1.5v battery drives LED

Discussion in 'General Electronics Chat' started by Razor Concepts, Nov 3, 2009.

  1. Razor Concepts

    Thread Starter Active Member

    Oct 7, 2008
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    On several dollar-store type lights I have seen them only use one AA battery, and a small circuit that contains one or two inductors and some other resistor/transistors to drive one LED. Does anyone have a basic schematic diagram of this?
     
  2. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    This site has a single transistor version:

    http://www.cappels.org/dproj/ledpage/leddrv.htm

    There are many versions around on the net to "do something" with AA batteries that are too dead to make an MP3 player run, but still have some juice in them.

    It is a very simple boost converter/unregulated switching power supply made with a home wound inductor, LED, a resistor or two, and one or two small signal transistors.
     
  3. Wendy

    Moderator

    Mar 24, 2008
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    Google "Joule Thief". I used the same principals in some of my CMOS 555 LED flashers, though I used 3V (2 AAA batteries).
     
  4. Xantor123

    New Member

    Nov 1, 2009
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  5. Razor Concepts

    Thread Starter Active Member

    Oct 7, 2008
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    Thanks for the help! I searched more on joule thief and I have found what I was looking for:
    http://www.joulethief.com/kit.php
    The other circuits use a hand-wound inductor, but that one above uses a normal inductor.
     
  6. BMorse

    Senior Member

    Sep 26, 2009
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    I had used the same circuit before and made some changes to where I can light 9 white LED's off of a single AAA battery, have not tested it to see how long one battery would last with 9 LED's but the circuit does work better than those hand wound inductor ones..... here is a video on youtube showing my initial power up test of the circuit >>> http://www.youtube.com/watch?v=6zVOcND-FuQ


    My .02
     
  7. Razor Concepts

    Thread Starter Active Member

    Oct 7, 2008
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    Cool! I didnt know they could drive that many LEDs.

    So I built that circuit:
    http://www.joulethief.com/images/boost-schematics.gif
    [​IMG]

    I did not have a .001uF capacitor, so I used a .1uf cap. It still works. How does the capacitance value affect the circuit?

    Also I did not have a 470uH inductor, so I used a 150uH. Still works. How does the inductance value affect the circuit?

    Thanks!
     
  8. Wendy

    Moderator

    Mar 24, 2008
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    The capacitor will probably affect the frequency, but that is a guess.

    The inductor will affect the amount of current generated and going through the LED. Basically you've reduced the efficiency a bit, but it doesn't matter much. Look for a choke coil, you can get closer to the inductance you need (again, if it works doesn't matter). They are about the size of a ½W resistor.

    You are looking at a primitive buck boost convertor.

    Did you look at my project?

    CMOS 555 Long Duration LED Flyback Flasher
     
  9. BMorse

    Senior Member

    Sep 26, 2009
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    I ended up using a 1000uf inductor and changed the 2.2 K to a 3.3K and used a .01 cap instead, and (when I first built the circuit I used all the same components, it could still light the 9 white LED's but they were a bit dim) after my alterations to the circuit (which wasn't much) all 9 lit up pretty bright! I have not tested it to see how long a AAA battery can sustain the circuit, but I have used the same circuit to boost my Solar LED lawn lights, and now I get a lot more than just 4 to 6 hours of light, and the lights seem much brighter than when I first purchased them....

    the cap value does affect the oscillation of the circuit (makes it slower) and the lower inductor value will lower your efficiency down to about half (or less) of what it should be.......

    My .02
     
  10. Razor Concepts

    Thread Starter Active Member

    Oct 7, 2008
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    Ok, so now I am experimenting with two variables - cap value and inductor value.

    Cap value, higher value (0.1uF) is a "litte bit" dimmer than .001uf.

    Inductor value, has a BIG impact on LED brightness. 150uH has a significant brightness increase compared to 470uH.

    So, the lower the inductance, the less efficient but more voltage?
     
  11. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    Both are adjusting the frequency and duty cycle. The LED isn't actually "ON" all of the time, but turning off and on between 20,000 and 50,000 times per second (20kHz-50kHz, roughly).

    Changing the capacitor and inductor values is changing both the frequency and the "Duty Cycle", or the portion of time that the LED is turned on compared to when it is turned off.

    When driving larger strings of LEDs, or when running a battery near the 0.6V level, the larger inductor is needed to create the needed voltage, while sacrificing duty cycle.
     
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