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Pulsed drive of white LEDs?

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

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  1. pimpom

    pimpom Guest

    Has anyone tried driving ordinary white LEDs - the kind used in
    cheap flashlights - with a pulsed current and done some kind of
    evaluation of their performance? Any idea if they can cope with
    frequencies of the order of a kHz?

    I'm thinking of using them with photosensors with a pulsed drive
    for discrimination against ambient light. It's for the drag
    racing christmas tree system that I asked about a few days ago.
    I've considered laser pointers and infrared devices, but a tight
    schedule and possible problems with on-site alignment makes me
    think that a pulsed white LED with a general-purpose photodiode
    might be the best option.

    I'm always open to alternative suggestions, but please remember
    that quick procurement of anything but the most common
    general-purpose devices is next to impossible from where I live.
  2. pimpom

    pimpom Guest

    P.S.: I'll be doing some search on the net, but I'm really
    pressed for time and will appreciate inputs from this group.
  3. RHRRC

    RHRRC Guest

    the problem with the leds used in cheap, and not so cheap, flashlights is
    particularly the life of the leds.
    By comparisonnthe rated life of flashlight some bulbs 50 hours (some higher
    some lower) so the leds dont have much to compete with.

    Infra red is much the better/cheaper/ easier.

    Look at the (many) integrated IR receivers built for the job

    As I stated:- better/cheaper/easier
  4. Nico Coesel

    Nico Coesel Guest

    A few years ago I made a PWM for intensity control at a few kHz to
    drive a white led.
  5. pimpom

    pimpom Guest

    Thanks for your interest. The laser is attractive and I had
    already considered using a blackened tube, but what made me
    reject it for the time being is having to align the thin beam
    precisely with the long narrow tube within a short time. The
    event is to be held in an abandoned airfield almost an hour's
    drive outside town. Everything will have to be set up on the day
    of the race itself - four lasers and their receivers, the
    christmas tree and control electronics.
  6. pimpom

    pimpom Guest

    The beam won't carry any data. It's to be a simple
    blocked/not-blocked switching action. I thought of using a
    pulsed drive to eliminate the effect of ambient light. The
    receiver will have a loosely tuned circuit (perhaps an RC twin-T)
    or just a high-pass filter.
  7. pimpom

    pimpom Guest

    Thanks. There's no modulation involved with this application.
    It's to act as a switch and driven with a pulses of constant
    rate, width and amplitude. The main purpose of using pulse drive
    is to enable filtering of the effects of ambient light.
  8. Yes, they certainly can (at least the ones I've worked with).

    Best regards,
    Spehro Pefhany
  9. _

    _ Guest

    Cigarrette - blow smoke so you can see the laser, line it up and fix. Two
  10. pimpom

    pimpom Guest

    Thanks for the reply. I have not fully rejected the idea of IR.
    The complicating factor is that the whole setup procedure,
    including the christmas tree and control electronics, will have
    to be done within a very limited time frame, outdoors, on the day
    of the race itself. The path of detection has to be made narrow
    (at emitter or receiver) because its purpose is to determine the
    correct positioning of the racers before and right at the moment
    of the start. Working within such constraints, I'm afraid using
    an invisible IR will be a major hindrance. Four emitter-receiver
    pairs are needed.
    I do have such parts in stock and in fact, I'd intended to use a
    555 to drive the LEDs.
    I've already tested the compatibility of a flashlight LED with
    reel sensor photodiodes from an old VCR.
    I'm not sure yet, but I'm guessing 8 to 15 feet. I'm thinking of
    placing the four emitters between the two drag racers, two each
    pointing to left and right.
    In one of the remotest corners of eastern India. So suppliers
    like Radioshack, Digikey, Farnell, etc. are out of the question.
    The way things are done in India, ordering something even from
    another Indian city is a cumbersome process.
    The organisers have tentatively targetted the second week of
    December for the race, but the date is not yet firmly fixed. I
    have to make decisions quickly because I'll have to do everything
    myself with materials I have or can get locally. No readymade
    modules, no outsourcing service.
  11. pimpom

    pimpom Guest

  12. pimpom

    pimpom Guest

    As I exlained in reply to another poster, my main worry about
    using IR is that the detection path has to be quite narrow and
    four beams have to be aligned outdoors within a limited amount of
    time using an invisible beam. However, this does not mean that
    I've rejected IR completely.
    All noted. Thanks for the input.
  13. krw

    krw Guest

    That doesn't mean the path has to be narrow at all. The only reason
    for a narrow beam is S/N. Even if you had an omnidirectional (point)
    source and detector only something directly in the path will interrupt
    the signal. Noise may be an issue and the size of the detector and
    source are important. Garage door opener safeties come into mind as
    an example of your application. They're not very directional and easy
    to set up but work well enough.

    I certainly wouldn't try to use a narrow source and detector. You'll
    never get that aligned right.
  14. pimpom

    pimpom Guest

    I didn't get your point at first, but I do now. You correctly
    pointed out that it's only the line of sight between source and
    receiver that matters - well, up to a point. Some care will still
    be needed to limit the angle of detection for two reasons. One is
    that reflections could cause problems. The other reason is that
    there will be two virtual lines in parallel working
    independently, only 7 inches apart over a distance of several
    feet. Cross detection is unacceptable.

    I suppose I could pulse the two beams at different frequencies
    and filter the signals at the receivers, but that adds complexity
    and could result in unforeseen problems..
    Correct. It's why I rejected the laser option, at least for the
    upcoming race. The only sensor with an area large enough to ease
    alignment would be an LDR, but it's too slow. Anyway, I doubt if
    I can get one an inch wide or larger in a short time.
  15. pimpom

    pimpom Guest

    That's an idea. I'd intended to place the four emitters as
    unidirectional transmitters between the two racers, battery
    powered without any electrical connection to the receiving units.
    Running a cable to the middle of the track from the side, even
    behind the starting line, might be a problem. But it's worth
    thinking about.
  16. pimpom

    pimpom Guest

    One thing I'm pretty sure of is that there won't be any
    fluorescent ballasts spewing EMI. The venue is an abandoned
    airfield and the starting line will be quite some distance from
    the old terminal building. There won't be any electrical supply
    at that point even if there's one in the building, unless we
    string a long power cable. This is why I decided to power
    everything with batteries, including the christmas tree lights.

    Thanks for the webcam idea. I still haven't completetely
    abandoned the IR route yet.
  17. pimpom

    pimpom Guest

    I made a trial batch of street light controllers for my state's
    electricity board nearly 20 years ago. Control was from
    distribution points rather than at each lamp post. The problem
    was that practically the whole state, including the capital, is
    made up of small jagged hills. Daylight varies from point to
    point and the controllers had to be set individually. They worked
    fine when set properly, but the engineers left the setup to
    unskilled workers who had no idea what they were doing. That
    turned the experiment into a failure.
  18. Baron

    Baron Guest

    Would not red laser pointers work.
  19. krw

    krw Guest

    You can limit the view with a simple shield. I used "Omnidirectional"
    just to try to get the point across.
    Even simpler, shut down one transmitter when the opposite receiver is
    in use. Alternate at some sufficiently high rate.
    A laser pointer would be simple to point accurately (kinda the purpose
    of the thing ;). Get the interest of all the neighbor cats too. ;-)
  20. ehsjr

    ehsjr Guest

    pimpom wrote:

    To avoid cross detection, you can do this:



    Each detector can "see" only its own emitter.


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