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Are white LEDs amenable to use as a strobe?

Discussion in 'Electronic Basics' started by Eric R Snow, Dec 9, 2004.

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  1. Eric R Snow

    Eric R Snow Guest

    Would a white LED, a Luxeon Star for example, be useful as a strobe
    light for checking engine timing etc.?
    Thanks,
    ERS
     
  2. 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
     
  3. Rich Grise

    Rich Grise Guest

    If you pulse it properly, I see no reason why not. Look up the pulse spec
    on the LED. The pulse width translates into how wide of a smear the timing
    mark makes. I've seen backyard mechanics use white "metal-marker" to
    emphasize the timing mark on their unit. Not with an LED timing light, but
    the principle is the same. The challenging part would be the pulse-forming
    network to get the pulse as narrow as possible.

    Then again, a one-millisecond pulse is probably not that hard to
    accomplish electronically, and it is without question narrow enough to get
    your timing within less than a degree.

    Good Luck!
    Rich
     
  4. 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.

    --
    Regards,
    Robert Monsen

    "Your Highness, I have no need of this hypothesis."
    - Pierre Laplace (1749-1827), to Napoleon,
    on why his works on celestial mechanics make no mention of God.
     
  5. Don Bruder

    Don Bruder Guest

    I'd throw an eyeball over that...
     
  6. John Larkin

    John Larkin Guest

    3600 RPM (we're checking the advance here) spins 360 degrees in 16.6
    milliseconds. So a millisec flash is 22 degrees. Even at 1000 rpm, 1
    msec is 6 degrees.

    John
     
  7. John Larkin

    John Larkin Guest

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

    John
     
  8. 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
     
  9. John Larkin

    John Larkin Guest

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

    John
     
  10. Rich Grise

    Rich Grise Guest

    Well, I'll be.

    I stand corrected, and will present myself as an object lesson in the
    risks of seat-of-the-pants design. :)

    Then again, one of the most effective ways to get accurate information on
    USENET is to post something obviously wrong. You _will_ be corrected. ;-)

    Thanks!
    Rich
     
  11. 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
     
  12. 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.


     
  13. John Larkin

    John Larkin Guest


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

    John

    who has a krytron on the shelf over there.
     
  14. 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?

    --
    Regards,
    Robert Monsen

    "Your Highness, I have no need of this hypothesis."
    - Pierre Laplace (1749-1827), to Napoleon,
    on why his works on celestial mechanics make no mention of God.
     
  15. 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
    http://home.comcast.net/~rcmonsen/strobe/circuit.gif
     
  16. colin

    colin Guest

    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 =^.^=
     
  17. Eric R Snow

    Eric R Snow Guest

    Greetings Robert,
    I for one, am interested. Thanks.
    Eric R Snow
     
  18. When I typed in the model, I apparently transposed the first two digits;
    its 2N5366. http://www.fairchildsemi.com/ds/2N/2N5366.pdf

    --
    Regards,
    Robert Monsen

    "Your Highness, I have no need of this hypothesis."
    - Pierre Laplace (1749-1827), to Napoleon,
    on why his works on celestial mechanics make no mention of God.
     
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