Traffic Light Trigger for Your Bike





Introduction: Traffic Light Trigger for Your Bike

About: 5,000+ miles a year on two wheels!

Not feeling ferrrous enough to trip the induction loops that trigger green lights? No problem - just epoxy a rare earth magnet to your shoe! Inspired by a product marketed to motorcyclists, which is basically a big neodymium magnet to stick under your ride. I thought it might be better to get a slightly smaller magnet closer to the road.

Step 1: Dremel As Needed Between the Lugs of One Heel

I always put my right foot down, so I ground out a little extra space on my right heel. Luckily, my Sidis have tall, widely-spaced lugs, so I didn't have to remove much material.

Step 2: Epoxy the Magnet, Backed With a Washer, to the Shoe

I used PC-7 heavy-duty epoxy paste and coated the entire magnet, both to protect it and to stick it firmly to the heel. Backing the magnet with a washer helps to focus the magnet field, aiming it down (and not up into your heel, not that it should do anything to you anyhow).

Step 3: Using the Magnet to Trigger Lights

Look for the round or diamond-shaped cuts in the asphalt that show where induction loops are buried at intersections, and set your heel down near the tar lines. Here in Portland, some lights helpfully have a small bike between two hashmarks which you're supposed to line your wheels up with, which is a the perfect spot (and more sensitive, too).



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    81 Discussions


    1 year ago

    Will it work if I attach it on bottom of bike? I do not want to dig a whole in my shoe.

    heres another idea check out yr local flea markets u might find a whole computer that up 4 sale

    I have a bunch of eraser - on - the - end - of - a - pencil size earth magnets. I almost always wear work boots on bike. If I stuck a few of those in the space just in front of the heel, do you think that'll work?

    2 replies

    or better yet with hard drive idea but cable tie the them to the spoke of the wheel near the rims.

    More images would help with this, but it's such a cool idea.

    I am quite amazed at the amount of comments this posting has generated. I have seen intersections where I live in Southern California that have the sensors in the bike lane, sized smaller (about 18" across) for bicycles. They even have a pictogram of a bicycle painted in the center. I hope this is a trend that continues to grow.

    1 reply

    I'm also from SoCal. I've seen those bike lane sensors, one up on PV and some in Santa Monica. Here in Torrance where I live, new traffic light installations have the camera type triggering and I've found these to work very well with bicycles. No magnets!

    i didnt read all the comments, so this may be repetitive info: if you put the magnet in your shoe, then you will be picking up all types of metal shavings and what not. these may come off as you walk into your home, and reside in your carpet. and then some how they make their way into your skin as a splinter. or scratch up a nice wood floor. if you get an old hard drive, crack it open and get those magnets out. tape it to one of your spokes as far away from the hub as you can. put one one each wheel, or two on the rear wheel, spaced 180 degrees apart, for maximum coverage. depending on your wheeels/tires, these magnets will come within 1 to 3 inches away from the road, for those magnets, that is close enough to trigger the loop. i had this work when i stacked 2 of these magnets together, and hot glued them to my frame, below the bottom bracket.


    12 years ago

    The sites for the motorcycles talk about ABS housings to protect the magnet from the road and giving separation from the motorcycle to create an optimum field effect - I am thinking about using your epoxy coating or even just duct tape around the magnet to use with my old steel frame commuter bike. 2 questions though for the physics majors - is it really better to separate the magnet from the metal of the (motor/bi)cycle - did I misread that? One of the magnet sites said the strongest field would be with the magnet sandwiched between 2 pieces of metal. Is there any advantage to disc, plate or ring magnets for inducing the sensor?

    9 replies

    I'm not a physics major, but I assume an engineering major will do :) The essence of a magnet is that it contains two poles, and magnetic 'flux' wants to go from one pole to the other. Some materials -- especially iron -- conduct magnetic flux better than others. If you sandwich a magnet between 2 pieces of metal (I assume iron is meant), almost all the flux will go through the iron, so the outside field will be much weaker. And in this context, we want the field to extend as far outside as possible, so the sandwich is not a good idea. For the same reason it may be better to keep the magnet away from the frame of your bike, because the magnetic field may find a shortcut through the metal of the bike instead of extending down to the ground. But this depends on where you mount the magnet. Depending on the shape of the underside of the bike, the iron might guide the lines in such a way that you'll get a nice concentrated field at the right place. I don't think the shape of the magnet matters much, it's the strength of the field that's important. Bigger will of course be better, because a large magnet made from a weak magnetic material may produce more flux than a small strong magnet. By the way, if you have a broken hard disk, you can find some very strong and often quite large neodymium magnets in the mechanism that controls the write heads.

    Adding iron would actually increase the magnetic field (it is ferromagnetic). For example, you're probably familiar with the practice of wrapping current carrying wire around an iron nail to make an electromagnet.

    Not really, an electromagnet and a permanent magnet are different things. An electromagnet works better with an iron core because a ferromagnetic material allows to generate more flux with the same electric current. A permanent magnet doesn't generate, it has a fixed amount of flux. Attaching iron to it will only enable you to channel this flux, not increase it.

    A wire coil carrying a *constant* electric current behaves exactly like a permanent magnet, so there would be no difference. The magnetic field of either a permanent magnet or a coil of current-carrying wire will cause the electrons in the iron to align their spins, from which an additional magnetic field will arise.

    There is a fundamental difference between a coil and a permanent magnet, even if the coil carries a constant current.

    When adding iron inside an empty coil, the current will align the spins inside the iron, increasing the magnetic field. This requires energy, hence if you would hook the coil to a Wattmeter, you would see a short bump in power consumption while the iron is added. This continues as long as the added iron is sufficiently near the coil to be influenced by it. At a certain point, the coil will be completely filled and surrounded by iron to such a degree that it's impossible to add extra iron that is influenced by the coil to any measurable degree.

    The permanent magnet on the other hand behaves like such an electromagnet that already has the absolute maximum amount of iron added to it. Producing additional magnetic flux would require an energy source of some kind, and there is none inside a permanent magnet. Even the most ideal ferromagnetic material would still only be able to guide the magnetic field much like an electric wire can carry current from a battery, not increase it.
    If your theory would be correct, it would be possible to keep on sticking iron coins indefinitely to a permanent magnet, or to build a perpetuum mobile of some kind.

    That doesn't make any sense. When you bring iron near a permanent magnet, energy would be released when the electron spins in the iron align with the permanent magnet, since this is the state with lower potential energy. Likewise, when putting an iron nail in a current carrying coil, you would also be releasing energy.

    The equilibrium state for the spins in a piece of iron at room temperature, when considered at a sufficiently large scale, is a random distribution. It takes energy to align the spins. The magnetized state has higher potential energy, otherwise all iron would become magnetic by itself. But as we all know, any magnetized piece of iron slowly demagnetizes on its own.

    The energy release you're speaking of, is the kinetic energy of the iron being pulled towards the magnet because indeed the equilibrium state for molecules with aligned spins is as close together as possible. But that can only happen after the spins have already aligned. The pulling force of magnets is a consequence of the alignment, not a cause. The total magnetic energy of the combination magnet+iron cannot be higher than of the magnet itself (plus any stray magnetism that was already in the iron). With each piece of iron added to it, the magnet will have less energy to spare to magnetize additional iron. The coil on the other hand can draw current at leisure from its power source.

    There is no direction to "align" to unless the iron is in the presence of the magnetic field. An initial random distribution means half spin-aligned, and half spin-anti-aligned in the direction of the magnet's field. Spin-anti-aligned is a higher potential energy, so the spins will all align since this is a lower potential energy state. In other words, in terms of potential energy, spin anti-aligned > spin-randomly aligned > spin-aligned. This is why when you put iron next to a permanent magnet it becomes magnetized. Try it with a permanent magnet and a nail.

    I wasn't suggesting sandwiching the magnet between two ferrous plates, just using one (the washer) to shape the field. See fig. 2 here:,42363#6

    The rare-earth magnets in the hard drives I've taken apart were about 1/3 the size of the 1 inch disc I've got on my shoe, so you'd want to stack several. They do make good fridge magnets, too.