Electrical & Electronic Measurements: Class Notes (15EE36) Module-5. Display Devices

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1 Module-5 Display Devices Syllabus: Introduction Character formats Segment displays Dot matrix displays Bar graph displays Cathode ray tubes Light emitting diodes Liquid crystal displays Nixies Incandescent displays Fluorescent displays Liquid vapour and Visual displays Display multiplexing and zero suppression Dr SM Shashidhar Class Notes RYMEC Page 1

2 1. Write a note on display devices. (04 Marks) A display device is an output device for presentation of information in visual] or tactile form (the latter used for example in tactile electronic displays for blind people). When the input information is supplied has an electrical signal, the display is called an electronic display. The display system acts as a final link between the measuring process and the user. If the display is not easy to see and easy to understand then that process is compromised. The user s sensory capabilities and cognitive characteristics, therefore, must both be addressed in display-system selection. Display devices are designed to project, show, exhibit or display softcopy information. Some of the display devices are Cathode Ray Tube (CRT) Nixie tubes Liquid Crystal Display (LCD) Light Emitting Diode (LED) 2. Explain with a suitable circuit diagram working of seven segment displays. (06 Marks) Some displays can show only digits or alphanumeric characters. They are called segment displays, because they are composed of several segments that switch on and off to give appearance of desired glyph. The segments are usually single LEDs or liquid crystals. The 7-segment display, also written as seven segment display, consists of seven LEDs (hence its name) arranged in a rectangular fashion as shown. Each of the seven LEDs is called a segment because when illuminated the segment forms part of a numerical digit (both Decimal and Hex) to be displayed. Each one of the seven LEDs in the display is given a positional segment with one of its connection pins being brought straight out of the rectangular plastic package. These individually LED pins are labelled from a through to g representing each individual LED. The other LED pins are connected together and wired to form a common pin. So by forward biasing the appropriate pins of the LED segments in a particular order, some segments will be light and others will be dark allowing the desired character pattern of the number to be generated on the display. This then allows us to display each of the ten decimal digits 0 through to 9 on the same 7-segment display. The displays common pin is generally used to identify which type of 7-segment display it is. As each LED has two connecting pins, one called the Anode and the other called the Cathode, there are therefore two types of LED 7-segment display called: Common Cathode (CC) and Common Anode (CA). Dr SM Shashidhar Class Notes RYMEC Page 2

3 In the common cathode display, all the cathode connections of the LED segments are joined together to logic 0 or ground. The individual segments are illuminated by application of a HIGH, or logic 1 signal via a current limiting resistor to forward bias the individual Anode terminals (a-g). Common Cathode 7-segment Display LED pattern for displaying digits using 7-segment display 3. Explain with a suitable circuit diagram working of a cathode ray tube (CRT). (06 Marks) Dr SM Shashidhar Class Notes RYMEC Page 3

4 It consists of a glass envelope made from a neck and cone. All air has been extracted so that it contains a vacuum. At the narrow end are pins which make connection with an internal electron gun, as shown in Figure above. Voltages are applied to this gun to produce a beam of electrons. This electron beam is projected towards the inside face of the screen. Different basic component of CRTs are electron gun, electron accelerating anode, horizontal and vertical electric field coils, electron beam and a screen coated with phosphor. The electron gun generates a narrow beam of electrons. The anodes accelerate the electrons. Deflecting coils produce an extremely low-frequency electric field that allows for constant adjustment of the direction of the electron beam. There are two sets of deflecting coils: horizontal and vertical. (In the figure, 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. The screen is covered with a fine layer of phosphorescent elements, called phosphors, which emit light by excitation when electrons strike them, creating a lit-up dot called a pixel. 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 a pattern similar to the way your eyes move when you read a 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-colour CRTs. However, virtually all CRTs today render colour images. These devices have three electron guns, one for the primary colour red, one for the primary colour green, and one for the primary colour blue. The CRT thus produces three overlapping images: one in red (R), one in green (G), and one in blue (B). This is the socalled RGB colour model. 4. What are the advantages and disadvantages of CRT? Advantages Fast response and high resolution possible Full colour (large modulation depth of E-beam) Saturated and natural colours Inexpensive, matured technology Wide angle, high contrast and brightness Large and heavy (typ. 70 x 70cm, 15kg) High power consumption (typ. 140W) Harmful dc and ac electric and magnetic fields Flickering at Hz Geometrical errors at edges 5. Write a note on Dot matrix display. (06 Marks) Light emitting diodes aligned in a form of matrix constitute a dot matrix display. It is commonly used to display time, temperature, news updates and many more on digital billboards. Dot Matrix Display is manufactured in various dimensions like 5x7, 8x9, 128x16, 128x32 and 128x64 where the numbers represent LEDs in rows and columns, respectively. Dr SM Shashidhar Class Notes RYMEC Page 4

5 Dot matrix LED display contains the group of LEDs as a two dimensional array. It is a display device used to display information on many devices like machines, clocks, railway departure indicators etc. They can display different types of characters or a group of characters. Dot matrix display is manufactured in various dimensions. Arrangement of LEDs in the matrix pattern is made in either of the two ways: Row anode-column cathode or Row cathode-column anode. By using this dot matrix display we can reduce the number of pins required for controlling all the LEDs. An LED dot matrix consists of an array of LED s which are connected such that the anode of each LED are connected together in the same column and the cathode of each LED are connected together in the same row or vice versa. This is done to minimize the number of pins required to drive them. For example an 8X8 matrix of LEDs would need 64 I/O pins, one for each LED pixel. By connecting all the anodes of LEDs together in a column and all the cathodes together in row,, the required number of input and output pins reduced to 16. Each LED will be addressed by its row and column number. An LED dot matrix display can also come with multiple LEDs of varying colours behind each dot in the matrix like red, green, blue etc. Diagram of 8X8 LED Matrix using 16 I/O pins 6. Write a note on bar matrix display. (04 Marks) The bar graph display is an analog display. In this display, the display elements are arranged in a column, in closely packed linear array. The dot LEDS are independently driven so that the Length of the array corresponds to the voltage or current measured. These are used in the panel meters which accept analog input signals and produce equivalent display of input signal. Dr SM Shashidhar Class Notes RYMEC Page 5

6 Analog meter using bar graph of LEDs 7. Explain Light Emitting Diode as a display device. (04 Marks) One of the cheapest and convenient ways to display information electronically is by using Light-Emitting Diodes (LEDs). It is basically a p-n junction photodiode when excited at forward-bias condition emits light. It can be easily interfaced with a simple electronic circuit and is durable and reliable. These LEDs are often arranged in different formats to display information. Among these, the seven segments configuration and dot matrix display are very common and widely used. The seven-segment configuration of an LED arranged in the form of the digit 8 can be restrictive in that it does not adequately allow the display of some alphanumeric characters. By contrast, the versatility of a dot-matrix arrangement allows an LED unit to display more complicated shapes. 8. Explain LCD as a display device. (06 Marks) The liquid crystals are one of the most fascinating material systems in nature, having properties of liquids as welll as of a solid crystal. The terms liquid crystal refers to the fact that these compounds have a crystalline arrangement of molecules, yet they flow like a liquid. Liquid crystal displays do not emit or generate light, but rather alter externally generated illumination. Their ability to modulate light when electrical signal is applied has made them very useful in flat panel display technology. The crystal is made up of organic molecules which are rod-like in shape. e.. The orientation of the rod like molecule defines the direction of the liquid crystal. The different arrangements of these rod-like molecules lead to three main categories of liquid crystals. Dr SM Shashidhar Class Notes RYMEC Page 6

7 Liquid crystals can be trapped between two parallel sheets of glass, with a matching pattern of transparent electrode on each sheet. Figure shows different layers of a typical LCD display. When a voltage is applied to the electrodes, the optical character of the crystal changes and the electrode pattern appears in the crystal. A huge range of LCDs has been developed, including those based on seven segment digits or dot matrix formats, as well as a variety of graphical forms. Many general-purpose displays are available commercially. A sheet of polarising material is bonded to the outside surfaces of both the front and rear glass covers. As incident light of random polarisation enters the top polarizer, it is stopped except for that which is polarised in the proper direction. With no electric field applied, the light is twisted or its polarisation is changed by the spiral pattern of the crystals. The bottom polariser is aligned opposite of the top one but the twisted light is now aligned with the bottom polariser and passes through. The display is now transparent and appears light. A simple black-or-white LCD display works by either allowing daylight to be reflected back out at the viewer or preventing it from doing so in which case the viewer sees a black area. The liquid crystal is the part of the system that either prevents light from passing through it or not. Since LCD panels produce no light of their own, they require external light to produce a visible image. In a transmissive type of LCD, this light is provided at the back of the glass stack and is called the backlight. While passive-matrix displays are usually not backlit (e.g. calculators, wristwatches), active-matrix displays almost always are. Over the last years, the LCD backlight technologies have strongly been emerged. 9. Explain Give the advantages and disadvantages of LCDs (06 Marks) Advantages of LCDs 1. Less power consumption 2. Low cost 3. Uniform brightness with good contrast 4. Low operating voltage and current Dr SM Shashidhar Class Notes RYMEC Page 7

8 Disadvantages of LCDs 1. Poor reliability 2. Limited temperature range. 3. Poor visibility in low ambient temperature. 4. Slow speed 5. Requires an ac drive. 10. Write a short note on Nixie tube. (04 Marks) A Nixie tube or cold cathode display is an electronic device for displaying numerals or other information using glow discharge. The glass tube contains a wire-mesh anode and multiple cathodes, shaped like numerals or other symbols. Applying power to one cathode surrounds it with an orange glow discharge. The tube is filled with a gas at low pressure, usually neon and often a little mercury or argon. Although it resembles a vacuum tube in appearance, its operation does not depend on thermionic emission of electrons from a heated cathode. It is therefore called a coldcathode tube (a form of gas-filled tube), and is a variant of the neon lamp. Such tubes rarely exceed 40 C even under the most severe of operating conditions in a room at ambient temperature. i) The numerals are large, typically mm high. ii) The Nixie tubes are single digit devices with or without decimal point. iii) They are available in two types: Side viewing and top viewing etc. iv) DC supply requirement for the nixie tube is V. While the current requirement of the selected cathode is 1-5 ma. Dr SM Shashidhar Class Notes RYMEC Page 8

9 v) Can be used as multiplexed displays. vi) Nixie tubes are also used to display alphabetical symbols. 11. Give the advantages and disadvantages of Nixie tubes Advantages: (i) Nixie tubes are bright and easy-to-read, with properly rounded numbers (unlike the squared-off "compromise" digits made by LEDs, LCDs. (ii) Although essentially obsolete, they're still widely available and relatively inexpensive (iii) They're also extremely reliable and, because each number is lit separately, Disadvantages: (i) Compared to more modern displays, they're powered by inconveniently high voltages (making them hard to use with low-voltage integrated circuit computer chips) and relatively large amounts of power and they can fail for a variety of reasons. (ii) Since they're made of glass and contain mercury, they can represent a health hazard if and when they break (iii) Nixies are also quite large compared to tiny modern displays, so they're hard to use in the kind of compact gadgets where we'd now mostly use LCD displays 12. Write a short note on Liquid Vapour Displays (LVD). (04 Marks) These are economical displays based on the principle of the pressure depends on the ambient light. The construction of LVD is as shown in the Fig. It consists of two glass plates with a transparent volatile liquid enclosed in it. The background of a rear glass is totally black. While the front surface is roughened so that the liquid can wet it. A voltage drive is used to heat a transparent electrode. Under OFF condition of the display, only the black background can be viewed through transparent glass electrode. To make display ON, a voltage is applied to the electrode, generating heat in electrode. This evaporates liquid which is in contact with it. Thus around the roughened surface of glass the vapour films and vapour bubbles are formed. With this vapour bubbles formation, there is a discontinuity between the glass plate and liquid interface. Due to which light scattering takes place. While selecting organic liquids the precaution must be taken such as the refractive index of the liquid should be close to that of glass, less requirement of energy for vaporising the liquid. The drawback of LVD is its speed of operation is low. Dr SM Shashidhar Class Notes RYMEC Page 9

10 13. Write a short note on Display multiplexing. Multiplexed displays are electronic display devices where the entire display is not driven at one time. Instead, sub-units of the display (typically, rows or columns for a dot matrix display or individual characters for a character oriented display, occasionally individual display elements) are multiplexed, that is, driven one at a time, but the electronics and the persistence of vision combine to make the viewer believe the entire display is continuously active. A multiplexed display has several advantages compared to a non-multiplexed display: fewer wires (often, far fewer wires) are needed simpler driving electronics can be used both lead to reduced cost reduced power consumption Multiplexed displays can be divided into two broad categories: 1. Character-Oriented Displays 2. Pixel-oriented displays Character-oriented displays: Most character-oriented displays (such as seven-segment displays, fourteen-segment displays, and sixteen-segment displays) display an entire character at one time. The various segments of each character are connected in a twodimensional diode matrix and will only illuminate if both the "row" and "column" lines of the matrix are at the correct electrical potential. Pixel-oriented displays: In dot-matrix displays, individual pixels are located at the intersections of the matrix's "row" and "column" lines and each pixel can be individually controlled. Here, the savings in wiring becomes far more dramatic. For a typical 1024x768 (XGA) computer screen, 2,359,296 wires would be needed for non-multiplexed control. That many wires would be completely impractical. But by arranging the pixels into a multiplexed matrix, only 1792 wires are needed; a completely practical situation. Dr SM Shashidhar Class Notes RYMEC Page 10

11 Book References I. A Course in Electronic Measurements and Instrumentation by A.K. Sawhney II. Electrical and Electronics Measurements and Instrumentation by Prithwiraj Purkait, Budhaditya Biswas, Chiranjib Koley III. Electrical Measurements and Measuring Instruments by Rajput R.K. IV. Electrical measurements book by P.M.Chandrasekharaiah V. Electrical Measurement & Measuring Instruments by A.V.Bakshi Bakshi, A.V.Bakshi VI. VII. VIII. IX. July 2017 Display Devices Questions from VTU CBCS Question Papers 1. Explain with a suitable circuit diagram working of. an cathode ray tube (CRT ). (06 Marks) 2. Write a note on display devices. (04 Marks) 3. Write a note on: (i) Dot matrix display. (ii) Bar matrix display. (06 Marks) Jan Explain LED and LCD displays. (08 Marks) 5. Write a short note on Nixie tube. (04 Marks) Other Questions 6. Explain with a suitable circuit diagram working of seven segment displays. (06 Marks) 7. Explain with a suitable circuit diagram working of a cathode ray tube (CRT). (06 Marks) 8. What are the advantages and disadvantages of CRT? (06 Marks) 9. Write a note on Dot matrix display. (06 Marks) 10. Write a note on bar matrix display. (04 Marks) 11. Explain Light Emitting Diode as a display device. (04 Marks) 12. Explain LCD as a display device. (06 Marks) 13. Explain Give the advantages and disadvantages of LCDs (06 Marks) 14. Write a short note on Nixie tube. (04 Marks) 15. Give the advantages and disadvantages of nixie tubes 16. Write a short note on Liquid Vapour Displays (LVD). (04 Marks) 17. Write a short note on Display multiplexing. (06 Marks) Dr SM Shashidhar Class Notes RYMEC Page 11