Simple LED 3.7V circuit?

Discussion in 'The Projects Forum' started by Ludilo, Mar 19, 2015.

  1. Ludilo

    Thread Starter New Member

    Mar 19, 2015
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    Hello everybody.
    I'm complete newbie to electronics, yet I would like to build simple and effective bicycle light.

    The components are:
    1. 1 x 3.7V, 350mAh, 25C LiPo battery
    2. 3 x 3.4V, Ultra-bright, 5mm, 18000mcd, 20mA LEDs
    3. 1x 22 Ohms resistor
    4. 1 x latching switch

    My questions are:
    1. What is the optimal resistor value and setup (Serial, Parallel.. common R, separate R)?
    2. How to keep the voltage steady for 3.4V LEDs (which should be 3.4V, I ques)?

    The battery is 3.7V, but when full, it is almost 4.2V, and off course,when connected to LEDs, voltage drops after some time well bellow 3.7V.
    Also, with 4.12V, full battery and 22 Ohms resistor, the voltage across the resistor and one LED is 4.06V, more or less.

    So, my question is how to keep the voltage steady and keep LEDs working on right voltage while having optimal battery efficiency?

    Thanks.
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    You operate LEDs at constant current, not constant voltage.

    An appropriate series resistance is (4.2V - 3.4V)/20mA = 40Ω

    If the battery voltage drops to 3.8V, then (3.8V - 3.4V)/20mA = 20Ω

    So use a separate 22Ω resistor in series with each LED. Then connect each resistor and LED combo in parallel cross the battery.

    Three LEDs will draw 3 x 20mA = 60mA.
    Your 350mAh battery will run for 350mAh/60mA = 6 hours.
    The LEDs might stay bright for less than that, about 3 hours.
     
  3. Ludilo

    Thread Starter New Member

    Mar 19, 2015
    16
    1
    Hi MrChips.
    Thank you for the reply.
    I have to buy some 22Ω resistors and try them.
    Should I be worried about the initial 4.2 voltage?
     
  4. MrChips

    Moderator

    Oct 2, 2009
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    If the battery voltage is truly 4.2V then try two 22Ω resistors in series.
     
  5. MrChips

    Moderator

    Oct 2, 2009
    12,405
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    For a bicycle light, consider using a simple CMOS 555 timer circuit using LMC555 or TLC555.
    Your battery will last twice as long and you wouldn't need series resistors.

    Mind you, just using series resistors for continuous illumination is simple enough.
     
  6. Ludilo

    Thread Starter New Member

    Mar 19, 2015
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    Well, the battery is 3.7 volts, but once full it "starts", with 4.2 volts.
     
  7. wayneh

    Expert

    Sep 9, 2010
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    This will be plenty bright for a "be seen" light, but not adequate for illumination of the path in front of you. I hope that is your plane?

    Since the goal is to be seen, I'd consider MrChips suggestion to flash the LEDs. This tends to be attention-getting and saves power by virtue of the LEDs being off when not actually flashing. I believe flashing at 10Hz gives a higher perceived brightness because of the way our eyes and brains work.
     
  8. Ludilo

    Thread Starter New Member

    Mar 19, 2015
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    Hi wayneh.
    Thank you for the advice.

    Actually, the purpose should be to illuminate the path.
    I have tested 3 LEDs with 3.7V LiPO battery and it seemed OK (in a dark room), but I haven't tried it in the "action", on the road...
     
  9. wayneh

    Expert

    Sep 9, 2010
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    Hmmm.... I think ~200mW of LED lighting is not considered enough for that application. It would be far better than nothing, but still not what is considered safe. I'd look at the commercial products to see what they use, as a guideline. More. Still more.

    Looks to me like 1-2W is more typical for a headlight.
     
    Last edited: Mar 19, 2015
  10. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    I'm with wayne on this..
    You need more light to be of any real use..
     
  11. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Powering a 3.7V LED from a 3.7V is problematic.
     
  12. Ludilo

    Thread Starter New Member

    Mar 19, 2015
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    Hello Mike.

    The LED is 3.4V, 20mA.
     
  13. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Ok, powering a 3.4V LED from a battery that has this discharge curve is problematic. There is not enough voltage drop (headroom) across the current-limiting resistor to provide proper current regulation.
     
  14. Ludilo

    Thread Starter New Member

    Mar 19, 2015
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    So true...

    With LiPo battery fully charged, almost 4.2V and with 3 x 3.4V LEDs, 2 x 22Ω resistor I still get 4.12V in the circuit.
     
  15. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    you could use a J112 fet, in constant current mode, works from 3- 30 v. Or buy a ready made one on the web...
     
  16. Ludilo

    Thread Starter New Member

    Mar 19, 2015
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    Hello Dodgydave.
    Thank for the reply.

    However, I have to ask you: What is J112 FET and what does it do..?
    As stated earlier, I'm complete newbie with LEDs and battery.
     
  17. Dodgydave

    Distinguished Member

    Jun 22, 2012
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  18. RichardO

    Well-Known Member

    May 4, 2013
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    The low power setting on my bike light is about 3 watts. This is enough light to be usable but not at full speed. To ride at full speed (about 15 MPH) or in areas where I am riding in and out of pools of light from street lights I use about 12 watts. 12 watts is a lot of power for extended periods from batteries. That is why I have 3, 6 and 12 watt settings. For instance, your battery would last less than an hour.

    Just as important as the power to the LED's is the optics. The beam has to be tight -- just a few degrees wide. And, don't waste light higher or lower than what you need in order to see far enough away to avoid hazards.
     
  19. Ludilo

    Thread Starter New Member

    Mar 19, 2015
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  20. BobTPH

    Active Member

    Jun 5, 2013
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    Have fun. I hope there are not potholes in front of you. I broke my collarbone that way, riding at night with insufficient light. This was when I was eighteen and could still see well.

    Bob
     
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