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  1. #1
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    Fscking magnets, how do they work?

    Put this little guy together for a 500km road ride earlier this month, and to test a few ideas.

    Lightweight helmet light that will run all night, easy on and off the helmet, front and rear low power lights always on. Main beam is not particularly bright but enough to see over my main light (B&M Cyo) which has a sharp cutoff, and enough to limp home on if needed (or to the gas station for more AAs!).

    I had most of the stuff hanging around. Main LED is a low bin XPG, very warm colour. 10mm narrow carclo optic. 350mA micropuck driver wired in buck configuration will run from just under 4 to 9v. Current drops off significantly around 4v and turns off around 3.6V so it's ideal for a 4 cell NiMH pack (won't toast the batteries), but will run off a MS pack too.
    Main front led is switched. There is also dual 3mm LEDs front and back which are on all the time. Had planned to do a flashing rear with a bloody bright XPE but realised how blinding this would be for anyone behind me, plus no room for a 555 circuit in there. Resistors chosen to keep the LEDs within spec at 9v. Reds are in series and whites parallel. Front light is bright enough for reading computer and looking for bottles etc and wont blind fellow riders too badly.

    Mount is magnetic. 4 strong little guys from Jaycar set in pairs with araldite. I'm using a cateye mount but might glue the magnets direct to the helmet. Really happy with how well it works, snaps on and holds really well. I have a Velcro loop on the cable as a backup but can't see it coming off on the road (branches would be a problem on the MTB).
    The driver doesn't like being too close to the magnets, I had to sand the driver casing off a little so I could jam it behind the switch to give a little distance.

    Current draw with 5V battery is 35mA low/300mA high so on 4 AAs its good for all night if you don't use the main beam all the time. Drained about 1500mAh from my pack using the light from 9PM to 7AM. With a MS battery good for a few nights.

    Weight... about 50gms on the helmet but half of that is cateye mount. Hmmm, time to get the crabon cloth out maybe.
    Battery and wire add around 140gms.
    Attached Thumbnails Attached Thumbnails Fscking magnets, how do they work?-c54.jpg  

    Fscking magnets, how do they work?-c53.jpg  

    Fscking magnets, how do they work?-c52.jpg  

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  2. #2
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    Buck/boost etc drivers utilise an inductor for power conversion. The inductor DOES not like being next to strong magnets that will basically saturate/detune etc the inductor.

    NEVER put strong magnets near inductors unless you want to potentially destroy your driver.

    cheers,
    george.

  3. #3
    Light freak
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    Digging the magnet idea!


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  4. #4
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    Quote Originally Posted by georges80
    Buck/boost etc drivers utilise an inductor for power conversion. The inductor DOES not like being next to strong magnets that will basically saturate/detune etc the inductor.

    NEVER put strong magnets near inductors unless you want to potentially destroy your driver.

    cheers,
    george.
    Yep! As I said I'm testing a few ideas. Might do a weekend long bench test
    Will be interesting to see how it works out long term. The micro puck is 1.5-2cm away from the magnets.
    I did a little bench testing and found that the output visibly dropped off within 1cm of the magnets, more so with the micropuck rotated one way. Mounted accordingly.
    Next build will be 4-5cm... and I'm using a nicer driver
    DIY LED Bike Lights:
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  5. #5
    www.hahntronix.com
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    I'd have to agree with georges80, the magnet could very well saturate the core of your driver's inductor. Everything else will go to hell after that. Inductor heats up. Resistance of inductor may change. You get really weird uncontrolled amounts of current thru the LED.

    I think Chester Gould (author of original Dick Tracy comics) said it best: "The nation that controls magnetism will control the world."

    I'll have to try the experiment of holding a Neodymium magnet near the inductor of one of my buck drivers some time and see how the current thru the current sense resistor changes. I suspect it will mess things up.

    I've been experimenting with the Dual-Lok velcro that Scar uses lately. Will have to post some of my ideas soon, since I'm getting out of the little lights that go on a helmet market.

    Mark
    Nimium est melior!

  6. #6
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    yeah, that's an awesome idea! I commute a lot so I want something that I can easily take off and take with me when I make small stops for errands. With a nice light, you just want to be sure that some jackass doesn't just swipe it off your bike, but it makes each stop more of a hassle. I've begun carrying all my repair kit things in my messenger bag so that I don't have to unclamp a saddle bag. Magnet lights- killer idea!

    The magnets out of hard drives are incredibly strong. Maybe too strong for this application (both interfering with driver, and too strong to easily remove), so maybe some from a 2.5" or even a 1.8" drive.

    I'm not too familiar with magnetic fields, but could you put a magnetic strip between the mounting magnet and the driver to counteract the mounting magnet so that the driver is not interfered with?

  7. #7
    Light freak
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    EMI shielding tape?


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  8. #8
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    Quote Originally Posted by scar
    EMI shielding tape?


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    brilliant

  9. #9
    Light freak
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    If George says it is not a good idea, then it is not a good idea.

    I just like the "thinking outside of the box"!


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  10. #10
    removing nudity
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    Love the idea of both the light and the magnets but I have to ask, just how much heavier was the other half of the cateye mount compared to the 4 magnets and epoxy?

    Nice work znomit

  11. #11
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    Did some somewhat more formal bench testing this afternoon. Attached is the charts showing output vs distance to the magnet (I'm at 12mm with this build).
    I set up the four magnets 2S2P like in my build and also 4S, pointing at the front and side of the micropuck.
    Front on theres a definite nasty band where the output drops significantly.
    Magnets here.
    Attached Thumbnails Attached Thumbnails Fscking magnets, how do they work?-c5mag.jpg  

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  12. #12
    Five is right out
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    Some materials shield magnetic fields. I have no idea what or how they work, but I've ripped apart a dozen or so hard drives recently. All of them use neodymium magnets, and sometimes these magnets have a metal backing on one side which completely stops the field of the magnet.

    The ones that I pulled out of my hard drives are probably a bit heavier than the OP requires though. The magnet is the smaller piece of metal. The shielding is the larger piece under it.
    Attached Thumbnails Attached Thumbnails Fscking magnets, how do they work?-magnet.jpg  


  13. #13
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    For the errand runner, weight isn't a large factor but all the time dismounting everything is.

    Will pull HD magnets. Very interesting idea. Wonder if there is significant field leakage around the shield, enough to interfere with the inductors in Georges drivers?

  14. #14
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    Quote Originally Posted by scar View Post
    EMI shielding tape?
    EMI Shielding tape is usually made of non-magnetic conductor (copper). It will only shield from alternating magnetic fields, not permanent ones.

    You could mount a ferromagnetic shield plate between the magnets and the aluminum housing. That will shield your driver from most feld lines, and also increase the holding power of the magnets (if you put one magnets North and one magnets South pole to the ferromagnetic plate). This is called flux shunting - your force the field lines to go through the ferromagnetic plate instead of the space behind it (where your diver is).


    PS: Be careful when playing with the forces of magnets, or else...

  15. #15
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    Quote Originally Posted by pe2erwin View Post
    EMI Shielding tape is usually made of non-magnetic conductor (copper). It will only shield from alternating magnetic fields, not permanent ones.

    You could mount a ferromagnetic shield plate between the magnets and the aluminum housing. That will shield your driver from most feld lines, and also increase the holding power of the magnets (if you put one magnets North and one magnets South pole to the ferromagnetic plate). This is called flux shunting - your force the field lines to go through the ferromagnetic plate instead of the space behind it (where your diver is).


    PS: Be careful when playing with the forces of magnets, or else...
    He's only got a mild case, your pic made me look up the kid in the metro paper from a couple of weeks ago..
    He can't even cook or he could get burnt..won't be long until he can take on magneto from xmen

  16. #16
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    Quote Originally Posted by womble View Post
    Some materials shield magnetic fields. I have no idea what or how they work, but I've ripped apart a dozen or so hard drives recently. All of them use neodymium magnets, and sometimes these magnets have a metal backing on one side which completely stops the field of the magnet.

    The ones that I pulled out of my hard drives are probably a bit heavier than the OP requires though. The magnet is the smaller piece of metal. The shielding is the larger piece under it.
    I'd be exceptionally careful about putting HDD magnets on top of your laptop as they're more than capable of interfering with the bits on your HDD.

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