Construction and Working Of LCD

Liquid Crystal Display (LCD)

LCD are passive display devices characteristics by very low power consumption and good contrast ratio. The liquid crystal display does not emit light or generate light. It requires an external or internal light source. Although LED give off light but LCD are not light sources but controls light. LCD a reflect part of surround by light.

Principle

The molecules in ordinary liquids normally have random orientations. In liquid crystals, the molecules are oriented in liquid crystals, the molecules are oriented in a definite crystal pattern when an electric field is applied to these crystals the molecules which are approximately cigar shaped, tend to aligns themselves perpendicular to the field. Charge carriers flowing through the liquid disturb the molecular alignment and cause a turbulence within the liquid, this is illustrated in below fig

.                                              Molecules In Liquid Crystels

When not activated, the liquid crystal is transparent.
When activated, the molecular turbulence causes the light to be scattered in all directions, so that the activated areas appear bright. The phenomenon is known as ‘dynamic scattering’ and is shown in fig.

The actual crystal material may be one of several organic compounds which exhibit the optical properties of a solid while retaining the fluidity of a liquid. Examples of such compound are

1.       Cholestryl nonanoate

2.       P – azoxyanisole

Construction and Working

The constructional features consists of a layer of liquid crystal material sandwitched between the glass sheets with transparent electrodes deposited on the inside surface and is shown in below fig.

                                                            Construction Of LCD

With both glass sheets transparent the cell is known as ‘transmitive type cell’. When only one glass sheet is termed as ‘reflective type’. The liquid crystal cells do into generate light but transmit or reflect light from external sources. Thus only energy required by the cell is to produce ‘dynamic scattering effect’.

LCD are usually seven segment type or dot matrix type displays. In the displays LCDs are activated by applying voltage between the segments and common electrode. Segments on the LCD are driven by low frequency ac signals typical driving voltage of 5 V rms. When segment is not activated, the transmitive type cell will simply transmit rear side lighting through the segment in straight line. In this condition the corresponding segment will not appear bright. In reflective type the light is reflected in usual way from mirror surface and corresponding segment will not appear bright.

When the segment is activated, the incident light is diffusely scattered forward and the corresponding segment appear bright.