WEBVTT Kind: captions Language: en 00:00:05.074 --> 00:00:06.040 [ambient noise] 00:00:06.040 --> 00:00:08.818 My car knows when it’s wet. 00:00:08.818 --> 00:00:11.508 That’s right, it’s got automatic windshield wipers. 00:00:11.508 --> 00:00:15.002 When they’re set to automatic, and the car gets wet, they wipe. 00:00:15.002 --> 00:00:16.668 How it know that? 00:00:16.668 --> 00:00:18.397 It’s actually really simple. 00:00:18.397 --> 00:00:19.704 If you look at the windshield, 00:00:19.704 --> 00:00:22.915 you’ll notice an area near the top center which is black. 00:00:22.915 --> 00:00:28.332 And if you look closelier you’ll see the component the car uses to determine windshield wetness. 00:00:28.761 --> 00:00:30.126 Oh, the camera? 00:00:30.126 --> 00:00:33.027 No, it's not the camera - that’d be silly! 00:00:33.080 --> 00:00:39.295 Then you’d have to devote some precious clock cycles away from the already wildly complicated computer vision tasks 00:00:39.295 --> 00:00:42.220 which help keep the car centered in its lane. 00:00:42.220 --> 00:00:43.459 That seems like a weird decision 00:00:43.459 --> 00:00:47.739 and I don’t even know how well that could possibly work no matter how many software updates you throw at it. 00:00:48.614 --> 00:00:51.596 No, I’m not alluding to anything specific, why on Earth would you think that? 00:00:51.596 --> 00:00:59.767 Right above that camera which looks forward at the road you’ll see a little circular spot with some spots within the spot and that, viewers, 00:00:59.767 --> 00:01:01.712 is the rain sensor. 00:01:01.712 --> 00:01:04.841 Look, I get it wet - and the car wipes! 00:01:04.841 --> 00:01:06.469 But two can play at that game. 00:01:07.721 --> 00:01:11.882 You may think this is wildly complicated and expensive technology 00:01:11.882 --> 00:01:18.912 but actually it’s just two of the simplest electronic components you can imagine working together in a very clever way. 00:01:18.912 --> 00:01:25.207 Below the dark portions of the rain sensor are infrared LEDs shining light up into the glass, 00:01:25.207 --> 00:01:29.190 and adjacent to them are photodiodes which detect that light. 00:01:29.190 --> 00:01:31.731 Now, that bright spot at the bottom? 00:01:31.731 --> 00:01:33.842 That’s not actually part of the rain sensor. 00:01:33.920 --> 00:01:38.576 It’s an ambient light sensor used to control the car’s headlights automatically. 00:01:38.576 --> 00:01:41.515 The two components just live in the same assembly. 00:01:41.515 --> 00:01:44.734 The parts that make up the rain sensor aren’t really visible 00:01:44.734 --> 00:01:50.433 because they’re below a filter which blocks visible light and thus it appears black to our eyes. 00:01:51.771 --> 00:01:55.143 That’s strange, didn’t we just encounter that with stovetops? 00:01:55.143 --> 00:01:57.527 Weird connections happening lately. 00:01:57.527 --> 00:02:02.772 To demonstrate how that detects rain, I’ll use a piece of glass and a laser pointer. 00:02:02.772 --> 00:02:06.614 If I shine the laser up at the glass at about a 45 degree angle, 00:02:06.614 --> 00:02:11.420 you’ll notice that a spot of laser light is reflected back down towards the table. 00:02:11.420 --> 00:02:14.515 I’ll put a piece of paper there to make it more visible. 00:02:14.515 --> 00:02:17.408 Now watch what happens when I get the glass wet. 00:02:18.334 --> 00:02:19.786 Not a whole lot. 00:02:20.386 --> 00:02:21.648 That’s a bummer. 00:02:21.648 --> 00:02:27.507 Ah, but this isn’t actually an accurate demonstration of the principles involved in the rain sensor. 00:02:27.507 --> 00:02:31.393 Rather than look at the light reflected back downward by the glass, 00:02:31.393 --> 00:02:36.554 let’s look at the spot produced above the glass by  the laser light which makes it through. 00:02:36.554 --> 00:02:38.014 Now watch closely. 00:02:38.460 --> 00:02:41.036 Did you see that? It moved. 00:02:41.036 --> 00:02:48.991 That happened because the water on top of the glass changed the angle of refraction as light left the glass. 00:02:48.991 --> 00:02:53.397 Now, like some politicians, I’m really not really good at the whole optics thing 00:02:53.397 --> 00:02:55.858 so I’m not getting too deep into the weeds here. 00:02:55.858 --> 00:03:01.042 But as light passes from one medium to another, its speed actually changes 00:03:01.042 --> 00:03:05.768 and because of… reasons that means it appears to change direction. 00:03:05.768 --> 00:03:07.967 That concept is known as refraction. 00:03:07.967 --> 00:03:12.485 And the refraction index of water is different from that of air. 00:03:12.485 --> 00:03:22.502 That means the change in speed and thus change in refraction angle is different between light passing from glass into air and light passing from glass into water. 00:03:22.880 --> 00:03:25.946 The rain sensor, though, is entirely under the glass 00:03:25.946 --> 00:03:29.779 so it’s not as if it can directly detect that change. 00:03:29.779 --> 00:03:34.080 Instead, it leverages the effect of total internal reflection. 00:03:34.080 --> 00:03:40.090 If you’ve ever gone swimming in a pool with goggles on and looked up at the surface of the water while submerged, 00:03:40.090 --> 00:03:44.061 you may have noticed that it looks like a mirror from shallow angles. 00:03:44.061 --> 00:03:46.487 Here’s some stock footage to illustrate. 00:03:46.487 --> 00:03:54.067 That’s total internal reflection at work - the water directly above you transmits light just fine and you can see through it, 00:03:54.067 --> 00:04:00.377 but there’s a critical angle at which the air-water interface will reflect all light back down 00:04:00.377 --> 00:04:04.481 and since it’s smooth like glass, it looks like a mirror. 00:04:04.481 --> 00:04:07.975 So now - how can the rain sensor take advantage of that? 00:04:07.975 --> 00:04:11.374 Well, first, since the module isn’t part of the glass, 00:04:11.374 --> 00:04:20.067 we need an optical interface material to couple the sensor to the glass and remove the air-to-glass optical interface on the bottom. 00:04:20.067 --> 00:04:26.184 That takes the form of a sticky gel which, now that it’s in-place, you can’t see. 00:04:26.184 --> 00:04:29.082 Which, really, is the point - when it’s doing its job, 00:04:29.082 --> 00:04:31.842 the sensor and the glass become one 00:04:31.842 --> 00:04:33.854 (from an optical perspective). 00:04:33.854 --> 00:04:37.788 Now the key is that the infrared LEDs that we talked about earlier 00:04:37.788 --> 00:04:44.476 are shining up at the glass just shallow of the critical angle of total internal reflection. 00:04:44.476 --> 00:04:51.893 So, all the light they produce will simply bounce off the top surface of the the glass and head back downward. 00:04:51.893 --> 00:04:55.884 And, at the other end of that beam of light, is a photodiode. 00:04:55.884 --> 00:04:59.982 That will then detect the light being generated by the LED. 00:04:59.982 --> 00:05:03.291 And to make sure it’s detecting that light and not something else, 00:05:03.291 --> 00:05:09.157 the LEDs are actually rapidly blinking and the photodiode is looking for that pattern of blinks. 00:05:09.157 --> 00:05:16.948 That’s a simpler way of saying the LEDs are producing a specific modulated signal that the photodiode is tuned to detect. 00:05:16.948 --> 00:05:24.925 But because the angle between the photodiode and  the LED is so close to the critical angle of total internal reflection, 00:05:24.925 --> 00:05:33.133 if conditions should change the intensity of that signal being received by the photodiode will also change. 00:05:33.133 --> 00:05:34.472 What condition? 00:05:34.472 --> 00:05:36.184 Condition wet. 00:05:36.184 --> 00:05:44.455 Water on the glass, as we saw in the earlier demo, changes the refraction angle because its refractive index is different from air. 00:05:44.455 --> 00:05:50.056 And it will change it just enough to break the effect of total internal reflection. 00:05:50.056 --> 00:05:58.015 Wet glass will allow some of the light produced by the LEDs  to actually escape and be transmitted through to the outside surface. 00:05:58.015 --> 00:06:03.132 Which in turn means there’s less light bouncing down to the photodiode to be detected, 00:06:03.132 --> 00:06:06.582 so the signal it was receiving begins to diminish. 00:06:06.582 --> 00:06:14.330 And, with a little logic circuitry and programming, you can use that loss of signal as a measurement of windshield wetness. 00:06:14.330 --> 00:06:17.058 And honestly… that’s it! 00:06:17.058 --> 00:06:18.491 It’s that simple. 00:06:18.491 --> 00:06:24.111 The sensor starts to see the signal from the LED drop out so it says to the windshield wiper motor - 00:06:24.111 --> 00:06:25.321 wipe. 00:06:25.321 --> 00:06:29.738 And the wetter the glass, the more the signal drops, so the more it wipes. 00:06:29.738 --> 00:06:37.064 And you’ll notice the sensor is deliberately placed in the path of the windshield wipers so they clear the glass with every wipe. 00:06:37.064 --> 00:06:45.155 That allows the logic to get some sense of how hard it might be raining based on how quickly the signal drops out after every wiping event. 00:06:45.240 --> 00:06:48.594 The only other complexity here is calibration. 00:06:48.594 --> 00:06:55.251 Every time the sensor powers up, it’s got to pick a baseline  signal level which it deems “dry.” 00:06:55.251 --> 00:07:00.880 Yet, if you are out and about in the rain and you turn your car on with the windshield already wet, 00:07:00.880 --> 00:07:05.095 it’s going to start out with some lower signal level than typical. 00:07:05.095 --> 00:07:12.080 So, somebody smarter than me probably established a minimum signal dropout level which always causes a wipe,   00:07:12.080 --> 00:07:17.121 and in fact I’m certain I’ve encountered the  car do a single wipe after turning it on 00:07:17.121 --> 00:07:19.889 when it didn’t really need to. 00:07:19.889 --> 00:07:23.680 But then it can reset its calibration after that wiping event. 00:07:23.680 --> 00:07:28.117 And, there’s another trick up its sleeve, and it's my very favorite one: 00:07:28.117 --> 00:07:29.520 having two of them. 00:07:30.160 --> 00:07:33.109 There are actually two sensor assemblies under here. 00:07:33.109 --> 00:07:37.654 You can barely make out the infrared light in this smartphone camera footage. 00:07:37.654 --> 00:07:39.751 You'll see there's two spots. 00:07:39.751 --> 00:07:44.223 If the windshield is dry, the two sensors should see very similar signals 00:07:44.223 --> 00:07:50.352 but if there’s even just a tiny bit of water on top of one of them, or differing amounts between the two, 00:07:50.352 --> 00:07:56.250 then the two signals will be very different and it will know immediately that the windshield is wet. 00:07:56.250 --> 00:07:58.988 Which calibration method is it actually using? 00:07:58.988 --> 00:08:00.438 I do not know. 00:08:00.438 --> 00:08:09.360 And if you’ve never lived with a car with automatic wipers, you may wish to know they are great but not exactly perfect. 00:08:09.360 --> 00:08:15.646 You are afforded a sensitivity adjustment on the wiper stalk, so you can fine-tune its behavior to your liking. 00:08:15.646 --> 00:08:19.883 But while the wipers pretty much always come on when they need to, 00:08:19.883 --> 00:08:27.665 sometimes they go absolutely nuts when it’s barely raining or they get a little lazy in a sudden downpour. 00:08:27.665 --> 00:08:34.252 Lucky for me, if that happens all I have to do is just slap this stalk which is right next to my hand on the steering wheel 00:08:34.252 --> 00:08:37.167 and the wipers will come on full-steam! 00:08:37.167 --> 00:08:39.287 Handy user interface, those stalks. 00:08:39.287 --> 00:08:42.173 They’re always there and muscle memory takes over! 00:08:42.173 --> 00:08:45.788 I’m pro-stalks, is what I’m saying. It’s a good idea. 00:08:45.788 --> 00:08:53.623 What’s less of a good idea, though, is the Konami-code like trick I had to look up just to change my car’s wiper blades. 00:08:53.623 --> 00:09:00.993 The hood blocks you from lifting the wiper arms so you need them to stop in the up position in order to change them. 00:09:00.993 --> 00:09:06.199 and despite digging through all the settings menus I couldn’t find a “change wipers” option. 00:09:06.199 --> 00:09:12.399 So I tried just switching the car off with the wipers in the up position, y'know just time it right, using the power button, 00:09:12.399 --> 00:09:15.527 but it outsmarted me and parked them anyway. 00:09:15.527 --> 00:09:23.154 It turns out what you have to do is turn the car off and then hold the wiper stalk up in the mist position for a few moments 00:09:23.154 --> 00:09:26.102 and then the car will do this. 00:09:26.102 --> 00:09:28.640 Is this procedure in the owner’s manual? 00:09:30.240 --> 00:09:33.640 Yes, but who looks there for information? 00:09:33.640 --> 00:09:37.824 I usually like to end my videos with something akin to a moral. 00:09:37.824 --> 00:09:41.015 In that vein, the first lesson today is for me 00:09:41.015 --> 00:09:43.778 and that is to read the freaking manual! 00:09:43.778 --> 00:09:49.557 There’s even an index in here and finding the appropriate  information would have taken me mere moments. 00:09:49.557 --> 00:09:53.471 People worked hard on this thing and I just… completely ignored it! 00:09:53.471 --> 00:09:56.848 So don’t ignore the manuals in your life. 00:09:56.848 --> 00:09:58.556 And the second moral is… 00:09:58.556 --> 00:10:01.519 don’t ignore what you can do with a few simple components. 00:10:01.519 --> 00:10:03.514 Software isn’t everything. 00:10:04.286 --> 00:10:07.310 That’s not directed at any organization in particular, mind you. 00:10:07.310 --> 00:10:08.992 Just something to keep in mind. 00:10:09.609 --> 00:10:11.118 Did I get to 10 minutes yet? 00:10:11.958 --> 00:10:14.503 ♫ internally smooth jazz ♫ 00:10:15.326 --> 00:10:20.857 Thanks, by the way, to Jason, a supporter on Patreon, for recommending this video topic. 00:10:20.857 --> 00:10:27.793 That happened a long time ago and I’m pretty certain at least one other person has suggested this in the meantime 00:10:27.793 --> 00:10:30.549 so if I forgot who you were, I’m sorry. 00:10:30.549 --> 00:10:33.046 There’s just too many bimetallic strips getting in the way. 00:10:34.675 --> 00:10:37.528 That’s right, it’s got automatic… the thing didn’t start. 00:10:37.528 --> 00:10:40.997 You’ll notice that the spot of laser light is re… oh. 00:10:40.997 --> 00:10:44.038 …conditions should change, the intensity of a- 00:10:45.530 --> 00:10:48.841 Ooh, I needed a comma for my sake 00:10:48.841 --> 00:10:51.883 ...critical angle of total internal reflection 00:10:51.883 --> 00:10:52.948 [annoyance at outside noise] 00:10:52.948 --> 00:10:55.956 ...it’s got to pick a baseline signal lever. 00:10:55.956 --> 00:10:56.806 Signal. 00:10:56.806 --> 00:10:57.619 Level. 00:10:57.619 --> 00:11:00.113 ...had to look, oh yeah I forgo ohyeahIgottagetthething! 00:11:01.176 --> 00:11:05.751 You mean to say all you need is an LED, a photodiode, and a bit of adhesive to take the task of detecting rain away from a sophisticated computer vision system? 00:11:05.751 --> 00:11:08.288 And that this was a technology available back in the nineties? 00:11:08.288 --> 00:11:12.797 That sounds unpossible, I mean if there were a better way to do things, surely it would only come from the halls of one particular company and not anyone else. 00:11:13.003 --> 00:11:15.188 This section is usually for puns but today I chose vicious sarcasm. 00:11:15.188 --> 00:11:17.076 Not directed at any particular organization, of course :)