Electronic Displays Chapter 5.11

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Questions and Answers

What is the purpose of the phosphor coating on a CRT screen?

To prevent secondary emission

To stabilize the electron beam during operation

To enhance the resolution of the display

To convert electrons’ energy into visible light (correct)

What is a consequence of secondary emission in a CRT?

It enhances the brightness of the display

It distorts the displayed image (correct)

It improves color accuracy

It creates a positively charged barrier

Which material is used in the aquadag coating to control secondary emission?

Copper oxide

Sodium chloride

Silica

Graphite (correct)

How does the aquadag coating function within the CRT?

It attracts and removes secondary emitted electrons Signup and view all the answers

Which statement best describes the relationship between the electron beam and the CRT screen?

The beam activates phosphor materials that produce visible light upon impact Signup and view all the answers

What is necessary to limit current when using LEDs with power supplies rated over 2 V?

A resistor Signup and view all the answers

What does the peak wavelength of an LED indicate?

The specific color of the emitted light Signup and view all the answers

What range of wavelengths corresponds to the red color emitted by an LED?

620 nm to 700 nm Signup and view all the answers

What components are inside a multi-coloured LED package?

Two separate reverse-parallel semiconductor chips Signup and view all the answers

What is a common misconception about the factors that affect an LED's peak wavelength?

It is influenced by the packaging color Signup and view all the answers

How many series resistors are required for bi-coloured LEDs?

One for both colors Signup and view all the answers

What color does a bi-coloured LED emit when both primary colors are combined?

Another separate color Signup and view all the answers

What aspect of LED light emission differs from incandescent lamps?

Distribution of wavelengths Signup and view all the answers

What is the result of operating a red and green bi-coloured LED with an AC voltage source?

The human eye perceives it as a constant yellow light. Signup and view all the answers

What is the primary function of the common lead in tri-coloured LEDs?

It allows the user to switch between colors. Signup and view all the answers

What is essential when using both semiconductor chips in a tri-coloured LED simultaneously?

Each chip must have its own dedicated resistor. Signup and view all the answers

In a seven-segment LED display, what effectively controls the displayed number?

The voltage applied to the segments. Signup and view all the answers

What kind of voltage is applied to the cathodes to illuminate a segment in a common-anode seven-segment display?

Negative voltage to all cathodes except the target segment. Signup and view all the answers

Which type of tri-coloured LED configuration exists to prevent mixing of colors?

Common anode configuration. Signup and view all the answers

Why are LEDs favored in devices like calculators or digital voltmeters?

They are cost-effective and have a long lifespan. Signup and view all the answers

What configuration connects the anodes in a common-anode seven-segment display?

All anodes are connected internally. Signup and view all the answers

What is required for an LCD to function properly?

An external light source Signup and view all the answers

Which input state of the XOR gate results in the segment being turned off?

When CONTROL input is LOW Signup and view all the answers

What kind of wave is typically used to control the voltage in a segment of an LCD?

Square wave Signup and view all the answers

What happens to the segment voltage when the CONTROL input is HIGH?

It alternates between +5 V and -5 V Signup and view all the answers

Which of the following is the primary advantage of using LCDs over LEDs in certain devices?

Lower current consumption Signup and view all the answers

In a seven-segment display, what does applying the defined control method affect?

The on/off state of individual segments Signup and view all the answers

How is the required AC voltage typically generated for an LCD segment?

By using out-of-phase square waves Signup and view all the answers

In the context of an LCD, what role does the Complementary Metal-Oxide-Semiconductor (CMOS) XOR gate play?

To determine the phase of the signal Signup and view all the answers

What is the maximum number of colors that a color LCD can display?

16.8 million colors Signup and view all the answers

What constitutes the 'additive primary colors' in additive color mixing?

Red, green, blue Signup and view all the answers

What role do the gaps between pixels in an LCD serve?

Accommodate drive circuits and wires Signup and view all the answers

What color is produced by the overlap of all three additive primary colors?

White Signup and view all the answers

What is considered the absence of light in the context of additive color mixing?

Black Signup and view all the answers

Which of the following best describes the arrangement of sub-pixels in a color LCD?

Regular matrix arrangement Signup and view all the answers

What colors are obtained in the overlaps of additive secondary colors?

Cyan, magenta, yellow Signup and view all the answers

Which of the following statements regarding voltage control in LCDs is correct?

It can control the intensity of each sub-pixel Signup and view all the answers

What is the primary function of the control grid in a CRT?

To control the number of electrons emitted Signup and view all the answers

Which component in a CRT is responsible for ensuring the emitted electrons are mostly unidirectional?

Cathode Signup and view all the answers

How do vertical-deflection plates in a CRT operate?

They move the electron beam up and down the screen Signup and view all the answers

What principle does the aquadag coating in a CRT help to eliminate?

Secondary emission Signup and view all the answers

What occurs when the electron beam is directed horizontally using horizontal-deflection plates?

The electron beam traces a horizontal line across the screen Signup and view all the answers

In terms of safety, what is a critical aspect of working with CRTs due to their design?

They contain a high vacuum which can implode Signup and view all the answers

Which of the following statements is true regarding the deflection of the electron beam in CRTs?

The deflection can be controlled by polarities of deflection coils Signup and view all the answers

What is the role of the focusing anode in a CRT?

To attract electrons and focus them into a beam Signup and view all the answers

Study Notes

Electronic Displays (5.11)

Learning objectives include describing cathode ray tubes, light-emitting diodes (LEDs), and liquid crystal displays (LCDs) in modern aircraft (Level 2).

LED Fundamentals

Light-Emitting Diodes (LEDs) are optoelectronic devices used to replace fragile incandescent bulbs for on/off indications.
LEDs produce visible light when forward biased, in various colours (red, orange, yellow, green, blue, white, ultraviolet, and infrared).
LEDs are valuable in infrared detection applications.
Typical forward voltage is about 1.6 V.
Typical operating current is about 10 mA.
LEDs have a long lifespan, exceeding 100,000 operating hours.
LEDs are susceptible to damage if connected to voltage supplies exceeding 2 V; resistors are needed to limit current at higher voltages.

Peak Wavelength Single-Coloured (Monochromatic) LEDs

Light colour perception is related to wavelength.
LEDs emit light over a specific narrow band of the spectrum, unlike incandescent lamps which produce a wide spectrum.
Peak wavelength defines the colour of emitted light.
Examples of peak wavelengths include 450nm (blue), 535nm (green), 585nm (yellow), 620nm (orange), 700nm (red), and 950nm (infrared).
Light intensity distributions are peaked at the specified wavelengths.

Multi-Coloured and Bi-Coloured LEDs

Multi-coloured LEDs contain multiple reverse-parallel semiconductor chips, each emitting a single colour.
Only one chip emits light at a time, the choice dependent on current direction.
A single series resistor is suitable for calculating the required resistance.
Bi-coloured LEDs combine red and green chips to generate yellow light. Alternating current operation results in seemingly constant yellow light.

Tri-Coloured LEDs

Tri-coloured LEDs combine three semiconductor chips emitting different colours to form a three-terminal component.
Common anode and common cathode types are available.
Switching voltages between the two semiconductor chips allows selection of a two-colour light source.

Seven-Segment LED Display

Seven-segment LEDs are common in calculators and digital voltmeters as indicators or displays.
The seven segments, represented by letters A-G, can be combined to form numbers 0-9.

Common Anode/Common Cathode Displays

Common anode displays have all anodes connected together.
Common cathode displays have all cathodes connected together.
Appropriate voltage/ground applications, are required to illuminate segments.

Alphanumeric LED Display

Alphanumeric displays function similarly to seven-segment displays, often employing 16 segments.

Dot Matrix LED Display

A more flexible display choice for alphanumeric representations using an array of LED dies (typically 7x5 arrangement).
Offers greater application possibilities compared to seven-segment displays.

Organic LEDs (OLEDs)

OLEDs are organic light-emitting diodes, formed by placing organic thin films between conductors.
Electrical current triggers light emission.
OLEDs do not require backlights, thus making them thinner and more efficient than LCDs.

OLED vs LCD

OLED displays generally exhibit lower power consumption, faster refresh rates, better contrast, and greater brightness.
Also offering better durability in a wider operating temperature range, and lighter weight.

Liquid Crystal Displays (LCDs) - Polarisation

Light is composed of electromagnetic waves that can travel in any orientation.
A polarised filter allows light waves of a specific orientation to pass
Polarisation restricts light waves to vibrate in a single plane.
Using cross-polarising lenses can block light completely.

Liquid Crystal (LC)

Liquid crystals exhibit characteristics between solid crystals and liquid, within specific temperature ranges.
They display a crystalline structure.
Refraction, depending on the crystalline state, shows variations.

Liquid Crystal Reorientation

LC molecules align with the direction of an applied electrical field.
This field causes the molecules to align parallel to the field.

LCD Construction

LCDs consist of two glass plates with a layer of liquid crystal fluid separating them.
Transparent-conductive electrodes, which shape segments, pixels, or symbols, are deposited onto the inner glass surfaces of the LCD.
Polarising films are layered perpendicularly onto the outer surfaces of the glass plates.

LCD Operation

Applying voltage between the segment and backplane causes the LCD segment to appear dark. With no voltage, the LCD appears clear.
Usually employs AC voltage for operation.

Backlit LCDs

Backlit LCDs, as opposed to reflective LCDs, employ internal light sources (like fluorescent tubes) to illuminate the display.
White diffusion panels behind the LCD are used to spread light evenly.

Greyscale LCDs

Greyscale displays use varying levels of voltage supplied to components, to control the light level and transition through shades.

Colour LCDs

Colour LC displays utilise sub-pixels (red, green, and blue) with colour filters.
Varying voltages produce varied shades within each sub-pixel to create combinations of colours.

Additive Colour Mixing

This method combines coloured light to form other colours.
Using red, green, blue (RGB) combinations results in various shades.

Cathode Ray Tube (CRT) - Thermionic Emission

Thermionic emission describes the escape of electrons from a heated material (such as a filament) in a vacuum.
This effect was discovered by Edison.
The released electrons move from the heated filament to the positively charged plate.
Materials with high temperatures release electrons more readily.

CRT - Electron Gun

The cathode is a small-diameter nickel cap coated with emitting material.
Used to produce an electron beam, which is highly concentrated.
A grid is used to concentrate and focus the emitted electrons into a beam.
Additional accelerating anodes (electrodes) ensure the electron beam reaching the screen is focused and high speed.

CRT - Screen

CRT screens comprise a phosphor-coated interior.
Phosphors give off light when struck by electrons.
A coating of aquadag, sometimes present on the screen, mitigates secondary electron emission effects.
Multiple phosphors within the display generate colour.

CRT - Operation Review

CRT elements (cathode, grid, focusing and accelerating anodes, deflection plates, aquadag-coating, and screen) work to control electron beam movement for creating the image.
Various voltages control the brightness of each spot or area.
Deflection coils (or plates) are used to control horizontal and vertical beam movement across the screen.

CRT - Deflection Coils/Plates

Deflection coils/plates function similarly, controlling the electron beam for horizontal and vertical sweeps across the screen.

###CRT Handling and Disposal Safety

Specific safety precautions are essential for handling CRTs.
Risk of implosion when damaged or broken.
The internal phosphor-coatings are often hazardous.
Specific precautions to minimize damage during removal or handling of damaged units.
Proper disposal procedure is crucial.

Simultaneous Picture Formation

Light-sensitive devices capture the image; the data is processed, amplified, and sent to the output displays.
Signal strength corresponds to light intensity.
System uses an array of amplifiers to ensure the quality and consistency in the output.

Sequential Scanning

Eliminates the drawbacks of simultaneous scanning by sequentially sending signals in rapid succession, providing a complete image.
Scanning techniques use the phenomenon called persistence of vision.
Sending all the odd-numbered lines first, then all even-numbered lines, this method creates a fluid image or raster.

Scanning Raster

The spot/electron beam moves across the screen in a predetermined sequence of horizontal lines and rows.
-The spot then rapidly returns to the starting point for each subsequent line.
The flyback period (returning the spot) is invisible.

Interlace Scanning

Images are broken into two fields (odd and even lines), which are consecutively displayed, creating a smoother image for viewers.
Allows for more video channels to be accommodated with a lower bandwidth.
The persistence of vision blurs the transition between the two fields.

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Description

Explore the fundamentals of electronic displays including cathode ray tubes, LEDs, and LCDs in modern aircraft. This quiz covers LED characteristics, their applications, and the science behind light emission. Test your knowledge on these essential display technologies.