WEBVTT Kind: captions Language: en 00:00:00.160 --> 00:00:01.724 Motion sensors. 00:00:01.724 --> 00:00:02.946 You’ve seen ‘em. 00:00:02.946 --> 00:00:05.511 But have you ever wondered how they work? 00:00:05.511 --> 00:00:10.874 There are several kinds out there but the most common one is the passive infrared sensor. 00:00:10.874 --> 00:00:16.063 You might already know that these detect your body heat, but how does it do that? 00:00:16.063 --> 00:00:23.243 And what’s going on inside here to, y’know, actually determine that something is moving and not just warm? 00:00:23.243 --> 00:00:29.943 Like so many of the ubiquitous devices in our lives, the fundamental thing this is doing is incredibly simple. 00:00:29.943 --> 00:00:34.823 But look at how all its pieces fit together and you’ll start to uncover something amazing. 00:00:34.823 --> 00:00:37.336 First, let’s talk about you. 00:00:37.336 --> 00:00:39.094 You’re glowing today! 00:00:39.094 --> 00:00:41.402 Quite radiantly, I might add. 00:00:41.402 --> 00:00:42.485 We all are! 00:00:42.485 --> 00:00:44.431 In the infrared. 00:00:44.431 --> 00:00:46.304 To quote some website I found, 00:00:46.304 --> 00:00:51.409 “All matter with a temperature greater than absolute zero emits thermal radiation” 00:00:51.409 --> 00:00:55.084 and we matter! And greater than absolute zero. 00:00:55.084 --> 00:00:58.399 So our bodies radiate some infrared light. 00:00:58.399 --> 00:01:04.361 Now, fun fact about infrared light, when it lands on stuff it heats that stuff up. 00:01:04.361 --> 00:01:11.109 So you literally make everyone and everything around you just a little bit warmer simply by being there. 00:01:11.109 --> 00:01:12.921 And that’s affirmative! 00:01:12.921 --> 00:01:14.784 And also useful! 00:01:14.784 --> 00:01:18.946 Because of a little phenomenon called pyroelectricity. 00:01:18.946 --> 00:01:21.101 Turns out there are these crystals — 00:01:21.101 --> 00:01:23.179 no not like that actual crystals — 00:01:23.179 --> 00:01:26.935 which generate a little bit of voltage when they are heated up. 00:01:26.935 --> 00:01:29.723 Gallium nitride is one such crystal. 00:01:29.723 --> 00:01:36.559 It’s a really, really small effect producing only a wee bit  of voltage so it’s not like they’re useful for doing work 00:01:36.559 --> 00:01:41.081 but thanks to the extremely sensitive field-effect transistor or FET, 00:01:41.160 --> 00:01:45.977 pyroelectric crystals can be used to detect changes in heat. 00:01:45.977 --> 00:01:50.381 Hook one of these crystals up to the gate of a sufficiently sensitive transistor 00:01:50.381 --> 00:01:56.719 and you can produce a large electrical signal when the crystal heats up even just a tiny little bit. 00:01:56.719 --> 00:02:05.539 Since your body is constantly radiating infrared light, that means a pyroelectric crystal could be used to detect your presence. 00:02:05.539 --> 00:02:08.458 Except… no. 00:02:08.458 --> 00:02:12.562 If you glitched into existence right in front of a pyroelectric crystal 00:02:12.562 --> 00:02:17.309 [fwoop] it would produce a blip of voltage as your body heat raised its temperature 00:02:17.309 --> 00:02:20.123 but then… well it would stop. 00:02:20.123 --> 00:02:26.469 It’s only the change in temperature that produces the voltage, otherwise we’d have something of a free energy device on our hands. 00:02:26.469 --> 00:02:30.231 So once your body heat brought it to a new equilibrium temperature, 00:02:30.520 --> 00:02:32.531 the voltage would go away. 00:02:32.531 --> 00:02:35.251 But also, we don’t want that anyway. 00:02:35.251 --> 00:02:41.456 Such a sensor would react to anything near it  that quickly changes temperature - say a heater switching on. 00:02:41.456 --> 00:02:43.729 And that’s not what we’re trying to detect. 00:02:43.729 --> 00:02:45.468 But what if you had… 00:02:45.468 --> 00:02:46.874 two of them? 00:02:46.874 --> 00:02:49.795 If one crystal warmed up but the other didn’t, 00:02:49.795 --> 00:02:54.601 there'd be an imbalance in voltage between the two which could be measured. 00:02:54.601 --> 00:03:04.945 So if you can figure out a way to cause a person moving in front of a device with a pair of pyroelectric crystals to heat only one of them at a time, 00:03:04.945 --> 00:03:07.881 you could detect that person’s movement. 00:03:07.881 --> 00:03:11.023 Which brings us to these: 00:03:11.023 --> 00:03:17.155 here I have two common types of passive infrared sensor, both used in lighting products. 00:03:17.155 --> 00:03:23.944 The translucent piece of plastic you find on the front of both sensors is actually a complicated lens. 00:03:23.944 --> 00:03:31.709 They look cloudy since they’re not transparent to visible light, but this material affects infrared light just as if it were glass. 00:03:31.709 --> 00:03:38.964 And sitting behind the lens (quite far behind it as a matter of fact) is our pair of pyroelectric crystals. 00:03:38.964 --> 00:03:45.123 They’re inside this metal can and hiding underneath a filter which rejects visible light so we can’t see them, 00:03:45.123 --> 00:03:48.745 but I've got a Dremel tool with a cutoff wheel so that won’t stop me! 00:03:48.745 --> 00:03:50.541 And what’s inside? 00:03:50.541 --> 00:03:52.987 The letter H. 00:03:52.987 --> 00:03:58.569 This may not look like much but this is in fact a pair of pyroelectric crystals. 00:03:58.569 --> 00:04:02.639 OK, well the H isn’t - at least, I don’t think it is. 00:04:02.639 --> 00:04:08.444 Full disclosure, I don’t really understand how this is  constructed or indeed what the crystals are - 00:04:08.444 --> 00:04:11.669 it may be gallium nitride, but it could be something else. 00:04:11.669 --> 00:04:14.862 Pyroelectricity is exhibited in lots of stuff. 00:04:14.862 --> 00:04:18.543 And I’m not even sure this is only two crystals. 00:04:18.543 --> 00:04:25.826 If you look closely, you’ll see that there’s a translucent, vaguely shiny and stripy pattern on top of the H. 00:04:25.826 --> 00:04:32.063 That could be a series of crystals, and my intuition tells me the H we see below them 00:04:32.063 --> 00:04:39.649 is a printed pattern on their support which helps maximize  the temperature differential between different areas of the crystal surface 00:04:39.649 --> 00:04:44.615 by absorbing more IR in some spots and reflecting more in others. 00:04:44.615 --> 00:04:49.957 I was looking through some data sheets for similar  parts and this arrangement isn’t always the same, 00:04:49.957 --> 00:04:55.813 which leads me to believe that the actual sensor  element can be tuned for different applications. 00:04:55.813 --> 00:05:02.267 Regardless of the specifics, look closely and  you’ll see that the crystal is sort of floating above… 00:05:02.267 --> 00:05:03.505 something. 00:05:03.505 --> 00:05:06.326 So I broke it to see what that something was. 00:05:06.326 --> 00:05:10.436 And it’s a small transistor package containing a FET. 00:05:10.436 --> 00:05:15.005 Now that we can see everything,  we can figure out how this component works. 00:05:15.005 --> 00:05:20.222 Its three pins which attach it to a circuit board  are labeled V, G, and S. 00:05:20.222 --> 00:05:23.080 That’s voltage, ground, and signal. 00:05:23.080 --> 00:05:31.630 Then if we look back at the component’s topside, we see that the voltage and signal pins are connected to the source and drain of the FET. 00:05:31.630 --> 00:05:36.584 But the ground connection - that went to one side of the crystal element. 00:05:36.584 --> 00:05:42.353 And, before I destroyed it, the other side of the crystal was connected to the gate of the FET. 00:05:42.353 --> 00:05:44.840 Just like I said a few paragraphs ago! 00:05:44.920 --> 00:05:49.760 Now, you might be a bit confused here  because I said there were two crystals   00:05:49.760 --> 00:05:54.900 and yet there’s only one structure connected to one transistor. 00:05:54.900 --> 00:05:57.389 Well, here’s the really clever bit. 00:05:57.389 --> 00:06:02.669 The two crystal elements found on this structure have opposite polarity. 00:06:02.669 --> 00:06:07.721 While one produces a positive voltage,  the other produces a negative voltage. 00:06:07.721 --> 00:06:15.542 This means that if the whole structure heats up, while both crystals will produce some voltage, they perfectly oppose each other. 00:06:15.542 --> 00:06:19.154 And the net effect is… nothing happens. 00:06:19.154 --> 00:06:21.044 Why do we want this? 00:06:21.044 --> 00:06:25.363 Well, let’s say there was a bright flash of light - maybe from lightning. 00:06:25.363 --> 00:06:34.187 That will actually cause a voltage spike in a pyroelectric material because that visible light also becomes heat when it’s absorbed by it. 00:06:34.187 --> 00:06:36.923 But we don’t want the sensor to react to that. 00:06:36.923 --> 00:06:41.033 Thanks to the opposing polarity of the crystals it won’t. 00:06:41.033 --> 00:06:44.479 If the entire sensor area is heated uniformly, 00:06:44.479 --> 00:06:51.542 the equal and opposite voltages produced in the two halves of the pyroelectric element cancel each other out. 00:06:51.542 --> 00:06:52.989 But… 00:06:52.989 --> 00:06:59.793 If it’s only partially heated, then only one crystal produces voltage while the other doesn’t. 00:06:59.793 --> 00:07:05.689 That means there’s an imbalance, so there’s a voltage potential across the entire structure. 00:07:05.689 --> 00:07:10.418 And, since that structure is connected across  ground and the gate of the transistor, 00:07:10.418 --> 00:07:18.120 that difference in heat (and thus difference in potential) will trigger the FET and it will in turn produce a blip on the signal pin. 00:07:18.120 --> 00:07:20.851 Which brings us back to the lens. 00:07:20.851 --> 00:07:23.060 I mean, lenses. 00:07:23.060 --> 00:07:29.551 If I remove this and we look at the backside, you’ll see that this is actually a bunch of Fresnel lenses. 00:07:29.551 --> 00:07:36.128 Counting each section with a circle bit as one lens  means there are twenty-eight lenses in total. 00:07:36.200 --> 00:07:41.720 Unfortunately because this is designed to  bend infrared light and not visible light   00:07:41.720 --> 00:07:46.237 I can’t demonstrate what this lens is actually  doing all that well. 00:07:46.237 --> 00:07:51.636 But if I stick my phone’s flashlight behind it at roughly the same position the crystals sit, 00:07:51.636 --> 00:07:57.000 you might be able to see that only one section of the lens lights up brightly, 00:07:57.000 --> 00:08:01.841 and which section lights up changes as I move the camera around. 00:08:01.841 --> 00:08:07.516 Of course, in practice,  this is designed to collect light and focus it on the sensor 00:08:07.516 --> 00:08:10.404 so what I’m demonstrating here is backwards. 00:08:10.404 --> 00:08:18.401 But hopefully this helps you understand that each section of this lens is looking at a narrow slice of what’s in front of it. 00:08:18.401 --> 00:08:20.437 And what goes in front of it? 00:08:20.437 --> 00:08:21.663 You! 00:08:21.663 --> 00:08:25.820 The infrared light your body is emitting will be collected by these lenses 00:08:25.820 --> 00:08:32.414 and projected onto the inside surface of the enclosure as a series of fuzzy spots. 00:08:32.414 --> 00:08:38.342 Since it’s infrared light, wherever those spots land heats up just a tiny bit. 00:08:38.342 --> 00:08:46.773 And here’s the key: as you move in front of the lenses, those fuzzy spots of infrared light will move, too. 00:08:46.773 --> 00:08:53.825 And some of them will end up sweeping across the crystal face, heating one side first, and then the other. 00:08:53.825 --> 00:09:00.308 That temperature imbalance as the warm spot sweeps across means there’s a voltage imbalance in the crystal, 00:09:00.308 --> 00:09:03.513 and that imbalance is what triggers the transistor. 00:09:03.513 --> 00:09:13.384 With each lens collecting light from a small slice of its field of view, these sweeps of IR across the sensor repeat as you continue moving. 00:09:13.384 --> 00:09:15.320 Do you understand how that works? 00:09:15.320 --> 00:09:18.320 It's tough because I can’t show you this but 00:09:18.320 --> 00:09:21.486 I can show you stock footage of a disco ball! 00:09:21.486 --> 00:09:25.734 As the ball turns (this clip is only seven seconds long, so it’s gonna loop a lot) 00:09:26.066 --> 00:09:28.972 as the ball turns (this clip is only sev… 00:09:28.972 --> 00:09:30.079 sorry couldn’t resist). 00:09:30.079 --> 00:09:37.901 As the ball turns, thanks to the fog machine in the room, we can see the moving beams of light that sweep across the space. 00:09:37.901 --> 00:09:45.218 The same thing would happen if the ball were motionless but the light shining on it was moving around the ball. 00:09:45.218 --> 00:09:51.509 And if we look at the walls of the room, you’ll see that those spots of light are sweeping across the walls. 00:09:51.509 --> 00:09:56.520 This motion sensor is basically exactly that, but in reverse. 00:09:56.520 --> 00:10:03.629 The lens is collecting your body heat and creating hot spots inside this enclosure, focused on the sensor. 00:10:03.629 --> 00:10:07.462 As you move in front of this thing, those spots move, too. 00:10:07.462 --> 00:10:16.059 When they sweep across the two halves of the pyroelectric crystal, it generates enough voltage to trigger the transistor and produce a signal blip. 00:10:16.059 --> 00:10:21.560 Then, the electronics inside the device will…  do whatever it is they’re supposed to do. 00:10:21.640 --> 00:10:27.456 And by the way, infrared motion sensors can be more like disco balls than you may realize. 00:10:27.456 --> 00:10:33.909 When the front face of one of these looks uniform rather than segmented, sometimes it isn’t a lens. 00:10:33.909 --> 00:10:37.868 Instead it might be a simple window that’s transparent to infrared, 00:10:37.868 --> 00:10:45.053 and behind it is a segmented mirror focusing and reflecting light onto the pyroelectric sensor. 00:10:45.053 --> 00:10:49.029 Those mirror segments reflect your body heat onto the face of the crystal, 00:10:49.029 --> 00:10:56.982 and just like the segmented lens, the spots of heat your body generates on the crystal move across it as you move around. 00:10:56.982 --> 00:11:03.280 A sweep across produces a voltage imbalance, which triggers the FET, and the electronics react accordingly. 00:11:03.280 --> 00:11:09.626 Speaking of electronics, this module is the  kind of module you typically find on an outdoor floodlight. 00:11:09.626 --> 00:11:12.520 And it has a distance adjustment. 00:11:12.520 --> 00:11:19.246 These knobs turn potentiometers on the board: one for the time the light should stay on after being triggered 00:11:19.246 --> 00:11:21.609 and the other labeled distance. 00:11:21.609 --> 00:11:31.493 What that distance adjustment does is change how many signal blips from the sensor are required in a given time for the module to interpret movement. 00:11:31.493 --> 00:11:36.110 Set at its furthest distance, it may react to just a couple of blips. 00:11:36.110 --> 00:11:44.794 But you can dull its sensitivity by requiring the logic to see more blips from the sensor before it switches the lights on. 00:11:44.794 --> 00:11:47.364 How does that change the distance it sees? 00:11:47.364 --> 00:11:53.866 You probably noticed already that some of the lens elements are smaller and they’re aimed in different directions. 00:11:53.866 --> 00:12:01.034 The smaller lenses at the bottom won’t produce as bright of an infrared spot on the inside as their larger neighbors, 00:12:01.034 --> 00:12:04.015 and since they’re aimed in slightly different directions, 00:12:04.015 --> 00:12:12.019 you have to be closer to this (with your body taking up more of its angular view) for those lens segments to trigger the sensor. 00:12:12.019 --> 00:12:16.912 So, if you require more sensor blips in a given period to trigger this, 00:12:16.912 --> 00:12:28.044 the thing it’s reacting to needs to be closer to it so it’s seen by more of the lens elements and causes more IR sweeps and thus more signal blips more quickly. 00:12:28.044 --> 00:12:34.711 Of course, there are plenty of other designs you can play with, but like everything in life, there are tradeoffs. 00:12:34.711 --> 00:12:43.442 The golf ball lookin' lens on this solar light, since it’s a hemisphere, is able to detect movement across a wider field of view. 00:12:43.442 --> 00:12:48.416 You often see this kind of sensor on ceilings to detect room occupancy. 00:12:48.416 --> 00:12:51.974 But since each lens element is very small, 00:12:51.974 --> 00:12:57.698 you have to be pretty close to this for your body heat to affect the pyroelectric crystal. 00:12:57.698 --> 00:13:03.596 That gives it a limited detection range, and since the lens elements are mostly uniform, 00:13:03.596 --> 00:13:12.372 while you could in theory adjust its sensitivity that won’t correspond as nicely to distance as it does with this design. 00:13:12.372 --> 00:13:20.291 You can also use the lens elements and their size to  calibrate how large of a living creature you want this to react to. 00:13:20.291 --> 00:13:25.977 Lots of home security systems  have motion sensors that don’t react to pets, 00:13:25.977 --> 00:13:32.427 and one way that's is accomplished is by using  lens elements that are simply too small to generate enough heat on the crystal 00:13:32.427 --> 00:13:35.870 unless it's a human-sized thing walking in front of it. 00:13:35.870 --> 00:13:40.565 It may also use smaller lens elements for areas facing the floor. 00:13:40.565 --> 00:13:49.350 Since the actual sensor in these is so simple, tweaks to the lens and careful calibration is really the name of the game here. 00:13:49.350 --> 00:13:52.670 And speaking of names, we've reached the end of the video. 00:13:52.670 --> 00:13:55.761 And as you might have noticed, at the end of every video 00:13:55.761 --> 00:13:58.015 (well almost every video sometimes I’m lazy) 00:13:58.015 --> 00:14:00.492 this scrolling list of names appears. 00:14:00.492 --> 00:14:04.852 These are some of the lovely people who are supporting my work via Patreon. 00:14:04.852 --> 00:14:11.008 I haven’t really talked about Patreon in a long time because, to be honest, I’m doing just fine. 00:14:11.008 --> 00:14:19.834 But in the video I just made where I was fawning over awnings, it was dawning on some folks that I’ve never done a sponsored segment or ad read. 00:14:19.834 --> 00:14:21.481 And that’s true! 00:14:21.481 --> 00:14:28.667 I’ve never explicitly said that since I don’t like tooting my own horn, and also that's a weird thing to brag about: 00:14:28.667 --> 00:14:31.541 it's not like I begrudge anyone for doing ad reads. 00:14:31.541 --> 00:14:33.743 Especially if they’re funny. 00:14:33.743 --> 00:14:38.109 But truly, the reason I'm not doing that is these folks! 00:14:38.109 --> 00:14:44.417 I consider myself very lucky that I don’t have to  read words somebody at a PR firm wrote for me, 00:14:44.417 --> 00:14:48.991 and so long as I can keep going as I am now, I won’t. 00:14:48.991 --> 00:14:57.762 So as the jazz fades in, I want to give a heartfelt thanks not just to the people you see here but everyone who supports my work on Patreon. 00:14:57.762 --> 00:15:02.803 I’ve got a lot of cool plans lined up and they’re all made possible by people like you. 00:15:02.803 --> 00:15:03.682 And thank you! 00:15:05.268 --> 00:15:06.678 What is this, PBS? 00:15:07.523 --> 00:15:10.180 ♫ pyroelecrtrically smooth jazz ♫ 00:15:10.716 --> 00:15:12.750 But look at how all its… beh. 00:15:13.698 --> 00:15:17.003 It may be gallium nitride but it could be something else. 00:15:17.003 --> 00:15:19.984 Pyroelectricity is exhibited in… a lot. 00:15:19.984 --> 00:15:21.836 What? What am I doing? 00:15:22.660 --> 00:15:25.068 STOP CHANGIN’ THE WORDS! 00:15:25.068 --> 00:15:26.849 And what goes in front of it? 00:15:26.849 --> 00:15:27.807 [confusion] 00:15:27.807 --> 00:15:28.960 Oh. 00:15:30.360 --> 00:15:31.767 Well that’s a problem. 00:15:34.466 --> 00:15:35.839 Are you gon… are you gonna stop? 00:15:37.899 --> 00:15:41.108 If you… I need you to stay… I need you to not do that. 00:15:41.108 --> 00:15:45.078 And one way this is accomplished is by using smalllllllll what? 00:15:45.078 --> 00:15:47.328 When it lands on the stuff... 00:15:48.317 --> 00:15:50.370 Well I skipped a skipped some words. 00:15:54.841 --> 00:15:55.719 oops! 00:15:57.800 --> 00:16:02.275 Speaking of oops, I'm sitting at my desk with the top half of the flood light sensor enclosure and there's a little mirror in there I didn't show you! 00:16:02.275 --> 00:16:06.392 It's angled to help the IR coming in from the extreme side areas of the lens make it onto the sensor. 00:16:06.392 --> 00:16:10.431 And it looks like it's also throwing the light coming up from the small segments at the bottom back downward. 00:16:10.431 --> 00:16:12.738 Neat.