Rainwater sensors in the car are used to detect rainfall and activate the wipers when needed. Under normal conditions, the driver can independently monitor the weather conditions and the operation of the brush, which distracts attention on the road. Still, the automatic system can react to the precipitation itself. Depending on the intensity of rain or snow, the sensor generates a control signal and regulates the brushes' operating mode and speed.

Usually, the sensor is located on the windshield and does not block the driver's view. The space behind the mirrors is suitable for this.

The sensor looks like a small reading device located on the back of the windshield. Depending on the design features, it can turn on the wipers and recognize the level of illumination to turn on the headlights. The device is fixed to the windshield using a special compound.

Rain sensor

With a rain sensor, the windscreen wiper is automatically activated and is controlled according to the rain intensity.

The sensor wiper can sense the size of the raindrops through the rain sensor, automatically adjust the running speed of the wipers, and provide the driver with a good vision, thereby greatly improving the convenience and safety of driving in rainy weather.

The rain sensor in the base of the interior mirror consists of several infrared LEDs and a centrally located photodiode. The light emitted by the LEDs is reflected from the windshield onto the photo sensor. The more water droplets on the windshield, the less light the sensor receives. This information is transmitted to control electronics, which adjust the wiping frequency of the windscreen wiper accordingly.

In addition, the sensor's sensitivity can be individually adjusted via a selector switch. The rain sensor is designed so that neither minor damage, dirt, nor aging of the windscreen impairs its function.

The driver can concentrate fully on traffic without adjusting the wiping frequency. This is a clear advantage, especially in the case of sudden visual obstructions, such as overtaking trucks on wet roads. The sun sensor works similarly.


Increasingly, automatic wiper systems eventually became almost universal equipment in all ranges, becoming a common accessory and, accordingly, commonplace. However, although most people understand that this kit consists of sensors connected to electronics, only some know how the sensors work. We are going to talk about one of the most commonly used processes.

Currently, the two mainstream sensors that are widely used are optical and capacitive.

●Optical sensor

Optical sensors work according to the principle of refraction of light. In the optical sensor, a light-emitting diode emits a cone-shaped beam of light that passes through the front windshield. When there is no rain on the windshield, and it is in a dry state, almost all the light will be reflected by an optical sensor. When it rains, there will be rain on the windshield, and part of the light will deviate, which causes the total amount of light received by the sensor to change, thereby detecting the presence of rain. The larger the area where an optical sensor can receive reflected light, the more detailed the information will be. Optical sensors are so accurate that it is possible to accurately determine the number of raindrops falling on the sensed area.

●Capacitive sensor

The other is a capacitive sensor, mainly designed to take advantage of the huge difference in the dielectric constant of water and glass, where the dielectric constant of water is 80, and the dielectric constant of glass is 2. It is common practice to place two parallel finger metal plates between the inner and outer layers of the windshield. A group of finger metal plates intersect without touching the other finger metal plates. When the windshield is dry, a dielectric is formed between the outer surface of the windshield and each set of finger-shaped metal plates. When the windshield becomes wet, the dielectric constant of the windshield changes depending on the amount of water in contact with the windshield. Suppose the sensor is mounted on the windshield's surface or against the windshield's lower surface. In that case, this is advantageous for the operation of the sensor because such an installation allows the sensor to exert its optimal sensitivity. On the downside, mounting a capacitive sensor on the outer surface of the windshield creates the same problems as a resistance sensor, and its metal plating will be quickly scraped off the windshield under long-term wiper operation.


As shown in the figure, the diode (Led) sends an infrared beam to the windshield. The windshield bounces off the inside of the windshield in a serrated pattern.


Infrared radiation was chosen because it is invisible, or rather, invisible to our eyes, which cannot detect light of this frequency. Light is divided into many different frequencies, which form colors. It is also the principle of the rainbow to break down white light (through rainwater, which acts as a prism) into different colors. All these colors are like well-separated and scattered components of light. But our eyes don't see all of them, including infrared (clearly visible in the rainbow), which is an extension of red. Thus, the latter's advantage is that it is invisible, making it possible to hide the beam passing through part of the windshield. Otherwise, you'll get a beam that's a little too customized It should also be noted that plants absorb all the colors of white light, not green reflections in crops. As a result, they seem green to us. When they are not there, they do not have the pigment of this color.

It should also be noted that the area where the spokes move is limited, and the length and width of the entire windshield do not overlap. Therefore, it may not work if bird droppings or insects accumulate, so it is necessary to thoroughly clean the outside where the detector is located (where the rear-view mirror is installed).


After figuring out what a car rain sensor is, you need to understand what the device is for and its main features:

  1. identify rain and snow.
  2. Windshield contamination analysis.
  3. control of wipers, as well as adjustment of its operating mode.
  4. Turn on the light automatically in case of insufficient illumination (in the case of combined sensors).

Precipitation sensors also have significant drawbacks, including false triggering when water enters the analysis area or malfunctions when dust in a glass or water floods an adjacent compartment. Similarly, the control circuitry of the car may not be able to open the gasket, which will cause the smudge on the glass to become dirty and affect visibility. No automatic system will rule out defects and errors. For example, the activation of the brush is usually slightly delayed, and during this time, the driver can independently clean the glass.


At the end of the beam path (bouncing off the serrated windshield wall), there is a sensor that measures the intensity of the infrared beam. If the intensity is reduced compared to when starting (where the diode is turned on), it means that it is raining (or dirt sticks to it ...


Because the droplets absorb part of the beam and send it to the "nothingness" outside the car, instead of bouncing off the wall, the beam continues its course within the drop as if it were an extension of the windshield. However, the latter does not return to the windshield and is lost in the air. As a result, the original rays lose their feathers and, accordingly, their strength.

The less traffic on arrival, the more rain it will have. The electronic mechanism can adjust the rhythm of the wipers accordingly.


This requires plain water and spray bottles. Step-by-step instructions on how to check the sensor with both hands:

  • Turn on the rain sensor
  • spray water onto the windshield with a spray
  • Wait for the system to work for 20-30 seconds

All systems are protected against false alarms, so it is necessary to sprinkle water evenly on the windshield for 20 seconds. Otherwise, the auto-assembly system will not work, and the brushes will not be turned on. Alternatively, you can use computer diagnostics.

The precipitation sensor allows you to monitor weather conditions automatically and turn on the cleaner when it rains or snows. Although this system has several drawbacks, it makes driving much easier.