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If I had say a dozen or so small bar magnets and I wanted to used them to trip the traffic light sensors (which are a coil embedded in the road where a vehicle would stop for the lights), what would be the best layout? Would it be best to put them all end-to-end to create one long 'super' magnet? Or should I separate them into say a 3 by 4 grid with them all orientated the same way? Or is there some other optimal layout?

By 'optimal layout' I mean, 'most likely to trip the sensor', which I'm guessing is the same as, 'would induct the most current in the sensor coil'.

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"small Magnets" will not trip such sensors at all, because those measure the self inductance of the coil. Tripping is done by either increasing inductance from a big ferroelectric car body, or from lowered inductance caused by some eg big/extended enough aluminium mass. Magnets, even when "big" are borderline. The magnetic field is useless, and the response to the test field (AC) is very small, because the coercitive field strenth of a magnet is high. A big metallic magnet might act like a non-ferromagnetic metal.

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The road coils at stoplights very in diameter and number if windings. They work bay sensing the change of inductance in the coils caused bay the car within the field. Now how to calculate the max effect of putting magnets on the road depends on many local factors. Can’t see a way to do this unless you have some more data or do some trial and error.

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It is good practice to write only in cases when one knows something. –  Georg Jun 20 '11 at 10:27
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