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Old 09-25-07, 05:15 PM   #1
jnb-rare
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Trigger Green Traffic Lights

http://www.metacafe.com/watch/828731...raffic_lights/

If this really works, then it seems like a good idea for bicycles, too. Beats rolling up on the sidewalk to press the pedestrian cross button.
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Old 09-25-07, 06:24 PM   #2
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Several years ago I placed a magnet sold for motorcycles to trip induction loops on one of my recumbents. It made no difference as far I could tell..
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Old 09-25-07, 06:32 PM   #3
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Originally Posted by recumbenteer View Post
Several years ago I placed a magnet sold for motorcycles to trip induction loops on one of my recumbents. It made no difference as far I could tell..
Rats! It's a good thing my folder is so manouverable. I deliberately use a couple of routes where I cross major roads from small, llightly-trafficked side streets. In a couple of cases the pedestrian button is essential unless one is very, very patient and waits for a car to come to the light.
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Old 09-25-07, 07:40 PM   #4
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If I may chip in...

These detector loops don't work by detecting magetic material, they work by changing inductance. This is achieved by any conductive material being inside the high frequency magnetic field generated by the loops. The field induces circulating (eddy) currents in the car and this is detected.

The strongest part of the field is concentrated at the corners and along the middle line. The magnetic field leaves the surface in one loop, crosses over the center line and enters again in the other loop.

For bicycles to be detected reliably, place the wheels along the center line, not in the middle of a block. Along the center line, the field goes neatly through the wheels and maximum effect is obtained since the wheels are at right angles to the field.

So forget about the magnet idea. Snake oil.
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Old 09-25-07, 08:10 PM   #5
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For stubborn traffic lights, lay your front wheel down almost flat along one of the corners of the inductive loop. (This obviously isn't practical with a heavy motorocycle.)
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Old 09-26-07, 07:25 AM   #6
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Good to know. To be honest, I've never even noticed the loops before. I'll have to look more carefully.
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Old 09-28-07, 12:05 PM   #7
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. . .These detector loops don't work by detecting magetic material, they work by changing inductance. This is achieved by any conductive material being inside the high frequency magnetic field generated by the loops. The field induces circulating (eddy) currents in the car and this is detected . . .
Are you sure? I have a comment and a question. The comment: I have noticed that when I am driving, cars with steel wheels will trip the sensor more reliably than cars with alloy wheels, and that when driving on alloys I have better luck approaching the limit line with the steel oil pan lining up over the "sweet spot" rather than the wheels. Once I was on a moped and couldn't get the light to change at all. I happened to see a steel hub cap at the curb, retrieved it, and waved it over the sensor with immediate results. This leads me to believe that it's not only the conductivity of a metal that makes the difference, but also whether or not the metal is ferrous.

And now the question: if I'm right about ferrous metals making a difference, and you are right about the sensor being triggered by a disruption of its magnetic field by a vehicle, would it not follow that a scooter, made mostly of alloy, might have trouble tripping the sensor? If a hubcap strapped to the bottom of the scooter would increase the scooter's "magnetic foot print" (that's layman, not lame man) couldn't it be possible that an exceptionally strong magnet attached to the underside of the scooter could provide a similar "footprint" to the larger but non-magnetized hubcap?
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Old 09-29-07, 03:58 AM   #8
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I admit this is not my own knowledge; I once read a short paper from an engineer who designs those things, explaining how they work and how best to trigger them. The principle is the same as for induction heating stoves. The explanation made perfect sense to me since I also work in designing magnetic components. I haven't yet failed to trigger a light by positioning the wheels of my bike right on the center line of the 2 side-by-side blocks, as the paper recommended. The paper specifically also mentioned that small magnets will have very little effect, if at all.

That of course does not preclude the possibility that there are other designs out there that do work by detecting ferro-magnetic material.
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Old 09-29-07, 04:05 AM   #9
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Here, I found it:

http://www.humantransport.org/bicycl.../detection.htm

http://www.humantransport.org/bicycl...nals/green.htm
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Old 09-29-07, 05:20 AM   #10
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Sounds like a good way to mess up a cadence sensor on a bike, since the BB area is the
only place we could attach a magnet like that.
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