Sensors bring advancements in automotive electronics

Thirty years ago the first cadmium sulfide photo resistor was used in a passenger car to automatically turn on and off the headlights by detecting the ambient light. Today, the base technology used to detect light has changed to silicon photodiodes and phototransistors, and as a result body electronic applications have expanded significantly.

In most cases, photodiodes are used in automotive applications because the photo current and wavelength sensitivity is linear over typical body electronic temperatures of -40°C to +85°C. Compared to photodiodes, the DC current gain and dark current of phototransistors have higher temperature dependence. The advantage of a phototransistor over a photodiode is that for the same amount of light, the output current is much higher, possibly eliminating the need for amplification.RAIN DETECTION

Figure 1 shows Valeo’s rain sensor mounted against the windshield becoming part of the rear view mirror assembly. Figure 2 illustrates the general principle of rain detection. The sensor consists of an infrared light emitting diode and a photodiode for detecting the amount of emitted light that is reflected off the glass. The infrared light is emitted through the sensor assembly at a precise angle, reflected inside the glass of the windshield, and back to the photodiode. When it begins to rain, droplets fall on the glass causing some of the light to be refracted resulting in less light reflected back on the photodiode. As the amount of rainfall increases, the amount of light reflected back on the detector surface decreases.

Eventually, the output current falls below a defined threshold and the sensor indicates “rain.” With this input to a microcontroller, the sensor turns on the wipers and regulates the wiper speed.

AMBIENT LIGHT DETECTION

In body electronic applications, ambient light sensors are being used to adjust the back light intensity of instrument panels and LCD backlights found in navigation (GPS), climate control, and DVD screens. This is especially important for displays like those found in BMW’s iDrive and Prius’s multi-info display. For example, as daylight gives way to twilight and darkness, the instrument panel backlight will variably adjust for optimal visibility and reduce potential glare for the driver. The use of these sensors can eliminate the annoying auto-dimming feature of displays when the headlights are turned on during the day. The key performance feature of an ambient light sensor is to replicate the sensitivity of the human eye with sensitivity between 380nm to 780nm – the visible wavelengths.

CLIMATE CONTROL

Photodiodes play a role in climate control by determining the angle of the sunlight and, in conjunction with a thermistor, adjust the fan speed and temperature. Determining the angle of the sun is a function of the illuminance on the photodiode where a peak illuminance translates to the sun at its apex. A photodiode with integrated NTC thermistor is best for this type of application.

TUNNEL DETECTION

Tunnel detection requires the input of two sensors. The first sensor has a wide field of view, oriented to “look upward” with a relatively long moving average time period. The long time period prevents the lights from cycling on and off. The second sensor – tunnel sensor – has a narrow field of view, oriented to “look forward” with a relatively short moving average time period. This allows the tunnel sensor to react quickly to sudden changes in light and to turn on the headlights, and possibly dim the display backlights when entering a tunnel. The forward looking sensor eliminates the lights turning on and off when going under a bridge or a canopy of trees. In these cases, the sensor will still “see” light ahead.

When entering a tunnel, the tunnel sensor signal will drop while the wide field of view sensor will remain high; the headlights will turn on. When exiting the tunnel, the tunnel sensor signal will be high while the wide field of view sensor will be low; the headlights will turn off. A clear distinction can be made by the controller thanks to the different moving average periods.

CONCLUSION

The use of optical sensors, particularly photodiodes and phototransistors, in body electronic applications is adding to the safety and convenience demanded by consumers. These features may only be found on high-end cars today but, as is typical for automotive applications, they will soon find their way to vehicles in every price bracket. To learn more go to www.vishay.com/optoelectronics.

About the author

Jim Toal is the senior manager of product marketing for the Sensor and IrDA division of Vishay Optoelectronics. Vishay is a major supplier of photo detectors to automotive manufacturers and offers a broad selection of passive and active components.

Click here for Illustrations:

Figure 1, Figure 2