EEVblog #67 – Hacking the Princeton Tec EOS LED Headlamp with a Cree XPG

Dave does a teardown of the Princeton Tec EOS headlamp and mods it with a new Cree XPG LED.
Throw in a quick reverse engineer of the circuit, gratuitous scope shots, torturing a thermistor, a practical demonstration of why you really do need 5 multimeters on your bench, and as usual Dave finds no problems filling up 25 minutes.

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  1. Where do you buy the cree leds?

  2. What happened before 22:01 Dave? You suddenly look either a bit distraught or just plained knackered.

  3. You draw bipolar transistors but you refer to the gate.
    What kind of transistor do you actually think it is?
    Would a synchronous converter actually be more efficient than a diode when using bipolar transistors?

  4. Ack! You didn’t show the mod. What actual resistor value did you use on the high beam?

  5. I have to say this is my favourite blog so far, I really like the concept of taking a product finding out how it works and improving it.

    Although more details on the actual mod would’ve been great it was an enjoyable watch and I hope to see more like this in the future.

    Thanks Dave keep up the good work 😀

  6. I like those cree xpg leds and you should give me one. Is it just me though or did the old leds have a more neutral white compared with the cree xpg’s bluish fringe when you were showing them on the wall.

    • I noticed that as well, although it’d hard to tell from this recorded footage what “actual white” is since we don’t have a metric for the camera’s white balance.

      Great vid! It’s always exciting to see a product reverse engineered.

      • The cutting edge, most efficient LEDs always have quite a cool white tint. As the technology matures they add neutral and warm white tints. The XP-G is known to range between bluish white and greenish white currently, although the neutral and warm variants are on their way!

    • The XP-Gs are only just becoming available in warm/neutral white colours.

  7. Dave,
    Best blog yet! I love taking things apart to see how they work and then learning how to mod it to my own liking. You MUST do more of these videos showing your evaluation of the how the circuits work and how to measure and mod things. SUPER!

  8. Nice episode! 🙂

    I have a question thought, I’ve always wondered about these on chip ICs that looks like they are melted onto the board.

    Are they “normal” surface mounted ICs under there? Do they use a standard PIC,AVR,MSP430 under there or is that some ASIC?

  9. Awesome, this was most informative. Please do more like these.

  10. If you are interested in very wide beam headlamps/flashlights (120

  11. Another lame video from Dave. tsk tsk. LEDs? WTF man, this place is turning into hackaday. First dave was crushing on the arduino, then he shilled for microchip pic products. Now he’s playing with LEDs. It’s all down hill from here.

    Most people would spill a little bit from a 40Oz bottle of malt liquor for all the dead homies, dave would just pull the leds out his bug box and stick them in a block of cheddar cheese.

    • Clearly LED technology is way over your head buddy. You’re ignorant, at best.

      The point of this blog entry is to share a thought or two on consumer products, personal choice and how to enhance them should you need to, theres no holy grail — you’re expecting too much from a blog, clearly you need to get out more and live — theres a world out there, experience it for once.


      • Dude, do you always write some blabber like this every time you see a post you don’t like? Sheesh, looks like someone has breathed in too much zinc chloride at the bench.

  12. @billyg Nice try, troll

  13. Dave,

    This is something I thought you would be interested in… I also would like to know if it really would work. Perhaps you should try it! 🙂


  14. Looking forward for more blogs like this one.

    Something a little more technical would be nice as well.

    • I’m looking forward to more entries per week and Dave sharing more tips, perhaps starting a few community projects as well. There are some interesting fellas around eevblog, so it wouldn’t go to waste at all.

  15. @eraser Wow. How did I not make that connection!? DOH!

  16. Can you put up a white board shot on how you chose your resistors?
    Fantastic video! I love when you show me how to do something or explain a design.

  17. If your into this sort of modding you should check the candle power forums. Its a forum of flashlight nerds who do nothing but this sort of thing and I really do mean nothing but.

  18. Awsome blog.
    I love you ‘teaching’ blogs in the sense of when you explain stuff and why you think it was designed as it is, it’s pros and cons etc…
    This is so useful especially if you do electronics mainly as a hobby and not as a professinonal.

  19. WOW Nice job!

    I love hardware hacking, and when I saw what you were doing I had to check it out.

    Man, what excellent results too!
    A more even and “floody” kind of light really is much more useful in more generalized outdoor situations and this does the trick!

    Add to that perfectly logical reasons to own more meters and this whole thing is just absolute WIN!

    As always you rock, Dave!

  20. I love Dave’s Videos Blogs and agree this episode definitely among the ‘best of’ — probably because it’s both accessible and of value to wider audience.

    Dave is among a very small minority of people creating Video Blogs that are actually worth watching: he’s well qualified with respect to what he’s talking about; and his enthusiasm and presentation are fantastic — I’m surprised networks haven’t discovered him yet.

    The amount of time and effort that goes in to making a Video Blog is no small thing, I hope others appreciate how much goes into this.

    Dave reminds me of some of my best Teachers and Professors that made paying for a very expensive education worthwhile; I’d be donating to his show if I was currently employed!

    • Thanks for the support Hoak.
      Yes, the video blog takes up more time than I originally thought it would, and it would probably be almost a full time job if I actually put more effort into it. i.e. actually prepare and write scripts and material, and took care in filming, presentation, and editing etc instead of my current off-the-cuff approach.


  21. Great blog but you missed one of the best design features of this light. The adjustment bracket on the headband is designed to be used as a screw driver for loosening the thumb screw on the battery compartment. How cool is that.

  22. So what do you do to replace those welded plastic things? Epoxy? Hot glue? New plastic? Those have always bothered me.

    • I didn’t bother, I just the whole assembly sit loose in the housing. The battery door takes care of holding it all in place, and it doesn’t really pop out when I open it.


  23. One other trick that they designed into the thumbscrew: the buckle on the headband is the perfect size for turning the screw. No need for a screwdriver! (and I remember reading somewhere that this was on purpose, not just a coincidence)

  24. Dave,

    Did you do anything to the original reflector to get that floodish beam pattern? Was that an older design of headlamp or the newer more diffused style lens?


  25. Dave,

    Did you do anything to the original reflector to get that flood…ish beam pattern? Was that an older design of headlamp or the newer more diffused style lens?


    • No change to the reflector. I’ve modded both and old and new model and both gave very similar wide beam patterns. Then lens was obviously closely matched to the original LED shape and pattern. The XPG has a different shape and pattern, so obviously is not as effective with the original lens, but I do like the resultant pattern.

  26. Did you consider that a better buck converter would be worse?

    A “perfect” converter would maintain full light output untill you are somewhere halfway a trip in a dark cave and then suddenly die completely.

    An even better converter (such as this one) would keep an even light output untill the bateries are almost flat and then slowly get dimmer. And you’ll think “Well, I didn’t get as much light on my last trip, maybe I’ll put some fresh bateries in it.

    It also would have been nice if you did a measurement on the sleep current. The headlight I use has a real on/off switch and I use it on about 2 or 3 trips per year. I think I replaced the bateries only once in the 5 to 7 years I have this light.

  27. Nice blog! How does the modified headlamp behave on the long run ? (batteries last the same? no heat problems for longer usage?)

  28. At those low voltages (3V…) BJTs can be more efficient that MOS, for instance Zetex makes some ultra low Vcesat transistors which would work well here.

    Also the optics look like standard optics, possibly from Ledil. The holders actually need to hold the optic at a specific distance from the star PCB, and this distance depends on the LED. Some LEDs sit a little further from the star PCB than others and the holder needs to compensate for this. Alignment needs to be quite precise. Using the wrong holder (like you did) can yield interesting effects if you’re lucky (like widening the beam) but if the LED ends up in the wrong spot you can lose almost all power to internal reflections in the lens.

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