5V. The LEDs will easily handle that much current at such a low duty cycle. And the peak light output will be much higher and brighter. And that's what you're looking for: a lotta light for a short time, which is what a strobe puts out.

Dr. Flash, Harold Edgerton, used to set a sheet of newspaper on fire with his strobes. Yeah, not too shabby! They used them in airplanes to take pics of cities.

I don't know, exactly. But the stuff I use often enough comes from GE and places that make light bulbs for the compact fluorescents and other fluorescent bulbs used most everywhere. Cheap as dirt and of such extremely high purity and quality, too. I'm sure that someone spent some time on selecting the right mix for making white LEDs, but I'll wager it's not a complete home-brew. We do that, too, and it's difficult to maintain consistency and quality if you are making small batches -- or large ones, for that matter. So, if they were 'smart' about it, they'd piggy back on what's already being done in large scale fashion. If so, chances are it's about like what I see. But that's only a wild ass guess, to be sure.

Jon

There's a newish book of his pics, "Stopping Time." Beautiful stuff.

John

who has a krytron on the shelf over there.

Thanks for the analysis. Your comments are as sagacious as usual.

The transistors are used to control the flash from the PIC. Without the transistors, you don't have a flash. I guess you could use a big power transistor to control the 10 LEDs, but this was cheaper for me, since I had them laying around. They can easily handle the 20mA to 100mA per flash.

The current through the transistors during a flash is controlled by the pot, which enables the brightness to be changed between very dim to fairly bright without toasting the LEDs.

I bought a bag of those PNP transistor on ebay recently, so I have lots of them. They were a 3 cents apiece, but I'm guessing you could get them cheaper. They are similar in rating to 2N4403s.

There was an exhibit of his work at the SF MOMA a decade ago. I wonder if it's still there?

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Are you sure that the numbers on those transistors are 2N3566? My old Motorola manual shows the 2N3566 as being a silicon NPN, similar to the MPS6514.

Yeah, I forgot to mention that removing the ten transistors requires adding a single high current transistor to switch all the LEDs on and off. A suitable NPN choice might be the 2SD965 or NTE11, which is capable of switching up to 5 amps, in just a little TO-92 package. The complement to that is the NTE12, but I don't know what the 2N or 2S equivalent is. You can often find the 2SD965s in disposable cameras, which often use them in the flash charging unit. I made the mistake of not discharging the 150uF 330VDC capacitor, and got a really good 'bite' from it when I touched it! That was even tho the camera had been off and not used for more than a week.

BTW, some guy put an experiment on his website where he took a dispoosable camera flash and fed it with a power supply, and triggered it with repetitive pulses. The rep rate was much faster than normal, and the strobe got the reflector so hot that it started to melt. He came to the conclusion that if you try to convert a disposable camera flash to a strobe, the charging capacitor should be a much smaller value, just a few uF or so.

For those just tuning in, the schematis is at

Greetings Robert, I for one, am interested. Thanks. Eric R Snow

ive made very simple led strobes ages ago using the old red leds and a 555 timer, the discharge pin can sink quite a lot of curent and drive a led directly, or an emiter folower boosts it even more, the LED curent resets the voltage on the timing capacitor, peak curent is limited by the 555 and or LED resistance etc, the average LED curent is controled by suplying a constant curent to the whole circuit, leds can be put in series if you have a high enough coltage supply but the lower threshold voltage (1/3 supply) needs to be higher than the voltage drop of the leds of course, or u can use a level shifter.

its not teribly good with old leds but newer ones are so very much brighter i havnt tried with those but might do out of interest now.

it all depends how dark your environment is, ive found cheap neon engine strobes to be virtualy usless unless u can use them in a garage wich u can make dark or use at night of course, or unless your timing mark is in a very convenient place.

Colin =^.^=

When I typed in the model, I apparently transposed the first two digits; its 2N5366.