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long range. Thus in a given layer, the rods are all oriented in the same direction Also, in

the smectic liquid crystals the molecules of different layers are ordered as shown in

Fig. 10.15 (a). Thus both orientation order and positional order is present in the smectic

crystals.

Fig. 10.15 (b) shows nematic structure of liquid crystals. In the nematic structure the

positional order between layer of molecules is lost, but the orientation order is maintained.

Fig. 10 15 (c) shows cholesteric structure of liquid crystals. In these crystals the rod-lik e

molecules in each layer are oriented a diff erent angle within each layer. Orientation order

is maintained in each layer. The cholesteric liquid crystal is related to the nematic crystal,

with the difference being the twist of the molecules as one goes from one layer to another.

The optical activity of the crystal depends upon the orientation and the twist of  the

molecules as one goes from one layer to another.

10.6.1 Types of LeOs

There are two types of liquid crystal displays (LCDs) according to the theory of 

operation:

1. Dynamic scattering 2. Field effect.

10 .6 .1 .1 Dyn am ic Scatter in g T ype L eO s

Fig. 10.16 shows the construction of a typical liquid crystal display. It consists of twoglass plates with a liquid crystal fluid in between. The back plate is coated with thin

transparent layer of conductive material, where as front plate has a photoetched

conductive coating with seven segment pattern as shown in Fig. 10.16.

Fig. 10.16 Liquid crystal display construction

Fig. 10.17 shows the operation of liquid crystal display. In the absence of the electrical

signal, orientation order is maintained in the crystal allowing light to transmit. This makes

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Display Devices

"-Transparentconductive

film display

egment

i~  _1LCD display clear. The current through the liquid 

crystal causes orientation order to collapse. The

random orientation results scattering of light

which lights display segment on a dark 

background as shown in Fig. 10.18.

0.6.1 .2 F ie ld E f fe ct D isp la y

In these displays nematic liquid crystals are used. Fig. 10.19 shows operation of field

ffect liquid crystal display with nematic crystals. It consists of two glass plates, a liquid

rystal f  luid, polarizers and transparent conductors. The liquid crystal fluid is sandwichedbetween two glass plates. Each glass plate is associated with light polarizer. The light

polarizers are placed at right angle to each other. In the absence of  electrical excitation, the

ight coming through the front polarizer is rotated through / -O" in the fluid and passed

hrough the rear polarizer. It is then reflected to the viewer by the back mirror as shown

n Fig. 10.19 (a).

On the application of electrostatic field, the liquid crystal fluid molecules get aligned

nd therefore light through the molecules is not rotated by 90° and it is absorbed by the

ear polarizer as shown in Fig. 10.19 (b). This causes the appearance of dark digit on a

ight background as shown in Fig. 10.19 (c).

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10.6 .1 .3 A d va nta ges o f LC Os

1. Less power consumption

2. Low cost

3. Unif orm brightneE''' with good contrast

10.6.1.4 Disadvantages of LCOs

'(101' reliability

l,imited temperature range.

~. roar visibility in Jow ambient temperature.

- + Slow speed

, Requires an ac drive.

The operation of this display is based on the principle that under breakdown

condition, a gas near cold cathode gas filled tube emits light. The cold cathode indicators

<Ire called Nixie Tubes. These are based on the principle of glow discharge in a cold

cathode gas filled tubes. The construction of the nixie tube is as shown in the Fig. 10.20.I t

consists 10 cathode and one anode, all are made of thin wires. But only diff erence is anode

is in the f orm of thin frame.

When a gas near cathode breaks down, a glow discharge is produced. The guaze

electrodes with a positive supply voltage work as an anode. In generaL this voltage is

selected greater than the worst case breakdown voltage of the gas within tube. When the

cathode is connected to ground potential, the gas which is close to a cathode glows.

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Gas filledenvelopes

Numeral shapedelectrodes

Cathodes

Nixie tubeFrontview

Sideview

Generally, Neon gas is used to f ill the glass tube. The Neon gas gives orcmge-red glow.

For other colours, diff erent gases are used. Sometimes dot cathodes arc used to serve as

decimal point in the character. The important characteristics are as h]]ows :

i) The numerals are large, typically 15 - 30 mm high.

ii) The Nixie tubes are single digit devices with or without Liccimal point.

iii) They are aVClilable in two types, side viewing and top "iev \  ing etc.

iv) D.C. supply requirement f or the nixie tube is "'50-200 V. While the current

requirement of the selected cathode is 1-5 mA.

v) Can be used as multiplexed displays.

vi) Nixie tubes are also used to display alphabetical symbols.

Following table gives the comparison on the basis of advantages, disadvantages and

applications of digital display devices.

LED displays LCD displays Nixie tube

Advantages • Bright display • Good contrast in bright • Bright display

• Available in differentambient light

• More colour ranges thancolours such as green, • Low power element LEOsyellow, red and amber

• Low cost • Low cost• Small" size

• C:0mpatible with ICs • Compatible with ICs.

• Compatible with ICs