rain sensor

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Rain / twilight sensor behind the windshield, integrated in the adhesive surface of the inside rearview mirror of a passenger car
Cutaway model of a rain sensor

A rain sensor is a technical component that can determine whether and how much it is raining in order to initiate appropriate action. For this purpose, the wetting of a surface is recorded and a switching signal or a quantitative signal is generated.

The difference from the rain gauge is that not the exact rainfall is measured, but only secondary effects such as the wetting or the visual obstruction are recorded a disc irrigated. In a vehicle, the rain sensor is usually located on the mirror on the windscreen.

application areas

Areas of application for rain sensors are the automatic control of various processes, including the control of irrigation systems .

Control of windshield wipers on vehicles or ships: In 1994, the automobile manufacturer Peugeot offered a rain sensor as standard in one of its models for the first time. Since then, this development has been incorporated into vehicles by several automobile manufacturers. The sensor is attached behind the windshield, usually integrated in the base of the rearview mirror in the upper area of ​​the windshield, at least at the edge outside the field of vision. The rain sensor measures the wetting of a small measuring field and enables the control electronics to assess the wetting of the windshield and to activate the wiper when an appropriate wetting is achieved. The measuring field usually has a size of 2 cm² (size of a one-cent piece). In order to be able to determine the wetting after a wiping process, the measuring field is attached in the wiping area of ​​the windshield wiper.

Other areas of application are roof hatches and awnings on houses that are automatically closed or retracted when it rains.

Structure and functionality

Scheme of an optical rain sensor
1 - LED, 2 - photodiode
Evaluation electronics of a light / rain sensor (without optics)

Rain sensors work

  • optoelectronic: reflection measurement behind a pane of glass
  • electrical:
    • Change in conductivity between open electrodes due to the conductivity of the wetting water
    • Change in the capacitance between insulated electrodes
  • mechanical: Materials that swell under the influence of moisture are used to actuate a switching contact or a valve.

Optoelectronic process

Illumination of a pane of glass by means of a light emitting diode (LED) and registration of the reflection by means of a light receiver ( photo transistor ). The physical law of reflection at the boundary between the optically denser and the optically thinner material ( refractive index ) is used. The angle of the light beam, which is inclined to the surface of the glass pane, is chosen so that when the outer pane surface is dry, the entire amount of light is reflected and goes to the photo receiver ( total reflection : the light does not leave the optically denser medium "glass"). Drops of water - from rain, for example - on the outer surface of the pane change the reflection behavior, i.e. This means that it is reflected less, part of it is scattered by the uneven surface of the water and can escape.

In order for light to be totally reflected at the boundary between glass (with a refractive index around n = 1.5) and air (almost n = 1), the angle of incidence must be greater than 41 ° (critical angle for total reflection, measured away from the perpendicular; i.e. the greater the running flatter to the interface). About 45 ° would be possible. If there is water ( n = 1.33) on the glass surface , which has an intermediate position in its optical density, the relevant difference in refractive index (glass to water) is significantly lower and there is only partial reflection, i.e. less light is reached the detector. Total reflection occurs (from glass to water) only from a higher critical angle of 61 °.

The critical angle for total reflection (denser to thinner) is calculated exactly from the refractive index ratio ( n 1n 2 ).

The same also applies to the newly added boundary layer between water and air on the very outside: smaller refractive index difference - larger critical angle. The light beam is refracted towards a larger angle (= away from the perpendicular) and could now be reflected by the water boundary layer to the air. However, the surface of the water is not even, which is why a large part of the light escapes: The water lies as more or less spread, curved drops or as an uneven layer on the glass due to the wind and water currents. The more the glass surface is unevenly wetted by the water, the more the total reflection is disturbed, which in turn is registered by the sensor.

The measurand is the decreasing light intensity on the photo receiver when wetted.

A single small drop in the measuring field should not yet trigger the wiper, but a larger one or more should. In this way, the sensor should react to the incipient visual impairment caused by drops of water on the glass.

The proportion of the amount of light reflected can also serve as a control variable for the interval time of a windshield wiper, so that the rain sensor can also control the speed of the windshield wiper as a function of the amount of rain.

Usually infrared is used. The IR beam is widened by diffusing lenses and guided into the windshield at the required angle by means of a glued-on optical prism. The prism is necessary in order to be able to couple the radiation into the pane at the required angle - without a prism, no ray, which is then totally reflected, could reach the plane-parallel pane due to the refraction. The light beam is now reflected inside the windshield and reaches a decoupling prism at another point, which avoids total reflection on the inner surface, leaves the windshield and is focused on the photo receiver by means of converging lenses.

In heavy rain, a sensor system can automatically switch from intermittent to continuous wiping. During a sudden downpour or in the spray of a truck, the system immediately switches to the highest speed level. If only a few raindrops fall, the electronics control the wiper speed so that the driver always has a clear view. Dry rubbing of the wiper blades on the windshield and thus excessive wear and tear on the wiper blade are avoided. In order to prevent misinterpretations due to condensation moisture, an integrated heater can keep the measuring area dry from the inside.

Compared with the infrared radiation output of the sun or tunnel lighting, the intensity of the IR diode is extremely low, the quantity to be measured (scattered light) is even vanishingly small. In order to still achieve precise measured values, the signal is operated in a pulsed manner and evaluated with the aid of a digital lock-in amplifier .

Electrical procedures

Electric rain sensors, such as those used in building services or in horticulture, have an electrode arrangement. They use the change in the capacity of the irrigated arrangement, or they register the changed electrical resistance due to the conductivity of the rainwater.

Depending on the process, the electrodes are either covered or protected with an insulating layer or metallic-open. Open arrangements must have corrosion-resistant metal surfaces; one often chooses gold-plated conductor tracks made as a printed circuit board .

Other functions in vehicles

Today's (2012) rain sensors connected to the vehicle network (e.g. CAN or LIN bus ) often offer additional control options: The signals are e.g. B. used to automatically close windows and sunroof. Integrated ambient light sensors can control the driving lights in order to switch them on automatically when it is dark or when entering a tunnel. The automatic headlight cleaning system also works on the principle of the opto-electronic measurement used in the rain sensor .

The exposed position on the windshield is also suitable for integrating condensation and sun position sensors.


  • Karl-Heinz Dietsche, Thomas Jäger, Robert Bosch GmbH: Automotive pocket book. Friedr. Vieweg & Sohn Verlag, Wiesbaden 2003 (25th edition), ISBN 3-528-23876-3 .
  • Katja Bammel: A clear view in bad weather. In: Physik Journal, March 2007.

Web links

Commons : Rain sensors  - collection of images, videos and audio files
Wiktionary: Rain sensor  - explanations of meanings, word origins, synonyms, translations