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Vehicle detector

Discussion in 'Electronic Basics' started by Roger Dewhurst, Sep 12, 2006.

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  1. Would a multi turn coil, a metre or two in diameter, buried beneath the
    surfce of a driveway, with the ends of the coil connected to the input pins
    of a operational amplifier serve as a reliable vehicle detector?

  2. Maybe. These type of coil detectors are often used near traffic lights. They
    are connected to an oscillator to produce a magnetic field. When a vehicle
    approaches the coil, it absorbs some of that field which lowers the voltage
    on the detector coil. These change of voltage is measured and interpreted.
    Frequency, coil dimensions, available power are amongst the things you need
    to find out to build a reliable system.

    petrus bitbyter
  3. I seriously doubt it, if the opamp is connected as a simple gain block.

    Most vehicle detectors based on coils operate on the principle of
    exciting the coil with some frequency around 10kHz to 100kHz, (higher
    frequency for a single or double turn, lower frequency for higher
    number of turns) and react to either the change in inductance or
    change in loss of the coil when a vehicle gets inside the AC magnetic
    field. This takes a bit more than a gain block.
  4. I was thinking that the vehicle would be magnetic enough to induce a current
    in the coil and that the op-amp would amplify the voltage difference between
    the two inputs.


    This seems to be much like a metal detector.

  5. Sjouke Burry

    Sjouke Burry Guest

    Ah!! you never heard about a moving magnet(the car)
    over a coil??
    A bit of amplifier and a Schmitt trigger plus a oneshot
    for a dead time window ought to show interesting
    Things might improve,if 1 or two amps dc flowed in the
    coil,that way the car needs no remanent field of its own.
    Of course there are about @#$% other ways of doing it,
    but dont underestimate rogers idea.
  6. Hmm... The idea is not very new. Magnetic mines used the magnetic field (or
    better disruption of the earthmagnetic field) caused by a ship. But a ship
    is a very big thing compared to a car. Ships (iron ships) were often more or
    less magnetic which could be decreased by degausing. Navy vessels still have
    a degausing system installed. Most cars are lousy magnets. To induce
    something measurable the car has to pass the coil with pretty high speed. I
    guess you will not drive that fast in a driveway. (Which I suppose to be the
    place it is meant to be used.) Pushing a DC-current through the coil will
    not do any good. It's only wasting energy.

    petrus bitbyter
  7. I suppose if one makes the loop part of a frequency generator any change in
    the inductance caused by the vehicle will change the frequency in the loop.
    But that is a bit more fiddly.

  8. jasen

    jasen Guest

    that'd detect the motion of the car, but not its presence,

    the active loop ("metal-detector") way triggers on the presence of a vehicle
    and is more sensitive. (a loop of steel packing strap or a bicycle wheel is
    enough to trigger many)
    you could sneak up on it and it'd not trigger.

  9. It is only movement over the coil, onto it or off it, which are of interest.

  10. Rich Grise

    Rich Grise Guest

    No, unless the car is magnetized, then you'd get one long, slow, not very
    strong, pulse.

    But if you use the coil in the tuned circuit of an oscillator, and some
    kind of frequency discriminator, then yes, certainly, although depending
    on the sensitivity, it might not detect your bike. :)

    Good Luck!
  11. Rich Grise

    Rich Grise Guest

    Exactly. :)

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