LCD Display paper presentation
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09-08-2010, 01:45 PM
This is murali who is studying B.Tech.
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I want LCD Display Paper presentation also.
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23-09-2010, 11:54 AM
A liquid crystal display (LCD) is a thin, flat electronic visual display that uses the light modulating properties of liquid crystals (LCs). LCs do not emit light directly. Each pixel of an LCD typically consists of a layer of molecules aligned between twotransparent electrodes, and two polarizing filters, the axes of transmission of which are (in most of the cases) perpendicular to each other. With no actual liquid crystal between the polarizing filters, light passing through the first filter would be blocked by the second (crossed) polarizer. The surface of the electrodes that are in contact with the liquid crystal material are treated so as to align the liquid crystal molecules in a particular direction. This treatment typically consists of a thin polymer layer that is unidirectionally rubbed using, for example, a cloth. The direction of the liquid crystal alignment is then defined by the direction of rubbing. Electrodes are made of a transparent conductor called Indium Tin Oxide (ITO). Monochrome LCD images usually appear as blue or dark gray images on top of a grayish-white background. Color LCD displays use two basic techniques for producing color: Passive matrix is the less expensive of the two technologies. The other technology, called thin film transistor (TFT) oractive-matrix, produces color images that are as sharp as traditional CRTdisplays, but the technology is expensive.
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Part1. LIQUID CRYSTAL DISPLAY CONSTRUCTION
Liquid crystal displays (LCDs) typically have three groups of assembly components: the cell,the module, and the monitor. The cell comprises the glass plates that contain the liquid crystalmaterial and the front and back polarizer filters. The module comprises the cell plus displaydrivers that control light and deliver host computer data to the cell, and a backlight assembly consisting of fluorescent lamps, light pipes, and associated diffusers and reflectors; all containedwithin a rigid sheet-metal structure. The monitor consists of the module plus an inverter topower the lamps, a display interface to the CPU, a plastic bezel and stand, and a power supply.
1.1 Liquid Crystal Chemistry
Liquid crystals are a set of complex organic compounds composed of elongated, rod-shapedmolecules that in their natural state are arranged in a loosely ordered fashion with their long axes parallel. They exist in many phases, with the most common being smectic (gel-like), nematic (most common for computer displays), and cholesteric (naturally rotating liquid crystal structures). There are hundreds of liquid crystal types from which to choose, depending on thephysical, electrical, and optical properties the user desires in a display. Typically, a flat panel display will contain a mixture of 10 or more of these compounds. Liquid crystal materials have two important features that make them useful in display applications: their molecules are “polar,” with one end being more electrically positive or negative than the other, much like a compass needle, to use a magnetic analogy; and they are able to conduct, bend, or twist rays of light along their axes depending on their orientation.
More on this property will be discussed in Part 2, Liquid Crystal Display Operation. Simply put, we use electronic devices to control liquid crystals to make them manipulate light. Figure 1shows the structure of a typical biphenyl type of liquid crystal molecule.
1.2 LCD Cell Construction
1.2.1 Substrates with Patterned Electrodes
An LCD cell is composed of two glass plates that are commonly coated with a very thin, metallic oxide layer known as indium tin oxide (ITO). Because the layer coating each glass substrate is so thin (only a few hundred angstroms), it is transparent; because it is made of an oxide of two metals, it is conductive.
Using conventional semiconductor photoimaging and etching techniques, these layers canbe patterned to form electrode structures. The electrodes may be patterned into 7-segmented numeric designs, as those commonly found in liquid crystal watches, or into a series of lines arranged along an x-y grid. In passive matrix-addressed cells, the two layers of ITO are patterned into tightly spaced parallel vertical traces on the front glass (columns) and horizontal traces on the back glass (rows). Figure 11 shows the construction of a 6 row x 7 column passive matrix cell, and its operation is described in section 2.2.1, Passive Matrix LCDs.
1.2.2 Molecular Alignment Layers
After patterning of the ITO layer, the surface of each glass plate is coated with an alignmentlayer, usually polyimide. That alignment layer is first baked and then polished or buffed to create microscopic parallel grooves on the surface of each plate. Although the grooves are all parallel, it is important to note that each plate has its grooves oriented in a different direction.
In subsequent processing, these grooves will cause the molecules of the liquid crystal material not only to “sheet” or wet the surfaces, but to line up parallel along the buffing direction as shown in Figure 2. What is not shown in Figure 2 is that the molecules do not lie exactly flat on the alignment layer but point up slightly from the surface at an angle of 2° to 5°. This “pretilt angle” is critical to the proper function of the display, but is also the cause of certain optical inconsistencies, as will be discussed in section 3.3.1, Viewing Angle. The plates are now ready for spacer application and assembly.
Figure 2.Molecular Alignment to a Buffed Surface. In their natural
state, liquid crystal molecules are arranged in a loosely ordered
fashion with their long axes parallel. The alignment layer surface
can be finely grooved by a polishing or buffing operation. When
liquid crystals are flowed onto this layer, their molecules line up
parallel along the grooves.
1.3 LCD Cell Assembly
During assembly, a sealing material is applied alongthe perimeter of one of the glass substrates, leaving a gap of a few millimeters at one corner, and then prebaked. Cell gap spacers, usually glass or plastic beads, are then applied by dry or wet spraying techniques. These beads are critical to the ultimate function of the display because they must maintain the spacing of the gap separating the two glass substrates at an optimum thickness of 4 to 5 microns, about 1/15 the thickness of a human hair. (In the Silicon Graphics 1600SW monitor display, the gap must be uniform across a 17.3-inch diagonal width!) The two glass plates are then oriented as shown in Figure 3 so that their respective buffing directions are at right angles to one another; they are then clamped and baked or exposed to ultraviolet radiation to set the sealant. This forms an empty package with an open port at one corner that is ready for the injection of the liquid crystal material.
Figure 3.Twisted-Nematic Alignment. A cell can be
constructed so that liquid crystals are sandwiched
between upper and lower plates with grooves pointing in
directions “a” and “b,” respectively. The molecules along
the upper plate point in direction “a,” and those along
the lower plate point in direction “b.” This forces the
liquid crystals into an overall 90° twisted-nematic state.
1.3.1 Filling and Sealing
Several cells are placed in a vacuum chamberin a fixture that suspends them on edge over a container of liquid crystal material. Air is exhausted from the chamber and the cells equilibrate to the surrounding vacuum through their fill ports. After the cells and the liquid crystal (LC) material in the reservoir have outgassed sufficiently, the plates are remotely lowered so that the fill ports are submerged. The LC material is injected by backfill pressure between the glass plates through the gap in the perimeter seal, which is then plugged with epoxy or more UV-cured adhesive. The filled and sealed cells are now ready for the addition of external optical elements and display drivers.
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Liquid crystal displays (LCDs) have materials which combine the properties of both liquids and crystals. Rather than having a melting point, they have a temperature range within which the molecules are almost as mobile as they would be in a liquid, but are grouped together in an ordered form similar to a crystal.
An LCD consists of two glass panels, with the liquid crystal material sand witched in between them. The inner surface of the glass plates are coated with transparent electrodes which define the character, symbols or patterns to be displayed polymeric layers are present in between the electrodes and the liquid crystal, which makes the liquid crystal molecules to maintain a defined orientation angle.
One each polarisers are pasted outside the two glass panels. These polarisers would rotate the light rays passing through them to a definite angle, in a particular direction
When the LCD is in the off state, light rays are rotated by the two polarisers and the liquid crystal, such that the light rays come out of the LCD without any orientation, and hence the LCD appears transparent.
When sufficient voltage is applied to the electrodes, the liquid crystal molecules would be aligned in a specific direction. The light rays passing through the LCD would be rotated by the polarisers, which would result in activating / highlighting the desired characters.
The LCD’s are lightweight with only a few millimeters thickness. Since the LCD’s consume less power, they are compatible with low power electronic circuits, and can be powered for long durations.
The LCD s doesn’t generate light and so light is needed to read the display. By using backlighting, reading is possible in the dark. The LCD’s have long life and a wide operating temperature range.
Changing the display size or the layout size is relatively simple which makes the LCD’s more customer friendly.
The LCDs used exclusively in watches, calculators and measuring instruments are the simple seven-segment displays, having a limited amount of numeric data. The recent advances in technology have resulted in better legibility, more information displaying capability and a wider temperature range. These have resulted in the LCDs being extensively used in telecommunications and entertainment electronics. The LCDs have even started replacing the cathode ray tubes (CRTs) used for the display of text and graphics, and also in small TV applications.
This section describes the operation modes of LCD’s then describe how to program and interface an LCD to 8051 using Assembly and C.
In recent years the LCD is finding widespread use replacing LED s (seven-segment LED s or other multi-segment LED s).This is due to the following reasons:
1. The declining prices of LCDs.
2. The ability to display numbers, characters and graphics. This is in contrast to LED which is limited to numbers and a few characters.
3. Incorporation of a refreshing controller into the LCD, there by relieving the CPU of the task of refreshing the LCD. In the case of LED s, they must be refreshed by the CPU to keep on displaying the data.
4. Ease of programming for characters and graphics.
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