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.
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.
maybe some cheap microphone?
use it to sense only very low frequencies (I would say under 100 Hz? or
even lower...)
Bruno L. Albrecht
Eng. da Computação/06
Falker Automação Agrícola Ltda. http://www.falker.com.br
Em 15/05/2010 23:54, jim escreveu:
> 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 removed}
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.
At 11:17 PM 5/15/2010, you wrote:
>Not zero power, but could be very low power - piezo cable, same as
>goes into those speed loggers that are laid across the road.
>
>Colin
The ones around here are pneumatic tubes... like the old gas station
tubes that would alert an attendant by ringing a bell when the
customer drove over it to pull up to the gas pump.
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 removed}
>:: What's this piezo cable about? Do you know who makes it?
>
>Don't know who makes it, vaguely remember a company selling surplus
>reels in Every Day Electronics and Wireless World back in the 80's.
>
>I like the idea of the petrol station tubing.
>
>However here is a link to a supplier, and I note Digikey stock reels
>of it.
>
>http://www.meas-spec.com/piezo-film-sensors.aspx
Thanks, that could be useful.
MSI's catalog says $16.50 or 12.50 USD/meter (depending on construction)
for up to 500m.
Geophones can easily put out several hundred millivolts depending on the
strength of the movement.
If you're looking for a sort of annunciator for a property, a geophone is a
good choice because it
Requires no electrical power to run it. Only mechanical power, which you
don't have to supply anyway.
The output can be connected to almost anything for use as a signaling
device. If you were to connected
It to an oscope for instance, and with a little practice, you can tell what
is coming onto your property.
Like a car, a truck, motorcycle, bicycle, etc. You probably don't want to
get that sophisticated, but it
Can be done.
I just got back from a google search and got quite an education in a
short time.
My guess is that a motor vehicle passing within 5 to 8 feet of one
would be easily detectable and that I might just need a peak detector
to trip an alarm transmitter.
What I haven't found yet is someone selling them for a reasonable cost.
From my brief google tour, I think I need a relatively high
frequency unit, in the 5 to 20 Hz range which is better suited for
detecting more 'local' disturbances.
>Art,
>
>Geophones can easily put out several hundred millivolts depending on the
>strength of the movement.
>If you're looking for a sort of annunciator for a property, a geophone is a
>good choice because it
>Requires no electrical power to run it. Only mechanical power, which you
>don't have to supply anyway.
>The output can be connected to almost anything for use as a signaling
>device. If you were to connected
>It to an oscope for instance, and with a little practice, you can tell what
>is coming onto your property.
>Like a car, a truck, motorcycle, bicycle, etc. You probably don't want to
>get that sophisticated, but it
>Can be done.
>
>Regards,
>
>Jim
>
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.
>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"
>speffspam_OUTinterlog.com Info for manufacturers: http://www.trexon.com
>Embedded software/hardware/analog Info for designers: http://www.speff.com
>
>
>
Art wrote:
> 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.
>
>
Hall sensor? I've seen cheap electronic compass with LCD in local Lidl
supermarket.
OR
two identical oscillators to avoid temperature issues. One oscillator
uses coil under or beside road, the other a shielded coil. Use the NF
radio freq (8.5MHz?). Mixer for difference.
Change in audio pitch as car goes past.
A range of 1m (coil in road) is easy enough. Traditionally this
approach is used in Metal detectors using 455kHz (and an IFT for one
oscillator coil).
Traffic light sensors used to use tube/plate switches. they now seem to
cut a slot around rectangle slightly smaller than a car with angle
grinder, and I presume multiple turn loop of wire and then seal surface
slot with tar or bitumen. I think.
At 01:19 AM 5/16/2010, you wrote:
>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
It's possible to use local distortions in the earth's magnetic field caused
by the presence of a nearby ferromagnetic object to passively detect
vehicles with sufficient magnetic moment, but the sensitivity falls off
with the distance^3, IIRC.
As a side note, since OP was asking for a "no-powered" solution, I
wanted to talk about my case - which thing I was going to do soon :) and
telepathically I got anticipated!
I placed 3 circles of wire, 2mt diameter, of 4mm wire below the surface
of the entrance of my garage; I planned (not using them yet) to detect
presence of a car.
My idea was to use the old "dip" method i.e. putting into the wire a
100KHz - 1MHz frequency and look at its resulting height (once tuned)
and, when its amplitude lowers, then a metallic-mass "should" be there...
Look at surplus dealers. I've seen them for just a few bucks at some
surplus dealers on the web.
Try WWW.EBAY.COM. There is a lot of 3 for about $30.00. I may have one or
two laying around
here. I'm cleaning out my lab, so if I run onto any, I'll set them aside.
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.
> 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.
>
>
> ********************************************************************
> Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products
> (978) 742-9014. Gold level PIC consultants since 2000.
> 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.
Consider "reluctor" as per electrons motorcycle ignition.
U laminations with coil and magnet in series. U facing towards
vehicle. As V passes magnetic fieldi s somewhat closed. How well and
how close needed TBD.
I think you can use peizo sensors mainly cables or sheets, piezo power
harvesting ICs are there (ex:from leniar technologies), use some lower end
micro controller works in very nA range with internal eeprom. using the
energy generated when vehicle moves over the piezo electric material you can
make the controller work and write it to eeprom using the energy generated
from piezoelectric material.
i have done some work on this like harvesting power when u walk. so i think
it would be easier to get enough energy (2mJ) from a vehicle movement.
YOurs SaRIn....
> > 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.
>
> Consider "reluctor" as per electrons motorcycle ignition.
>
> U laminations with coil and magnet in series. U facing towards
> vehicle. As V passes magnetic fieldi s somewhat closed. How well and
> how close needed TBD.
>
>
> R
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..
>Art,
>
>Look at surplus dealers. I've seen them for just a few bucks at some
>surplus dealers on the web.
>Try WWW.EBAY.COM. There is a lot of 3 for about $30.00. I may have one or
>two laying around
>here. I'm cleaning out my lab, so if I run onto any, I'll set them aside.
>
>Regards,
>
>Jim
>
The units on ebay are shells, with no guts-the sensors are not included.
If you run across any surplus in your lab clean up, email me direct
at ky1k at myfairpoint.net.
Also keep in mind, the geophone will be located close to the driveway
itself and the car/truck that will trigger it is very heavy. So, a
high sensitivity geophone is probably not needed.
Also, the driveway is a dirt driveway, so the rocks and bumps in the
driveway probably means that the moving vehicle generates a much
higher amplitude than it would for a smooth blacktop type driveway.
Again, a high performance high sensitivity geophone isn't necessary.
I am concerned however.....the trees that surround the driveway have
roots in the soil. These roots could be a noise/interference source
when the wind blows and cause false alarms.
>Art,
>
>Look at surplus dealers. I've seen them for just a few bucks at some
>surplus dealers on the web.
>Try WWW.EBAY.COM. There is a lot of 3 for about $30.00. I may have one or
>two laying around
>here. I'm cleaning out my lab, so if I run onto any, I'll set them aside.
>
>Regards,
>
>Jim
>
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