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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 ambient light.
Today, the base technology used to detect
light has changed to silicon photodiodes
and phototransistors, and as a result
body electronics 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 electronics temperatures of
-40° to +85°C (-40° to +185°F). 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.
In one example, a Valeo rain 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 and 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.
In body electronics applications,
ambient-light sensors are being used to adjust the backlight
intensity of instrument panels and LCD backlights found in GPS
navigation, climate-control, and DVD screens.
This is especially important for
applications such as BMW’s iDrive and the Toyota Prius’ Multi-Info
displays. For example, as daylight gives way to twilight and
darkness, the instrument panel backlight adjusts for optimal
visibility and reduces 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 between 380 to 780 nm the visible
wavelengths.
Photodiodes play a role in climate
control by determining the angle of the sunlight and, in conjunction
with a thermistor, adjust
fan speed and temperature. Determining the angle of the sun is a
function of the illuminance on the photodiode, where a peak
translates to the sun at its apex. A photodiode with integrated NTC
thermistor is best for this type of application.
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 that prevents the
lights from cycling on and off. The second sensor, the tunnel
sensor, has a narrow field of view and is 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, 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 ASIC usage shows steady growth or a canopy of trees. In these
cases, the sensor will still “see” light ahead. When entering a
tunnel, the tunnel-sensor signal drops while the wide-field-of-view
sensor remains high; the headlights will turn on. When exiting the
tunnel, the tunnel sensor signal will be high while that for 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.
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 be found on only 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.
Jim Toal, Senior Manager of Product Marketing for the Sensor and
IrDA division of
Vishay Optoelectronics, wrote this article for AEI.
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