Dimming LED torch using LM3914

Discussion in 'The Projects Forum' started by foxonwheels, Jan 11, 2012.

  1. foxonwheels

    Thread Starter New Member

    Jan 11, 2012
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    Howdy. I am interested in beginning a project that I found at the following links:

    http://www.extremecircuits.net/2010/06/novel-white-led-torch_18.html

    http://hobby.dapj.com/2009/09/lm3914-dimming-flashlight.html

    Basically I want to use the LM3914 or something similar to create a white LED flashlight that in creases or decreases output by sequentially turning LEDs on or off. My only concern after reading the above links are that it doesn't seem the LM3914 is powerful enough to feed HPLEDs.

    What would be the best way to obtain a 500+ lumen output when all LEDs are lit? It doesn't matter to me if a few HPLEDs are used or a bunch (say 50-100) of less powerful LEDs are used. I was also thinking that a few LM3914's could be used together. I'm looking for a good place to start and my initial thoughts are that the LM3914 would be very helpful in this design. If I am going to wrong way with this, feel free to chime in. Thank you!
     
  2. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    My guess is that the LM3914 should work well with a driver for each
    output. How fine does the control need to be? ; another LM3914 could be added in series to give 20 steps. 'havent used an LM 3914 for several years so am a bit rusty on them.
     
  3. foxonwheels

    Thread Starter New Member

    Jan 11, 2012
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    The resolution, or amount of "steps" for adjustment, isn't terribly important at this point. I mean, at a minimum I would like 10 (1 LM3914 with an LED for each output pin). But more importantly, I am concerned with achieving a 500+ lumen brightness.

    It looks as if the LM3914 can output a maximum of 30mA per output. With 10 LEDs per IC, I would think I would need roughly 10 LM3914's to get near my desired lumen output with all LEDs in the ON state. Ideally, I would like to use no more than 3-4 LM3914 due to size constraints in the flashlight itself.

    So my main question is how can I use the LM3914 (or up to four of them) to power brighter, higher current LEDs and attain 500+ lumens?
     
  4. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    One IC will do with 10 P channel MOSFET's driving 10 bright LED's, assuming about 350 mA @ 3.6 to 4V giving 90 lumens ea. A possible FET is FDS9435, 35V, 5A, P ch, medium size surface mount, US $ .43 @ Electronic Goldmine. Power might be 3 " D " cells, or equiv. Current limiting resistors will depend on operating V selected.
     
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  5. foxonwheels

    Thread Starter New Member

    Jan 11, 2012
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    Wow, thanks for your help Bernard. I didn't think it would be so simple. I'm assuming the IC's outputs would go to the gate of each FET?
     
  6. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    The FDS9435 is for power supply for 10 to 15 V, with 4.4 to 6V,a logic level FET is needed, convenliently? suplied by praondevou ref to IRLML2246 TRPBF in SOT 23 pkg. 20V, 2.6A logic level from Digi-Key. The outputs on the LM3914 act like open collector transistors, so need a load to keep from having floating gates, R2-R11. Control of illumination is via variable resistor,VR, to allow a 0 to 1.2V signal to input. R22-R31 depends on LED's selected & operating V.
     
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  7. Bernard

    AAC Fanatic!

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    Might brouse the two posts on this page, at least for now.
    P Channel MOSFET Switch & pmos as a switch
     
  8. foxonwheels

    Thread Starter New Member

    Jan 11, 2012
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    Bernard, I can't thank you enough. So R12-R21 connect to the gates, correct? And those values are 100, not 100k?
     
  9. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    Yes & 100 Ω.
     
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