Introduction to rain/light sensor:
Nowadays, more light sensors are being used in cars. These sensors measure the amount of (sun)light shining on the car’s windshield. Based on this, the sensor switches the car’s lighting on or off.
The light sensor is mounted behind the rearview mirror. Cars with a light sensor usually also have a rain sensor. These are then placed together in one housing. Therefore, both sensors are discussed on this page. The rain sensor measures the light transmissivity on the windshield, which changes due to reflections when water droplets are on the windshield.
Operation of rain sensor:
The rain sensor detects if moisture is present on the windshield. As soon as moisture or droplets are detected, the rain sensor controls the windshield wipers.
The image shows two situations:
Situation 1: It is dry. The rain sensor emits infrared light through the windshield outward. It does not reflect back, so the rain sensor does not activate anything.
Situation 2: It is raining. The raindrops land on the windshield and the direction of infrared light beams is disrupted. The raindrops act like a kind of mirror/magnifying glass, causing the infrared light to reflect back through the windshield to the rain sensor. The rain sensor recognizes this and activates the windshield wipers.
The rain sensor is also able to measure the intensity of the moisture content; during a heavy downpour, the 2nd or 3rd setting of the windshield wiper motor is activated. However, there is also a downside. As soon as the wiper blades are somewhat worn, a moisture trail may remain on the windshield after wiping. This causes the rain sensor to keep detecting moisture on the windshield and continues to activate the windshield wipers even when it is no longer raining.
Operation of light sensor:
The light sensor is mounted together with the rain sensor behind the rearview mirror. The light sensor is also referred to as an LDR; Light Dependent Resistor. The LDR is an electrical sensor whose internal resistance changes with changes in light intensity.
The resistance decreases with a lot of light and increases with little light.
As discussed in the section on the Law of Ohm, the resistance changes the current of the electrical circuit. This means that when the resistance is lower, the current will increase. The LDR is connected to a control device that measures the amount of current coming from the LDR.
During the day with plenty of sunlight, the resistance will be high, so the current will be low. The control device keeps the car’s lights turned off. As soon as it becomes twilight, or when entering a dark area (a tunnel or a parking garage), the sensor’s resistance value changes. It decreases, the current increases, and based on this information, the control device determines whether the lights need to be turned on.
When exiting the tunnel, the control device intentionally keeps the lights on for 5 to 10 seconds, as otherwise the lights would constantly toggle on and off with changing light conditions. This would not only be annoying for the driver and other road users, but would also shorten the lifespan of the bulbs.