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.

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