Tech In Plain Sight: Rain-Sensing Wipers

While it is definitely a first-world problem that you don’t want to manually turn on your windshield wipers when it starts raining, it is also one of those things that probably sounds easier to solve than it really is. After all, you can ask a four-year-old if it is raining and expect a reasonable answer. But how do you ask that question of a computer? Especially a tiny cheap computer that is operating pretty much on its own.

You might want to stop here and try to think of how you’d do it. Measure the conductivity of the glass? Maybe water on the glass affects its dielectric constant and you could measure the resulting capacitance? Modern cars don’t do either. The problem is complicated because you need a solution that works with the glass and isn’t prone to false positives due to dirt or debris.


Instead, they use infrared light shot at an angle at the windshield. The glass reflects most of the light back to the sensor, but water causes the reflection to scatter. If the sensor sees less return light, it turns on the wipers. Where is the sensor? It depends on the car, but [Jeff] helpfully points out the location on Toyota vehicles in the video below.

Typically, the whole assembly sits behind the windshield somewhere near the rearview mirror. There’s a good writeup and the graphic used here on the Clemson Vehicular Electronics Lab website.

Of course, the car companies aren’t designing these from scratch. They buy the technology from other companies, for example, Hamamatsu and other companies.

There was a time when you could buy kits to add this to your car if you couldn’t stand manually operating your wipers. It shouldn’t be too hard to roll your own if you were so inclined.

Of course, there are other ways to do the same thing. Some Tesla vehicles can use their cameras to passively detect rain. Also, if you don’t need to sense the glass, it is pretty easy to measure the effect getting wet has on a PCB resistor.

It is amazing how many things are easy to figure out for humans but much tougher for computers. While we do enjoy our automatic wipers, we also don’t really mind having to turn them on if we had to. We also do get the occasional false positive.

There is a surprising amount of tech behind windshield wipers. Not to mention, potentially, rhythm.


Banner image: “Rain Rain Go Away” by Basheer Tome. Thumbnail: “Bank of America cash machines in a row, windshield wiper, rain, University Village, Seattle, Washington, USA” by Wonderlane.

37 thoughts on “Tech In Plain Sight: Rain-Sensing Wipers

  1. Cool tech. I did know it was optical sensors, but I didn’t know it was based on the way water on the surface changes the way light reflects internally. That’s a pretty ingenious way of going about it. A dry windshield will always have the same internal reflection, so it’s easy to detect the changes brought on by water.

    That being said, I’ve been in a few cars with auto detecting wipers that would fire off at random without any rain at all. One car would randomly go on 20-30 second bursts every 10 minutes or so. Another one my buddy owned would go off randomly and wouldn’t stop till you turned off the car and started it back up.

    I love new features on cars, I love new tech on cars, but I don’t think wipers should be automatic without the option of setting it to manual only. It becomes a distraction and an obstruction when they misbehave. I’ve also had many instances of very light rain that really didn’t need wipers. I like having the choice of when to engage a convenience feature, which is all automatic wipers are.

    1. Yes, cool if it worked reliably. I can’t say whether it was the hardware or the firmware tuning, but it the system in my car never worked robustly enough for me to embrace it. Polysiloxane coatings are my favorite as under certain conditions (above certain travel speed and droplet size) they greatly reduce the need to use wipers. Nonetheless, I wonder what environmental effect they may have.

    2. Funny thing, my Volkswagen Golf has the normal wiper switch which turns on or off the sensor and has two sensorless speeds – I think. You can select several sensitivities of the sensor resulting in more or less wipes. This has all the options I could ever want :-D

      1. Not sure what year yours is, but my 2003 Mk4 Golf has the same sounding setup. They’re more of a semi-automatic though as they don’t turn themselves on, you have to do that initially, but once on, they use the rain sensor to control the intermittent durations and/or speed of the wipers, but with the ability to switch to a manual override if needed.

        The auto mode effectively replaces the location of what would conventionally be the intermittent functionality on the wiper stalk and is always off by default every time you start the car, even if the stalk was left in that position when you last turned the engine off. You have to move the stalk to that first position to initiate the first swipe (or usually off then on again in my case as I tend to leave the stalk in that position) and they’re then in auto mode with about 4 sensitivities controlled in the same way you’d control the pause duration for intermittent. If you move the stalk past the auto/intermittent position, you get conventional style “slow on”, followed by “fast on” manual override.

        Seems to be a decent compromise between autonomy and manual functionality, in my opinion.

    3. How will they get $2000+ for a windshield replacement that way?

      Rain detecting wipers are just another part of planned obsolescence. Another (at best) marginally useful gadget to break and make the car uneconomical to repair. The Germans have gotten really really good at that. Never buy a water cooled German car.

  2. My truck has a pretty advanced rain-detecting mechanism. It actually has a biological component with immense computational capacity—or at least it used to before forty years of beers diminished it somewhat.

  3. Actually Al, this brings me back to a ‘fantasy’ I’ve never developed, not being a car designer. But once this baby gets up to speed, couldn’t you use the air speed from the front hood to make like, a ‘speed scoop’, or like ‘blow the rain’ off the windshield, before it even hits (?). I’ve never heard (seen) anyone trying, so until then I’ll deal with ‘wipers’.

    1. It certainly would be possible to implement,mbut I wonder what such a scoop would do to aerodynamics/fuel efficiency. Under about 30mph it doesn’t really matter what shape the car is. But above that where air starts behaving more like a liquid, around where you might expect your scoop to work, it might negatively effect the efficiency of the car enough to make it not worth it.

      I suggest a modification to you idea, you have air flowing into the engine compartment that creates drag already, if you put the scoop in there, or alternatively designed the engine bay to vent the excess air that enters it out in front of the windshield, you might be able to achieve the desired effect without adding any additional drag you might get from an external scoop.

      1. It would probably be difficult, if not impossible, to implement since the rain droplets are part of the “air” that you’re trying to move around the car. Patient that air through a moisture/water barrier would likely be impractical. Perhaps you could use exhaust, which is relatively “dry”, due to its temperature, but then you’d be quite literally swimming in your own … exhaust. I prefer to dilute the exhaust substantially before I breathe it in and give at least some of the particulates a chance to settle out. … In short, manually continuously adjustable windshield wipers have work well enough for me, but I’m open to new experiences so long as I have a fallback to the incumbent reliable solution. I can see, however, that the quasi-manual solution might not work for autonomous cars whose sensors rely on relatively unwetted surfaces.

        1. I have a version of RainX (some other brand, further developed by original RainX person, supposedly). While fresh, it clears the windshield even at 20mph. But it’s my experience, the main advantage of RainX is not clearing the glass — but in beading up the droplets. It’s amazing how much visibility remains even while the windshield is wet, as long as the droplets bead up.

          1. But, my wife’s 2019 Subaru Outback Owner’s Manual warns us no to apply such treatments to the windshield. It claims they mess up the cameras of the lane keeping and safe following distance functions.

          2. @The Commenter Formerly Known As Ren*:

            “Definitive answer re: Rain-X from by dealership (today!): there’s no problem using it as long as it’s been buffed clear and isn’t leaving a hazy finish.”

            The stuff I use has explicit instructions that it needs to be wiped until the window is clear, and that’s what I’ve been doing. I bet Rain-X has the same instructions, and most people are just shit at following instructions.

            * For some reason I don’t see a reply button under some posts…

  4. I guess the detecting rain part is the easy part. The hard bit is deciding when to start wiping, how fast and at what intervals. Borrowed a Renault once and that got it horribly wrong. Switched it back to manual. My current car has it almost nailed to perfection, save the occasional false positive wipe. Usually just one or two. This gets left on auto and I never need to bother with it. Car also switches on the lights after about a minute of rain.

  5. A large square grill will work. I had an 89 Mercury and the rain just went over the roof around 35 mph, when you slowed down it got wet. Works great if you like getting 19mpg fuel economy.

  6. Velux automatic windows I think have a piezo sensor. It produces very few false positives, though I’ve seen it trigger when workmen were cutting tiles on our roof. Which actually I was glad it closed to shut out the debris.

  7. I used to develop these sensors years ago.
    They serve a whole lot of other purposes as well:
    – light sensing: for switching your headlights on in tunnel and dark conditions
    – solar sensor: for increasing the aircondition output on the side of the car, depending from where the sun is shining
    – HUD sensor: for setting the brightness of the heads-up-display
    – fogging/condensation sensor: measuring the dewpoint and temperature of the windscreen to prevent fogging.

    I don’t know any car manufacturer developing this stuff, they are mostly produced by the suppliers like:
    Kostal, Valeo, TRW, Hella.
    Thats why you often find the same sensor (with slightly different software) in cars from completely different manufacturers

    Detecting rain with the front facing camera is very difficult because of the different focal points. For rain you’d like to have a macro-lens, for the street it is more like infinity.

    The biggest problem the manufacturers are facing is that the space near the rearview mirror is very limited. The rain sensor has been shrunk down over the years to give space for driver assistance camera (sometimes 2 of them). Now, they cover only the area of a 2€-Coin: One can imagine that extrapolation from such a small area to the whole windscreen makes the correct detection of the amount of rain very difficult.

  8. When it comes to a car or other tool that is mission critical, automation is a hinderance and not a feature. Especially when I want that tool to work for 30 or more years.

  9. I’ve got some advanced tech on my vehicles, but no auto wipers. Had them on a 2014 Ford and they worked extremely well. Wondered if their IP was tied up by Ford and that’s why I don’t have it on my Toyota or Nissan.

  10. Had a rental in Japan with only a single momentary push-button for the manual wipers. I thought I was going to have to press for each wipe, but it turned out much much cooler.
    First press of the button was the first wipe. Next press was the second wipe. Third and subsequent wipes did not require a button press and had the same cadence as the first two wipes. It would automatically turn off after two minutes of no turn signal or steering wheel activity. You could manually turn them off by holding down the button for two seconds. So simple and perfect. Two quick taps would be highest speed. Slowest speed was a crazy 90 second interval!

    1. Genius. Tesla should definitely implement something like this as it is missing proper sensors. I keep hitting the button on my model Y long before the top part of the windscreen(where the cam is) get wet.

    2. Weird, my `94 VW Passat does exactly this – it “learns” how often to wipe from the delay between the first two activations, and then it keeps doing it until turned off (and remembers it for the next time too). I’m not entirely sure though whether this is built into the car or I just happen to have some extra-fancy aftermarket wiper relay. After all, think about it – the whole learning thing can just be confined to / built into the relay then used in any car with a matching relay socket…

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