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PostPosted: Wed Jun 10, 2009 2:41 pm 
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Okay, so it sounds like I'm more or less right about the mechanism. :)

What I don't get is why attaching a permanent magnet should help at all. A moving permanent magnet will induce a small voltage in the loop as the motorcycle moves over it, but that can't possibly be what the sensor is looking for--it's looking for a change in inductance. The point is to pick up stopped objects, and a stopped magnet is indistinguishable from a stopped non-magnet insofar as inductance is concerned.

Put another way, I think you'd get the exact same performance if you attached an identically-sized block of non-magnetic material to the bike. Its own magnetism can't matter.

(One might be able to design the circuit to detect a magnet moving over it in addition to inductance change from stopped cars, but the effects are definitely separate.)

-D


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PostPosted: Wed Jun 10, 2009 2:50 pm 
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Interesting, wikihow even has a link to a discussion on snopes about the issue:

http://message.snopes.com/showthread.php?p=330814

In that discussion, an engineer for a company that builds the things seems to refute the idea that a permanent magnet should help at all.

I stand by the possibility of generating an active signal electronically to trigger the light. That ought to work if done properly.

-D


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PostPosted: Wed Jun 10, 2009 5:02 pm 
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Around Phoenix this is a big problem. (Maybe our road crews are still thinking in terms of burros and dirt roads) Anyway, those of us riding scooters around town have had some success by salvaging the magnet from a large base speaker and mounting it to the underside of the scooter/bike. You might be able to pick one up for free at a local repair shop.


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PostPosted: Wed Jun 10, 2009 5:15 pm 
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Wether it should work or not, a lot of us can verify that there are lights that we can't trip without a magnet, but can trip with it.

And you're right: it _shouldn't_ work. But to a degree, it actually does. I fought the idea for six or seven years before finally giving in (I didn't tell anybody at the time, of course ;) ). I was astonished at the results! :shock: While I couldn't trip every light, I was suddenly able to trip almost half of the "known to hate motorcycles" lights I put on my test route!

I came home, took the magnet off, and re-ran the route again (I had ridden it before as well), and again-- I couldn't trip any of them.

I went home, put the magnet back on, and I've run one ever since.

But again:

There is absolutely no reason by physics why it _should_ work...

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PostPosted: Wed Jun 10, 2009 5:49 pm 
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Hmm, interesting. I can think of two possible explanations why it might work despite the physics:

(1) It could be that the magnet material itself simply has a high magnetic permeability. In this case, the presence of the magnet would change inductance enough to trip the sensor--but not because it's magnetic! Alas, I know nothing about the permeability of these materials, so I can't really tell whether this is plausible or not. Or,

(2) The circuit is wired to detect voltage blips from a moving magnet in addition to detecting changes in inductance. This seems implausible to me, because it'd be very sensitive to electrical noise.

If it is permeability at work, I wonder if a thin sheet of something like Mu metal or permalloy (non-magnetic materials with very high permeability, usually used for magnetic shielding) would work even better...


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PostPosted: Wed Jun 10, 2009 5:55 pm 
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geoff101 wrote:
sounds like you need to contact the city you're in and let them know so they can send someone out to adjust the sensitivity


umm yeah, I'm sure that the city of Columbus OH will get right on that, checking nearly every traffic light in the city. They have so much money to spend... :P A ballot issue is up next election, to raise the income tax from 2 to 2.5%, if it fails they plan to lay off hundreds of police and firemen.


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PostPosted: Wed Jun 10, 2009 5:59 pm 
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You got me there.

I didn't even know you could insulate against magnetism! :shock:

I can't tell you _why_ it works. I _can_ tell you why it _shouldn't_. But I can also tell you that for whatever reason, it _does_ work,

BUT!!!

And this is rather important to note:

It works with _limited_ results. I am currently running on one bike the biggest magnet that I have ever run: a 750 pound pull magnet (I run a 500 pounder on the Valk). Even then, I still only get right around or just under half the lights that I cannot trip without the magnet.

It seems that there are always going to be those that are either damaged enough or 'out of whack' enough that they aren't going to register a bike, period.

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"Skills must be Learned"
------ Herb Christian


"Ask your doctor if medical advice from a television commercial is right for _you_."


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PostPosted: Wed Jun 10, 2009 6:15 pm 
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Duke wrote:
You got me there.

I didn't even know you could insulate against magnetism! :shock:

I can't tell you _why_ it works. I _can_ tell you why it _shouldn't_. But I can also tell you that for whatever reason, it _does_ work,

BUT!!!

And this is rather important to note:

It works with _limited_ results. I am currently running on one bike the biggest magnet that I have ever run: a 750 pound pull magnet (I run a 500 pounder on the Valk). Even then, I still only get right around or just under half the lights that I cannot trip without the magnet.

It seems that there are always going to be those that are either damaged enough or 'out of whack' enough that they aren't going to register a bike, period.


I wonder if the mass of the magnet has more to do with it than the fact that it is a magnet, rather than an inert chunk of iron or steel.

I have been running my own very unscientific study of placing my steel toed boots over the groove sometimes, and not others. Results so far have been unscientifically inconclusive, but as long as I'm standing there...

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PostPosted: Wed Jun 10, 2009 9:59 pm 
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Duke wrote:
I didn't even know you could insulate against magnetism!


Quite right, you can't! Not in the way people usually think about insulation, anyway; you can't block field lines. However, you can bend field lines to some extent, in this case, around whatever it is you're trying to shield by redirecting them along your shielding material. At least that's how I think it works.

It turns out that Mu metal can be bought relatively cheaply: http://www.aircraftspruce.com/catalog/inpages/magneticcompassshield.php. I might give it a try...


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PostPosted: Wed Jun 10, 2009 10:04 pm 
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If you do, let us know how it works! :shock:

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Duke
"Skills must be Learned"
------ Herb Christian


"Ask your doctor if medical advice from a television commercial is right for _you_."


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PostPosted: Wed Jun 10, 2009 11:27 pm 
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Interesting... googling around, it seems that strong permanent magnet materials like ferrite or certain ceramics do, in fact, have high relative permeability, some as high as 2000-4000 compared to about 700 for steel (according to Wikipedia). This would make a small magnet worth a far larger mass of steel insofar as the sensor is concerned, so that's a plausible reason why magnets would have an effect. Under this hypothesis, the fact that they're actually magnetic is irrelevant.

However, magnetic shielding alloys have still higher permeability, by a factor of 5-10, so reason has it that they would work better for a given mass of material. I think I'll give it a shot; I'll post results when I get them.

-Detour


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PostPosted: Wed Jun 10, 2009 11:55 pm 
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Hey, if it works, you better by enough to go into production! :lol:

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"Skills must be Learned"
------ Herb Christian


"Ask your doctor if medical advice from a television commercial is right for _you_."


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PostPosted: Thu Jun 11, 2009 8:01 am 
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Detour wrote:
Under this hypothesis, the fact that they're actually magnetic is irrelevant.



What you're measuring is change in lines of magnetic flux.

I suspect if you detonated a low-yield, completely un-magnetic, thermonuclear device over the inductive loop, you'd get the light to change...just from the EMP :shock:


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PostPosted: Thu Jun 11, 2009 12:19 pm 
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And in further thought...

Probably a large weak magnet would be better than a small strong one. It's area of disturbance being the important thing. While a small strong magnet is strong, it's gradient is over such a small area, it's pretty much negligible at say, 1 foot distance. At one foot distance, the magnetic flux of a small strong magnet and a large weak one...is about the same. But the large magnet has a lot bigger effective area.


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PostPosted: Thu Jun 11, 2009 3:20 pm 
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The magnet should change the induced magnetic field put forth by the wire loop. An easy test is to check a magnet and a piece of metal response to a hand held metal detector. The magnet should be detectable from further away than the same mass of steel.


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PostPosted: Thu Jun 11, 2009 5:35 pm 
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Tinker, I think that's the gist of it. The traffic sensor is basically a metal detector. If the magnet material happens to have higher permeability than steel, then it will be detectable from further away. But not because it's magnetic! And high-permeability shielding alloy (which is not magnetic) would be detectable from further still. Maybe I should try this first, as it wouldn't require having to wake up really early on a weekend and find a deserted stop light sensor...

smdata: well, that's one way to change the light! :lol:


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PostPosted: Sat Aug 08, 2009 7:40 am 
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This may have been touched on in the previous 13 pages, but I haven't read them all. If it has, forgive me. I may tick off a few folks here with this concept, but hear me out on this philosophical rumination.
Ask yourself- "What is the purpose of traffic lights?" I can only think of 2 possible ones.
1) To let people know when it's their turn to go, thereby preventing collisions.
2) The government wants to flex it's power over citizens and pointlessly yank them around.
I'd like to think it's #1. Therefore, when you're at a light and it's obvious that it isn't going to change, look around, be CERTAIN the way is clear (just as if you were at a stop sign, and be SURE no police are present!) then just GO! You are fully accomplishing purpose #1 above, safety.
An entertaining analogy for those of you who now think I'm a scoundrel- remember in "Blazing Saddles" when Sheriff Bart (Clevon Little) and the Waco Kid (Gene Wilder) needed to slow down the army of thugs that were pursuing them? They set up a toll booth in the middle of an empty desert! Slim Pickens starts fussing, cursing, and shouts- "Somebody go back and get a s***load of dimes!"
All the discussion of the physics of magnetism and high-tech alloys is intellectually interesting, but I think you'll find the above solution works quite well, and quite consistently!
Just one "knuckle-walker's" humble opinion.
(I am, however, going to try the "magnet solution" too. Can't hurt.)
Anybody wants to blast me, it's your turn. (Assuming the light changes...!)
8)


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PostPosted: Sat Aug 08, 2009 9:14 am 
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#2 is legal in some states. I have performed this maneuver in front of LEO's without getting a second glance.

Dan

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PostPosted: Sat Aug 08, 2009 11:50 am 
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And over the years I've amassed nine tickets doing it. Someone (during the course of this thread, maybe?) told a story here a while back about how he was stuck at a red light, and a cop stopped and just sat there, waiting for him to run it.

While the letter of the law and the spirit of the law do not always coincide, the larger problem comes when you consider the number of absolute jackasses scattered through the ranks of those in charge of making the determination.

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Duke
"Skills must be Learned"
------ Herb Christian


"Ask your doctor if medical advice from a television commercial is right for _you_."


Last edited by Duke on Sat Aug 08, 2009 12:43 pm, edited 1 time in total.

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PostPosted: Sat Aug 08, 2009 11:57 am 
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Safe and less costly to turn right and make a u-turn at the first legal, safe opportunity.

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