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Ignorant LED questions (low power LED usage)

Discussion in 'Electronic Design' started by Ignoramus21592, Feb 12, 2007.

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  1. So, I completed putting LEDs on my trailer. (LEDs are used to
    indicate power and braking status) They are current limited by simple
    500 ohm resistors, to approximately 25 mA. No big deal.

    That made me think: most of the power to LEDs is wasted worthlessly in
    those resistors. Only a small fraction of it is used to make
    light. (my guess is about 85% of power is wasted)

    Are there any LED driver kinds of chips that allow LEDs to be used in
    a low power usage kind of situation (as opposed to running from a
    truck alternator). That replace those resistors with something smarter.

    The reason for my question is that I have a mailbox that used to be
    often hit by vehicles. I have changed it to a higher visibility,
    swingaway mailbox:

    but I would like to also add solar powered LEDs to it based on a WHY
    NOT principle. (why not do it)

    For this thing to work properly, it has to use as little power as
    possible, so, I would like to know how LEDs are used in low power

  2. Tim Wescott

    Tim Wescott Guest

    Igor, juvenile hoodlums will thank you. Leaning out of a car window
    clutching a baseball bat while going fast in the dark makes it very hard
    to see mailboxes. Having one lit up by LEDs would be a blessing.

    So much for bad humor:

    A switching power supply would be the way to go. I doubt that any are
    made specifically for LED supplies, and finding one that'll be efficient
    at the low power levels you're looking at will be a challenge. So you
    have your work cut out for you -- but you're known to be energetic.


    Tim Wescott
    Wescott Design Services

    Posting from Google? See

    "Applied Control Theory for Embedded Systems" came out in April.
    See details at

  3. Well, you could have put some LEDs in series on that 12V, and create a
    constant current source with some FET or transistor.
    This is the classic low component count LED constant current source:

    k a k a k a k a
    ------ LED1--LED2--LED3..... LEDn--------- PLUS 12 V
    ---->| JFET
    | |---
    | | select
    | [ ] R for 25 mA
    | |
    ---------------------------------------------------- MINUS

    See above.
    More efficient even is a switchmode current regulator ....

    Well light at night, sun during the day, you need at least a battery of sorts.

    Long story, many solutions, late at night, whatdoyouknow.
    Over to somebody else.
  4. default

    default Guest

    There are some chips made for the purpose but they tend to be hard to
    find, costly, and are primarily for high power leds.

    Easiest way to save some power is to just pulse the LED with a much
    higher current for a much shorter duty cycle - or use one of the
    flashing LEDs or get a hold of a LM3904 and use that to flash your

    Solar powered - you'd want it to shut off while light outside.
  5. D from BC

    D from BC Guest

    Could one say that impedance matching is required?

    Match load and supply or use something acting like an imaginary DC to
    DC transformer (ex. smps).
    D from BC
  6. LED's are efficiently driven by switchmode current supplies.
    There are now dozens of different chips available from just about every
    major player.

    Many thanks,

    Don Lancaster voice phone: (928)428-4073
    Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
    rss: email:

    Please visit my GURU's LAIR web site at
  7. Readily available from a dozen manufacturers.

    Many thanks,

    Don Lancaster voice phone: (928)428-4073
    Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
    rss: email:

    Please visit my GURU's LAIR web site at
  8. Tim Wescott

    Tim Wescott Guest

  9. I know what you mean, but so far, all my mailbox accidents were NOT
    due to juvenile hoodlums. They were due to people not knowing how to
    drive. Once it was a snow plow, and another time it was some odiot who
    left his or her car mirror lying next to the mailbox.
    Thanks Tim... I am still hoping that this is a standard problem...

  10. That's a great idea, very simple and elegant. Thanks.
    Definitely, yes. Battery plus solar charger.

  11. Also very wrong.
    The efficiency would be negligible.

    The trick is to provide JUST ENOUGH VOLTAGE to allow a desired current
    to flow through both the LEDs and their (hopefully very small) current
    sensing resistor.

    Many thanks,

    Don Lancaster voice phone: (928)428-4073
    Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
    rss: email:

    Please visit my GURU's LAIR web site at
  12. Not really, a few volts drop with 12V and a few more when battery goes way up
    say to 14 V or more, will protect the LEDs, keep teh current constant very well.
    It is more constant and more efficient then each LED a resistor.
    It is not as efficient as _most_ switchmodes.
    And it is not wrong.
    The only thing that is wrong is that your prediction I would have LED lights
    in my house by now never happened.
    And will not happen in the coming years.
  13. James Arthur

    James Arthur Guest

    Greetings Don!
    Perhaps you missed that there were many LEDs in series? Although
    not as good as a switcher, it would be many-fold better than a single
    LED resistored to 12v.

    James Arthur
  14. Rich Grise

    Rich Grise Guest

    I've seen ordinary bicycle retroreflectors on mailboxes. They take zero
    power. :)

    However, if the vandal is driving with no lights, I guess it wouldn't
    help much. ;-)

    (and if you put a solar panel on top of your mailbox, it would probably
    get stolen. =:-O )

    Good Luck!
  15. Rich Grise

    Rich Grise Guest

    I don't know where you live, but in snow country I've seen mailbox posts
    set back 4-6' from the shoulder, with the box on a cantilevered arm, like
    a fence gate. It stays in place by ordinary friction, or maybe there's a
    piece of breakaway wire. This does two things - the snowplows don't plow
    the post down, and if the box gets hit, it just swings out of the way.

    Hmm - this suggests an additional scenario: the box bounces off the front
    of their car, swings around, and breaks their back window. ;-)

  16. it could be, in fact, much closer to 100% than to 0%. Depending on
    voltage drop per diode.

  17. That's exactly what I did.

  18. ehsjr

    ehsjr Guest

    Cat# SPL05 is a solar powered, 3 LED lite -
    charges in daylight, turns on automatically
    at night. $4.50

    I don't think you can beat that. Even if
    it is less power efficient than some pulser
    or switcher design, it's a cheap "done deal"
    that seems to fit your needs. Besides it
    provides the solar cell you'll need if you
    build one.

  19. Ed, looks very interesting (the number is actually SPL-05). Thanks.

  20. jasen

    jasen Guest

    some sort of "switcher" eg "joule thief" (google)

    those cheap solar led lights often use a similar approach too.
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