LIQUID CRYSTAL DISPLAY 

Liquid crystal cell displays (LCDs) are used in similar applications where LEDs are used. LCD displays are common in Engineering applications and play a vital role.  These applications are display of numeric and alphanumeric characters in dot matrix and segmental displays. Now a days Computer monitors and TV, Watches and advertisements  use  LCD technology for display. LCD display produces high quality images and thinner compared with CRT technologies. The basic idea behind the working of LCD is given here. 

LCDs are of two types: 

I. Dynamic scattering type 

II. Field effect type 

The construction of a dynamic scattering liquid crystal cell:

The liquid crystal material may be one of the several components, which exhibit optical properties of a crystal though they remain in liquid form. Liquid crystal is layered between glass sheets with transparent electrodes deposited on the inside faces. When a potential is applied across the cell, charge carriers flowing through the liquid disrupt the molecular alignment and produce turbulence. When the liquid is not activated, it is transparent. When the liquid is activated the molecular turbulance causes light to be scattered in all directions and the cell appeas to be bright.This phenomenon is called dyanamic scattering. 

source of image: Wikipedia

The construction of a field effect liquid crystal display 

This is similar to that of the dynamic scattering type,with the exception that two thin polarizing optical filters are placed at the inside of each glass sheet. The liquid crystal material in the field effect cell is also of different type from employed in the dynamic scattering cell. The material used is twisted nemayic type and actually twists the light passing through the cell when the latter is not energised. i. Transmittive type ii. Reflective type In the transmittive type cell, both glass sheets are transparent, so that light from a rear source is scattered in the forward direction when the cell is activated. In reflective type cell has a reflecting surface on one side of glass sheets. The incident light on the front surface of the cell is dynamically scattered by an activated cell. Both types of cells appear quite bright when activated even under ambient light conditions. The liquid crystals are light reflectors are transmitters and therefore they consume small amounts of energy (unlike light generators). 

The seven segment display, the current is about 25 micro Amps for dynamic scattering cells and 300micro amps for field effect cells. Unlike LEDs which can work on d.c. the LCDs require a.c. voltage supply. A typical voltage supply to dynamic scattering LCD is 30v peak to peak with 50 Hz