use a fluxgate magnetometer...any ferrous material passing over the
detector will cause a distortion in the local magnetic field and cause an
emf to be induced into a sense coil

On 26 Feb 1997, Affayroux wrote:

---------------------------------------------------------------------------­-
When you have learned what an explanation really is, you can then go on
to more subtle questions.
                                                                -Feynman

Does anyone out there have a schematic for a vehicle detection sensor?
The type I am looking for is a coil of wire, usually about 30-50 inches in
diameter.  To install the coil in blacktop, they take a saw, and cut a
grove about an inch deep, and then bury the coil in the cut made by the
saw.  

What I am looking for is a schematic that would detect when a vechicle
passes over the coil.  I'm not sure how they work, maybe by changing the
frequency of an oscillator and then  using a tone decoder chip to sense
that the frequency has changed??  What I am after, is to sound a chime in
the house when someone pulls in the driveway. I don't want to use motion
detector technology because deer wander around all night long.  Any ideas?
Thanks, John

In article <19970226014501.UAA22...@ladder02.news.aol.com>, affayr...@aol.com (Affayroux) says:

In general terms, your coil is an inductor, and if you drive a car over
it, the iron in the engine and chassis put an iron core into the inductor,
thus raising its inductance, and a shorted turn in parallel with the
inductor, lowering its Q.

A deer isn't a good conductor, so won't have much effect on the Q, and
doesn't contain much iron, so won't have much effect on the inductance.

I've never worked with these detectors, so I don't know how they are
usually driven.

They have to be excited with AC, and a low frequency like 50/60Hz ought
to work fine. Get a transformer to cut your mains voltage down to about
12V RMS before you apply it to the coil, if you do go for mains
excitation. In theory you can do without, but your question makes it
clear that you know too little electronics to tackle that sort of job
safely.

My inclination would be to set up a bridge circuit, with a big capacitor
in series with your coil as one arm of the bridge, and a much smaller
capacitor in series with a proportionately smaller inductor (bought from
Farnell or somelplace like that) as the other arm.

If you get the ratio's right (which means measuring or calculting -
ideally both -the inductance of your buried coil for starters) then there
will be no difference between the two sides of the bridge, in the
absence of car, and a significant difference in the presence of car. This
usually means that you have made one of the arms of the bridge
adjustable, and adjusted it for no signal with no car.

Measure the difference signal with a differential amplifier (Farnell lists
a bunch - AMP01, AMP04, INA101/2/4, AD524, AD625, LM725 (cheapest) - and
demodulate the 50/60Hz output signal with a phase-sensitive detector,
which will give you a Dc voltage which you can then use to drive a relay,
which you cna then use to turn on your chimes, or your lights, or whatever.

With any luck, somone in the news grup will have done it, and will tell
you how to do it easily - it isn't demanding electronics, but does
require some backaground information.

Bill Sloman (slo...@sci.kun.nl)        | Precision analog design
TZ/Electronics, Science Faculty,       | Fast analog design and layout
Nijmegen University, The Netherlands   | Very fast digital design/layout
                                       |  e-mail for rates and conditions.

: use a fluxgate magnetometer...any ferrous material passing over the
: detector will cause a distortion in the local magnetic field and cause an
: emf to be induced into a sense coil

: On 26 Feb 1997, Affayroux wrote:

: > Does anyone out there have a schematic for a vehicle detection sensor?
: > The type I am looking for is a coil of wire, usually about 30-50 inches in
: > diameter.  To install the coil in blacktop, they take a saw, and cut a
: > grove about an inch deep, and then bury the coil in the cut made by the
: > saw.  
: >
: > What I am looking for is a schematic that would detect when a vechicle
: > passes over the coil.  I'm not sure how they work, maybe by changing the
: > frequency of an oscillator and then  using a tone decoder chip to sense
: > that the frequency has changed??  What I am after, is to sound a chime in
: > the house when someone pulls in the driveway. I don't want to use motion
: > detector technology because deer wander around all night long.  Any ideas?
: > Thanks, John
: >

A fluxgate magnetometer is a little overkill. The commercial traffic
sensors are simply a tuned LC oscillator (I cannot remember the
frequency) where the buried sensor coil is an active inductor in the
circuit. The influence of a vehicle changes the resonant frequency
somewhat, which can be relatively easy to detect.

Bob.

In <19970226014501.UAA22...@ladder02.news.aol.com>, affayr...@aol.com (Affayroux) writes:

There are commercial driveway detector products which use a probe
buried by the side of the driveway.  The probe is sealed in a piece
of PVC pipe about a foot long.  I've never had the opportunity to
disassemble one of these, so I can't tell you the exact circuit
design.  However:  (a) installation on a residential driveway is a
lot simpler than using a loop detector, and (b) there is no need to
locate the signal processor near the sensor, as I believe is the
case with loop detectors.  You simply bury the probe by the side of
the driveway (even if the driveway is concrete) and run the cable
to the processor/annunciator unit in your house.

In article <5f1kku$...@wnnews.sci.kun.nl>, Bill Sloman
<slo...@sci.kun.nl> writes

- Prikaži navedeno besedilo -

 It's an intersting idea to use a very low frequency and a Maxwell
bridge (but you need resistive components to balance the coil losses, so
look up the complete bridge circuits), but most systems seem tu use
resonant circuits at around 100 kHz. Thse may, however, need a licence
from the Radiocommunications Authority (in UK). A 50 Hz system would be
a baseband induction system, which RA are unlikely to want to get
involved with (although they could).
--
Regards, John Woodgate Tel. +44 (0)1268 747839
Fax +44 (0)1268 777124. OOO - Own Opinions Only

cc: John Affayroux

AF>From: affayr...@aol.com (Affayroux)

AF>Does anyone out there have a schematic for a vehicle detection sensor?
AF>The type I am looking for is a coil of wire, usually about 30-50 inches in
AF>diameter.  To install the coil in blacktop, they take a saw, and cut a
AF>grove about an inch deep, and then bury the coil in the cut made by the
AF>saw.

AF>What I am looking for is a schematic that would detect when a vechicle
AF>passes over the coil.  I'm not sure how they work, maybe by changing the
AF>frequency of an oscillator and then  using a tone decoder chip to sense
AF>that the frequency has changed??  What I am after, is to sound a chime in
AF>the house when someone pulls in the driveway. I don't want to use motion
AF>detector technology because deer wander around all night long.  Any ideas?

The first technique you mention is used by municipalities to trigger
stop lights. The technique now uses a figure eight pattern (to make the
loop more sensitive to motorcycles).  Pretty much shifts the frequency
of a self oscillator.

However, a lot of work went into the techniques of properly installing
these gizmos. They cut the pavement with a diamond saw, place in the
coils based upon proper "polarity", and then glue the slots shut with
adhesive epoxies that match the expansion coefficients of the
pavement.

  If you do bury the cable without the proper stuff, you can get rocks
  "migrating" downward due to tire pressures and these rocks can
  actually sever your buried cables.

I would recommend purchasing a commercial unit (there's nothing worse
than spending time debugging a tool).  Or, make your own system with the
sensor(s) mounted above ground at the side of the driveway.

Either a "metal detector" loop at one side with the coils mounted inside
PVC, or use "interrupter" metal detector loops with one on each side of
the driveway.  Obvious advantage of the loop on only one side is that
there is no cabling required across the drive.

To get an idea of the size of your coil:  A metal detector should be
able to "see" more than 3 diameters away.  Place it at a constricted
spot in your drive where the vehicle will have to get closer to it.

Single coil metal detectors are usually beat frequency oscillators
creating a tone as the two oscillators shift away from matching
frequency.  This beat tone can be AC coupled and rectified to trip your
indicator.

Since a vehicle usually contains a lot of iron, you should be able to
work at audio frequencies where there is less mystery in the circuit's
performance.  ..in the 5KHz to 20KHz range.

Good luck.  Let us know what you end up doing.

                                         - Robert -
                                  robert.m...@engineers.com
                                       AJM Electronics

 * OLX 2.1 TD * Never provide it, but always demand it.

How about accomplising it with two optical sensors??

Use two so false triggering is minimized.

Then you dont have to cut up your driveway!!!

: Does anyone out there have a schematic for a vehicle detection sensor?
: The type I am looking for is a coil of wire, usually about 30-50 inches in
: diameter.  To install the coil in blacktop, they take a saw, and cut a
: grove about an inch deep, and then bury the coil in the cut made by the
: saw.  
:
: What I am looking for is a schematic that would detect when a vechicle
: passes over the coil.  I'm not sure how they work, maybe by changing the
: frequency of an oscillator and then  using a tone decoder chip to sense
: that the frequency has changed??  What I am after, is to sound a chime in
: the house when someone pulls in the driveway. I don't want to use motion
: detector technology because deer wander around all night long.  Any ideas?
: Thanks, John

John, you're asking about a VERY sensitive suject.

What you are describing is pretty much a standard type 'Traffic Detector'
loop.  While the properties of the loops themselved are described in
exhausting detail in various publications (including the 'Traffic
Detector Loop Handbook'), the electronic boxes that these loops drive
are a very competitive, large dollar business, and the workings of
these detectors are carefully guarded and rarely published.

I can only guess how the detector works, but several obvious solutions
cross my mind -- the most attractive of which to me would be to design
the circuit similar to that of an old-fashioned Beat Frequency Oscillator
metal detector.  (This is a venerable design, but may not be the one
actually used by detector manufacturers.)

In concept, you use the detector loop in the tank circuit of an
oscillator (the frequency of which will change when metal passes
over the loop).  A second, fixed-tuned oscillator is tuned to
the (unoccupied) frequency of the first.  A mixer cicuit combines
the two oscillator frequencies with the output going through a
high-pass filter and a threshold detector circuit that pulls in
a signaling relay whenever the beat frequency of the two oscillators
is higher than a pre-set limit.

Other than sensitivity, long-term drift is probably the biggest
technical concern.  With today's technology, however, it shouldn't
be difficult to device a simple digital adjustment servo that
performs long-term tracking compensation of the reference oscillator
frequency.

                                      Harry C.

Hello, John

Nice place you live in, with deers by night time!
It's been many years I've worked with these detectors, but the best
circuit I saw was:

The coil was part of a LC oscilator, at about 250Khz;
The oscilator was conneted to a PLL circuit;
In steady state, the PLL oscilator will be pulled by the phase
comparator so it's at the same frequency of the LC one.
When a car crosses the loop, the LC oscilator will change it's
frequency, and there will be an error signal at the output of the
phase comparator. That's your output.
The time constant of the PLL filter shall be some tens of seconds, so
a slow moving car will be detected.
If this circuit is implemented using say a 4046, it will be OK for
your use but will not detect an car that parks over the loop.
For that one must use a more sophisticated solution.

- Prikaži navedeno besedilo -

Jorge

j...@mpc.com.br*
remove the * when repliyng to this message
My employer agrees with all my opinions (-:

I've also heard of it being done with a length of hose, and a pressure sensor...Kinda like they do in a gas station when you pull in and the bell goes off, except instead of the bell, you are electronically coupled through a pressure sensor...False triggering is prevented by measuring the signal...Cars are obviously heavier than a deer, or a person crossing over one...Even a bike...So you could set it up to trip only under high-pressure.

Chris Savage/Knight Designs ... KnightD...@aol.com