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Magnetically detecting crossing of finish line

Discussion in 'Electronic Design' started by Arto Sarle, Nov 2, 2003.

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  1. Arto Sarle

    Arto Sarle Guest

    Hi all,

    I'm looking for advice on what would be the best method for a
    home-built transponder to detect when it has crossed over a certain
    point (the finish line, of course) on a kart car track.

    After passing over the finish line, the transponder needs to transmit
    timing information (via RF) to a PC. The reason I want an "active"
    transponder is that at some point in the future it can be used to
    transmit other telemetry as well.

    At the moment I'm thinking of creating a localized EM field at the
    finish line by burying a wire loop or permanent magnet there, and then
    including a magnetic (Hall effect) sensor or switch in the

    Provided that the transponder would be mounted in the car 10-20 cm
    above the track surface and the car may be moving up to 80 km/h, do
    you think this method would work reliably? Most uses for Hall sensors
    seem to be in the millimeter range.

    I'd much appreciate your thoughts or any suggestions for a better
    transponder activation method!

    Best regards,
  2. R.Legg

    R.Legg Guest

    Think optical.

  3. Arto Sarle

    Arto Sarle Guest

    I've given some thought to optical solutions, but couldn't come up
    with a satisfactory trackside solution that would work under all
    circumstances (such as when there are multiple cars crossing the
    finish line simultaneously) ?

  4. legg

    legg Guest

    Detection would have to be vertical. Either place emitter to shine
    through track, detector to sense through track, or use reflective tape
    on track to bounce back info to integrated emitter/receiver.

    Magnetic solutions have to be immune to noise (relatively massive near
    these crude motors and brushes) and variations in the devices being

    If you want to be archaic, then use a video/flash camera with
    playback, as might be the case in the real world. This could enhance
    whatever sort of vroom vroom effect you're getting out of this

  5. John Smith

    John Smith Guest

    Thanks for answering! I'm very intrigued about your suggestion to use
    reflective tape on the track, bouncing a beam of light back into a detector
    in the transponder. If this would work, it would be exactly the kind of
    low-budget, simple-yet-beautiful design that I'm interested in. It would
    also allow the detection point/line itself to be totally passive.

    I think, though, that instead of tape I'd probably use some sort of
    reflective white paint, as that would make maintenance easier (tape might be
    short-lived on the kart track).

    Could you offer any suggestion as to what kind of emitter/receiver might
    best work in this configuration, and how close to the track would the
    transponder have to be mounted? Do you think there might be problems because
    of false positives (other reflective surfaces such as a coin on the track)?

  6. Arto Sarle

    Arto Sarle Guest

    Oops! The above was posted by me but with an incorrect name ;-)

  7. R.Legg

    R.Legg Guest

    Could you offer any suggestion as to what kind of emitter/receiver might
    Reflective sensing is the most dificult and prone to interference,
    especially if three or four are operating in close viscinity.

    You'll find IR emmitter reciever packages with angled and focused
    devices. For the distance involved you'd probably have to modify them.

    As to reflectivity of target, I suggest useing the best possible to
    begin with. Reflective tape in .0025 thickness(including adhesive) is
    a commodity item.

    Simplest is reciever below track, just detecting ambient light - gets
    blocked off by traffic, to give your signal.

  8. Wong Sy Ming

    Wong Sy Ming Guest

    The commercial units for R/C racing can handle multiple "triggers" past a
    loop buried in the ground. The transponders are intelligent, having their
    own power source and such...

    If cost is not an object, you might want to have a look at
  9. Arto Sarle

    Arto Sarle Guest


    Thanks for the suggestion! I am familiar with AMB, as they seem to dominate
    this field.

    However, going for an existing system is out of the question as there are
    eventual special needs to be addressed. Right now I just need to figure out
    what is the best, most reliable way to actually implement the finish line
    crossing with an active transponder.

  10. Arto Sarle

    Arto Sarle Guest

    Simplest is reciever below track, just detecting ambient light - gets
    I agree it would likely be the simplest method, but I need to send out the
    ID from the transponder, so I need to figure out a way to trigger the
    transponder's RF transmitter instead of activating the detection loop itself
    (as most existing systems seem to do).

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