Display Devices & Recorders

Reference Book: Electrical Measurement & Instrumentation, By-A.K. Sawhney, Chapter:

28, 31

Data Acquisition System

Introduction• The last stage of instrumentation & measurement is display devices &

recorders. Because the results of a measurement system are meaningful, they must be displayed for instant observation of for storage for observation at a later stage. The first device is called a “display device” and the second a “recorder”. The data presentation devices may be called as output devices.

Choice to select display devices

The expected use of the output The information content of the output

Advantage of digital display system

• Faster• Lighter weight• Cheap• Low error probability• Compatible with other devices• Adjustable resolution gives better display• Low power consumption

Our Focus

• Seven segment display• Liquid crystal display• Cathode ray tube

• Digital Frequency Meter• Digital Voltmeter (DVM)

seven-segment display (SSD)• A seven-segment display (SSD), or seven-segment indicator, is a form of

electronic display device for displaying decimal numerals that is an alternative to the more complex dot-matrix displays. Seven-segment displays are widely used in digital clocks, electronic meters, and other electronic devices for displaying numerical information.

• The idea of the seven-segment display is quite old. In 1910, for example, a seven-segment display illuminated by incandescent bulbs was used on a power-plant boiler room signal panel.

Seven Segment Display: Basic Idea

Retro LED Watch

(Circa 1970s)

This presentation will demonstrate how• A seven-segment display can be used to display the decimal

numbers 0-9 and some alpha characters.• A common anode seven-segment display works.• A common cathode seven-segment display works.• To select the resistor value for a seven-segment display.

6

Segment Identification• A Seven-Segment Display (SSD) is simply a figure eight grouping of

LEDs {some include a decimal point (DP)}.• Each Segment is labeled (a) thru (g).• SSDs are available in two configurations

– Common Cathode (all LED cathodes are connected)– Common Anode (all LED anodes are connected)

a

b

c

d

e

g

dp

f

7

Basic LED Operations

To Turn an LED ON . . . • The ANODE must be at a higher

voltage potential (1.5v) than the CATHODE.

• The amount of current flowing through the LED will determine the brightness of the LED.

• The amount of current is controlled by a series resistor. (not shown)

To understand how a seven-segment display works, we must review how an LED works.

CATHODE (‒) (+) ANODE

← Current Flow

8

LED Configuration – Anode @ 5 VoltsSwitch @ 5v• Top Circuit• LED Off

Switch @ 0v• Bottom Circuit• LED On• ANODE @ 5v• CATHODE @ 0v (nearly)• The 220 resistor controls the

current.• A larger resistor . . . less current . . .

dimmer LED• A smaller resistor . . . more current .

. . brighter LED

Common AnodeConfiguration

(5v=Off / 0v=On)

9

Example #1: Common Anode SSDExample

What value would be displayed in the common anode seven-segment display shown?

10

Example #1: Common Anode SSDExample

What value would be displayed in the common anode seven-segment display shown?

SolutionCommon Anode:

• 0 volts = Segment On• b, c, f, & g

• 5 volts = Segment Off• a, d, & e

a

b

c

d

e

gf

11

LED Configuration – Cathode @ GroundSwitch @ 5v• Top Circuit• LED On• ANODE @ 5v (nearly)• CATHODE @ 0v• The 220 resistor controls the

current.• A larger resistor . . . less current . . .

dimmer LED• A smaller resistor . . . more

current . . . brighter LED

Switch @ 0v• Bottom Circuit• LED Off

Common Cathode SSD Configuration (5v=On / 0v=Off)

12

Example #2: Common Cathode SSDExample

What value would be displayed in the common cathode seven-segment display shown?

13

Example #2: Common Cathode SSDExample

What value would be displayed in the common cathode seven-segment display shown?

SolutionCommon Cathode:

• 5 volts = Segment On• a, b, d, e, & g

• 0 volts = Segment Off• c & f

a

b

c

d

e

gf

14

liquid crystal display (LCD )• A liquid crystal display (LCD) is a flat panel display, electronic visual display,

or video display that uses the light modulating properties of liquid crystals (LCs). LCs do not emit light directly.

• LCD displays are available to display arbitrary images (as in a general-purpose computer display) or fixed images which can be displayed or hidden, such as preset words, digits, 7-segment displays, etc., as in a digital clock. They use the same basic technology, except that arbitrary images are made up of a large number of small pixels, while other displays have larger elements.

• LCDs are used in a wide range of applications, including computer monitors, television, instrument panels, aircraft cockpit displays, signage, etc. They are common in consumer devices such as video players, gaming devices, clocks, watches, calculators, and telephones. LCDs have replaced cathode ray tube (CRT) displays in most applications. They are available in a wider range of screen sizes than CRT and plasma displays, and since they do not use phosphors, they cannot suffer image burn-in. LCDs are, however, susceptible to image persistence.

• The LCD is more energy efficient and offers safer disposal than a CRT. Its low electrical power consumption enables it to be used in battery-powered electronic equipment. It is an electronically modulated optical device made up of any number of segments filled with liquid crystals and arrayed in front of a light source (backlight) or reflector to produce images in color or monochrome. Liquid crystals were first developed in 1888.

1. Reflective twisted nematic liquid crystaldisplay.Polarizing filter film with a vertical axis to polarize light as it enters.2. Glass substrate with ITO electrodes. The shapes of these electrodes will determine the shapes that will appear when the LCD is turned ON. Vertical ridges etched on the surface are smooth.3. Twisted nematic liquid crystal.4. Glass substrate with common electrode film (ITO) with horizontal ridges to line up with the horizontal filter.5. Polarizing filter film with a horizontal axis to block/pass light.6. Reflective surface to send light back to viewer. (In a backlit LCD, this layer is replaced with a light source.)

Underlying technologies for full-area 2-dimensional displays include:• Cathode ray tube display (CRT)• Light-emitting diode display (LED)• Electroluminescent display (ELD)• Electronic paper, E Ink• Plasma display panel (PDP)• Liquid crystal display (LCD)• High-Performance Addressing display (HPA)• Thin-film transistor display (TFT)• Organic light-emitting diode display (OLED)• Surface-conduction electron-emitter display (SED) (experimental)• Laser TV (forthcoming)• Carbon nanotubes (experimental)• Quantum dot display (experimental)• Interferometric modulator display (IMOD)Three dimensional

Swept-volume displayVarifocal mirror displayEmissive volume displayLaser displayHolographic display

Cathode ray tube• Almost all TVs in use today rely on a device known as the cathode ray tube, or CRT, to display

their images. LCDs and plasma displays are sometimes seen, but they are still rare when compared to CRTs. It is even possible to make a television screen out of thousands of ordinary 60-watt light bulbs! You may have seen something like this at an outdoor event like a football game. Let's start with the CRT, however, because CRTs are the most common way of displaying images today.

A cathode ray tube consists of several basic components, as illustrated below. The electron gun generates an arrow beam of electrons. The anodes accelerate the electrons. Deflecting coils produce an extremely low frequency electromagnetic field that allows f or constant adjustment of the direction of the electron beam. There are two sets of deflecting coils: horizontal and vertical.(In the illustration, only one set of coils is shown for simplicity.) The intensity of the beam can be varied. The electron beam produces a tiny, bright visible spot when it strikes the phosphor-coated screen.

To produce an image on the screen, complex signals are applied to the deflecting coils, and also to the apparatus that controls the intensity of the electron beam. This causes the spot to race across the screen from right to left, and from top to bottom, in a sequence of horizontal lines called the raster. As viewed from the front of the CRT, the spot moves in pattern similar to the way your eyes move when you read single-column page of text. But the scanning takes place at such a rapid rate that your eye sees a constant image over the entire screen.

The illustration shows only one electron gun. This is typical of a monochrome, or single-color, CRT. However, virtually all CRTs today render color images. These devices have three electron guns, one for the primary color red, one for the primary color green, and one for the primary color blue. The CRT thus produces three overlapping images: one in red (R), one in green (G), and one in blue (B). This is the so-called RGB color model.

In computer systems, there are several display modes, or sets of specifications according to which the CRT operates. The most common specification for CRT displays is known as SVGA (Super Video Graphics Array). Notebook computers typically use liquid crystal display. The technology for these displays is much different than that for CRTs.

Digital Frequency Meter (Page-1295)

Data Acquisition System