An ultrasonic sensor module works ok but needs more power than your 
requirements. Maybe playing with the sampling frequency, in some parts  the 
current can be reduced down to an average of 2-3ma.
Or an optical interrupter, as a cheap keychain red laser pulsed by a 
microcontroller.
Or perhaps a light sensor, with an accurate selection of the site, sensing 
angle and sampling rate, can do the job.  If this can work, also a low-end 
microprocessor + phototransistor (or even a led) can do the job. Power can 
be in the microwatt range. However, IMHO the piezo cable solution, at a 
first look, seems the best for your needs..


regards,
Marco




----- Original Message ----- 
From: "Art" <KY1K@myfairpoint.net>
To: <piclist@mit.edu>
Sent: Sunday, May 16, 2010 4:40 AM
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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