Art wrote:
> 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.

How about a passive weight sensor?  Maybe you can go to Cambodia and strip a
few from (yet) unexploded land mines.


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