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'Driveway loop'
1997\08\26@015915 by David Duffy

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Does anyone know how the car detector loops at fast food drive thru's
and traffic lights work ? I saw a dud board from one and it seemed to
have a 18 pin micro on it (not PIC). The micro seemed to connect to the
loop via a small coupling transformer,etc. I assume it senses a change
in a HF field in the loop. Any ideas ?
Thanks...
__________
Dave Duffy

1997\08\26@082010 by Tom Handley

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  Dave, back in the 70's I was desinging these kind of detectors. We
buried a wire loop in concrete to detect vehicles. The loop was one part
of an LC tank whose frequency changed as a vehicle passed over. We simply
detected the frequency change. I have'nt thought about this since then so
I'm not sure what they use now days but I suspect it is similar.

  - Tom

At 03:51 PM 8/26/97 +1000, you wrote:
>Does anyone know how the car detector loops at fast food drive thru's
>and traffic lights work ? I saw a dud board from one and it seemed to
>have a 18 pin micro on it (not PIC). The micro seemed to connect to the
>loop via a small coupling transformer,etc. I assume it senses a change
>in a HF field in the loop. Any ideas ?
>Thanks...
>__________
>Dave Duffy

1997\08\26@083435 by Miller, Steve

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face
>Does anyone know how the car detector loops at fast food drive thru's
>and traffic lights work ? I saw a dud board from one and it seemed to
>have a 18 pin micro on it (not PIC). The micro seemed to connect to the
>loop via a small coupling transformer,etc. I assume it senses a change
>in a HF field in the loop. Any ideas ?
>Thanks...
>__________
>Dave Duffy

Dave,

These systems work on the principle that a large ferro-magnetic object will
detune a nearby inductor coil.  Typically one or two turns of wire are embedded
in the concrete.  With no car on the sensor, the system  auto-tunes the system
to resonant the coil.  When a car drives over the sensor the coil falls out of
resonance and this event is detected.  The systems are fairly robust in that
they can work with many different sizes of coils due to the auto-tune feature.
(The cable driving the coil is a twisted pair.)

---- Steve

1997\08\26@084224 by Douglas J.A.R.Sasse

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>   Dave, back in the 70's I was desinging these kind of detectors. We
>buried a wire loop in concrete to detect vehicles. The loop was one part
>of an LC tank whose frequency changed as a vehicle passed over. We simply
>detected the frequency change. I have'nt thought about this since then so
>I'm not sure what they use now days but I suspect it is similar.
>
>   - Tom
>
>At 03:51 PM 8/26/97 +1000, you wrote:
>>Does anyone know how the car detector loops at fast food drive thru's
>>and traffic lights work ? I saw a dud board from one and it seemed to
>>have a 18 pin micro on it (not PIC). The micro seemed to connect to the
>>loop via a small coupling transformer,etc. I assume it senses a change
>>in a HF field in the loop. Any ideas ?
>>Thanks...
>>__________
>>Dave Duffy
>
>
Dave and Tom, here in Mankato they still use the same system on the signal
light's and at the D Q shop's. in the Twincitys they also use a sensor for
the police and fire Veicles that respondes to there SOUND's also to control
the signals.

regards

http://ic.mankato.mn.us/~douglas
spam_OUTdouglasTakeThisOuTspamic.mankato.mn.us

1997\08\26@165446 by Martin McCormick

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       The same devices are used here to control traffic signals and they
tear the daylights out of AM radio reception as you drive over them.  What
one hears is a carrier with lots of 60 HZ modulation on it that drifts across
the station you are listening to as the car drives over the coil.  Some
motor cycles and bicycles don't have enough metal to register and cyclists
have been seen jumping off their bikes and hitting the crosswalk buttons
when there isn't enough traffic around to cycle the lights.:-)

       I think the emergency vehicles here have IR-based strobes that
can control the lights when a police car or fire truck needs to get somewhere
fast.

       I don't know what hardware is actually used, but traffic signals
always seemed to me like a perfect use for PIC's.

Martin McCormick

1997\08\26@192906 by sdattalo

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Martin McCormick wrote:

>                                                                  Some
> motor cycles and bicycles don't have enough metal to register and cyclists
> have been seen jumping off their bikes and hitting the crosswalk buttons
> when there isn't enough traffic around to cycle the lights.:-)

I don't know what effect if any my Titanium frame has on the inductance
of these sub-surface spiraled signal samplers. But my steel spokes and
chain certainly have a stronger effect when I lean my bicycle over. If
that doesn't work then I run the light.

>        I don't know what hardware is actually used, but traffic signals
> always seemed to me like a perfect use for PIC's.

That is, of course, someone other than the traffic light designers
were programming them. I mean if we left the PIC development up to the
same people that design the switching logic we would have an even
larger mess on our hands. I wish I could be paid a nickle an hour for
the time I've spent thinking about better ways to implement traffic
switching logic. It would sure buy a whole lot of pizza...

Scott
--
                                __o
 I buy pizza instead of gas.    \<
                             (*)/(*)

1997\08\26@233806 by Mike Keitz

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On Tue, 26 Aug 1997 15:11:04 -0500 Martin McCormick
<.....martinKILLspamspam@spam@DC.CIS.OKSTATE.EDU> writes:
>        The same devices are used here to control traffic signals and
>they
>tear the daylights out of AM radio reception as you drive over them.
>What
>one hears is a carrier with lots of 60 HZ modulation on it that drifts
>across
>the station you are listening to as the car drives over the coil.

I don't think the modulation is intentional.  Maybe it is though. Offhand
I think of two major approaches to building such a detector.  The first
one would make the loop part of an oscillator and detect changies in
frequency of oscillation.  The second would make the loop part of a
filter and detect changes in impedance to a signal from a
constant-frequency oscillator.  Most likely the loop characteristic is
tuned in when the loop is installed and any change assumed to be from a
vehicle near it.  Some sort of auto-calibration could be used but it
might get fooled by heavy traffic.

>Some
>motor cycles and bicycles don't have enough metal to register and
>cyclists
>have been seen jumping off their bikes and hitting the crosswalk
>buttons
>when there isn't enough traffic around to cycle the lights.:-)

Which naturally leads to a potentially marketable project idea: some sort
of active or passive device (other than a large block of Fe) that could
be mounted on a bicycle or motorcycle to help it trigger a traffic
detector.  Nearly all traffic detectors currently in use appear to be the
type with a large loop under the road, if the project  adapts to all the
various frequencies and circuits in use it would work most everywhere.

Obviously one would need to build up the actual detector or a reasonable
copy of it to develop this project, since testing in the street isn't
very practical or safe.

>
>        I think the emergency vehicles here have IR-based strobes that
>can control the lights when a police car or fire truck needs to get
>somewhere
>fast.

They use a light detector intended primarily for visible light and
precisely time the rate of the strobe to reject random flashes of light.
School and city buses are also often equipped with such strobes.  The
strobelight detector will extend the green light if already green, or
shorten the green light for the other way if the vehicle with strobe is
approaching a red light.

1997\08\27@031156 by mikesmith_oz

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On 26 Aug 97 at 20:22, Mike Keitz wrote:
<snip>

> They use a light detector intended primarily for visible light and
> precisely time the rate of the strobe to reject random flashes of
> light. School and city buses are also often equipped with such
> strobes.  The strobelight detector will extend the green light if
> already green, or shorten the green light for the other way if the
> vehicle with strobe is approaching a red light.
>

Wonder if there are any 'contra' versions of such things around? :)
The local fire service has complete override control of the lights in
the city here - they program the route before leaving.
MikeS
<mikesmith_ozspamKILLspamrelaymail.net>

1997\08\27@190810 by David Duffy

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picon face
Thanks to all who replied on this. I managed to get a working one yesterday
to play with. I made a detector coil (around 40cm diameter) from some 24/020
cable - seems that 4 turns works well but 6 is more sensitive. When observed
on the CRO, the coil voltage seems to drop by 50% near metal. The frequency
also changed from around 85 KHz to 90 KHz. Different metals made if change in
different direction. So... I still not sure whether it's a frequency or level
*change* that triggers the unit. The micro (?) seems to drive the coil via a
small coupling transformer - possibly to balance input for less interference.
The waveform on coil is a nice clean sinewave. Maybe the micro is really a
hybrid chip - just happens to use a 4.00MHz crystal ! (no xtal caps though)
Two of the chips have 'Detector Systems' stamped on them - hard to say if they
are 'house numbered' for DS or are actually custom chips. I tried a search on
that name,etc but drew a blank. I'm not sure how to implement the HF side of
things but maybe a PIC with A/D could process a F/V converter ?
Regards...
__________
Dave Duffy

1997\08\28@181156 by paulb

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I have a particular interest here. My pet PIC project is the control
of my house lights(, fans, solar water heating etc.) on a centralised
basis.  This includes the touch/ proximity sensors I propose to install
instead of conventional light switches, which will be PIC-based and
execute an algorithm similar to the popular "stud-finder"s.

 One of these latter, instead of using a little pot with an edge-wheel
to set the reference level, has a simple "zero" button.  Probably
analogue sample-and-hold circuitry, but a feature dead easy to implement
with a PIC.  A 12C508 would be perfect for this application?

 My proximity plates will be expected to auto-zero over a time constant
of some minutes (altough a shorter period would most likely do fine) to
compensate for humidity variation, presence of furniture, component
drift, ease of set-up and any other presently unquantified reason.

 Now I have ALSO had embedded in the driveway when it was built, three
arbitarily sized single-turn loops.  These are yet to be connected by a
conduit and boxes alongside (indeed, I have presently mislaid where the
wires emanate from the slab), but I have been thinking on the circuitry
to use.  Note that the loops are actually attached to the reinforcing
mesh in my case, but I think that is common enough practice.

Mike Keitz wrote:

> Offhand I think of two major approaches to building such a detector.
> The first one would make the loop part of an oscillator and detect
> changes in frequency of oscillation.  The second would make the loop
> part of a filter and detect changes in impedance to a signal from a
> constant-frequency oscillator.

 Getting back to the "stud-finder"s, these use a simple R-C time
constant determination directly comparable to the STAMP "pot" function.
This can be very sensitive indeed where long-term stability is not a
concern.  The equivalent L-R version should surely be a third, if not
first candidate for this job?

>  Most likely the loop characteristic is tuned in when the loop is
> installed and any change assumed to be from a vehicle near it.  Some
> sort of auto-calibration could be used but it might get fooled by
> heavy traffic.

 Obviously, a microcontroller can implement various self-calibration
algorithms digitally and these can be defined in a logic, rather than
exponential time domain.  In particular, a digital sensor could
distinguish and note the presence of a car parked near a sensor as well
as other cars passing over it.  Note that for loop sensors, both
increase (ferrous object) and decrease (conductive) in inductance
represent valid events.

> Which naturally leads to a potentially marketable project idea: some
> sort of active or passive device (other than a large block of Fe) that
> could be mounted on a bicycle or motorcycle to help it trigger a
> traffic detector.

 The only passive device coming to mind is a horizontal conductive
shorted turn.  This being rather inconvenient, perhaps a PIC-based
transponder which detects the search coil frequency and generates a
phasing pulse timed to "bend" it.

> if the project  adapts to all the various frequencies and circuits in
> use it would work most everywhere.

 Microcontroller-based for sure.

> Obviously one would need to build up the actual detector or a
> reasonable copy of it to develop this project, since testing in the
> street isn't very practical or safe.

 Safe?  Well, maybe not if you insist on sitting in the middle of the
road, but with some fancy talking, council approval and a quantity of
witches hats...  Practical?  Well, without access to the actual direct
sensor output in the control box, a very SLOW test process, even if you
do it after midnight with no traffic present.

 Cheers,
       Paul B.


'Driveway loop'
1997\10\06@214458 by Richard Wamser
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At 08:59 AM 8/28/97 +1000, you wrote:
>Thanks to all who replied on this. I managed to get a working one yesterday
>to play with. I made a detector coil (around 40cm diameter) from some 24/020
>cable - seems that 4 turns works well but 6 is more sensitive. When observed
>on the CRO, the coil voltage seems to drop by 50% near metal. The frequency
>also changed from around 85 KHz to 90 KHz. Different metals made if change in
>different direction. So... I still not sure whether it's a frequency or level
>*change* that triggers the unit. The micro (?) seems to drive the coil via a
>small coupling transformer - possibly to balance input for less interference.
>The waveform on coil is a nice clean sinewave. Maybe the micro is really a
>hybrid chip - just happens to use a 4.00MHz crystal ! (no xtal caps though)
>Two of the chips have 'Detector Systems' stamped on them - hard to say if they
>are 'house numbered' for DS or are actually custom chips. I tried a search on
>that name,etc but drew a blank. I'm not sure how to implement the HF side of
>things but maybe a PIC with A/D could process a F/V converter ?
>Regards...
>__________
>Dave Duffy
>
>Hi Dave:
I am reading your E-Mail message with great interest as I have buildt a "CAR
DETECTOR"
for my driveway. I buryed 4 turns of #14 Non-Metallic Sheathed Cable ( House
hold wire )
under my driveway actually I pounded a plastic pipe under the drive ... the
driveway is 19
feet across and I spaced the coils 10 feet apart under the drive. I ran 75
feet of wire to the
house. I made up a Osc. circuit that runs at about 60K Hz. Right now that
feeds a frequency
counter and the counter feeds a comparator. the Circuit changes about 600 Hz
when a Car
drives over it the time base for the counter circuit is 1 second. The only
problem I have is that
the frequency drifts over time and I can't figure out why it does not seem
to have anything to
do with temperature or time of day or moisture.

1997\10\07@082713 by Steven J Tucker

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On Wed, 5 Feb 1997, Richard Wamser wrote:

> the frequency drifts over time and I can't figure out why it does not seem
> to have anything to
> do with temperature or time of day or moisture.

Have you tried keeping track of the mean temprature vs. the drift?  The
temprature just a few inches underground do not swing so wildly as above.

Steve

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