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[ambient noise]

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My car knows when it’s wet.

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That’s right, it’s got automatic windshield wipers.

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When they’re set to automatic, and the car gets wet, they wipe.

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How it know that?

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It’s actually really simple.

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If you look at the windshield,

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you’ll notice an area near the top center which is black.

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And if you look closelier you’ll see the component
the car uses to determine windshield wetness.

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Oh, the camera?

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No, it's not the camera - that’d be silly!

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Then you’d have to devote some precious clock cycles
away from the already wildly complicated computer vision tasks

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which help keep the car centered in its lane.

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That seems like a weird decision

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and I don’t even know how well that could possibly work
no matter how many software updates you throw at it.

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No, I’m not alluding to anything specific,
why on Earth would you think that?

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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,

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is the rain sensor.

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Look, I get it wet - and the car wipes!

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But two can play at that game.

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You may think this is wildly complicated and expensive technology

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but actually it’s just two of the simplest electronic components you can imagine working together in a very clever way.

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Below the dark portions of the rain sensor are
infrared LEDs shining light up into the glass,

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and adjacent to them are photodiodes which detect that light.

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Now, that bright spot at the bottom?

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That’s not actually part of the rain sensor.

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It’s an ambient light sensor used to control the car’s headlights automatically.

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The two components just live in the same assembly.

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The parts that make up the rain sensor aren’t really visible

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because they’re below a filter which blocks visible light
and thus it appears black to our eyes.

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That’s strange, didn’t we just encounter that with stovetops?

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Weird connections happening lately.

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To demonstrate how that detects rain,
I’ll use a piece of glass and a laser pointer.

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If I shine the laser up at the glass at about a 45 degree angle,

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you’ll notice that a spot of laser light is reflected back down towards the table.

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I’ll put a piece of paper there to make it more visible.

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Now watch what happens when I get the glass wet.

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Not a whole lot.

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That’s a bummer.

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Ah, but this isn’t actually an accurate demonstration
of the principles involved in the rain sensor.

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Rather than look at the light reflected back downward by the glass,

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let’s look at the spot produced above the glass by 
the laser light which makes it through.

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Now watch closely.

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Did you see that? It moved.

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That happened because the water on top of the glass
changed the angle of refraction as light left the glass.

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Now, like some politicians, I’m really not really good at the whole optics thing

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so I’m not getting too deep into the weeds here.

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But as light passes from one medium to another,
its speed actually changes

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and because of… reasons that means it appears to change direction.

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That concept is known as refraction.

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And the refraction index of water is different from that of air.

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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.

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The rain sensor, though, is entirely under the glass

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so it’s not as if it can directly detect that change.

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Instead, it leverages the effect of total internal reflection.

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If you’ve ever gone swimming in a pool with goggles on and looked up at the surface of the water while submerged,

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you may have noticed that it looks like a mirror from shallow angles.

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Here’s some stock footage to illustrate.

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That’s total internal reflection at work - the water directly above you
transmits light just fine and you can see through it,

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but there’s a critical angle at which the
air-water interface will reflect all light back down

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and since it’s smooth like glass, it looks like a mirror.

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So now - how can the rain sensor take advantage of that?

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Well, first, since the module isn’t part of the glass,

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we need an optical interface material to couple the sensor to the glass
and remove the air-to-glass optical interface on the bottom.

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That takes the form of a sticky gel which, now that it’s in-place, you can’t see.

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Which, really, is the point - when it’s doing its job,

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the sensor and the glass become one

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(from an optical perspective).

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Now the key is that the infrared LEDs that we talked about earlier

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are shining up at the glass just shallow
of the critical angle of total internal reflection.

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So, all the light they produce will simply bounce off
the top surface of the the glass and head back downward.

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And, at the other end of that beam of light, is a photodiode.

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That will then detect the light being generated by the LED.

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And to make sure it’s detecting that light and not something else,

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the LEDs are actually rapidly blinking
and the photodiode is looking for that pattern of blinks.

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That’s a simpler way of saying the LEDs are producing
a specific modulated signal that the photodiode is tuned to detect.

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But because the angle between the photodiode and 
the LED is so close to the critical angle of total internal reflection,

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if conditions should change the intensity of that signal
being received by the photodiode will also change.

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What condition?

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Condition wet.

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Water on the glass, as we saw in the earlier demo,
changes the refraction angle because its refractive index is different from air.

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And it will change it just enough to break the effect of total internal reflection.

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Wet glass will allow some of the light produced by the LEDs 
to actually escape and be transmitted through to the outside surface.

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Which in turn means there’s less light bouncing down
to the photodiode to be detected,

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so the signal it was receiving begins to diminish.

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And, with a little logic circuitry and programming, you can use that loss of signal as a measurement of windshield wetness.

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And honestly… that’s it!

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It’s that simple.

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The sensor starts to see the signal from the LED drop out
so it says to the windshield wiper motor -

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wipe.

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And the wetter the glass,
the more the signal drops, so the more it wipes.

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And you’ll notice the sensor is deliberately placed in the path of the windshield wipers so they clear the glass with every wipe.

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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.

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The only other complexity here is calibration.

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Every time the sensor powers up, it’s got to pick a baseline 
signal level which it deems “dry.”

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Yet, if you are out and about in the rain
and you turn your car on with the windshield already wet,

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it’s going to start out with some lower signal level than typical.

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So, somebody smarter than me probably established
a minimum signal dropout level which always causes a wipe,  

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and in fact I’m certain I’ve encountered the 
car do a single wipe after turning it on

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when it didn’t really need to.

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But then it can reset its calibration after that wiping event.

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And, there’s another trick up its sleeve,
and it's my very favorite one:

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having two of them.

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There are actually two sensor assemblies under here.

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You can barely make out the infrared light in this smartphone camera footage.

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You'll see there's two spots.

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If the windshield is dry, the two sensors should see very similar signals

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but if there’s even just a tiny bit of water on top of one of them,
 or differing amounts between the two,

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then the two signals will be very different
and it will know immediately that the windshield is wet.

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Which calibration method is it actually using?

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I do not know.

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And if you’ve never lived with a car with automatic wipers,
you may wish to know they are great but not exactly perfect.

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You are afforded a sensitivity adjustment on the wiper stalk,
so you can fine-tune its behavior to your liking.

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But while the wipers pretty much always come on when they need to,

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sometimes they go absolutely nuts when it’s barely raining
or they get a little lazy in a sudden downpour.

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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

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and the wipers will come on full-steam!

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Handy user interface, those stalks.

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They’re always there and muscle memory takes over!

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I’m pro-stalks, is what I’m saying. It’s a good idea.

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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.

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The hood blocks you from lifting the wiper arms
so you need them to stop in the up position in order to change them.

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and despite digging through all the settings menus
I couldn’t find a “change wipers” option.

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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,

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but it outsmarted me and parked them anyway.

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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

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and then the car will do this.

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Is this procedure in the owner’s manual?

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Yes, but who looks there for information?

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I usually like to end my videos with something akin to a moral.

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In that vein, the first lesson today is for me

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and that is to read the freaking manual!

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There’s even an index in here and finding the appropriate 
information would have taken me mere moments.

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People worked hard on this thing and I just… completely ignored it!

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So don’t ignore the manuals in your life.

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And the second moral is…

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don’t ignore what you can do with a few simple components.

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Software isn’t everything.

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That’s not directed at any organization in particular, mind you.

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Just something to keep in mind.

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Did I get to 10 minutes yet?

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♫ internally smooth jazz ♫

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Thanks, by the way, to Jason, a supporter on Patreon,
for recommending this video topic.

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That happened a long time ago and I’m pretty certain at least one other person has suggested this in the meantime

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so if I forgot who you were, I’m sorry.

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There’s just too many bimetallic strips getting in the way.

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That’s right, it’s got automatic… the thing didn’t start.

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You’ll notice that the spot of laser light is re… oh.

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…conditions should change, the intensity of a-

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Ooh, I needed a comma for my sake

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...critical angle of total internal reflection

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[annoyance at outside noise]

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...it’s got to pick a baseline signal lever.

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Signal.

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Level.

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...had to look, oh yeah I forgo ohyeahIgottagetthething!

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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?

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And that this was a technology available back in the nineties?

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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.

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This section is usually for puns but today I chose vicious sarcasm.

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Not directed at any particular organization, of course :)