HowStuffWorks "How does a traffic light detect that a car has pulled up?" - 0 views
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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.
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il 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,
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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 becau
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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 ente
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ch more induct
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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
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5 feet in diameter, containing five or six loops of wire. You cut some grooves in a road and place the
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now" you are there -- the instant you pull up, they change! How do the
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traffic light detect that a car has pulled up and is
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The number of coils
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e material that the coils are wrap
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h the induc
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completely different. The light bulb is a resistor (the resistance creates heat to make the f
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mly. Most of the current should follow the low-resistan
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ulb burns brightly and then gets dimmer. When you ope
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the bulb burns very brightly and then quickly goe
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thing from lasers to rubber hoses filled with air! By far the most common
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are all sorts of te
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e lights don't have any sort of detectors. For example, in a large city, the traffic lights may simply operate on timers -- no matter
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e is something exotic about the tra
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detect your presence?
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r detecti
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In the subu
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nductive loop. An inductive lo
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face. To install the loop, they lay th
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the road
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e embedded
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work by detecting a change of inductance. To understand the process, let's first look at what inductance is. The illustration on this page is helpful.
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see here is a battery, a light bulb, a c
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e is a normal flashlig
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take the inductor out of this circuit, then w
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circuit as sho
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in the bulb glo
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, the behavior is
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he wire in the coil has much lower resistance (it's just wire), so what you would expect when you turn on the switc
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appens instead is that when you close the switch,
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n the switch,
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he capacity of an inductor is controlled by two factors
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ped around (the core)
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ance than air or any other non-magnetic core would.
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hat can measure the inductance of a coil,
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n inductor gives it mu
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what time of day it is,
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country roads,
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there is going to be a lot of tra
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bs and
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ed a turn lane (in order to activate the arrow light).
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asphalt
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nductive loops
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bvious.
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se the compound i
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the light to change? #brandscape{ position: fixed; text-align: center; margin:0px auto; top: 0px; z-index:0; cursor: pointer; background-image: url(http://static.ddmcdn.com/gif/hsw-sysk-brandscape-07-03-12a.jpg ); background-position: top center; } #root{ width:60px; }
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