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Triggering a trafic light on a bicycle.

  1. Jun 28, 2009 #1
    i'm a cyclist. Some traffic lights aren't triggered by bicycles. What is the frequency of the AC signal in the coils imbedded in the road to detect cars? Could I build an RLC circuit about the size of my foot that would trigger the light? Knowing the actual frequency is critical to choosing the correct capacitor and inductor. Would a small coil size capture enough magnetic flux? Is there any other way to trigger the signal that I'm not thinking about?
  2. jcsd
  3. Jun 30, 2009 #2
    A quick google search turned up the following link:

    Why do you think the sensor is related to frequency? I think it's more related to the amount of mass of metal than the frequency at which the sensing occurs...

  4. Jun 30, 2009 #3
    Well, I don't want a mass of metal. I want to be as light as possible. I would think an RLC tuned the same frequency of the induction coils would trigger the light without the need for a large mass of medal.
  5. Jun 30, 2009 #4
    Can you explain a bit more? Is the RLC creating some kind of magnetic field? I just don't see how the RLC is related... What physics concept is this working on? Do you know how these sensors work?
  6. Jul 1, 2009 #5


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    They're just inductive loops, like a metal detector (except they're calibrated to detect car-sized amounts of metal, and sometimes they're not calibrated well enough to detect lighter cars). The quick and dirty way is to have more mass. The brute force solution is probably to hook up a coil under your bike and drive a fair bit of AC through it in order to induce enough current in the inductive loop to trigger the sensor.
  7. Jul 1, 2009 #6


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    I once happened to catch a tech setting up the loops for a newly installed system. According to him the points (our loops are diamond shaped) were more sensitive. And yes this was about the loops in the bike path. For the most part I am totally unimpressed with the performance of these sensors. It is not clear that they actually ride a bike across them to set up the sensitivity. Unfortunately, I will bet that there is no standard for the operating frequency of the systems. So without more information there is no way we could tell you that info. You might contact your city or county maintenance shop, they should be able to give you a lead.
  8. Jul 1, 2009 #7


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    Then there's the practical solution: lay your bike down directly on top of the sensor; getting the metal mass of you bike closer to the sensor increases the size of its return signal. Pain in the ***, sure, but it usually works.
  9. Jul 1, 2009 #8


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    My riding-buddies and I had this problem with our Harleys and we always stopped at lights doubled up or even tripled up over the sensing coil, and even then some of those didn't trip, and we'd either have to wait until the traffic light timed out or else run a red light. I can't imagine tripping those coils with a bicycle with alloy frame, cranks, etc.
  10. Jul 1, 2009 #9


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    Wait a second, it just occurred to me: wouldn't the metal have to be in motion in order to induce any current in the loop?! Wait, they've probably got a current going in the loop, and looking to see how much of it gets coupled out to any metallic objects around.
  11. Jul 1, 2009 #10
    I suspect that they operate on the basic Faraday Law principle, with perhaps some imbellishments. The basic principle is inducing eddy currents in metallic objects, especially good conductors, so in this case being magnetic contributes only slightly. The best conductor per unit weight is aluminum. The loop may look for de"Q"-ing or for frequency shifts (i.e., like a good metal detector) resulting from the presence of the metal object. Next time you go through Airport Security, ask them how their system works.
    Last edited: Jul 2, 2009
  12. Jul 1, 2009 #11


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    What's the point in triggering a traffic light with a bike?

    Wait... are you saying there's somewhere in the world where bikes actually obey traffic lights and other rules of the road??? Gwaaaaan....
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