The simple, clever sensor behind automatic windshield wipers

Technology Connections
29 Apr 202411:17


TLDRThe video script explains the technology behind automatic windshield wipers in vehicles. It clarifies that the wipers are not controlled by a camera, but by a rain sensor located near the top center of the windshield. The sensor uses infrared LEDs and photodiodes to detect light reflected from the glass. When the windshield gets wet, the refraction of light changes, affecting the signal received by the photodiode. This change in signal intensity is used to measure the wetness of the windshield and activate the wipers accordingly. The script also humorously discusses the manual calibration process of the sensor and the importance of referring to the car's manual for troubleshooting. The video concludes with a reminder that simple components can achieve complex tasks and the value of consulting manuals for guidance.


  • šŸš— Automatic windshield wipers in cars are activated by a rain sensor, not a camera, to avoid overloading the car's computer vision system.
  • šŸ” The rain sensor is located near the top center of the windshield and is typically black, housing an infrared LED and a photodiode.
  • šŸ’” The sensor works by using infrared LEDs that emit light towards the glass, which is then detected by a photodiode.
  • šŸŒ§ļø Wetness is detected when water on the glass alters the refraction of light, breaking the total internal reflection and reducing the signal received by the photodiode.
  • šŸ“‰ As the windshield gets wetter, the signal to the photodiode diminishes, which triggers the wipers to activate and increase their frequency.
  • šŸ¤– The sensor needs to calibrate a baseline 'dry' signal level every time it powers up, adjusting for different starting conditions.
  • šŸ”„ The system compensates for inconsistencies in wetness by employing two sensor assemblies, which compare signals to determine the wetness state.
  • šŸ› ļø Manual adjustments can be made to the sensitivity of the wipers through a control on the wiper stalk to suit driver preferences.
  • šŸ“š The importance of reading manuals is emphasized, as they contain valuable information that can help avoid frustration and guesswork.
  • šŸ‘ The simplicity and effectiveness of using basic electronic components like LEDs and photodiodes to achieve complex tasks is highlighted.
  • šŸ˜… The video humorously addresses the challenges of changing wiper blades and the need to find the right procedure in the car's manual.
  • šŸŽµ The video concludes with a reminder that sometimes, simple solutions and reading manuals can save time and effort, wrapped up with a touch of humor and smooth jazz.

Q & A

  • How do automatic windshield wipers in a car detect when it's wet?

    -They use a rain sensor that consists of infrared LEDs and photodiodes. The LEDs shine light onto the windshield, and the photodiodes detect the light. When the windshield gets wet, the refraction of light changes, which alters the signal received by the photodiode, indicating wetness.

  • What is the role of the black area near the top center of the windshield?

    -The black area houses the rain sensor, which is responsible for detecting wetness on the windshield and activating the automatic wipers.

  • Why is it not ideal to use a camera for detecting windshield wetness?

    -Using a camera would divert valuable computational resources from other critical tasks like computer vision, which is used for features like lane keeping assistance. Additionally, it might not be as effective or efficient as the current sensor technology.

  • How does the rain sensor work in conjunction with the windshield wipers?

    -The rain sensor monitors the signal from the LEDs. When the signal diminishes due to wetness, the sensor triggers the wiper motor to activate. The sensor is also placed in the path of the wipers to allow for continuous calibration.

  • What is the purpose of the ambient light sensor in the same assembly as the rain sensor?

    -The ambient light sensor controls the car's headlights automatically, adjusting them based on the surrounding light conditions.

  • How does the rain sensor leverage the concept of total internal reflection?

    -The rain sensor uses infrared LEDs shining at an angle just below the critical angle for total internal reflection. When the windshield is dry, the light is reflected back towards the photodiode. When the windshield is wet, the change in refraction index disrupts this reflection, reducing the signal to the photodiode.

  • What happens when the windshield is wet and how does it affect the rain sensor's operation?

    -When the windshield is wet, the refraction angle changes due to the different refractive index of water compared to air. This causes some of the light to escape through the wet surface rather than being reflected back to the photodiode, resulting in a diminished signal that triggers the wipers.

  • Why are there two sensor assemblies under the windshield?

    -Having two sensor assemblies allows for a more accurate detection of wetness. If there's a difference in the signals from the two sensors, it indicates that the windshield is wet, providing a more reliable trigger for the wipers.

  • How does the automatic wiper system calibrate its baseline signal level for dry conditions?

    -Each time the sensor powers up, it establishes a baseline signal level that it considers 'dry.' If the windshield is already wet when the car is turned on, the sensor starts with a lower signal level and adjusts its calibration after the first wipe.

  • What is the user's recommendation for adjusting the sensitivity of the automatic wipers?

    -The user recommends using the sensitivity adjustment on the wiper stalk to fine-tune the wipers' behavior according to personal preference.

  • What is the user's opinion on the manual controls for windshield wipers in cars?

    -The user appreciates the manual controls, known as stalks, as they are always within reach and allow muscle memory to take over, providing a quick and convenient way to activate the wipers when needed.

  • What lesson does the user take away from their experience with the automatic wiper system?

    -The user emphasizes the importance of reading the manual for any device or system, as it contains valuable information that can help in understanding and effectively using the technology.



šŸš— Automatic Windshield Wipers Explained

This paragraph explains the technology behind automatic windshield wipers in cars. It clarifies that the car doesn't use a camera to detect rain, but rather a rain sensor located near the top center of the windshield. The rain sensor consists of infrared LEDs and photodiodes working together to detect the presence of water on the windshield. The sensor operates on the principle of total internal reflection, where water changes the refraction angle, thus affecting the light signal received by the photodiode. The paragraph also includes a demonstration using a laser pointer and a piece of glass to illustrate the concept of refraction and how the sensor uses this principle to detect rain.


šŸ’” How the Rain Sensor Works

This paragraph delves deeper into the functioning of the rain sensor. It describes how the sensor uses infrared LEDs and photodiodes to create a modulated signal that changes in intensity when the windshield becomes wet. The change in refraction due to water on the glass disrupts the total internal reflection, causing some light to escape and reducing the signal received by the photodiode. This change in signal intensity is used to measure the wetness of the windshield and control the automatic wipers. The paragraph also mentions the sensor's calibration process and the presence of two sensor assemblies for increased accuracy. It touches on the imperfections of the system and the manual adjustments available to the driver.


šŸ“š The Importance of Manuals and Simple Components

The final paragraph reflects on the importance of reading manuals and the power of simple components in technology. It humorously recounts the speaker's struggle to change the car's wiper blades and the discovery of the correct procedure in the owner's manual. The speaker emphasizes the value of the information contained within manuals and encourages viewers not to overlook them. Additionally, the paragraph highlights that software is not the only key to innovation and that simple components can be used in clever ways to solve complex problems, as demonstrated by the rain sensor technology.



šŸ’”Automatic Windshield Wipers

Automatic windshield wipers are a feature in modern cars that automatically activate and adjust their speed based on the amount of rain or moisture detected on the windshield. In the video, it is mentioned that when the car gets wet, the wipers wipe, showcasing the convenience and technology integration in vehicles.

šŸ’”Rain Sensor

A rain sensor is a device used in vehicles to detect the presence of rain or moisture on the windshield. The video explains that this sensor is located near the top center of the windshield and is composed of infrared LEDs and photodiodes. It plays a crucial role in the automatic wiper system by determining when and how often the wipers should operate.

šŸ’”Total Internal Reflection

Total internal reflection is an optical phenomenon that occurs when light strikes a medium boundary at an angle greater than the critical angle, causing all of it to be reflected back into the original medium. In the context of the video, the rain sensor uses this principle to detect changes in light reflection caused by water on the windshield, which indicates wetness.


Refraction is the change in direction of light as it passes from one medium to another due to a change in its speed. The video script uses the concept of refraction to explain how water on the glass changes the angle at which light exits, affecting the sensor's detection capabilities and, consequently, the operation of the automatic wipers.


Photodiodes are light-sensitive semiconductor devices that convert light into electrical current. In the video, photodiodes are used in the rain sensor to detect light from the infrared LEDs. The change in the detected light signal is used to measure the wetness of the windshield and trigger the automatic wipers.

šŸ’”Infrared LEDs

Infrared LEDs (Light Emitting Diodes) emit light in the infrared spectrum, which is not visible to the human eye. In the video, these LEDs are used in the rain sensor to shine light onto the windshield. The presence of water on the windshield alters the pattern of light received by the photodiodes, signaling the need for the wipers to activate.

šŸ’”Ambient Light Sensor

An ambient light sensor is a device that measures the intensity of light in the surrounding environment. In the video, it is mentioned alongside the rain sensor, used to control the car's headlights automatically. It is not part of the rain detection mechanism but shares the same physical assembly, highlighting the integration of multiple sensors in modern vehicles.

šŸ’”Signal Modulation

Signal modulation is the process of varying one or more properties of a periodic waveform, called the carrier signal, with a modulating signal. In the context of the video, the infrared LEDs produce a modulated signal that the photodiode is specifically tuned to detect. This allows the system to distinguish between the LED's signal and other light sources.

šŸ’”Critical Angle

The critical angle is the angle of incidence at which total internal reflection occurs. It is a key concept in the operation of the rain sensor, as the infrared LEDs are positioned to shine at an angle just below this critical angle. When the windshield becomes wet, the change in refraction can cause the light to no longer be totally internally reflected, signaling the sensor to activate the wipers.


Calibration in the context of the video refers to the process by which the rain sensor establishes a baseline signal level for 'dry' conditions every time it powers up. This is important for accurate detection of rain and subsequent activation of the automatic wipers. The video also mentions the need for recalibration if the car is turned on with a wet windshield.

šŸ’”Dual Sensor Assemblies

Having two sensor assemblies is a feature that allows for more accurate detection of windshield wetness. If there is a difference in the signals received by the two sensors, it indicates the presence of water. This is highlighted in the video as a clever approach to ensure reliability in the rain detection system.


Automatic windshield wipers in cars are triggered by a rain sensor, which is a simple yet effective technology.

The rain sensor is located near the top center of the windshield, often appearing as a black area.

Contrary to common belief, the camera is not used to detect wetness; instead, a rain sensor does the job.

The rain sensor uses infrared LEDs and photodiodes to detect changes in light reflection caused by water on the windshield.

Total internal reflection is a key principle exploited by the rain sensor to detect changes in light due to rain.

The sensor is calibrated to determine a baseline 'dry' signal level and adjusts its response based on the change in signal.

Having two sensor assemblies improves the accuracy of wetness detection by comparing signals from both sensors.

The sensor is strategically placed in the path of the wipers to ensure clear readings and effective wiping.

The system can adjust the frequency of the wiper operation based on the rate of signal drop, correlating to the intensity of rain.

Calibration is important for the sensor to accurately measure the 'dry' state and respond correctly to wet conditions.

The presence of two sensors allows for immediate detection of wetness by comparing the signals from both, accounting for uneven distribution of water.

The simplicity of the rain sensor, composed of just an LED, a photodiode, and adhesive, showcases the power of basic electronic components.

Despite being a reliable technology, automatic wipers may not always perfectly adjust to the rain conditions and may require manual adjustment.

The video emphasizes the importance of reading manuals for understanding the proper procedures and functionalities of devices.

The presenter humorously discusses their experience with changing wiper blades and the process required to do so.

The moral of the video is to not underestimate the capabilities of simple components and to always consult the manual for guidance.

The technology behind rain sensors has been available since the nineties, highlighting the long-standing effectiveness of this innovation.



[ambient noise]


My car knows when itā€™s wet.


Thatā€™s right,Ā itā€™s got automatic windshield wipers.


When theyā€™re set to automatic, and the carĀ gets wet, they wipe.


How it know that?


Itā€™s actually really simple.


If you lookĀ at the windshield,


youā€™ll notice an area near the top center which is black.


And if youĀ look closelier youā€™ll see the component the car uses to determine windshield wetness.


Oh, theĀ camera?


No, it's not the camera - thatā€™d be silly!


Then youā€™d have to devote some precious clockĀ cycles away from the already wildly complicatedĀ computer vision tasks


which help keep the carĀ centered in its lane.


That seems like a weird decision


and I donā€™t even know how well thatĀ could possibly work no matter how many software updates you throw at it.


No, Iā€™m not alluding to anythingĀ specific, why on Earth would you think that?


Right above that camera which looks forward at theĀ road youā€™ll see a little circular spot with some spots within the spot and that, viewers,


is theĀ rain sensor.


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


But two can play at that game.


You mayĀ think this is wildly complicated and expensive technology


but actually itā€™s just two of theĀ simplest electronic components you can imagine working together in a very clever way.


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


and adjacentĀ to them are photodiodes which detect that light.


Now, that bright spot at the bottom?


Thatā€™sĀ not actually part of the rain sensor.


Itā€™s an ambient light sensor used to control theĀ carā€™s headlights automatically.


The two components just live in the same assembly.


The parts thatĀ make up the rain sensor arenā€™t really visible


because theyā€™re below a filter which blocksĀ visible light and thus it appears black to our eyes.


Thatā€™s strange, didnā€™t we just encounter that withĀ stovetops?


Weird connections happening lately.


To demonstrate how that detects rain, Iā€™llĀ use a piece of glass and a laser pointer.


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


youā€™ll notice that a spot of laser light is reflected back down towards the table.


Iā€™ll put a piece of paper there to make it more visible.


Now watch what happens when I get theĀ glass wet.


Not a whole lot.


Thatā€™s a bummer.


Ah, but this isnā€™t actually an accurateĀ demonstration of the principles involved in the rain sensor.


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


letā€™s look at the spot produced above the glass byĀ  the laser light which makes it through.


Now watch closely.


Did you see that? It moved.


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


Now,Ā like some politicians, Iā€™m really not really good at the whole optics thing


so Iā€™m not getting too deepĀ into the weeds here.


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


and because ofā€¦ reasons that means it appears to change direction.


That concept is known asĀ refraction.


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


That means the changeĀ in speed and thus change in refraction angle is different between light passing from glass intoĀ air and light passing from glass into water.


The rain sensor, though, is entirely under theĀ glass


so itā€™s not as if it can directly detect that change.


Instead, it leverages the effect ofĀ total internal reflection.


If youā€™ve ever gone swimming in a pool with goggles on and lookedĀ up at the surface of the water while submerged,


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


Hereā€™s some stock footage to illustrate.


Thatā€™sĀ total internal reflection at work - the water directly above you transmits lightĀ just fine and you can see through it,


but thereā€™s a critical angle at which the air-water interface will reflect all light back down


and since itā€™s smooth likeĀ glass, it looks like a mirror.


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


Well, first, since the module isnā€™t part of the glass,


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


That takes the form of a sticky gel which, now that itā€™s in-place, you canā€™t see.


Which, really, is the point - when itā€™s doing its job,


the sensor and the glass becomeĀ one


(from an optical perspective).


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


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


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


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


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


And to make sure itā€™s detecting that light andĀ not something else,


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


Thatā€™s a simpler way of saying the LEDs are producing a specific modulatedĀ signal that the photodiode is tuned to detect.


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


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


What condition?


Condition wet.


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


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


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


Which in turn means thereā€™sĀ less light bouncing down to the photodiode to be detected,


so the signal it was receivingĀ begins to diminish.


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


And honestlyā€¦ thatā€™s it!


Itā€™s that simple.


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




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


And youā€™ll notice the sensor is deliberately placed in the path ofĀ the windshield wipers so they clear the glass with every wipe.


That allows theĀ logic to get some sense of how hard it might be raining based on how quickly theĀ signal drops out after every wiping event.


The only other complexity here isĀ calibration.


Every time the sensor powers up, itā€™s got to pick a baselineĀ  signal level which it deems ā€œdry.ā€


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


itā€™s going to start out with some lower signalĀ level than typical.


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


and in fact Iā€™m certain Iā€™ve encountered theĀ  car do a single wipe after turning it on


when it didnā€™t really need to.


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


And, thereā€™s another trick up its sleeve, and it'sĀ my very favorite one:


having two of them.


There are actually two sensor assemblies under here.


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


You'll see there's two spots.


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


but if thereā€™s even just a tiny bit ofĀ water on top of one of them, or differing amounts between the two,


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


Which calibrationĀ method is it actually using?


I do not know.


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


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


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


sometimes theyĀ go absolutely nuts when itā€™s barely raining or they get a little lazy in a sudden downpour.


LuckyĀ for me, if that happens all I have to do is just slap this stalk which is right next toĀ my hand on the steering wheel


and the wipers will come on full-steam!


Handy user interface,Ā those stalks.


Theyā€™re always there and muscle memory takes over!


Iā€™m pro-stalks,Ā is what Iā€™m saying. Itā€™s a good idea.


Whatā€™s less of a good idea, though, is theĀ Konami-code like trick I had to look up just to change my carā€™s wiper blades.


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


and despite digging through all the settings menus I couldnā€™t find a ā€œchange wipersā€Ā option.


So I tried just switching the car off with the wipers in the up position, y'know just time it right, using the powerĀ button,


but it outsmarted me and parked them anyway.


It turns out what you have to do is turnĀ the car off and then hold the wiper stalk up in the mist position for a few moments


and then theĀ car will do this.


Is this procedure in the ownerā€™s manual?


Yes, but who looks there for information?


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


In that vein, the first lesson today is for me


and thatĀ is to read the freaking manual!


Thereā€™s even an index in here and finding the appropriateĀ  information would have taken me mere moments.


People worked hard on this thingĀ and I justā€¦ completely ignored it!


So donā€™t ignore the manuals in your life.


And the second moral isā€¦


donā€™t ignore what you can do with a few simple components.


Software isnā€™t everything.


Thatā€™sĀ not directed at any organization in particular, mind you.


Just something to keep inĀ mind.


Did I get to 10 minutes yet?


ā™« internally smooth jazz ā™«


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


That happened a long time agoĀ and Iā€™m pretty certain at least one other person has suggested this in theĀ meantime


so if I forgot who you were, Iā€™m sorry.


Thereā€™s just too manyĀ bimetallic strips getting in the way.


Thatā€™s right, itā€™s got automaticā€¦Ā the thing didnā€™t start.


Youā€™ll notice that the spotĀ of laser light is reā€¦ oh.


ā€¦conditions should change, the intensityĀ of a-


Ooh, I needed a comma for my sake


...critical angle of total internal reflection


[annoyance at outside noise]


...itā€™s got to pick a baselineĀ signal lever.






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


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?


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


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.


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


Not directed at any particular organization, of course :)

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Automatic WipersRain SensorInfrared LEDsPhotodiodesTotal Internal ReflectionOptical InterfaceVehicle TechCar FeaturesTech ExplainedSarcasmDIY Car Maintenance