Traffic Lights detecting a car
that has pulled up..!
• 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
• 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
• 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
• These detectors are essentially the same as can be used to measure traffic
• Loop detectors can easily sense different lanes of traffic uniquely, which we
can’t really do with a radar device.
• 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:
• 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
• 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
• 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
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
• 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
• 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.
• 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