ON Semiconductor : Why an Automotive Rain and Light Sensor is a Safety Feature

by Dan Clement - 03-23-2022

Rain and light sensors are small optoelectronic modules typically located in the vehicle's rearview mirror. The sensors capture the current driving conditions based on incoming sunlight and reflected infrared light flashes and provide this information to the body control module over a LIN (Local Interconnect Network) interface. The control module can automate various systems such as headlight, windshield, and HVAC with this information.

Rain and light sensors were originally only installed in high-end vehicles as a convenience function to automatically turn on/off the headlights by sensing the ambient light. Legacy systems are known for false wipe triggers when entering a tunnel or on very sunny days, also known as ghost wipes.

Today's rain and light sensor modules, used in almost all vehicle classes, have significantly improved performance. These modules also evolved in safety-critical functions.

In addition to the basic turn on/off of the headlights, modern rain and light sensors also toggle the high beams automatically. By selectively operating the high beams, other drivers can see better, which increases their visibility and overall safety for everyone on the road.

Similarly, the newer rain and light sensors are also used beyond the basic on/off wiper function. Modern systems can also control the wiper speed, including intermittent delay, based on the amount of moisture on the windshield, greatly increasing safety because the driver is completely hands-off for the wiper function, allowing more focus on driving and watching the road.

Figure 1: Rain and Light Sensors Typically Located Behind the Rearview Mirror.

Modern rain and light sensor modules contain NIR (near-infrared) LEDs that illuminate the windshield through a special optical interface, like a lens in a camera. NIR-sensitive photodiodes measure the reflected light. Rain and snow produce a different signal reflection than a clear windshield, making moisture detection possible. A visible light photodiode measures the amount of daylight around the car and far in front of the vehicle-information used to decide the headlight settings going into and coming out of tunnels. It further helps mitigate false triggers. Additional infrared diodes allow measuring the radiated solar energy to control the HVAC system better. Implementing a last photodiode automatically controls head-up display (HUD) backlight dimming.

Figure 2: Illustration of Rain and Light Sensor Windshield Interface.

The NCV76124 rain and light sensor interface from onsemi is the only one on the market designed for functional safety, or ISO 26262. The device is ASIL B-capable, and a complete safety plan is available. Our rain and light sensor requires minimum software overhead with easy control via the Serial Peripheral Interface (SPI) interface. It provides more sensor channels with an extremely high dynamic range, creating a high-performance and safe solution for modern vehicles. The SPI programmability allows support for various applications with one hardware configuration.

Figure 3: Rain and Light Sensor Solution Block Diagram.

In addition to the rain and light sensor, onsemi offers solutions for battery protection, system basis chips (SBC), and the NPN bipolar transistors used in this system. Learn more about products and solutions:

• NCV76124: Rain and Light Sensor Interface with SPI
• NCV76124EVK Rain and Light Sensor Evaluation Kit
• ISO 26262 Functional Safety: Turning Compliance into Concrete Added Value