1 00:00:00,560 --> 00:00:03,600 Speaker 1: Welcome to brain Stuff from how stuff works dot com 2 00:00:03,600 --> 00:00:14,600 Speaker 1: where smart Happens. Hi, I'm Marshall Brain with today's question, 3 00:00:15,000 --> 00:00:17,840 Speaker 1: how does a traffic light detect that a car is 4 00:00:17,880 --> 00:00:20,439 Speaker 1: pulled up and is waiting for the light to change? 5 00:00:21,079 --> 00:00:24,439 Speaker 1: There's something exotic about the traffic lights that know you're 6 00:00:24,520 --> 00:00:27,920 Speaker 1: there the instant you pull up they change. How do 7 00:00:28,000 --> 00:00:32,080 Speaker 1: they detect your presence? Some lights don't have any sort 8 00:00:32,120 --> 00:00:35,080 Speaker 1: of detectors. For example, in a large city, the traffic 9 00:00:35,159 --> 00:00:38,520 Speaker 1: lights may simply operate on timers. No matter what time 10 00:00:38,520 --> 00:00:41,239 Speaker 1: of day it is, there's going to be traffic, so 11 00:00:41,280 --> 00:00:44,640 Speaker 1: the traffic lights just use their timers. In the suburbs 12 00:00:44,640 --> 00:00:48,639 Speaker 1: and out on country roads, however, detectors are common. They 13 00:00:48,680 --> 00:00:51,920 Speaker 1: may detect when a car arrives in an intersection, when 14 00:00:52,000 --> 00:00:54,800 Speaker 1: too many cars are stacked up at an intersection to 15 00:00:54,840 --> 00:00:57,480 Speaker 1: control the length of the light, or when cars have 16 00:00:57,560 --> 00:01:00,560 Speaker 1: entered to turn lane in order to activate that arrow light. 17 00:01:01,240 --> 00:01:05,040 Speaker 1: There are all sorts of technologies for detecting cars, everything 18 00:01:05,120 --> 00:01:08,880 Speaker 1: from lasers to rubber hoses filled with air, but by 19 00:01:08,959 --> 00:01:13,000 Speaker 1: far the most common technique is the inductive loop. An 20 00:01:13,000 --> 00:01:16,440 Speaker 1: inductive loop is simply a coil of wire embedded in 21 00:01:16,480 --> 00:01:20,520 Speaker 1: the road's surface. To install the loop, they lay the asphalt, 22 00:01:20,560 --> 00:01:22,640 Speaker 1: and then they come back and cut a groove in 23 00:01:22,680 --> 00:01:26,120 Speaker 1: the asphalt with a big circular saw. The wire is 24 00:01:26,160 --> 00:01:29,280 Speaker 1: placed in the groove and sealed with a rubbery compound. 25 00:01:29,680 --> 00:01:32,920 Speaker 1: You can often see these big rectangular loops cut in 26 00:01:32,959 --> 00:01:38,039 Speaker 1: the pavement because the compound is obvious. Inductive loops work 27 00:01:38,200 --> 00:01:42,360 Speaker 1: by detecting a change in inductance. Any coil of wire 28 00:01:42,440 --> 00:01:45,320 Speaker 1: creates an inductor, so the coil of wire in the 29 00:01:45,440 --> 00:01:49,520 Speaker 1: road is a giant inductor. The capacity of an inductor 30 00:01:49,600 --> 00:01:52,640 Speaker 1: is controlled by two things, the number of coils of 31 00:01:52,680 --> 00:01:56,520 Speaker 1: wire and the material that the coils are wrapped around, 32 00:01:56,680 --> 00:02:00,440 Speaker 1: also known as the core. Putting iron in the core 33 00:02:00,480 --> 00:02:03,840 Speaker 1: of an inductor gives it much more inductance than air would. 34 00:02:04,400 --> 00:02:07,640 Speaker 1: There are devices that can measure the inductance of a coil, 35 00:02:07,960 --> 00:02:12,040 Speaker 1: and the standard unit of inductance is the Henry. So 36 00:02:12,480 --> 00:02:14,919 Speaker 1: let's say that you take a coil of wire, perhaps 37 00:02:15,000 --> 00:02:18,720 Speaker 1: five feet in diameter, containing five or six loops of wire. 38 00:02:19,200 --> 00:02:21,440 Speaker 1: You cut some grooves in the road, and you place 39 00:02:21,520 --> 00:02:25,160 Speaker 1: the coil in the grooves. You attach an inductance meter 40 00:02:25,320 --> 00:02:28,040 Speaker 1: to the coil and see what the inductance of the 41 00:02:28,080 --> 00:02:31,280 Speaker 1: coil is. Now, you park a car over that same 42 00:02:31,320 --> 00:02:34,960 Speaker 1: coil and check the inductance again. The inductance will be 43 00:02:35,120 --> 00:02:39,080 Speaker 1: much larger because of the large steel object positioned in 44 00:02:39,160 --> 00:02:42,840 Speaker 1: the loops magnetic field. The car parked over the coil 45 00:02:43,280 --> 00:02:46,079 Speaker 1: is acting like the core of the inductor, and its 46 00:02:46,160 --> 00:02:50,239 Speaker 1: presence changes the inductance of the coil. A traffic light 47 00:02:50,320 --> 00:02:54,559 Speaker 1: sensor uses the loop in that same way. It constantly 48 00:02:54,720 --> 00:02:57,600 Speaker 1: tests the inductance of the loop in the road, and 49 00:02:57,639 --> 00:03:00,760 Speaker 1: when the inductance rises, it no is that there's a 50 00:03:00,840 --> 00:03:04,760 Speaker 1: car waiting. Do you have any ideas or suggestions for 51 00:03:04,800 --> 00:03:07,800 Speaker 1: this podcast? If so, please send me an email at 52 00:03:07,880 --> 00:03:11,000 Speaker 1: podcast at how stuff works dot com. For more on 53 00:03:11,040 --> 00:03:13,400 Speaker 1: this and thousands of other topics, go to how stuff 54 00:03:13,400 --> 00:03:16,160 Speaker 1: works dot com and be sure to check out the 55 00:03:16,160 --> 00:03:18,639 Speaker 1: brain stuff blog on the how stuff works dot com 56 00:03:18,680 --> 00:03:19,239 Speaker 1: home page.