How
does a traffic light detect that a car has pulled
up and is waiting for the light to change?
There
is something exotic about the traffic lights that "know" you
are there -- the instant you pull up, they change!
How do they detect your
presence?
Some lights don't have any sort of detectors. For
example, in a large city, the traffic lights may
simply operate on timers -- no matter what time of
day it is, there is going to be a lot of traffic.
In the suburbs and on country roads, however, detectors
are common. They may detect when a car arrives at
an intersection, when too many cars are stacked up
at an intersection (to control the length of the
light), or when cars have entered a turn lane (in
order to activate the arrow light).
There are all sorts of technologies for detecting
cars -- everything from lasers to rubber hoses filled
with air! By far the most common technique is the
inductive loop. An inductive loop is simply a coil
of wire embedded in the road's surface. To install
the loop, they lay the asphalt and then come back
and cut a groove in the asphalt with a saw. The wire
is placed in the groove and sealed with a rubbery
compound. You can often see these big rectangular
loops cut in the pavement because the compound is
obvious.
Inductive loops work by detecting a change of inductance.
To understand the process, let's first look at what
inductance is. This figure is helpful:
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continues below
What you see here is a battery, a light bulb, a
coil of wire around a piece of iron (yellow), and
a switch. The coil of wire is an inductor. If you
have read How Electromagnets Work, you will also
recognize that the inductor is an electromagnet.
If you were to take the inductor out of this circuit,
then what you have is a normal flashlight. You close
the switch and the bulb lights up. With the inductor
in the circuit as shown, the behavior is completely
different. The light bulb is a resistor (the resistance
creates heat to make the filament in the bulb glow).
The wire in the coil has much lower resistance (it's
just wire), so what you would expect when you turn
on the switch is for the bulb to glow very dimly.
Most of the current should follow the low-resistance
path through the loop. What happens instead is that
when you close the switch, the bulb burns brightly
and then gets dimmer. When you open the switch, the
bulb burns very brightly and then quickly goes out.
The reason for this strange behavior is the inductor.
When current first starts flowing in the coil, the
coil wants to build up a magnetic field. While the
field is building, the coil inhibits the flow of
current. Once the field is built, then current can
flow normally through the wire. When the switch gets
opened, the magnetic field around the coil keeps
current flowing in the coil until the field collapses.
This current keeps the bulb lit for a period of time
even though the switch is open.
The capacity of an inductor is controlled by two
factors:
Putting iron in the core of an inductor gives it
much more inductance than air or any other non-magnetic
core would. There are devices that can measure the
inductance of a coil, and the standard unit of measure
is the henry.
So... Let's say you take a coil of wire perhaps
5 feet in diameter, containing five or six loops
of wire. You cut some grooves in a road and place
the coil in the grooves. You attach an inductance
meter to the coil and see what the inductance of
the coil is. Now you park a car over the coil and
check the inductance again. The inductance will be
much larger because of the large steel object positioned
in the loop's magnetic field. The car parked over
the coil is acting like the core of the inductor,
and its presence changes the inductance of the coil.
A traffic light sensor uses the loop in that same
way. It constantly tests the inductance of the loop
in the road, and when the inductance rises, it knows
there is a car waiting!
