# How do VW wipers "know" how much rain is falling?

**URL:** <https://boards.straightdope.com/t/how-do-vw-wipers-know-how-much-rain-is-falling/307798>\
**Category:** Factual Questions\
**Created:** [June 10, 2005, 9:40pm UTC](https://boards.straightdope.com/t/how-do-vw-wipers-know-how-much-rain-is-falling/307798 "2005-06-10T21:40:05Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![trupa](https://avatars.discourse-cdn.com/v4/letter/t/aeb1de/32.png) [@trupa](https://boards.straightdope.com/u/trupa)\
**Post date:** [June 10, 2005, 9:40pm UTC](https://boards.straightdope.com/t/how-do-vw-wipers-know-how-much-rain-is-falling/307798/1 "2005-06-10T21:40:05Z")

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This is something that has amazed and intrigued me ever since we took possession of our VW Passat wagon: when I have the winshield wipers on “intermittent”, the wiper speed adjusts automatically to the amount of rain on the windsheild. More rain, faster wiper actuation. Less rain, wipers slow down.

How the dickens does the car sense the amount of water on the windshield? I can’t see any sensors or electrical contacts on the glass, but that doesn’t mean they aren’t there… Is it an optical sensor looking at light through the glass? some type of electrical conductivity measurement?

How’d they do that?

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**Author:** ![Scruloose](https://avatars.discourse-cdn.com/v4/letter/s/e68b1a/32.png) [@Scruloose](https://boards.straightdope.com/u/Scruloose)\
**Post date:** [June 10, 2005, 9:44pm UTC](https://boards.straightdope.com/t/how-do-vw-wipers-know-how-much-rain-is-falling/307798/2 "2005-06-10T21:44:43Z")

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> [@](#):
>
> The majority of the rain sensors are mounted on the inside surface of glass near the rear view mirror and are attached directly to the glass with an adhesive or a mechanical clip-on locking mechanism to a bracket that is attached to the glass. This keeps the sensor out of the primary vision area of the driver while giving it the opportunity to be in the standard wiping path of the traditional windshield wiper.
> 
> However, the system is flexible enough that the sensor can be placed in various alternate windshield areas when used with the non-standard wiper configurations and travel paths.
> 
> The rain sensor is actually based on an optical sensor unit that emits infrared (IR) light to the windshield. The system, when turned on, monitors for changing conditions constantly. Some of the light is reflected back to the IR detectors. The amount of light reflected back to the sensors changes depending on the outside-surface conditions of the windshield. When the unit detects moisture, less light is reflected back to the photo cells in the unit. The system detects a change in the intensity of the reflected light and that change is interpreted by a unique microprocessor contained in the control module. This activates the wipers as needed. These systems are sensitive enough to pick up mist, wet snow and rain.

From here: [http://www.usglassmag.com/AGRR/Backissues/0202/0202rainsensors.htm](http://www.usglassmag.com/AGRR/Backissues/0202/0202rainsensors.htm)

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**Author:** ![trupa](https://avatars.discourse-cdn.com/v4/letter/t/aeb1de/32.png) [@trupa](https://boards.straightdope.com/u/trupa)\
**Post date:** [June 10, 2005, 10:14pm UTC](https://boards.straightdope.com/t/how-do-vw-wipers-know-how-much-rain-is-falling/307798/3 "2005-06-10T22:14:54Z")

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Wow, that was quick. Thanks **Scruloose**.

Trupa, _off to look behind the rear view mirror._