LCD Construction and Working

A liquid crystal is a material (usually, an organic compound) which flows like a liquid at room temperature but whose molecular structure has some properties normally associated with solids. As is well-known, the molecules in ordinary liquids have random orientation but in a liquid crystal they are oriented in a definite crystal pattern.

Normally, a thin layer of liquid crystal is transparent to incident light but when an electric field is applied across it, its molecular arrangement is disturbed causing changes in falls on an activated layer of a liquid crystal, it is either absorbed or else is scattered by the disoriented molecules.

As shown in Fig. 1, a liquid crystal ‘cell’ consists of a thin layer (about 10 mm) of a liquid crystal sandwiched between two glass sheets with transparent electrodes deposited on their inside faces.

With both glass sheets transparent, the cell is known as transmittive type cell. When one glass is transparent and the other has a reflective coating, the cell is called reflective type.

The LCD does not produce any illumination of its own. It, in fact, depends entirely on illumination falling on it from an external source for its visual effect.

The two types of display available are known as

field-effect display and
dynamic scattering display.

When field-effect display is energized, the energized areas of the LCD absorb the incident light and, hence give localized black display. When dynamic scattering display is energized, the molecules of energized area of the display become turbulent and scatter light in all directions.

Consequently, the activated areas take on frosted glass appearance resulting in a silver display. Of course, the unenergized areas remain translucent.

As shown in Fig. 2, a digit on an LCD has a segment appearance. For example, if number 5 is required, the terminals 8, 2, 3, 6 and 5 would be energized so that only these regions would be activated while the other areas would remain clear.

Advantage of LCDs

An LCD has the distinct advantage of extremely low power requirement (about 10-15 micro watt per 7-segment display as compared to a few mili watts for a LED). It is due to the fact that it does not itself generate any illumination but depends on external illumination for its visual effect (colour depending on the incident light).
They have a life-time of about, 50,000 hours.
Visibility of LCD is superior to that of led even in poor ambient light.
Low power consumption characteristic of LCD makes it useful for watches and small portable instruments.

Disadvantages of LCDs

LCD has some disadvantages also, they are given below:

This is very slow device compared to LED. The turn ON and OFF times are quite large. The turn ON time it typically of the order a few milliseconds while the turn OFF is 10 milliseconds.
Due to chemical degeneration, life of LCD is limited.
They occupy a large area.
Operating range of temperature form 0 to 60°C only

Applications of LCDs

The liquid crystal display is a matured technology these days. So it is being used in practically all areas where sunlight does not directly fall on the display. Following are some of the important applications of LCDs:

Field-effect LCDs are normally used in watches and portable instruments such as digital thermometers, blood pressure, blood sugar, pressure monitoring instruments etc. where source of energy is a prime consideration.
Recent desk top LCD monitors.
Notebook computer display.
Cellular phone display.
To display data on personal digital assistant (PDAs) such as Palm Vx and pocket PCs etc.

The liquid crystal display (LCDs) commonly used on notebook computers and handheld PADs are also appearing on desktop. These flat panel displays promise great clarity at increasingly high resolutions and are available.

The LCD monitor offers benefits and drawbacks. The first benefit is size. Because of the need to house the tube itself, cathode-ray tube (CRT) monitors are big and heavy.

Another problem is the viewing angle. The optimal viewing angle of an LCD is from straight in front and as you move further to the side the screen becomes harder to read.