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  1. #1
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    Custom 1A Boost Driver for 1400 Lumen Light

    I've been meaning to build a bike light for a while, and I finally did as an entry to the Texas Instruments Analog Design Competition. The requirements state that you must use three "analog" parts from TI, which is almost anything except digital logic. I based my design around a TPS61500 LED driver. It is boost driver, so it steps the voltage up from 12V to the ~19V the series of LEDs needs.

    I haven't put everything in the enclosure yet, but the board layout and assembled board for the driver are shown below. It is small and measures just 2.2x1.15 inches and could be made smaller with a real 2-sided board.

    I am using six Luxeon Rebel neutral white LEDs, rated for about 237 lumens each at 1000ma. The light right now is bright, but I am a little skeptical that the LEDs are being driven at the full 1000ma. I will have to take more measurements, but I can say right now that it is close.

    The power source for now is a 12V 8Ah sealed lead acid battery. It's heavy, but it fits in my camelbak and will run the light for more than four hours at full power. The current draw at full power at 12V is 1.67A, which works out to about 87% electrical efficiency.

    Other features, accomplished with other TI ICs, are a 50% power setting and a low battery indicator (which might not be working).

    I will post more pictures when it's done.
    Attached Thumbnails Attached Thumbnails Custom 1A Boost Driver for 1400 Lumen Light-p1020735_e.jpg  

    Custom 1A Boost Driver for 1400 Lumen Light-layout_edited.png  

    Matt

  2. #2
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    Good bit of work Matt. Have you made the unit programmable, or is it fixed?

  3. #3
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    that's really impressive! Just wait until you step up to some newer LEDs with higher Lm/W and you'll be even more amazed at the light output. You might even be able to get by with using the 50% setting more and carry a smaller 12V battery around to save some weight.

  4. #4
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    It used fixed resistors to set the 50% level. I could replace the resistors if I want.
    Matt

  5. #5
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    This is really cool! What seems really cool about that driver is the integrated mosfet rated for 3A. Most of the drivers I have seen require a mosfet or two external to the chip, though I have not seen that many. On your design, I see the driver, a 555 timer (PWM) and the other chip must be a comparator or similar for the battery gauge. If you did not require 3 TI components you could get by with one PIC to do the job of the 2 components and have some extra pins for other stuff like a temp monitor and tri-color status led.

  6. #6
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    The MOSFET is rated for 3A, but it can't actually drive LEDs at 3A. Using the component values I have, the max current should be about 1.6A, and even under ideal conditions it would be hard to get the output current over 2A. I originally wanted this to be a Cree XML driver, but I was confused by the 3A rating. Also, because this is a boost converter and the output voltage has to be higher than the input voltage, I would have needed a series of 5-6 XMLs to run off my 12V battery. I think that would just be too much light.

    A TI microcontroller is actually an option as one of the qualifying chips, but I didn't have time to learn how to program it. I had about a week and a half to design, build, and write a report about this project. I agree that a microcontroller would allow me to add some nice features.

    The other chips are a 555 for dimming and an op-amp for the low battery indicator.
    Last edited by m85476585; 05-16-2011 at 09:34 PM.
    Matt

  7. #7
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    This light is going to be awesome! When I plugged it in during testing I was skeptical about the light output because I had nothing to compare it to, but I just tried it next to my old 180 lumen LED light, and the difference is incredible!

    I have it assembled enough to do some test rides with, but I don't have the dimming or battery indicator hooked up yet. Unfortunately that means I have to drill another hole in the case for the indicator LED since I forgot about it. Dimming isn't hooked up because my one mistake in the design was not putting a pull-up resistor on the enable pin, so I need to do that (replaces the yellow wire) then I should be able to directly connect the PWM to the enable pin as long as it can still drive it low with the pull-up resistor on there.

    For $42 worth of LEDs I could upgrade it to 1920 lumens, but I don't see the need to do so right now.

    I'll try to get some beam shots later, but it's 35 degrees and raining outside right now.

    Does anyone have a suggestion for a way to mount this on my bike? I was just going to use one of these
    http://www2.giant-bicycles.com/EN-PA...ies/329/26300/
    since my LBS sells them, but it would be nice to find something better. A stem mount would actually be ideal since I have carbon bars.

    For size reference, the box is 4"x2"x1" and the power connector is standard BNC.
    Attached Thumbnails Attached Thumbnails Custom 1A Boost Driver for 1400 Lumen Light-imag0237es.jpg  

    Custom 1A Boost Driver for 1400 Lumen Light-imag0242es.jpg  

    Matt

  8. #8
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    I was gonna suggest the cateye mounts, but their small parts store seems to be down or gone. Since the case has a nice flat bottom there should be plenty of options for mounting, right down to epoxying some of the square cable tie mounts with the double sided tape and just using zip ties to hold it on - I'd wrap part of an old tube around the bars, then the cable ties so it does not slip if going that route.

    Are you gonna put optics on that or run it au' natural?

  9. #9
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    Just been reading the technical data on that TPS61500 driver - very useful. You say it won't run an LED chain at 3A though? The datasheet does include a fairly complex formula for calculating the output current.

    Shame they don't do a buck version of this driver.

  10. #10
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    If the output voltage is very close to the input voltage is is possible to get close to 3A, and it may be possible to reach it. The formula looks complex, but all the values are known or can be estimated worst-case (like efficiency).

    I'll probably run it without optics. One of the LEDs is really lacking optics since the little silicone lens fell off. In previous experience I found Rebel lenses to be somewhat robust, so I'm not sure what happened.
    Matt

  11. #11
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    I just soldered in the switch and pull-up resistor for PWM dimming, so now I lave a low and high mode. Low looks at least half as bright as high, but only draws half the current-- 0.78A instead of 1.6A. It will also help a lot with heat dissipation. At that rate, the light will last nearly 10 hours on low with the battery I have. I could get a smaller battery that would be half the size and weight and still have plenty of run time on either setting, or I could rig up a helmet light. The battery I have weighs 8lbs but surprisingly enough it can be shoved into my Camelbak Lobo. I don't mind carrying it since I carry a lot more weight when I commute.

    It's still cold and raining, and it might snow later, so no beam shots or test rides today.
    Matt

  12. #12
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    nice job !
    it's on my todo list, for LED drivers.
    TI makes some nice stuff, but national, seams to be easier on the design.
    suggestions welcome. it's like mice, everybody got a different one, some last longer than others.

  13. #13
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    Quote Originally Posted by m85476585
    I'll probably run it without optics. One of the LEDs is really lacking optics since the little silicone lens fell off. In previous experience I found Rebel lenses to be somewhat robust, so I'm not sure what happened.
    Optics will transform the light completely. Without optics you will be cycling inside a small bubble of bright light, unable to see very far at all. The nearfield around the front wheel and for a few metres in front will be very bright but with no real light permeating outside this bubble. Put on an optic and things will be completely different - you will be putting the light where it is needed, more lumens projected away from you giving an even transition of constant light from near to far. Maybe get one and hold it in front of the LEDs to see the difference?

  14. #14
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    I went for a test ride today and you are right, it needs optics. The light is so bright that the beam spill on the ground directly in front of me ruined my night vision, and not enough light was projected farther away.

    I bought one elliptical lens with the light (although I thought I meant to get two). If I get more lenses, what angle should I get? Here are the angles available:
    18
    22 (frosted)
    27 (frosted)
    44 (frosted)
    41x19 elliptical
    Matt

  15. #15
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    Well, now that DST ended I figure it was time to get this light working. I bought two of each lens in 18, 27, 44, and 41x19 elliptical. The elliptical lens has a nice beam pattern on the wall but I would either have to rotate the LED stars or mount the light vertically.

    I'll try to do some beam shots later tonight if I have time and if I can find a dark location. For reference I have a Minewt 250 and a Princetontec Apex.

    What's the best way to secure these lenses for riding? I want it to not be permanent, but I need them to not wall out while I'm riding. Maybe some RTV silicone? Or hot glue (or would it melt from the heat of the lights)?
    Matt

  16. #16
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    I took some beam shots as promised. My camera lacks manual control, so the best I could do was F3.3, 1 second, ISO 400. I also couldn't find a large dark area. In order of brightness, the shots are: Reference, Niterider Minewt 250, My light (low), My light (high).

    I'm using the narrow lens.
    Attached Thumbnails Attached Thumbnails Custom 1A Boost Driver for 1400 Lumen Light-p1030988.jpg  

    Custom 1A Boost Driver for 1400 Lumen Light-p1030993.jpg  

    Custom 1A Boost Driver for 1400 Lumen Light-p1030991.jpg  

    Custom 1A Boost Driver for 1400 Lumen Light-p1030989.jpg  

    Matt

  17. #17
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    It looks like my camera setting is in fact equivalent to the MTBR standard exposure:

    1 sec F3.3 ISO 400 = 4 sec F3.3 ISO 100 = 6 sec F4 ISO 100.

    Only difference is auto white balance, but I could change that.
    Matt

  18. #18
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    That's a good amount of light! Nice work.

  19. #19
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    I went for a ride with this light on Friday. I kept it on half power the whole time because full power was too bright. It worked well, but it could use more beam width to see around corners. The light just about overwhelmed my Princetontec Apex headlamp. I think I'll try to install one or two of the elliptical lenses I have.
    Matt

  20. #20
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    Great scot!! What a nice set up and powerful beam !
    I'd JB weld the lenses, modify the holders to fit on the stars and never worry about losing a lens during your ride.

    Good job, what about waterproofing ?

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