Low V led?

Discussion in 'General Electronics Chat' started by Bobbed750, Nov 2, 2013.

  1. Bobbed750

    Thread Starter New Member

    Nov 2, 2013
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    I am needing to know how I could make an led light turn on when voltage on a 18650 battery goes below 3.2v

    The battery will be at 4.0 volts fully charged.

    Thanks in advance guys!!
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    There are many LEDs that have a Vf of ~1.9V
     
  3. Bobbed750

    Thread Starter New Member

    Nov 2, 2013
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    I'm pretty new to this whole electrical thing, Could you put it in layman's terms for me?

    I'm building an electronic cigarette "mod" and need the led to indicate when my internal battery drops below 3.2v, so I know when to charge it without having to pull it out and put a multi-meter on it. Will be a sealed unit with usb charging,
     
  4. bertus

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  5. Bobbed750

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    Nov 2, 2013
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    The led that I'm tryng to use has a forward voltage rating of 1.7-2.6V
     
  6. MikeML

    AAC Fanatic!

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    Then it should be possible to choose a current limiting resistor such that you have less than a two-to-one change in LED current as the battery discharges.

    What is the current rating of your LED? How bright do you want it to be? How long do you want the battery to last?
     
  7. Bobbed750

    Thread Starter New Member

    Nov 2, 2013
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    The current is rated 28mA, Near full brightness would be good since its only a 0.8MCD LED.

    I really appreciate your help guys
     
  8. tubeguy

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    Nov 3, 2012
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    If you want an LED to turn ON below a certain voltage, a comparator circuit should be used.
     
  9. Wendy

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    Mar 24, 2008
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    Another possibility is a joule thief, which will light any LED from 1.5VDC. They are very common, just Google around for the plans. They use one transistor, a couple of caps and resistors, and a coil.

    LEDs, 555s, Flashers, and Light Chasers
     
  10. Bobbed750

    Thread Starter New Member

    Nov 2, 2013
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    Pardon my ignorance, But I have no idea how to design said circuit.
     
  11. Bobbed750

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    Nov 2, 2013
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    Thanks Bill, I will check it out
     
  12. Wendy

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    If you need more help just ask. No one is born knowing this stuff, and I like what I call do nothing circuits.
     
  13. GopherT

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    Mechanical equivalent of a "do nothing circuit".

    [​IMG]
     
  14. Austin Clark

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    Dec 28, 2011
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    It may be easier to string together a couple LEDs to get close to 3.2 Vf, and then connect that, through a resistor, to your battery. Then, when the batter falls under 3.2V and the LEDs go out, or when the LEDs rapidly start to go dim, you'll know you need to charge. Maybe connect a "test switch" so that the LEDs aren't always on when the battery is charged and needlessly draining it.
     
  15. John P

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    I believe that this circuit will work if you choose the resistors properly, but it's crude and inefficient, which may be a problem if you're using battery power. Also, the LED would light up gradually, getting brighter as the battery voltage drops.

    https://dl.dropboxusercontent.com/u/28291527/circ131102.jpg
     
  16. Wendy

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    There are much better out there. For example, a quick search on Google found this...

    Make a Joule Thief
     
  17. Wendy

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    Really? Given that newer generation LEDs start at 2.5V Vf, and go as high as 3.6V Vf or more it is really impractical. Older LEDs drop a lot less due to different materials, but trying to match a voltage to LEDs suggests you think you do not need a current limiting resistor, which is far from the truth. The old generation of LEDs are rare too.

    I would suggest reading the link in post #9.
     
  18. John P

    AAC Fanatic!

    Oct 14, 2008
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    Note that the assignment here is to turn on an LED when the voltage drops, not turn it off! The Joule Thief is fine, but what's its function here?
     
    tubeguy likes this.
  19. tubeguy

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    Nov 3, 2012
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    I second that. :)
     
  20. ian field

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    The shops around my way have Vapourlires or E-lites, the Vapourlites already have a function for the blue LED in the end to flash when the battery needs charging - the E-lites effectively cost double because you have to buy the USB charging bit separately, so I haven't tried those.

    There is possibly a question mark over the reliability of the suction sensor in the Vapourlite - especially if you refill from bottles of the fluid instead of buying expensive refill tips (the fluid can leak into the battery compartment and contaminate the suction sensor) I have stripped a couple and found that the battery charge control chip can be used with bigger/other lithium cells.

    A quick google came up with various third party suppliers offering tips with various different resistance - allegedly affects the flavour. I'd be interested to see any discussion that develops on which tips/batteries etc are compatible with what.
     
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