Vehicle detector

"Roger Dewhurst" schreef in bericht news:ee76mf$blg$ snipped-for-privacy@lust.ihug.co.nz...

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

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.

pins

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.

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. 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.

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

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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.

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.

Bye. Jasen

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

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! Rich

Exactly. :-)

Cheers! Rich