Page 326 - Understanding Automotive Electronics

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AUTOMOTIVE INSTRUMENTATION 9

Electromechanical and Recent developments in solid-state technology in the field called
simple electro-optical optoelectronics have led to sophisticated electro-optical display devices that are
displays are being capable of indicating alphanumeric data. This means that both numeric and
replaced by sophisticated alphabetic information can be used to display the results of measurements of
electronic displays that automotive variables or parameters. This capability allows messages in English
provide the driver with or other languages to be given to the driver. The input for these devices is an
numeric or alphabetic electronic digital signal, which makes these devices compatible with computer-
information. based instrumentation, whereas electromechanical displays require a D/A
converter.
Automobile manufacturers have considered many different types of
electronic displays for automotive instrumentation, but only four have been
really practical: light-emitting diode (LED), liquid crystal (LCD), vacuum-
fluorescent (VFD), and the cathode ray tube (CRT). It now appears that the
VFD will be the predominant type of instrumentation for at least the near
future. Each of these types is discussed briefly to explain their uses in
automotive applications.
LED
LEDs are solid-state The light-emitting diode is a semiconductor diode that is constructed in a
devices that emit light manner and of a material so that light is emitted when an electrical current is
when current is passed passed through it. The semiconductor material most often used for an LED
through the diode. that emits red light is gallium arsenide phosphide. Light is emitted at the
LEDs are difficult to diode’s PN junction when the positive carriers combine with the negative
view in bright sunlight. carriers at the junction (see Chapter 3 for a discussion of PN junctions). The
diode is constructed so that the light generated at the junction can escape from
the diode and be seen.
An LED display is normally made of small dots or rectangular segments
arranged so that numbers and letters can be formed when selected dots or
segments are turned on. The configuration for these segments is described in
greater detail later in this chapter in the section on VFD. A single LED is not
well suited for automotive display use because of its low brightness. Although it
can be seen easily in darkness, it is difficult to impossible to see in bright
sunlight. It also requires more electrical power than an LCD display; however,
its power requirements are not great enough to be a problem for automotive use.
LCD
LCDs use a liquid that The LCD display is commonly used in electronic digital watch displays
possesses the ability to because of its extremely low electrical power and relatively low voltage
rotate the polarization of requirements. The heart of an LCD is a special liquid that is called a twisted
polarized light. LCD nematic liquid crystal. This liquid has the capability of rotating the polarization of
displays have low power linearly polarized light. Linearly polarized light has all of the vibrations of the
requirements. optical waves in the same direction. Light from the sun and from most artificial
light sources is not polarized, and the waves vibrate randomly in many directions.

UNDERSTANDING AUTOMOTIVE ELECTRONICS 313