ELECTRONIC
INSTRUMENTATION
(WLE-304)
UNIT-1
DISPLAY DEVICES
Inroduction to Display Devices
• Display devices are used to provide visual display of numbers,letters and symbols in response to the electrical input.
• It is an output device for presentation of information in visual or tactical form.
• One of the earliest electronic displays is the CRT(Cathode Ray Tube),which was made commercial in 1922.
• Since then electronic display devices based on various principles are developed.
Classification of Display Devices
Display Devices are broadly classified in a number of ways:
1. According to the methods of conversion of electrical data into a visible light:
• Active Display Devices(e.g. CRTs,LEDs,Gas Discharge Plasma)
• Passive Dispay Devices(e.g. LCDs)
2.According to the application
• Analog Display(e.g. Oscilloscope Tube,TV-CRT)
• Digital Display[e.g. LEDs including Organic LEDs,LCDs,PDP(Plasma Display Panels),ELDs(Electro Luminescence Display)]
3. According to the display size and physical dimension
• Symbolic Display(e.g. alphanumeric displays,Nixie Tube Display)
• Console Display(e.g. CRTs,LEDs)
• Large Screen Display(e.g. Projection System)
4. According to the display format
• Direct View Type or Flat Panel Planar(e.g. segmental,Dot Matrix Displays,CRTs)
• Stacked Electrode or Non-Planar Type(e.g. Nixie Tube Displays)
5. According to resolution and legibility of characters
• Simple Signal Indicator
• Multi Element Display
Luminescense
• Active Display Devices are based on luminescense.
• It is a general term used to describe the emission of electromagnetic radiation from a substance due to non-thermal process.
• Luminescense occurs from a solid when it is supplied from some form of energy and is just described as a form of cold body radiation.
• It can be caused by chemical reaction,electrical energy,sub-atomic motions or stress on a crystal.
• This distinguishes luminescense from incandescence in which the light emitted by a substance as a result of thermal process.
The following are the types of luminescense:
• Chemoluminescence:The emission of light as a result of chemical reaction.
• Electroluminescence:The emission of light as a result of an electric passed through a substance.Cathodoluminescence comes under this category of luminescence.
• Mechanoluminescence:The emission of light as a result of mechanical action on a solid or stress on a crystal.
• Photoluminescence:The emission of light as a result of absorption of light.Phosphorescence comes under this category of luminescence.
• The segmental displays may be either seven or fourteen segments depending upon whether numeric or alphanumeric displays are required.
Seven Segmental Display:
• This is used for numeric display and consists of seven segments a,b,c,d,e,f and g.
• It displays the digits from 0 to 9 by illuminating proper segment from the group.
• This display is incandescent and operate on low voltage ranging from 5 to 12V.
• It requires about 5 to 50mA of current when using LEDs.
Segmental Display
• It is not necessary that only LEDs are used,LCDs can also be used for segmental display.
Fig 1.Ckt for 7-segment display
Fourteen Segment Display:
• A fourteen-segment display sometimes referred to as starbust display is a type of display based on fourteen segments that can be turned on or off to produce letters and numerals.
• It is an expansion of the more common seven-segment display,having an additional four diagonals and two vertical segments with the middle horizontal segment broken in half.
• This display may use LEDs or LCDs.
• Often a character generator is used to translate 7-bit ASCII character codes to the 14-bits that indicate which of the 14 segments to turn on or off.
• They were used to produce alphanumeric characters on calculators and other embedded systems.
• Application today include displays fitted to telephone Caller ID units,gymnasium equipment,VCRs,car stereos,microwave ovens,slot machines and DVD players.
Fig 2.14-Segment Display
Dot Matrix Display
• It may be used for display of numeric and alpha numeric characters.
3x5 Dot Matrix Display:
• It may be used to display numeric characters .
• The dot of these displays can be made with the help of LEDs or LCDs depending upon the requirement.
• These dots may be round or swapped with 0.5mm side or diameter.
Fig 3.3x5 Dot Matrix Display
Dot Matrix Display using 27 dots:
• Another system using 27 dots displays the numeric characters .
• The dots may be square or round with 0.4mm side or diameter.
• LEDs or LCDs are used for display of dots.
Fig 3.Dot Matrix Display using 27 dots
5x7 Dot Matrix Display:
• It consists of 5 dots in horizontal row and 7 dots in vertical column.
• These dots may be made up of LEDs or LCDs depending on our requirements.
• It is used for the display of alphanumeric characters.
Fig 4.5x7 Dot Matrix Display
• It is a vaccum tube that contains one or more electron guns and a fluorescent screen mainly made up of phosphor material and is used to display images.
• It modulates,accelerates and deflects electron beam onto the screen to create the images.
• The images may be either electrical waveforms(oscilloscope),pictures(television,computer,m onitor),radar targets or others.
• CRTs have also been used as memory devices in which the visible light emitted from the fluorescent material which forms visible pattern on the screen represent the stored data.
CRT Display
• In Oscilloscope CRTs,electrostatic deflection is used,rather than the magnetic deflection commonly used with television and other large CRTs.
Fig 5.Cathode Ray tube
Advantages:
• They operate at any resolution,geometry and aspect ratio without the need for recreating the image.
• CRTs run at the highest pixel resolution generally available.
• CRTs have fast response times and no motion artifacts.Best for rapidly moving or changing images.
• CRTs are less expensive than comparable displays using other display technologies.
• Produce a very dark black and the highest contrast levels normally available.Suitable for use even in dimly lit or dark environments.
Disadvantages:
• Subject to geometric distortion and screen regulation problems.Also affected by magnetic fields from other equipments including other CRTs.
• Relatively bright but not as bright as LCDs.Not suitable for very brightly lit environments.
• It looses 15% of its brightness after 10,000hrs.
• They are large,heavy and bulky and consume a lot of electricity and produce a lot of heat.
• The rear glass tube of modern CRTs may be made from leaded glass,which represent environmental hazard if disposed improperly.
Q.Draw the waveforms of A and X on CRT Display?
• A projection display is an optical device that projects an image onto a surface,commonly a projection screen.
• Most projectors create an image by shining a light through a small transparent lens,but some newer types of projectors can project the image directly,by using lasers.
Rear Projection Display:
• A typical block diagram of a rear projection display system is shown in Fig 6.
• In a rear projection display system ,the projector is on one side of a translucent screen and the viewer is on the other.
Projection Display
• Each of the 12 incandescent lamps inside the projector light source when energized by the input signal illuminates a different part of filmstrip.
• The lens system(optics) projects the illuminated part of the film onto a viewing screen.
Projector Image Signal
Source Controller
Image Engine
Light Source
optics
optics
Screen
Fig 6.Projector Schemetic
Nixie Tube Display
Salient Features:
• Nixie stand for Numeric Indicator Experiment No.1.
• It is an electronic device which is non-planar in nature. It is use for displaying number in the form of glass tube containing multiple cathode and wire mesh anode.
• The glass chamber is filled with Neon gas and a little Mercury or Argon gas at a small fraction of atmospheric pressure.
• Most common forms of Nixie tube has 10 cathodes in the shape of the numerals and an occasional decimal point.
• In its normal operation positive dc supply is applied to anode (150V-220V) through a suitable current limiting resistor.
• The value of the supply voltage is greater than the worst case breakage down voltage of the gas within the tube.
Fig 7.Nixie Tube
• The gas in the viscinity in appropriate cathode glows when the cathode is switched to ground.
• Since 10 cathodes have to be associated with single anode inside the glass bulb, they have necessarily to be stacked in different planes making it as a non- planer device.
• This require different voltage for different cathode to enable the low discharge for which the worst case supply voltage is chosen.
Characteristics Of Nixie Tube:
• The numerals are usually large typically (15-30mm height).
• They are the single digit devices with or without the decimal point. The selected cathode carries a current in the range of (1-5mA).
• Nixie tube display can be pulse operated and hence can be use in multiplexed display.
LED (Light Emitting Diode)
LED is a diode that gives a visible light when it is biased properly and the mechanism of electromagnetic radiation (injection luminescence) takes place, this occurs in two steps:
1. Injection of minority carriers across junction.
2. Radiative recombination of minority carriers .
• Two types of radiative recombination mechanism are commonly encountered in diode depending upon the characteristics of band gap of the semi conductor material used that is, direct and indirect energy band gap.
• The emission of protons as a result of recombination electrons and holes possible only when both energy and momentum are conserved.
• The simplest and most probable recombinations will be that where electrons and holes have the same value of momentum which is in the case of direct band gap material and the released energy falls in the visible spectrum.
GaAs , GaP ,GaAsP and GaN are direct band gap materials.
• In indirect band gap material example-Si,Ge and SiC,the conduction band minima and valance band maxima occurs at different value of momentum and the recombination of electron and hole results into the emission of energy into that phonons that conserve the momentum and the photons which do not fall in the visible range.
• The probability of recombinations is higher direct band gap material is obviously much higher than indirect band gap . GaP is may be used for emission in the visible spectrum.
• LED’s are also housed in plastic bulb (round end) that concentrate the light in the particular direction. They are intensively used in segmental and dot matrix display for numeric and alpha numeric character.
Advantages:
• LEDs require moderate power (1.2V) and 20mA of current for there operation, full brightness.
• LEDs are very fast in operation and have a switching time less than 1ns.
• They are rugged in nature therefore can with stand with shock and vibration and also operate on a wide range of lamp.(0-70 degree centigrade)
• LED don’t have filament that will burn out so they last much longer more than 1lakh hours.
Segmental Gas Discharge Display
• It works on the principle of gas discharge glow similar to the Nixie Tube.
• They are mostly available in 7 or 14 segments to display numeric or alpha numeric characters.
• In this setup each segment including decimal point of a seven segment display formed on a base as a separate cathode.
• The anode is common to each member of seven segment group which is deposited on the covering face plate.
• The space between the anode and cathode contains gas.
• For each group of a segment a”keep alive”cathode is also provided
• This cathode improves the switching speed of display by passing a small constant current of few microamperes through it and also act as a source of ion.
• A grid of tiny cells lined with phosphor that are full of inert ionised gas (typically a mixture of Neon and Xenon). Three cells make up one pixel (one cell has red phosphor, one green, one blue).
• The cell are sandwiched between X and Y axis panels, and a cell is selected by charging the appropriate X and Y electrodes.
• The cell are sandwiched between X and Y axis panels, and a cell is selected by charging the appropriate X and Y electrodes.
• The charge causes the gas in the cell to emit ultra-violet light, which causes the phosphor to emit colour.
• The amount of charge determines the intensity and combination of different intensities red, blue and green produce all the colours required.
Fig 8.Gas Discharge Display
Advantages:
• Power requirement in such devices are more or less in the same way as those for Nixie tube that is the power consumed is extremely small.
• A major disadvantage of this gas discharge tube is the high voltage operation. Typically in the range of 150- 220V.
Therefore, high voltage transistors are required as switches for the cathodes.
LCD(Liquid Crystal Display)
• We always use devices made up of Liquid Crystal Displays(LCDs) like computers,digital watches,DVD and CD Players.
• A liquid crystal material is a material (normally organic for LCDs) that flows like a liquid but whose molecular structure has some properties associated with the solid i.e. liquid crystals are substances that exist in an odd state:sort of liquid and sort of like solid ;we normally considered it as 4th matter .
Construction:
• A liquid crystal cell consists of a thin layer (about 10um) of a liquid crystal sandwiched between two glass sheets with transparent electrodes deposited on their inside face.
Working:
• The main principle of LCs is that when an electric current is applied to them ,they tend to untwist.
• This causes a change in light angle passing through them as well as change in the angle of the top polarizing filter with respect to it .
Fig 9. Working of LCD
• The working of LCD is shown in fig 9.
• It has a mirror A in the back which makes it reflector .
• Then ,there is glass B with a polarizing film on the bottom side.
• And a common electrode plane C made of Indiumtinoxide on top .
• Above this is layer of liquid crystal substance D .
• Next come another piece of glass E with an electrode in the shape of rectangle on the bottom.
• And on top there is another polarizing film F is placed.
• At right angle to the first one.
• The electrode is hooked up to a power source like a battery ,when there is no current light entering through the front of the LCD will normally hit the mirror and bounce right back out .
• When battery supplies the current to LCs between the electrode and electrode shape like rectangle untwist and block the light in that region from passing through.
In twisted nematic displays the LCD molecules lie parallel to the glass plates and glass is specially treated so that the crystal is forced to point a particular direction near one of the plate and perpendicular to the direction of another plate .
• This forces the director to twist by 90 degree from back to the front display forming a helical structure.
• The thin film of twisted nematic crystal is circularly birefringent.
• This induces the rotation of the polarization of the incident light, and the device appears gray.
• If the applied voltage is large enough, the liquid crystal molecules in the center of the layer are almost completely untwisted and the polarization of the incident light is not rotated as it passes through the liquid crystal layer.
• This light will then be mainly polarized perpendicular to the second filter, and thus be blocked and the pixel will appear black.
• By controlling the voltage applied across the liquid crystal layer in each pixel, light can be allowed to pass through in varying amounts thus constituting different levels of gray.
Fig 10.Liquid Crystal Display
Advantages:
• Produces very bright images due to high peak intensity.
Very suitable for environments that are brightly lit.
• Produce considerably lower electric, magnetic and electromagnetic fields than CRT.
• Consume less than 1/3rd the power of a comparable CRT.
• Life span : 50,000 - 100, 000 hours
• Screen size : 13-57 inches
• Power Consumption : Require less power to operate compared to plasma, but more than OLED TVs
• Viewing Angle : Up to 165°, Picture suffers from the side
Disadvantages:
• The aspect ratio and resolution are fixed.
• Lower contrast than CRTs due to a poor black-level.
• Slow response times and scan rate conversion result in severe motion artifacts and image degradation for moving or rapidly changing images.
References:
• http://www.slideshare.net/abhishekpal1991/pal- 17140724
• https://en.wikipedia.org/wiki/Liquid-crystal_display
• https://en.wikipedia.org/wiki/Electronic_visual_displ ay
• http://www.explainthatstuff.com/how-nixie-tubes- work.html
• http://electronics.howstuffworks.com/tech
• https://en.wikipedia.org/wiki/Seven-segment_display
• https://www.tricksway.com/advantage-disadvantage- crtledlcdplasma-display/
