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homemade LED flashlight/torch

Discussion in 'Electronic Basics' started by Seb, Dec 31, 2004.

  1. Seb

    Seb Guest

    Hello,
    Tried to google but didn't quite find the info I wanted.
    After reading on at the LED FAQ pages from the links in the Wikipedia
    article on LEDs, I thought it should be fun to make my own LED
    flashlight. Unfortunately the FAQs seem to assume more knowledge than I
    have, being a non-scientist. I know how a simple circuit works and how
    to solder wires together, but not much else (certainly not the peculiar
    requirements of LEDs). From one of the help pages I got the impression
    that you should use lower current (15 mAh, by using 130 ohm resistor)
    and higher voltage than the LED's specs require.

    What I still need to know but couldn't quite find out: what size LED do
    I need to get approx. light of a 2xAA pocket incandescent flashlight? I
    want to use yellow, maybe orange (cheaper and more user friendly than
    white, OK match for incandescent even though the most common yellow is
    greenish)--I know that different colors need different voltage. And is
    2xAA a suitable power source? 9V? What resistor to go with those batts?
    Maybe if it works out great, I'll try a "red night vision" flashlight.

    Seb
     
  2. smpowell

    smpowell Guest

    Unfortunately the FAQs seem to assume more knowledge than I
    have, being a non-scientist.

    The key is to understand that you must control the current
    flow through the led to keep it from burning out.
    The resistor acts as a crude current regulator.

    Here are some good sources:

    USING LED's
    http://ourworld.compuserve.com/homepages/g_knott/elect112.htm

    Current limiting resistor calculator
    http://metku.net/index.html?sect=view&n=1&path=mods/ledcalc/index_eng

    LED Apps
    http://www.plasma-ireland.com/lp/applications.html

    LEDs 101
    http://members.misty.com/don/ledd.html
    I need to get approx. light of a 2xAA pocket incandescent flashlight?

    I'd get a catalog from a place like
    http://www.hosfelt.com/
    and pick some led's with the high MCD ratings.
    Stay away from super duper LEDs ("Luxons") for
    the time being.

    As a practical matter, for these simple circuits you will need at
    least
    3 volts to run red or yellow leds. At least 6 volts to run white ones.

    Stephen Powell

    Electronic Hobby Information
    Stuff that should be easy to find, but isn't
    http://octopus.freeyellow.com/
     
  3. Lots of alternatives to consider. If you want to keep this dead-simple, you'll
    need to provide a voltage source that exceeds the required voltage needed for
    the LED and use a dropping resistor to help establish a predictable operating
    point.

    LEDs require different voltages depending on their color, so reds will need
    about 2V while blues will be more in the 3.5V to 4V area. You need to find a
    specification sheet for the LED you want to consider and if you are going to
    consider several, you'll need several such sheets. Yellow and orange are
    probably in the roughly 2.5V to 3V (just guessing, as I haven't seen a sheet on
    these.) You will need at least this voltage for those to operate. Whether or
    not you'll be satisfied with the results is entirely a different question. But
    incandescent bulbs provide blackbody distributions in their light, which is
    quite different from what you will get from a yellow LED, so you may not like
    the LED as a replacement. That's why the white LEDs are often worth the bother.
    On the other hand given your 'red night vision' point, you may like it better.

    The actual resistor you'd use can vary. The calculation is something like this,
    though. Assume you read from the data sheet that a certain LED has 2.15V when
    40mA is running through it. Also, assume that you are using 2 AA batteries for
    your supply and that you want to guess that the combined voltage will average
    about 2.8V during most of its normal operation. Then:

    R = (2.8V - 2.15V) / 40mA = 16.25 Ohms

    This is an estimate, based on assumptions and you don't need to have an
    absolutely exact value like that. There are two nearby standard values of
    resistors, one at 15 Ohms and one at 18 Ohms. Either of these may be just fine
    to try. If your estimate of 40mA for running the LED was a bit on the high
    side, then try the 18 Ohms. If you really wanted a little more current, then
    try the 15 Ohms. Also, keep in mind that the specification sheet tells you
    typical values, usually. So the 2.15V mentioned (hypothetically) would be a
    typical number. The actual voltage for some LED you actually try may be a
    little different, as well. The point here is that these calculations are
    approximate. They get you into the ballpark, but you may want to have several
    nearby values to try out and see what you get in practice.

    You can, however, operate an LED entirely well from a single alkaline AA. But
    you will need three components in addition to the LED. The circuit can operate
    any of the LED types, including the higher voltage ones. It does so through the
    use of a small transformer (coupled inductors) to get the voltage up. The nice
    thing about it is that it will drain the battery to near-dead state, so your
    lifetime of operation is pretty good. It doesn't regulate well, though, so as
    the battery gets lower your light output will similarly decline. The problem
    here is that to understand it you need to know more about ideas in electronics.

    Jon
     
  4. Seb

    Seb Guest

    Thanks for your help people. FYI I'm in Taiwan. I used to think this
    stuff was cheaper here than the US but it looks like you got good deals
    if you look at the right places.
    I am aware that yellow or orange LEDs aren't great matches for
    incandescent color but will be adequate for finding my way in a
    blackout. Now if it's not a dumb question: what are the
    volt/current/other specs for an LED (let's say orange) that should have
    roughly the same light output as an 2xAA incandescent flashlight? Unless
    I missed it, don't think there was this info in the FAQs.
    Otherwise now I do know how to calculate what resistor I need for a
    given setup. Since higher volatge tends to be safer, should I use a 9v
    batt instead? No thanks, don't feel like playing with transformers yet. :)

    Seb
     
  5. I don't know exactly. I have the CIE curves describing human responses to
    colors and brightness and I could probably calculate this from the spectral
    emissions of some LED. But I believe if that the usual measure in candelas
    already incorporates this detail of human perception -- so, as a layperson on
    this subject, I'd suggest that you simply compare the candelas and also at the
    same time look at the dispersion angle of the LED, too. You can get specs from
    various incandescent lamps in candelas, I think, for comparison purposes, but
    you will also have to realize that most flashlights use a parabolic reflector so
    that you will get an apparent concentration. I'm guessing here, but I believe
    you need to get an idea of the effective dispersion beam of a typical
    flashlight, get the candelas for the lamp itself from a spec sheet, and then
    somehow increase the effective candelas on the spec sheet to take into account
    of the fact that the reflector puts a spherical radiation pattern into a tight,
    forward beam (which should increase the effective spec.) It's that figure you
    can compare with the LED values for a similar dispersion pattern, I think.

    Jon
     
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