Hi Spehro,

When I mentioned low mass, I was referring strictly to a compass like 
device, that would indicate a slightly different direction as the 
large steel vehicle interacted with the Earth's magnetic field. Since 
the Earth's magnetic field is relatively weak, I was trying to say 
that the rotating mass should be kept as low as possible in order to 
allow the needle in the compass to move enough in the short time 
periods that it would take the vehicle to move past the detector.

The sensor can weigh 10 pounds, but the internal rotating mass of the 
magnet in the compass should be kept low.

Regards,

Art


>At 10:40 PM 5/15/2010, you wrote:
> >Hi All,
> >
> >I need suggestions for a low power (or zero power) detector for
> >automobiles. It is for a rural site that is wooded, so all vehicles
> >must pass along a relatively narrow driveway with lots of trees on
> >both sides. So, it is relatively easy to conceal detection hardware.
> >Since vehicles are relatively large masses of steel, the thought of a
> >magnetic sensor came to mind. As a vehicle drives by the sensor, a
> >change in the static magnetic flux is detected. I did some tests in
> >my own driveway, and I can see a compass on the edge of the driveway
> >move slightly as my John Deere lawn mower is driven past.  But, I
> >can't figure out how to build something that is practical as anything
> >I could build would be relatively high mass and I don't have a clue
> >regarding how to get from 'concept' to a practical sensor.
>
>Why do you care about the mass of the sensor?
>
>Best regards,
>
>Spehro Pefhany --"it's the network..."            "The Journey is the reward"
>speff@interlog.com             Info for manufacturers: http://www.trexon.com
>Embedded software/hardware/analog  Info for designers:  http://www.speff.com
>
>
>
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