B1-05.11 ELECTRONIC DISPLAYS

Electronic Displays

Light Emitting Diodes (LEDs)

• LEDs are optoelectronic devices used to replace fragile incandescent bulbs for on/off indications.
• When forward biased, LEDs produce visible light in various colors (red, orange, yellow, green, blue, white, ultraviolet) depending on the material.
• LEDs emitting non-visible infrared light are used in detection with infrared detector components.
• In diagrams, LEDs are represented by a standard diode symbol with two arrows pointing away from the cathode.
• LEDs operate at a low voltage (about 1.6V forward bias) and 10 mA.
• Their lifespan can exceed 100,000 hours.
• To turn one on, the flat spot on the plastic base must connect to the negative side of the power supply.
• LEDs can be damaged by power supplies rated over 2 V.
• Resistors are required to limit current for power supplies rated at over 2 V.

Peak Wavelength Single-Coloured (Monochromatic) LEDs

• The color of light is perceived by its wavelength.
• The light radiation spectrum is measured in nanometers (nm).
• LEDs emit light over a small part of the radiation spectrum, unlike incandescent lamps.
• Peak wavelength defines the color emitted by the LED.
• Typical figures range from 450 nm (blue) to 950 nm (infrared).
• Output is generally distributed over a narrow range, with a peak at the specified wavelength.
• The peak wavelength is determined by the chemical makeup of the semiconductor substrate instead of a current or power.

Multi-Coloured and Bi-Coloured LEDs

• Multi-coloured LEDs have two separate reverse-parallel semiconductor chips producing different single colors within the epoxy package.
• Only one chip emits light at a time, depending on current direction.
• Only one series resistor is required for current calculation.
• Bi-coloured LEDs can create a third color by mixing two primary colors.
• A red and green bi-coloured LED can produce yellow light. Operating a bi-coloured LED from an AC voltage source results in each primary color chip operating during alternating current cycles.
• The human eye perceives the flickering red and green lights as constant yellow.

Tri-Coloured LEDs

• Tri-coloured LEDs have two separate semiconductor chips that produce a different color in one epoxy package.
• A common lead internally connects the two chips, forming a three-terminal component.
• Common cathode and common anode types are available.
• A selectable two-color light source can be obtained by switching the voltage between semiconductor chips.
• Both chips can be operated simultaneously to mix primary colors. One series resistor is needed if the chips cannot be operated simultaneously.
• Separately dedicated resistors are essential if both chips are operated simultaneously.

Seven-Segment LED Display

• LEDs are used as power-on indicators and as displays for devices like pocket calculators.
• In calculators, LEDs are placed in seven-segment displays.
• Seven LED segments or bars (A through G) can be lit in different combinations to form numbers (0-9).
• The displays can be common cathode or common anode.
• All anodes in a display are connected in a common-anode display
• A negative voltage must be applied across the proper cathodes to form a number.
• Applying negative voltage to all cathodes except LED E displays the number 9.
• Applying negative voltage to all cathodes except LED B displays the number 6.
• Common Cathode seven-segment displays have all cathodes of the seven segments connected directly.
• Common Anode seven-segment displays have all anodes of the seven segments connected directly.
• Power must be applied externally to the anode connection that is common to all segments when working with a common anode (CA) seven-segment display.
• Grounding segments A-G lights the segment with resistors to reduce the current through each LED segment.
• The instance when power is applied to the CA connection and segments B and C are grounded will cause these two segments to light up.
• To use the common cathode seven-segment display, the common cathode connection must be grounded and power must be applied to the segments to illuminate them.

Alphanumeric LED Display

• Alphanumeric LED displays use 16 segments and operate like seven-segment displays.
• A 35-dot matrix is commonly used to produce a full alphanumeric range with LED dies mounted in a 7 x 5 array.
• The applications increase for a dot matrix compared to those of seven-segment displays.
• An alphanumeric LED display has a lower cost because it does minimal maintenance while using such a simple design.
• Their simplicity requires that they are easily programmed.

Organic LEDs (OLEDs)

• OLEDs are flat, light-emitting technologies that are made by placing organic thin films between two conductors.
• Electrical current emits bright light.
• OLEDs can be used for displays and lighting.
• OLEDs emit brighter light than the backlight of LCDs.
• OLEDs are thinner and more efficient than LCD displays which require a white backlight.
• OLEDs use organic molecules within conventional diodes and LEDs to emit their light.
• Simple OLEDs consist of six layers: top protective layer called the seal, bottom layer called the substrate, a negative terminal (cathode), and a positive terminal (anode).
• Between them are two layers of organic molecules: the emissive layer (where the light is produced) and the conductive layer.

OLED vs LCD

OLED displays compared to LCD displays:

• Consume less power
• Have faster refresh rates and better contrast
• Have greater brightness and a fuller viewing angle
• Can be ultra-thin, flexible, and transparent
• Can be used to make exciting new displays
• Tend to be more durable
• Better operation in a broader temperature range
• Lighter weight
• Can be printed on flexible surfaces

Liquid Crystal Displays

Polarisation

• Light waves are made up of electromagnetic radiation and can travel in any direction or have any orientation.
• A polarized filter allows light waves with only one position to pass through.
• The filter has parallel micro-sized slits that block out all but one position of wave.
• Polarisation causes the light to vibrate in one plane only.
• Cross-polarising lenses will stop light altogether.

Liquid Crystal

• Liquid crystal has solid crystalline and liquid characteristics within temperature ranges.
• Unlike liquid substances, liquid crystal demonstrates a crystalline structure and related refraction behaviors.
• The crystalline state determines various refractions.
• Calculators, digital watches, portable word processors, notebook PCs all use nematic liquid crystals that change their structure via electric voltage.
• Molecules with such a characteristic brought into a strong electrical field align themselves in the direction of the field.
• When applying an electrical field with direction E, there is a force T that aligns the molecule parallel to the field.

Liquid Crystal Displays (LCDs)

• An LCD consists of two glass plates sealed around the perimeter with liquid crystal fluid between them.
• The liquid crystal layer is a few microns thick.
• The thickness of the liquid crystal layer is about 1/10 the thickness of an average human hair.
• Transparent, conductive electrodes are deposited on the inner surfaces of the glass plates defining segments, pixels, and special symbols.
• A thin polymer layer is applied on top of the electrodes that is etched with channels to align the twist orientation of the LC helix-shaped molecules followed by polarising films laminated to the surfaces of the glass plates at 90° angles.
• Two polarising films at 90° prevent light transmission.
• LC can rotate polarized light to make the display clear.
• When AC voltage passes through the LC, the crystals align so that the light is not twisted.
• Therefore the light is blocked by the crossed polarisers, making the activated segment or symbol appear dark.
• LCDs operate from AC voltage.
• LCDs use low-voltage (typically 3 to 15 V RMS), low-frequency (25 to 60 Hz) signals and draw very little current.
• Segments are turned on and off between segment and backplane.

LCD Display

• A backplane is common to segments.
• A segment and backplane create a capacitor drawing very little current as long as the AC frequency remains low.
• It it is generally not lower than 25 Hz because this would produce visible flicker.
• LCDs draw less current than LED displays.
• LCDs are widely used in battery-powered devices like calculators and watches.
• LCDs need an external source of light since it doesn't emit any light.
• Applying an AC voltage between the segment and backplane turns on the LCD.
• No voltage turns it off.
• The required AC voltage is obtained by applying out-of-phase square waves to the segment and backplane as part of generating an AC signal
• A complementary Metal-Oxide-Semiconductor (CMOS) 4070 exclusive-OR gate is applied for one segment.
• Whether this works is controlled using an extra control input.
• When the CONTROL input is LOW, the XOR output matches the 40-Hz square wave to make equal signals.
• The segment will be off because there is no difference in voltage.
• When the CONTROL input is HIGH, the XOR output is the INVERSE of the 40-Hz square wave.
• As a result, the segment voltage will alternatively be at +5 V and -5 V relative to the backplane, therefore turning the segment on.
• The idea can be extended to a complete seven-segment LCD that has the Binary Coded Decimal (BCD) segment either ON or OFF.
• A BCD-to-seven-segment decoder/driver supplies the CONTROL signals to each of seven XORs for the seven segments
• CMOS devices drive LCDs for power and voltage reasons where the TTL LOW state is not exactly zero, that a DC component of voltage between the segment and backplane is produced.
• It is highly recommended for LCDs because lifetime is impacted

Reflective LCD

• Liquid crystal materials do not emit light.
• Small and inexpensive LCDs are often reflective and must reflect light to display anything.
• Small electrodes charge the liquid crystals to untwist the layers so light cannot transmit through the polarised film to display numbers.
• LCDs are common in watches due to the low electrical power demands.
• Each panel has two polarisers, a mirrored surface and a layer of twisted nematic liquid crystal material sandwiched between two electrically conducting glass plates.
• Entire panels have aligned liquid crystal molecules is precisely aligned, thus being silvery.
• Light passes through both polarisers, reflects off the mirrored surface and then passes back.
• Ribbing the surfaces of the glass plates orients glass contact while molecules are oriented in direction.

Operation of a LCD

• There cannot be any application of voltage to molecules.
• Polarised light results in a direct relationship of molecules' alignment, thus being easily lost if its required orientation is to align with the second polarizer.
• The second polarizer blocks light that creates that segment as black on the panel.

Backlit LCD

• Backlit LCDs function like normal LCDs (including the reflective type).
• Most computer displays use built-in fluorescent tubes above, beside or behind the LCD to provide light.
• They use a white diffusion panel behind the LCD that redirects/scatters light to ensure a uniform display.

Greyscale LCDs

• By controlling the voltage supplied to a crystal, it can be made to untwist enough to allow light through.
• LCDs can create a greyscale in very small increments on displays today offer 256 levels of brightness per pixel.

Color LCDs

• An LCD that can show colors must have three sub-pixels that have color filters for red, green and blue.
• The intensity of 3 sub-pixels can range for 256 shades in order to carefully control and variation in color.s
• Combining produced sub-pixels have roughly 16.8 possible palette colours
• Combining red, green, and blue sub-pixels arranged in a regular matrix creates all known colors

Additive Color Mixing

• This type of color mixing combines projected beams of colored light to form other colours.
• The light creates color shining
• Red, green and blue combine to create a white color
• White light converts into different types of light (e.g., cyan, magenta and yellow)

Color LCDs

• Millions of transistors make up modern Color LCDs.
• Common resolutions for monitors is 1024X768 pixels.
• Sub-pixels (magnified) are controlled to obtain color and clarity.

Cathode Ray Tube

Thermionic Emission – Edison Effect

• Thomas Edison's experiment with the effect was to prevent soot, using heated filament electron that had a deflected reading using a series circuit and a high positive charge that attracts negative-charged (Electrons) filament back to the positively charged plate
• heating a conductor (filament) to a point gives off electrons from the heating process, also known as thermionic emission

Cathode Ray Tube (CRT)

• Older television technology used the CRT to present a picture on a screen, unlike new LED and plasma screens.
• All CRTs consist of the main elements: an electron gun, a deflection system and a screen.
• Their unique capability is to convert electronic signals to visual displays.
• The electron gun's cathode emits electrons in various directions.
• Using the cathode and grid, it creates a focused electron beam is fired in one direction.
• Its path gets curved in its direction that can result as positive potential, that is attracted towards the high positive potential on the anode.
• Brightness that strikes results displays intensity on the numbers of amount used.
• If there is not control it will glow. The purpose of electric gun the tight area of the beam.

Construction of the CRT

• Cathode: The negatively charged electrode in the electron gun is made from the heated filament inside the gun that releases the electrons through the wire to the anode.
• Grid: The circular device is charged negatively that repulses on the cathode to reduce electron scattering, in which controls the electric beam.
• Brightness Control: Control the flow of electricity that comes from the potentiometer to vary the potential from from the grid.
• Focus Anode: Charged electrodes focus on another tight angle.
• Accelerating Anode: Charged elements that allows electrons moving from the front wall.

CRT Screen

• coated screens display different colored images or screens when the elements strike it, as known to emit light from the coating.
• coated materials require a vacuum seal with a coating called aquadag, because the material itself emits invisible rays.
• aquadag's two responsibilities are that is to attract secondary emitted electrons to the second element

CRT Operation Review

• Electrons are emitted from a specially made cathode and are shot to the front of the screen.
• The amount of electrons are the area of the electron from a certain potential.
• The grid will concentrate the electrons into a beam, and then is focused and accelerated towards the acceleration anode.
• The beam then strikes where a bright phosphorus occurs from the elements used.

CRT Electron Beam Deflection

• the electrons have a charged spot at that spot: with either moving from the magnetic or electric field.
• electrostatic deflection is a charged plate that has a CRT screen.
• the travel goes left to right or right to top with the ability the deflection of electron.

Horizontal Deflection of CRT Electron Beam

• A spot of light in general does not mean much to the screen. It must in effect be a brighter line known as a sweep element on screen.
• This is done from a sequence of 5, with the proper adjustments applied to the proper angles with electron and resistors.

Horizontal Deflection of CRT Electron Beam

• One type is graphical that provides the visual image of the system.
• Deflection uses vertical plates that sweep electrons, unlike the horizontal deflection above.