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Pulsed drive of white LEDs?

Discussion in 'Electronic Design' started by pimpom, Nov 22, 2009.

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  1. In <>,
    Thanks for the info - now I know phosphor persistence is pretty small,
    as I suspected.

    This data point appears to me to indicate that phosphor persistence
    counts for something, since unphosphored LEDs usually have rated rise/fall
    times of 30 nanoseconds.

    - Don Klipstein ()
  2. Jon Kirwan

    Jon Kirwan Guest

    Emergency vehicles already signal at intersections. The receivers sit
    high on the wires or poles across an intersection and are IR-based.
    They use optical baffling to narrow the acceptance angle downward
    towards oncoming traffic at the right spot and optical wavelength
    filtering to narrow the accepted IR color bandwidth, as well. They
    use electronic receivers that are tuned with extremely narrow notches
    so that only a very precise flicker rate of the emitter is accepted.
    The sun doesn't yield much flicker in that band (circa 15Hz, if I

    I haven't been reading the thread and I apologize if I've duplicated
    something already said. My thought though is for you to use a tube,
    baffling and possibly some optics, a narrow-band wavelength filter
    (thin film?), and a receiver/emitter pair that transmit/receive in an
    extremely narrow bandpass. The combination appears to work,
    elsewhere. I believe I've also seen similar things designed to count
    traffic, replacing those hoses that lay across the road. Maybe
    there's no time to screw with this stuff, though.

  3. ehsjr

    ehsjr Guest

    If you can get laser pointers, your outdoor quick setup problem
    can be easily solved. When building the equipment, install the
    detector into a fairly large white surface. Set things up (at
    home) such that the emitter and detector are perfectly aligned,
    and are separated by the same distance they will be on race
    day. Mount a laser pointer on the emitter assembly such that it
    shines someplace on the detector white surface, but not on the
    detector itself. Draw an X where the laser pointer dot is when
    the emitter & detector are perfectly aligned. Make sure everything
    is firmly mounted so that it cannot move when you bring the equipment
    to the race. Then, when you are setting it up in the field for the
    race, set it up so that the laser dot hits the X and it will be
    properly aligned.

    Your biggest problem may turn out to be getting coplanar level
    spots at the race start line to place your emitters & detectors.

  4. pimpom

    pimpom Guest

    You hit it with that last sentence. No time, *plus* procuring
    things quickly from where I live is next to impossible. I have to
    rely on what I already have, can get locally (which is of very
    limited range) or can scrounge from discarded stuff.
  5. pimpom

    pimpom Guest

    That should indeed work except for two things. I had intended to
    place the emitters between the two racers, battery powered
    without any cables leading to the sides, two beams each emitting
    outwards unidirectionally. The second factor is that an inward
    beam could spill over to the other side and hit the receiver for
    the other lane. That could be avoided by placing a baffle in the
    center, but I thought it would be best to make the center piece
    small, sturdy and low-profile.

    Reflections could also be a problem. But I'm not rejecting the
    idea out of hand. It might just be the best solution under the
  6. pimpom

    pimpom Guest

    I'd already considered laser pointers but, as I explained
    elsewhere, I anticipate problems with on-site alignment - four
    narrow beams hitting tiny sensors at a distance of more than 10
    ft over uneven ground. And once aligned, both emitters and
    sensors would have to be fixed firmly enough against accidental
    knocks and vibration.

  7. Put a single tuned circuit in the receiving amplifier and feed its
    output back to the light source. When the beam is unblocked, the whole
    system will oscillate, but you don't need anything more complicated than
    a diode to detect it.

    There is only one tuned circuit , so temperature drift doesn't matter
    and alignment isn't necessary.

    I built a system like this to work over a reflected path across a wide
    south-facing doorway at my local garage; the source and detector are in
    the same box but carefully screened from each other. It has been
    operating without problems for more than 10 years.
  8. pimpom

    pimpom Guest

    Thanks for all your interest and helpful replies. Even the
    suggestions I can't use directly were helpful in narrowing down
    possible solutions.

    I've tentatively decided to go for a pulsed IR system. What
    particularly lit a light bulb in my head was when krw pointed out
    that the emitter and receiver do not have to operate with narrow
    beams. I had been stuck with the idea of collimating a thin laser
    beam on to a tiny sensor.

    I think it will be feasible to guard against false detection due
    to reflections by limiting the angle of incidence with blackened
    tubes at both ends. I don't think there will be a problem
    aligning the path within a couple of degrees. A laser pointer
    beam impinging on a sensor 2mm wide from 15 ft would require an
    accuracy of something like 0.025º and that's simply not
    practicable in the given circumstances.

    The remaining problem is interference between the two parallel
    paths, 7 inches apart, for each rider. Coded pulsing would
    obviate the problem, but I don't think I'll have time to try that
    out or get the parts. For the time being, I may just decide to
    use only one beam for each rider to mark the starting line (and
    detect false starts), and do without the less essential
    pre-staging marker for now.

    Refinements can be added for later events - the full two-beam
    system, measurement of speed at regular intervals along the race
    track, elapsed time, reaction time, etc.
  9. [...]

    It is much easier to keep all the electronics together in one place and
    just fold the light path. The self-oscillating system I have described
    in another post will work perfectly well over a double 10ft path in
    bright sunlight (as long as it doesn't fall directly on the sensor).

    Use a reflexor at the far side and keep the source and detector close
    together in the same box on the near side. Test a few easily-obtainable
    reflexors (such as number plate background material and the reflectors
    for bicycles and the sides of lorries) to check that they work at your
    chosen wavelength.
  10. Jon Kirwan

    Jon Kirwan Guest

    Rare earth phosphors I use have taus on the order of milliseconds.

    The term 'phosphor' refers to both fluorescence and phosphorescence,
    now. Centuries ago, to any substance that seemed to emit light on its
    own, without combustion taking place. (So a radium watch dial, if sent
    backwards in time a few centuries, would probably be included by some
    earlier texts as a phosphor.)

    Some phosphors (usually those that do NOT exhibit a nice exponential
    decay) can have decay tails that last well into minutes of time. One
    of the earliest recorded phosphors were of the this very persistent,
    phosphorescent kind. Some paintings in Japan or China used materials
    that came from volcanic activity acting on seashells and sulfur (with
    rare earth included, of course.) Although these have non-exponential
    decays in the microseconds, they also have long emission tails lasting
    well into seconds or even a minute or two.

    Saying "phosphor persistence is pretty small" when talking about 200ns
    periods of time cuts off at least half of the modern meaning of the
    term, 'phosphor.'

  11. Baron

    Baron Guest

    Actually I don't think it would. I checked a couple of red laser
    pointers I have here. Both produce spot diameters of around 6 or 7 mm
    at 12 feet (the longest distance I checked). I also tried pointing one
    at my car number plate. The reflected light was just a blinding blob at
    that distance.
  12. Uwe Hercksen

    Uwe Hercksen Guest


    I drived red, green and blue LEDs with pulses of 10 to 100 microseconds,
    no problem at all. But white LEDs may have a problem with the yellow
    flourescent dye, so I would prefer single color LEDs for short defined

  13. JosephKK

    JosephKK Guest

    Modulate the LED and look for the modulated signal. Great aid to
    {semi-)manual alignment. Plus it improves S/N a bunch. All
    monochrome LEDs are good for MHz, whites may be good for many kHz,
    depends on the phosphor.
  14. JosephKK

    JosephKK Guest

    Or any other fine particulate source, like superfine flour in a
    eductor type sprayer, or just kick up some dust.
  15. JosephKK

    JosephKK Guest

    Design case point. What do the maintainers think? How will they act?
    There may be more to the customer than the representatives you meet.
  16. JosephKK

    JosephKK Guest

    On Mon, 23 Nov 2009 10:17:54 +0000,
    Ah yes, the three corner reflector trick. Reflects the incoming beam
    parallel to and adjacent to itself. Then the detectors are adjacent
    to the emitters and no wire to run and most alignment issues reduced
    or eliminated.
    Thanks Adrian, i had missed this.
  17. I have what I consider to be "major experience".

    My experience is in 2 parts, both disfavoring pulsing white LEDs in
    attempt to increase ratio of visual perception of brightness to amount of
    current or power delivered to the LEDs.

    One is personal experimentation into this bit of LEDs appearing brighter
    when pulsed. I found that human vision is a "good integrator" when pulse
    rate is fast enough to achieve lack of human-perceptible flicker.

    On that line, I also found that the legend of benefit from pulsing arose
    from the benefit being applicable to a widely-used kind of LED having a
    nonlinearity favoring efficiency and more-important-still ratio of
    lumens to average-mA varying directly with instantaneous current
    throughout at least most of the range of instantaneous current under

    I discuss this a bit more in a web page of mine:

    (slightly outdated due to excessive consideration to low-power LEDs at
    the times when I wrote that long ago, enough long-ago for high-power LEDs
    350 mA-plus to be either not-yet-in-existence or something quite new.)

    Principles when translated to such higher currents of modern high power
    LEDs still apply.

    - Don Klipstein ()
  18. This physical efect has a name that I suspect is good enough to
    web-search on:

    "corner cube reflector"

    It appears to me that plenty of bicycle reflectors including most other
    than "aftermarket", some bicycle taillights, and some automotive
    taillights use this principle.

    In most vehicle-mounted retroreflectors (one more keyword), there is at
    least one area of clustered small-sizede corner-cube inits with "cell
    size" around 3 mm IIRC, and depending on (successfully) that a refractive
    surface towards the incoming light source (boundary between air
    and plastic) refracts the incoming and outgoing light rays equally so as
    to maintain retroreflective property of the array of "corner cubes".

    Many of these vehicle retroreflectors rely on arrays of small "corner
    cubes", whose surfaces are angled sufficiently close to parallel to
    incoming light rays desired to be retroflected, so as to achieve great
    retroreflection from air-plastic boundary via "total internal reflection".
    That principle of physics has its presence being a function of angle of a
    ray being incident to a boundary from higher refractive index material
    (such as plastic) to a lower refractive-index adjacent material (such as

    There are also retroreflective objects marketed to cyclists (and
    elsewhere) involving very small glass spheres made of glass of whatwever
    variant has refractive index that favors a significant and notable bit of
    retroreflection. One thing that comes to my mind is "Scotchlite" (tm) by

    - Don Klipstein ()
  19. JosephKK

    JosephKK Guest

    Alas, that does not mean that any of this (with the possible exception
    of bicycle reflectors) is currently readily available to pimpom (OP).
  20. baron

    baron Guest

    JosephKK Inscribed thus:
    There is always a reflective car number plate.
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