Hi Jim,

A large coil with a magnet suspended in it is also called a 
seismometer, single axis. It still needs power to run the 
electronics-which is costly because low power op amps tend to be 
noisy at the lower frequencies.

The seismometers I've seen and read about need amplifiers-is the coil 
on a geophone large enough so that an amplifier is not needed??? If 
so, it must be one wonkin' big mother of a coil!!!! By 'slightly' I 
think you must mean pico amps per volt-which is a bit more than 'slightly'.

I'll google geophone and see what I come up with.

Piezo cable and air filled hoses are not stealthy and are problematic 
in the winter, especially during plowing operations.

Regards,

Art




.
>Art,
>
>If you want a zero power ground tremor sensor, use a geophone.
>These are basically a coil suspended around d a magnet.
>Any movement causes the coil to move, generating a voltage.
>You can amplify the voltage slightly, and go from there.
>
>Jim
>
>-----Original Message-----
>From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf Of
>Art
>Sent: Saturday, May 15, 2010 9:40 PM
>To: piclist@mit.edu
>Subject: [TECH]low power motor vehicle sensor
>
>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.
>
>A sensor for detecting ground tremors might work, but I think those
>need more power than I'd prefer to use. If I have to power a sensor,
>I'd probably need it to draw less than 1 ma.
>
>The obvious answer is PIR, but in a wooded area, they false all the
>time and they do use much more power than I'd prefer. I'm not sure if
>the field of view for a PIR can be made narrow enough to minimize
>false alarms or not-but I have some PIR;s that have a 10 degree FOV,
>and it's not nearly narrow enough.
>
>Any suggestions from the group?
>
>Thanks,
>
>Art
>
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