Finding a loop detector

Many traffic lights are triggered by loop detectors in the pavement - coils of wire in various configurations, carrying high frequency alternating current. When a suitable conductive shape is present on the road above the coil, it changes the circuit inductance and its resonant frequency. The light controller senses this and (eventually) gives a green light to the vehicle that's been detected.

Bicycles (and sometimes motorcycles) are sometimes not detected. Most loop designs have spots that are more sensitive, where a two-wheeler has the best chance of getting detected. There are MUTCD-approved markings to indicate these spots, and I'm working to get them applied in our village.

But it gets more difficult when the road's been repaved and the cuts for the coils are no longer visible. I don't have that problem here in town, but I know of one problem light a few miles away where the sweet spot really needs to be marked. Also, when marking roads for a club's century ride, it might be handy to do some guerrilla marking at certain intersections, to reduce the chance of riders running red lights.

So: Anyone know the easiest way to determine the exact location of such a buried coil? I wonder if - by chance - a carpenter's electronic stud locator might work. If not, what? Perhaps a coil of wire to act as a transformer's secondary coil, connected to an LED? Other ideas? I'd like something small enough to fit in a bike bag.

- Frank Krygowski

On Monday, October 14, 2013 10:10:57 AM UTC-7, Frank Krygowski wrote:
Many traffic lights are triggered by loop detectors in the pavement - coils of wire in various configurations, carrying high frequency alternating current. When a suitable conductive shape is present on the road above the coil, it changes the circuit inductance and its resonant frequency. The light controller senses this and (eventually) gives a green light to the vehicle that's been detected.



Bicycles (and sometimes motorcycles) are sometimes not detected. Most loop designs have spots that are more sensitive, where a two-wheeler has the best chance of getting detected. There are MUTCD-approved markings to indicate these spots, and I'm working to get them applied in our village.



But it gets more difficult when the road's been repaved and the cuts for the coils are no longer visible. I don't have that problem here in town, but I know of one problem light a few miles away where the sweet spot really needs to be marked. Also, when marking roads for a club's century ride, it might be handy to do some guerrilla marking at certain intersections, to reduce the chance of riders running red lights.



So: Anyone know the easiest way to determine the exact location of such a buried coil? I wonder if - by chance - a carpenter's electronic stud locator might work. If not, what? Perhaps a coil of wire to act as a transformer's secondary coil, connected to an LED? Other ideas? I'd like something small enough to fit in a bike bag.

How about a compass. Doesn't the loop create a magnetic field?

-- Non Engineer

On 10/14/2013 12:15 PM, Jay Beattie wrote:
On Monday, October 14, 2013 10:10:57 AM UTC-7, Frank Krygowski wrote:
Many traffic lights are triggered by loop detectors in the pavement - coils of wire in various configurations, carrying high frequency alternating current. When a suitable conductive shape is present on the road above the coil, it changes the circuit inductance and its resonant frequency. The light controller senses this and (eventually) gives a green light to the vehicle that's been detected.



Bicycles (and sometimes motorcycles) are sometimes not detected. Most loop designs have spots that are more sensitive, where a two-wheeler has the best chance of getting detected. There are MUTCD-approved markings to indicate these spots, and I'm working to get them applied in our village.



But it gets more difficult when the road's been repaved and the cuts for the coils are no longer visible. I don't have that problem here in town, but I know of one problem light a few miles away where the sweet spot really needs to be marked. Also, when marking roads for a club's century ride, it might be handy to do some guerrilla marking at certain intersections, to reduce the chance of riders running red lights.



So: Anyone know the easiest way to determine the exact location of such a buried coil? I wonder if - by chance - a carpenter's electronic stud locator might work. If not, what? Perhaps a coil of wire to act as a transformer's secondary coil, connected to an LED? Other ideas? I'd like something small enough to fit in a bike bag.

How about a compass. Doesn't the loop create a magnetic field?

It detects a change in the magnetic field caused by metal passing over.

I think the easiest way to detect the metal in the coil would be with a
metal detector.

On Mon, 14 Oct 2013 12:15:03 -0700, Jay Beattie wrote:

On Monday, October 14, 2013 10:10:57 AM UTC-7, Frank Krygowski wrote:
Many traffic lights are triggered by loop detectors in the pavement -
coils of wire in various configurations, carrying high frequency
alternating current. When a suitable conductive shape is present on
the road above the coil, it changes the circuit inductance and its
resonant frequency. The light controller senses this and (eventually)
gives a green light to the vehicle that's been detected.



Bicycles (and sometimes motorcycles) are sometimes not detected. Most
loop designs have spots that are more sensitive, where a two-wheeler
has the best chance of getting detected. There are MUTCD-approved
markings to indicate these spots, and I'm working to get them applied
in our village.



But it gets more difficult when the road's been repaved and the cuts
for the coils are no longer visible. I don't have that problem here in
town, but I know of one problem light a few miles away where the sweet
spot really needs to be marked. Also, when marking roads for a club's
century ride, it might be handy to do some guerrilla marking at certain
intersections, to reduce the chance of riders running red lights.



So: Anyone know the easiest way to determine the exact location of
such a buried coil? I wonder if - by chance - a carpenter's electronic
stud locator might work. If not, what? Perhaps a coil of wire to act
as a transformer's secondary coil, connected to an LED? Other ideas?
I'd like something small enough to fit in a bike bag.

How about a compass. Doesn't the loop create a magnetic field?

-- Non Engineer

Not usefully - since a compass responds to an essentially static field,
and the excitation is, as has been stated, at a high frequency. And the
presence of power line wiring (with likely iron-containing support cables for
wire bundles and pole/supports) frequently causes problems at intersections.

I've had some success bugging local authorities to put a painted marker on
the roadway to indicate the most sensitive pickup spot. Mostly these have
been done already in this city.

Jay Beattie writes:

On Monday, October 14, 2013 10:10:57 AM UTC-7, Frank Krygowski wrote:
Many traffic lights are triggered by loop detectors in the pavement - coils of wire in various configurations, carrying high frequency alternating current. When a suitable conductive shape is present on the road above the coil, it changes the circuit inductance and its resonant frequency. The light controller senses this and (eventually) gives a green light to the vehicle that's been detected.



Bicycles (and sometimes motorcycles) are sometimes not detected. Most loop designs have spots that are more sensitive, where a two-wheeler has the best chance of getting detected. There are MUTCD-approved markings to indicate these spots, and I'm working to get them applied in our village.



But it gets more difficult when the road's been repaved and the cuts for the coils are no longer visible. I don't have that problem here in town, but I know of one problem light a few miles away where the sweet spot really needs to be marked. Also, when marking roads for a club's century ride, it might be handy to do some guerrilla marking at certain intersections, to reduce the chance of riders running red lights.



So: Anyone know the easiest way to determine the exact location of such a buried coil? I wonder if - by chance - a carpenter's electronic stud locator might work. If not, what? Perhaps a coil of wire to act as a transformer's secondary coil, connected to an LED? Other ideas? I'd like something small enough to fit in a bike bag.

How about a compass. Doesn't the loop create a magnetic field?

It does, but AC is used to generate the field. The frequency
is around 20kHz. The average magnetic field, which a compass
would respond to, is zero. So a compass won't work.

--
Joe Riel

On 15/10/13 04:10, Frank Krygowski wrote:
Many traffic lights are triggered by loop detectors in the pavement -
coils of wire in various configurations, carrying high frequency
alternating current. When a suitable conductive shape is present on
the road above the coil, it changes the circuit inductance and its
resonant frequency. The light controller senses this and
(eventually) gives a green light to the vehicle that's been
detected.

Bicycles (and sometimes motorcycles) are sometimes not detected.
Most loop designs have spots that are more sensitive, where a
two-wheeler has the best chance of getting detected. There are
MUTCD-approved markings to indicate these spots, and I'm working to
get them applied in our village.

But it gets more difficult when the road's been repaved and the cuts
for the coils are no longer visible. I don't have that problem here
in town, but I know of one problem light a few miles away where the
sweet spot really needs to be marked. Also, when marking roads for a
club's century ride, it might be handy to do some guerrilla marking
at certain intersections, to reduce the chance of riders running red
lights.

So: Anyone know the easiest way to determine the exact location of
such a buried coil? I wonder if - by chance - a carpenter's
electronic stud locator might work. If not, what? Perhaps a coil of
wire to act as a transformer's secondary coil, connected to an LED?
Other ideas? I'd like something small enough to fit in a bike bag.


Possibly with another coil.

A metal detector might be "confused" by the alternating magnetic field
generated by the loop detector if it is in use at the time, so it
*might* be necessary to turn off the loop detector energization.

It might be possible to detect with a hand wound coil and an AC microamp
meter while it's energised.

Without knowing the excitation current, number of turns, depth in the
road and frequency, etc, it's hard to give a definitive answer.

As a 5th year project I designed, built and tested a device for
measuring magnetic fields in 3 dimensions, generated by mains currents
(50-60Hz). It had 3 coils and 3 separate amplifier channels, each with
switchable gain up to 10,000, and an ADC. There was a microprocessor to
record waveforms and software grab the data from the micro and plot the
results. Something similar would work for this, and allow you to "see"
the magetic field lines, I expect.

--
JS

James wrote in :

Without knowing the excitation current, number of turns, depth in the
road and frequency, etc, it's hard to give a definitive answer.

As a 5th year project I designed, built and tested a device for
measuring magnetic fields in 3 dimensions, generated by mains currents
(50-60Hz). It had 3 coils and 3 separate amplifier channels, each with
switchable gain up to 10,000, and an ADC. There was a microprocessor to
record waveforms and software grab the data from the micro and plot the
results. Something similar would work for this, and allow you to "see"
the magetic field lines, I expect.

Is it possible to manafacture a low cost battery operated device which
triggers the induction loops? If this is possible, it would save
cyclists fom the engineers who don't make the detectors very sensitive.

gotta tamper ?

tamp tamp tamp tamp LIGHT

BEING AN ADVANCED Homo Sapiens 2 ( AND FROM Louisiana), I feeeeel the circuit changes n jump into green zoooooooooooooooming off into the distance while lesser muggles sit on the line.

OFF course knowledge of my ability is known and we have fun with it....THE LIGHT CHALLLENGE..a few false starts and once in a while a solid strike by another driver.

Several show talent but.....aha ! move out when the turn lane circuit clicks green.

try it.

no cheating looking for the adjacent yellow.

On Monday, October 14, 2013 7:48:24 PM UTC-4, Peter Gordon wrote:

Is it possible to manafacture a low cost battery operated device which
triggers the induction loops? If this is possible, it would save
cyclists fom the engineers who don't make the detectors very sensitive.

I've wondered about that, and proposed such a project to an Electrical Engineering masters degree student for her project. Unfortunately, she chose something else.

But in looking around, I learned that in the U.S., at least, there is a patent on this concept that I consider illegitimate, one that exists only to pry royalties from an inventor who might really work this out and market it. The patent (as I recall) is for a metal plate of suitable thickness, attached to a bicycle by a cord or other device, that the bike rider would throw down on the ground to trigger a traffic light, then haul back up by the cord. The patent specifically states that it's for this OR ANY OTHER METHOD of triggering a traffic light sensor.

The fact that such a patent can exist is pretty clear indication that our patent laws are broken.

- Frank Krygowski

datakoll wrote:
gotta tamper ?

tamp tamp tamp tamp LIGHT

BEING AN ADVANCED Homo Sapiens 2 ( AND FROM Louisiana), I feeeeel the circuit
snip

Where in Louisiana? Don't recognize the accent. lol.
--
duane