B1-05.11 ELECTRONIC DISPLAYS

Electronic Displays

Light Emitting Diodes (LED) in Aircraft

• LEDs are optoelectronic devices
• LEDs were developed as replacements for fragile incandescent bulbs in on/off panel indicators
• When forward biased, LEDs emit visible light that can be red, orange, yellow, green, blue, white, or ultraviolet, depending on the material composition
• LEDs emitting non-visible infrared light are also available and suited for detection with infrared detector components
• The LED is designated by a standard diode symbol with two arrows pointing away from the cathode
• Operating voltage is low, around 1.6 V with forward bias
• Operating current is generally about 10 mA
• LED life expectancy can exceed 100,000 hours
• To activate an LED, anode and cathode connections must be determined
• The wire beside the flat spot on the LED's red plastic base must connect to the negative side of the power supply and the other wire to the positive side
• LEDs will be damaged if connected to power supplies rated above 2 V; resistors are needed to limit current when higher rated power supplies are used
• Always consult the supplier's or manufacturer's literature to avoid damaging components

Peak Wavelength Single-Colored (Monochromatic) LEDs

• Color is how wavelength is perceived, and light radiation spectrum is measured in nanometers (nm)
• LEDs emit light over a small part of the radiation spectrum, unlike incandescent lamps
• Peak wavelength is the technical method of defining the color emitted
• Typical figures range from 450 nm (blue) through 535 nm (green), 585 nm (yellow), 620 nm (orange), and 700 nm (red), up to 950 nm (infrared)
• Output is distributed over a narrow range, with intensity peaks at the specified wavelength
• Light radiation spectrum is expressed in "nanometers" (nm)
• An LED component's peak wavelength is set by the semiconductor substrate's chemical makeup rather than current or power dissipation
• Color or clarity of LED package color makes no difference

Multi-Coloured and Bi-Coloured LEDs

• Multi-colored LED epoxy packages contain two separate reverse-parallel semiconductor chips, each for a different single color
• Only one LED chip emits light at any instant, dependent on the direction of current in the component
• Only one series resistor is required, calculated using the same formula as series resistors for LEDs
• Bi-colored LEDs can mix two primary colors to produce a third
• Red and green bi-colored LEDs can produce yellow light
• Simplest way to operate relies on an AC voltage source
• During respective half cycles of alternating current, each of the primary color chips operates
• Rapidly flickering red and green lights is perceived as constant yellow

Tri-Coloured LEDs

• Tri-Colored LEDs are composed of two separate semiconductors that each produce a different color
• A common lead is connected internally to the two semiconductor chips to product a three-terminal component
• Common cathode and anode types exist
• Selectable two-color light source via voltage switching on the semiconductor chips
• Both semiconductor chips can mix two primary colors
• A single resistor is required in cases where both chips are never operated simultaneously
• Individual resistors are required when both chips are operated simultaneously

Seven-Segment LED Display

• Used in pocket calculators, digital voltmeters, frequency counters, etc.
• LEDs are placed in seven-segment displays
• Uses seven LED segments or bars
• Segments are labeled A through G
• Segments can combine to form any number from 0 to 9.

7-Segment LED display: Common Anode vs Common Cathode

• All anodes are connected internally in a common-anode display
• A number is formed when a negative voltage is applied to the proper cathodes
• All cathodes are connected directly in a common-cathode display
• CA seven-segment display requires external power to the anode connection common to all segments
• Applying a ground to a A-G segment connection will cause the segment to light up
• An additional resistor is used to limit the amount of current flowing through each LED segment
• A common cathode connection must be grounded to use a seven-segment display
• Power must be applied to the appropriate segments to illuminate them

Alphanumeric LED Display

• Operate similarly to seven-segment displays
• Typically use 16 segments

Dot Matrix LED Display

• Used to produce the full alphanumeric range
• Features a 35-dot matrix, in which LED dies are mounted in a 7 × 5 array
• The range of applications increases to a much higher level compared to use cases for the seven-segment display

Organic LEDs

• OLED is a light-emitting technology placing organic thin films between two conductors
• Bright light is emitted when electrical current is applied
• Used to make displays and for lighting
• OLEDs emit light, so require no backlight
• Thinner and more efficient that LCDs which require a white backlight
• Simple OLEDs have six layers: top and bottom layers of protective glass or plastic, a top seal layer and bottom substrate layer, and a negative terminal cathode and positive terminal anode which has next to it two layers of organic molecules, the emissive (where the light is produced) and conductive layers
• Key advantages include: lower power consumption, faster refresh rate and better contrast, greater brightness and fuller viewing angle, ultra-thin, flexible or transparent displays, better durability and broader temperature operation, lighter weight due to ability to be printed onto flexible surfaces

Liquid Crystal Displays

• A type of display made of two plates of glass sealed around the perimeter and sealed around a layer of liquid crystal
• The liquid crystal layer is a few microns thick (around 1/10 the thickness of an average human hair)
• This device depends on polarization to manipulate light

Polarization

• Light is composed of electromagnetic radiation (waves)
• The waves travel in all directions
• Polarized filters have parallel microsized slits that block all but ones position of wave
• Lenses vibrate in one place only because of polarization
• Cross-polarizing lenses will stop light

Liquid Crystal Reorientation

• Organic substance with solid crystalline and liquid characteristics between certain temps
• Demonstrates crystalline structure and refraction, depends on the crystalline state
• Calculators, digital watches, portable word processors and notebook PCs use nematic liquid crystals
• The Nematic liquid crystals change their structure with the application of electric voltage
• Molecules align themselves in direction of a sufficiently strong electrical field
• Orientation is almost flat originally
• When an electrical field (E) with direction is applied (represented in red), force (T) (represented in green) aligns the molecule parallel to the field
• The molecule becomes parallel when the field is strong enough

Liquid Crystal Displays (LCD) Construction

• Transparent, conductive electrodes are deposited onto inner glass plates
• Electrodes define the segments, pixels, or special symbols of the display
• A thin polymer layer is applied to the electrodes
• The polymer is chemically etched with channels to align the orientation of the liquid crystal (LC) helix-shaped molecules
• Polarizing films are then laminated to the outer surfaces of the glass plates at 90° angles

LCD Function

• Typically two polarizing films aligned at 90° prevent light transmission
• LC's ability to rotate polarized light allows the display to appear clear
• Passing AC voltage through the LC realigns crystal so polarized light is not twisted, which is blocked by polarizers, making segments appear dark
• Operation voltage is low (3 to 15 V RMS), with a low frequency (25 to 60 Hz) AC signal that draws very little current
• AC voltage to turn on a segment is applied between the segment and backplane

LCD Backplane and Control

• Backplane is common to all segments
• Segment and backplane form a capacitor
• The capacitor draws only a little current as long as the AC frequency is low
• The AC frequency is generally not lower than 25 Hz, because 25 Hz will produce visible flicker
• LCDs require an external light source because they do not emit their own light
• The common AC signal generation method applies out-of-phase square waves to the segment and backplane
• Turning on an LCD segment requires AC voltage to be applied between the segment and the backplane
• Controlled with a Complementary Metal-Oxide-Semiconductor (CMOS) 4070 exclusive-OR gate and a control input
• When the control input is high, the XOR output will be the INVERSE of the 40Hz square wave

Seven-Segment LCD Control

• A BCD-to-seven-segment decoder/driver is a common way to supply control signals to each of the seven segments through XOR gates
• CMOS devices are generally used to drive LCDs for low power (battery-powered devices) and the low state voltage (avoids DC leakage)
• DC leakage will reduce the life of the LCD

Reflective LCDs

• Liquid crystal materials do not emit light
• Inexpensive LCDs are reflective, needing light from an external source
• Applied electrodes charge the liquid light, causing them to untwist and block light

Backlit LCDs

• Similar to a normal LCD, but uses backlighting with built-in fluorescent tubes
• Uniform display from a white diffusion panel that redirects and scatters the light
• Light is lost due to through filters, liquid crystal and electrode layers

Greyscale LCDs

• The amount of voltage supplied to a crystal controls the light able to pass through
• Precise voltage control allows LCDs to create shades of grey
• Typically, most displays today offer about 256 levels of brightness per pixel

Colour LCDs

• Three sub-pixels with red, green and blue colour filters creates the colour pixel in a LCD
• Sub-pixel intensity is controlled and varied by voltage, for 256 shades
• Combining the sub-pixels creates 16.8 million colors (256 shades of red × 256 shades of green × 256 shades of blue)

Additive Colour Mixing

• Beams of projected coloured light combine to create other colours
• Light creates colours - shining red, green and blue, the additive primary colors, results in white in the overlap of all three
• White light splits into colours (e.g. prisms, rainbows), and a yellow, cyan and magenta creates overlaps
• Additive secondary colors occur here
• Black is light absence

Limitations of LCDs

• Red, Green and Blue stripes are arranged in a regular matrix
• The gaps between pixels, aka shadows, are for drive circuits and wires
• The black lines cannot have their colour changed
• Computer resolution of 1024 X 768 etched on the glass means about 2, 359, 296 transistors are used

Cathode Ray Tube

Edison's discovery of Thermionic Emission

• Thomas Edison found by putting metal plate into evacuated bulb, and placing battery between electron plate and filament: electric current flowed
• Electrons boiled off from filament, were attracted to plate because of opposite charges
• This phenomenon only worked because the electrons were already free from the hot filament (would destroying the filament)
• Causing electrons to flow across filament space became known as Edison effect
• Temperature reached where electrons escape, "boil" from surface leaving boiling water
• Called Thermionic Emission

Cathode Ray Tubes

• Used older television technology (prior to plasma and LED screens)
• Function cannot be duplicated, can convert electronic signals to visual displays such as electronic wave forms, pictures, and radar sweeps
• Three main elements
• Electron gun: generates electron beam
• Deflection system: positions electron beam onto screen
• Screen: displays in the form of a illuminated location

Electron Gun Composition

• The gun cathode is a cylinder that emits electrons in all directions along its entire length
• Electrons are fired in one direction through a small-diameter nickel cap with coated emitting material
• Emitted Electrons leave cathode at different trajectories
• The grid is negatively charged relative to the cathode
• Electron repulsions forces the emitted electrons through the hole in the grid
• The negative potential of the grid with the positive potential of the anode curved the path of electron

Electron Gun Components

• Brightness control has a potentiometer used to vary the potential applied to grid (electronic lens)
• The electronic lens focuses beam at point P
• Additional positive charged anodes provided for tight beam focus
• Focusing anode is changed a few hundred volts relative to the cathode
• The process of applying to the cathode leads them to travel through grid
• The acceleration anode is several thousand volts (greater electrostatic pull/acceleration the electron through tube center
• Deflects them from the wall with the attracting force of screen, passes mid-point

CRT Screens

• The interior of CRT is coated with florescent material, it gives off light, invisible electrons/ energy -> visible
• A special costing called Aqua Dag, has the same high positive potential
• Aqua Dag helps in the acceleration of electrons forward the screen, attract all secondary emission
• Deflecting electron can manipulate
• A beam creates bright spot the center
• Position varies to produce a picture on screen
• Continuous is produced when a phosphor glow and speed match up with the electron beam

CRT Operating Principles

• A specially constructed cathode emits electrons fired towards the front
• The potential of the grid determine amount of leaving area of cathodes, control brightness
• Emitted electrons of focused/concentrated by a grid
• Electrodes direct focal point of accelerated anodes toward the screen, strikes is the electron beam that causes brigthness
• Secondary Emission that are released, leave through Dag coating

Electrostatic Deflection

• Moves an electrom (both move)
• Magnetic or electrostatic
• All three ways are used to move
• Opposites attract, called deflection plates
• The spot of lighting will be able to shown, is called face
• Horizontal deflection = same concept as above

CRT Electron Beam Deflection

• The beam, aka the sweep is controlled by resistors r1 and r2 controlling potential on the deflection plates
• In view A, five electrons are emitted by the gun. Right Deflector, (H2) has + potential, and is negative, and is applied to the center
• The amount potential = deflection
• A large amount of deflection leads one side
• View B, the electron is still deflected, adjust to make that the right is less positive
• Result -> electrons are more towards their center
• View C with all the applied electrons are equal towards the center
• Result -> non exististed to simply travels the CRT Screen

Vertical Electronic Beam

• The way CRTS function - can see electrical and vertical. (Sine Wave)
• Electrons sweeping light for left is where apply

Magnetic Deflection

• Allows "trace out" a luminescent display (deflection)
• Deflection coils provided (horizonal and vertically)
• Signal that is connected depends on the deflection/polatiry

CRT Summary of

• A schematic diagram of cathode
• Heater = heat source for tube
• Control grid = electrical amount released
• Electrode will have attract/focus point
• Aquadag = aids to the screen
• 2 general principles that it operates - > electrical attraction and repulsion / therm ionic emissions

CRT Handling

• Safe precautions must follow
• Encloses volume ( atmospheric volume)
• 10-in can exceed 40-lb
• Breaking occurs = the side contains something toxic
• Pre-Caution
  • DO not scratch or strike
  • Don't use excessive force force
  • Neck should not hold
  • Always be on the side of safety
  • Do not try to use yoke with high voltage high
• The safest process - > make sure to have the correct equipment
• Place = face = the empty carton (wear safety Google always)

Colored CRT Screens

• Three electrons one the screen. + coating (blue and green and red) - this shows the color type

CRT Screen Shadow Mask

• Each electrons must be one dot
• Triad -> different electrons
• Thin sheet of material = the shadow for each is hole one type or can view

Simultaneous Picture Formation

• Display holding light sensitive
• Amps allows signals to the more intense the signal . The Amps wire up and transfer more power to the 25 line setup the is near the receiver.
• Expensive because both need adjusted the the same characteristic has
• If not all wires and communication is disturbed if any.
• The high amplitude helps brightness

Picture Resolution & Scanning

• Increase number of phots
• The picture tube are by the process by Inter laced Scanning
• Transmitter and and Switch needs to the synced at all
• There are a lot of horizontal lies ( that increase the picture.
  • A scanning the screen on a faster time to time
  • Scanning and Raser = simimlar to red point page line by line
  • Each image are set as series if of parrallel to be at close to the line on the CRT

Inerlace Scanning

• Odd then even = a 15 or 25 to each is the picture. The one line top is a picture
• The reason this happen because can’t to much info or will bounce like small “F” letter
• Synchro = signal transfer , and be the line is the top the each can be on small angel that they all appear to be straight . The small that are on the sides = “overscan” area

Aircraft CRT (modern type)

• The image generator what paints and fires electrons in color (50 times a second)
• Instruments include electronic attitude indicator, Electronic Indicator, central device or heading.