Traffic Lights detecting a car that has pulled up..!

Vaishnavi Chigarapalle

Traffic Lights

• Traffic lights, also known as traffic signals, traffic lamps, and signal lights, and

also known technically as traffic control signals.

• These are signaling devices positioned at road intersection, pedestrian crossings

and other locations to control competing flows of traffic.

• Traffic lights alternate the right of way accorded to road users by displaying

lights of a standard color (red, orange, and green) following a universal color

code.

• Green light: Allows traffic to proceed in the direction denoted.

• Orange Light: Denoting prepare to stop short of the intersection.

• Red Light: Prohibits any traffic from proceeding.

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).

• They may also detect when cars have entered a turn lane ( in order to activate

the arrow light).

Technologies for Detecting cars

• There are all sorts of technologies for detecting cars.

• They range from lasers to rubber hoses filled with air.

• By far the most common technique is the inductive loop.

• They can be detected using the rubber hoses, filled with air, which can be

squashed by a car running over it.

• Some traffic lights use a Doppler Radar module.

• These use radio waves at 10GHz to 30 GHz and detect the radio waves

reflected back off the vehicle.

• Because they beat the received signal against a portion of that transmitted, they

only detect motion.

• And by using quadrature receivers, can distinguish between towards and away

motion.

• These detectors are essentially the same as can be used to measure traffic

speed.

• Loop detectors can easily sense different lanes of traffic uniquely, which we

can’t really do with a radar device.

Common Technology: 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 then cut a

groove in the asphalt with a saw.

• The wire is placed in the groove and sealed with a rubbery compound.

• We 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, we need to look at what an inductance is.

• The illustration is as follows:

Explanation

• What we 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.

• The inductor is an electromagnet.

• If we were to take out the inductor out of this circuit, then what we have is a

normal flashlight.

• We close the switch and the bulb lights up.

• With the inductor in the circuit as shown, the behavior is completely different.

• The light bulb here is a resistor.

• The resistance creates heat to make the filament in the bulb glow.

• The wire in the coil has much lower resistance.

• So what we would expect when we 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.

• When we close the switch, instead of getting dimmer the light first burns

brightly and then gets dimmer.

• When we open the switch, the bulb burns very brightly and then quickly goes

out.

Reason for the strange behavior

• 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.

• The 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:

– The number of coils.

– The material that the coils are wrapped around the coil.

• Putting iron in the core of an inductor gives it much more inductance.

• Much more than air or any other non-magnetic core wound.

• There are devices that can measure the inductance of a coil.

• The standard unit of measure is the henry.

Example

• Let’s say we take a coil of wire that is 5 feet in diameter.

• This coil of wire consists of five or six loops of wire.

• We cut some grooves in a road and place the coil in the grooves.

• We then attach an inductance meter to the coil and see what the inductance of

the coil is.

• Now 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.

• When the inductance rises, it knows there is a car waiting!

• So next time you are stopped at a red light and are in a hurry, try putting the

car in reverse.

• This helps to trip up the detector below you.

• Or if we are at a traffic light that has a headlight sensor, try flicking your high

beams on and off to trip the light sensor.

• We never know .. We may speed up the process to getting the light to change

green.

Sources

• http://en.wikipedia.org/wiki/Traffic_light

• http://auto.howstuffworks.com/car-driving-safety/safety-regulatory-d

• http://images.google.com/

• http://www.thenakedscientists.com/forum/index.php?topic=13783.0

• http://www.curiousread.com/2008/01/its-go-time-how-do-traffic-light.html