Electronic Displays (PDF) - Aviation Australia Training

## Electronic Displays ### Learning Objectives * 5.11.1.1 Describe the principles of operation of cathode ray tubes used in modern aircraft (Level 2). * 5.11.1.2 Describe the principles of operation of light emitting diodes used in modern aircraft (Level 2). * 5.11.1.3 Describe the principle...

## Electronic Displays ### Learning Objectives * 5.11.1.1 Describe the principles of operation of cathode ray tubes used in modern aircraft (Level 2). * 5.11.1.2 Describe the principles of operation of light emitting diodes used in modern aircraft (Level 2). * 5.11.1.3 Describe the principles of operation of liquid crystal displays used in modern aircraft (Level 2). ### Light Emitting Diodes in Aircraft #### LED Fundamentals The Light-Emitting Diode (LED) is one type of optoelectronic device. It was developed to replace the fragile, short-life incandescent light bulbs used to indicate on/off conditions on panels. An LED, when forward biased, produces visible light. The light may be red, orange, yellow, green, blue, white or ultraviolet, depending on the material used to make it. LEDs emitting a non-visible light in the infrared part of the radiation spectrum are also available. These LEDs are invaluable in detection applications when used in conjunction with infrared detector components. The LED is designated by a standard diode symbol with two arrows pointing away from the cathode, indicating light leaving the diode. The LED operating voltage is low, about 1.6 V forward bias and generally about 10 mA. The life expectancy of the LED can be very long, over 100 000 hours of operation. The document includes an image of the LED symbol Description: The first image shows a side view of an LED and illustrates the "shortest" lead going to the cathode on the left, and the anode on the right. It is labeled "Side View". The second image shows a bottom view of an LED and illustrates the cathode having a "flat" on the body. It is labeled "Bottom View". There is also a generic real photograph and symbol of an LED. The LED symbol shows a diode symbol with two arrows pointing away from the diode. To turn on an LED, you must determine the anode and cathode connections. If you look closely at the red plastic base of an LED, you will notice a flat spot. The wire that comes out beside the flat spot must connect to the negative side of a power supply, and the other wire to the positive side. LEDs will be damaged if connected to power supplies rated at over 2 V. LEDs require resistors to limit current when used with power supplies rated at over 2 V. To avoid damaging your components, always check the supplier's or manufacturer's literature for this information. #### Peak Wavelength Single-Coloured (Monochromatic) LEDs The colour of light is the way we perceive its wavelength. The light radiation spectrum is expressed as the wave length in nanometres (nm). Unlike incandescent lamps that produce light over a wide spectrum (of which visible light is only a small segment), LEDs emit light over only a relatively small part of the radiation spectrum. Peak wavelength is the technical method of defining the colour emitted by the LED (the wavelength of the emitted light). 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). Generally the output is not at one precise wavelength, but is distributed over a narrow range; a graph of intensity against wavelength would show a peak at the specified wavelength. The document contains the following image that depicts color wavelengths. Description: The image displays a horizontal spectrum of light wavelengths, ranging from 450nm to 700nm. The document contains the statement "Light radiation spectrum is expressed in 'nanometres' (nm)". The peak wavelength for any LED component is determined by the chemical make-up of the semiconductor substrate rather than the current or power dissipated. It makes no difference if the LED is in a coloured or clear package. #### Multi-Coloured and Bi-Coloured LEDs Within the multi-coloured LED epoxy package are two separate reverse-parallel semiconductor chips, each producing a different single colour. At any instant of time, only one of the LED chips can emit light, and which one depends on the direction of current flowing through the component. Only one series resistor is required, and is calculated using the same formula for calculation of series resistors for LEDs. The document includes two images of Bi-colour LEDs Description: One image displays a schematic of the red and green diodes facing opposite of each other. The second image displays a real photograph of the bottom of a bi-colour LED. Bi-coloured LEDs can produce a third colour that is a product of mixing the two primary colours. For example, a red and green bi-coloured LED can produce a yellow light. The simplest method to achieve this is to operate the LED from an AC voltage source. This results in each of the primary colour chips operating during their respective half cycles of the alternating flow of current; however, the human eye perceives the rapidly flickering red and green lights as a constant yellow. #### Tri-Coloured LEDs Within the LED epoxy package are two separate semiconductor chips that each produce a different colour. A common lead from the two semiconductor chips is connected internally to produce a three-terminal component. Both common cathode and common anode types are available. The document includes the schematic of a tri-colour LED with "Common Anode" and with "Common Cathode" Description: Two images are displayed: 1. The first photograph displays a "Common Anode" tri-colour LED with a "Cathode" to the left and "Cathode +" to the right. 2. The second photograph displays a "Common Cathode" tri-colour LED with a "Anode +" to the left and "Anode +" to the right. Used simply, these components provide a selectable two-colour light source by switching the voltage between the two semiconductor chips. Alternatively both semiconductor chips can be operated simultaneously to mix the two primary colours. Only one series resistor is required provided that both semiconductor chips are never operated simultaneously. Otherwise it is essential that each chip is protected by its own dedicated resistor. #### Seven-Segment LED Display LEDs are used widely as power-on indicators of current and as displays for pocket calculators, digital voltmeters, frequency counters, etc. In calculators and similar devices, LEDs are typically placed together in seven-segment displays. This type of display uses seven LED segments, or bars (labelled A through G in the diagram), which can be lit in different combinations to form any number from 0 through 9. The document has the following images of a "seven-segment LED display": 1. The first image displays the numbers "0 1 1 1 0 1 1" on a "seven-segment LED display" with the individual segments labelled g, f, e, d, c, b, and a. 2. The second image displays the letter F, B, G, E, C, and D to compose the number "8" 3. The third image displays the letter A, F, B, G, E, C, and D to compose the number "8" Description: The first image displays the numbers 0, 1, 1, 1, 0, 1, 1 on a "seven-segment LED display" with the labels g, f, e, d, c, b, and a. The second image displays the letters A, F, B, G, E, C, and D, to compose the number "8" within two different schematics #### Common Anode seven-segment display with Common Cathode (CC) and Common Anode (CA) schematic diagrams The schematic shows a common-anode display. All anodes in a display are internally connected. When a negative voltage is applied to the proper cathodes, a number is formed. For example, if negative voltage is applied to all cathodes except that of LED E, the number 9 is displayed. If the negative voltage is applied to all cathodes except LED B, the number 6 is displayed. The corresponding image is included in the document: Description: The schematic shows a common-anode display. All anodes in a display are internally connected. When a negative voltage is applied to the proper cathodes, a number is formed. For example, if negative voltage is applied to all cathodes except that of LED E, the number 9 is displayed. If the negative voltage is applied to all cathodes except LED B, the number 6 is displayed. There are two types of LED seven-segment displays: Common Cathode (CC) and Common Anode (CA). In the common cathode, all the cathodes of the seven segments are connected directly, and in the common anode, all the anodes of the seven segments are connected. Shown above is a common anode seven-segment display. When working with a CA seven-segment display, power must be applied externally to the anode connection that is common to all the segments. Then, applying a ground to a particular segment connection (A-G) will cause the appropriate segment to light up. An additional resistor must be added to the circuit to limit the amount of current flowing through each LED segment. The Common Anode seven-segment display diagram shows the instance when power is applied to the CA connection and segments B and C are grounded, causing these two segments to light up. A common cathode seven-segment display is different from the common anode version in that the cathodes of all the LEDs are connected. To use this seven-segment display, the common cathode connection must be grounded and power must be applied to the appropriate segments in order to illuminate them. #### Alphanumeric LED Display Alphanumeric LED displays operate similarly to seven-segment displays and typically use 16 segments. The document contains the following image Description: Three alphanumeric LED displays. The left display renders a rotated/mirrored 'N' character. The center display is disconnected. The right display contains the text "ENGAGE PRIMARY DRIVE". #### Dot Matrix LED Display The more flexible display which is commonly used to produce the full alphanumeric range is the 35-dot matrix in which LED dies are mounted in a 7 × 5 array. With these types of displays, the range of applications increases to a much higher level compared to use cases for the seven-segment display. The following image is contained in the document: Description: An image displaying a dot matrix LED display, the schematic of the LED Display, and a display mounted on a 7x5 array. The alphanumeric LED display may be an older technology, but its uses are still numerous, even with the wide array of new technologies available for displaying information. Its biggest advantage is cost. It is hard to find a cheaper design solution for displaying small amounts of information. Another very good quality is its durability. The only pitfall is the possibility of a disconnection because, in effect, the diode is nothing more than an electrical connection. These displays are generally very simple to program. The logic to decode incoming data for the display is so basic that a designer can buy a desired decoder fairly cheaply if it does not come with the display. Also, because of its simplicity, required maintenance is minimal. #### Organic LEDs OLED (Organic Light-Emitting Diode) is a flat light-emitting technology made by placing a series of organic thin films between two conductors. When electrical current is applied, a bright light is emitted. OLEDs can be used to make displays and lighting. Because OLEDs emit light, they do not require a backlight and so are thinner and more efficient than LCD displays (which do require a white backlight). The document contains an image of an OLED Description: The image indicates the basic OLED design on how light is emitted. * Schematic of a bilayer OLED 1. Cathode (-) 2. Emissive Layer 3. Emission of radiation 4. Conductive Layer 5. Anode (+) OLEDs work in a similar way to conventional diodes and LEDs, but instead of using layers of n-type and p-type semiconductors, they use organic molecules to produce their electrons and holes. A simple OLED is made up of six different layers. On the top and bottom are layers of protective glass or plastic. The top layer is called the seal and the bottom layer the substrate. Between those layers are a negative terminal (sometimes called the cathode) and a positive terminal (called the anode). Finally, between the anode and cathode are two layers made from organic molecules called the emissive layer (where the light is produced, which is next to the cathode) and the conductive layer (next to the anode). The document contains the following figure: Description: This schematic illustration shows the different layers of an OLED display from top to bottom: a Protective Seal, a Cathode, an Emissive Layer, a Conductive Layer, an Anode, and a Substrate. #### OLED vs LCD OLED displays have the following advantages over LCD displays: * Lower power consumption * Faster refresh rate and better contrast * Greater brightness and a fuller viewing angle * Exciting new types of displays that we do not have today, like ultra-thin, flexible or transparent displays * Better durability and operation in a broader temperature range * Lighter weight due to a very thin screen and even the ability to be 'printed' on flexible surfaces. ### Liquid Crystal Displays #### Polarisation Light is made up of electromagnetic radiation (waves). Light waves can travel in any direction or have any orientation. The document contains an image of light passing thru polarization strips Description: The illustration demonstrates how a polarized filter allows light waves of one orientation to pass through. When two polarized filters are in line with the same orientation as each other, light will travel through and when the polarization lenses are perpendicular to each other, light will be stopped. A polarised filter passes light waves with one orientation A polarised filter allows only light travelling in one position to pass through. It is made of parallel micro-sized slits that block out all but one position of wave. Polarisation is the process of causing light to vibrate in one plane only. Cross-polarising lenses will stop light altogether. #### Liquid Crystal Liquid crystal is an organic substance that has both solid crystalline and liquid characteristics within certain temperature ranges. Unlike liquid substances, liquid crystal demonstrates a crystalline structure and related refraction characteristics. Depending on the crystalline state, various refractions are possible. The document contains an image of molecules passing through the liquid crystal Description: It demonstrates how light passes through liquid crystals when power is applied. The liquid crystals become aligned in a crystalline structure, allowing light to pass through. Calculators, digital watches, portable word processors and notebook PCs all use nematic liquid crystals which change their structure with the application of electric voltage. #### Liquid Crystal Reorientation When molecules with such a characteristic are brought into a sufficiently strong electrical field, they tend to align themselves in the direction of the field. Originally the orientation is almost flat. When an electrical field with direction $E$ is applied (represented in red), there is a force $T$ (represented in green) that tends to align the molecule parallel to the field. When the field is strong enough, the molecule will be almost parallel to the field. The document contains the image of the liquid crystal reorientation Description: The image models four displays of crystals aligning based on electrical flow. #### Liquid Crystal Displays A Liquid Crystal Display (LCD) consists of two plates of glass, sealed around the perimeter, with a layer of liquid crystal fluid between them. The liquid crystal layer is a few microns thick. The layer thickness of liquid crystal is approximately 1/10 the thickness of an average human hair. The document display the cross-section of a liquid crystal display Description: Image illustrates the construction of a liquid crystal display, which includes transparent electrodes deposited on glass plates, polymer layer for twist orientation, and polarizing films laminated at 90 degree angles. Transparent, conductive electrodes are deposited on the inner surfaces of the glass plates. The electrodes define the segments, pixels or special symbols of the display. Next a thin polymer layer is applied on top of the electrodes. The polymer is etched with channels to align the twist orientation of the Liquid Crystal (LC) helix-shaped molecules. Finally, polarising films are laminated to the outer surfaces of the glass plates at 90° angles. The document displays the process in which lighting passes thru liquid crystals Description: Schematic diagrams of the construction of a liquid crystal showing cross-sectional views on and off. Normally two polarising films at 90° should be dark, preventing any transmission of light, but due to the ability of LC to rotate polarised light, the display appears clear. When AC voltage is passed through the LC, the crystals within this field align so that the polarised light is not twisted. This allows the light to be blocked by the crossed polarisers, making the activated segment or symbol appear dark. Basically, LCDs operate from a low-voltage (typically 3 to 15 V RMS), low-frequency (25 to 60 Hz) AC signal and draw very little current. They are often arranged as seven-segment displays for numerical readouts. The AC voltage needed to turn on a segment is applied between the segment and the backplane. The image below contains a seven-segment LCD display with common back plane. Description: Image displays a cross-section of a 7-segment LCD display with a common back-plane. The backplane is common to all segments. The segment and backplane form a capacitor that draws very little current as long as the AC frequency is kept low. It is generally not lower than 25 Hz because this would produce visible flicker. LCDs draw much less current than LED displays and are widely used in battery-powered devices such as calculators and watches. An LCD does not emit light energy like an LED, so it requires an external source of light. An LCD segment will turn on when an AC voltage is applied between the segment and the backplane, and will turn off when there is no voltage between the two. Rather than generating an AC signal, it is common practice to produce the required AC voltage by applying out-of-phase square waves to the segment and backplane. For one segment, a 40-Hz square wave is applied to the backplane and also to the input of a Complementary Metal-Oxide-Semiconductor (CMOS) 4070 exclusive-OR gate. The other input to the XOR is a CONTROL input that will control whether the the segment is ON or OFF. The following images illustrate output voltage Description: Illustrates the process of applying alternating current (AC) waves to segment to control on/off states When the CONTROL input is LOW, the XOR output will be exactly the same as the 40-Hz square wave, so that the signals applied to the segment and backplane are equal. Since there is no difference in voltage, the segment will be off. When the CONTROL is HIGH, the XOR output will be the INVERSE of the 40-Hz square wave, so that the signal applied to the segment is out of phase with the signal applied to the backplane. As a result, the segment voltage will alternatively be at +5 V and -5 V relative to the backplane. This AC voltage will turn on the segment. #### Driving a Seven-Segment LCD This same idea can be extended to a complete seven-segment LCD. The Binary Coded Decimal (BCD) segment is ON or OFF. A BCD-to-seven-segment decoder/driver supplies the CONTROL signals to each of seven XORs for the seven segments. In general, CMOS devices are used to drive LCDs for two reasons: (1) they require much less power than Transistor-Transistor Logic (TTL) and are more suited to battery-operated applications where LCDs are used, and (2) the TTL LOW-state voltage is not exactly 0 V and can be as much as 0.4 V. This produces a DC component of voltage between the segment and backplane that considerably shortens the life of an LCD. The document displays the following circuit diagram: Description: The image illustrates BCD circuit with external all 40708 (X-OR) and BCD to the LCD with pin connections labeled. #### Reflective LCD Liquid crystal materials emit no light of their own. Small and inexpensive LCDs are often reflective, which means to display anything they must reflect light from external light sources. Look at an LCD watch: The numbers appear where small electrodes charge the liquid crystals and make the layers untwist so that light is not transmitting through the polarised film. LCDs are common in watches due to the low electrical power demands of the LCD panel. This panel is composed of two polarisers that transmit light in perpendicular directions, a mirrored surface and a layer of liquid crystal material that is sandwiched between two electrically conducting glass plates. The liquid crystal material used is of the so-called twisted nematic type. Description: The graphic highlights the alignment of liquid crystals in panels with polarized light. When light passes through the panel aligned, a segment becomes clear due to lack of voltage on any of the electrodes. The liquid crystal molecules in all segments of the panel are precisely aligned in the absence of an applied voltage. Therefore, the entire panel appears silvery because light passes through both polarisers, reflects off the mirrored surface and then passes back through both polarisers again. If the surfaces of the glass plates that will be in contact with the liquid crystal molecules are ribbed, the liquid crystal molecules orient in the direction of the ribbing. Molecules have been oriented in the direction in which the adjacent polariser transmits light, and the intervening molecules gradually rotate their relative orientation to accommodate the 90° change from one polariser to the other. This gives a silvery appearance to the panel. Description: The diagram shows a schematic of liquid crystals turning on and turning off The initial alignment of the liquid crystal molecules is lost when a voltage is applied to a segment of the panel. That segment will then appear black against a silver background. When a voltage is applied to a segment of the display, the precise alignment of the liquid crystal molecules is lost. This results in the polarised light from the first polariser not being rotated by the required 90° to align with the second polariser. The second polariser blocks the passage of light and causes that segment of the panel to appear black. #### Backlit LCD A backlit LCD functions in the same manner as normal-type LCDs (including the reflective type) except it uses backlighting. Most computer displays are lit with built-in fluorescent tubes above, beside and sometimes behind the LCD. A white diffusion panel behind the LCD redirects and scatters the light evenly to ensure a uniform display. On its way through filters, liquid crystal layers and electrode layers, a lot of this light is lost - often more than half. The document includes an image of an LCD screen with the following text. * 160x128 Optrex LCD LED Backlit * 480x128 Scrollable Image #### Greyscale LCDS If we carefully control the amount of voltage supplied to a crystal, we can make it untwist only enough to allow some light through. By doing this in very exact, very small increments, LCDs can create a greyscale. Most displays today offer 256 levels of brightness per pixel. The document contains the following image Description: Image shows a digital camera LCD monitor. #### Colour LCDS An LCD that can show colours must have three sub-pixels with red, green and blue colour filters to create each colour pixel. Through the careful control and variation of the voltage applied, the intensity of each sub-pixel can range over 256 shades. Combining the sub-pixels produces a possible palette of 16.8 million colours (256 shades of red × 256 shades of green × 256 shades of blue). The image below contains the components in the LCD monitor Description: Cross-section of an LCD display with color filters. #### Additive Colour Mixing Additive colour mixing is the combination of projected beams of coloured light to form other colours. Many find this model hard to understand, simply because it does not work like anything you have learned about before. But since additive colour mixing is how the eye (and electronic displays) produce colour, it is important to know about. It uses light to create colours - shining red, green and blue (additive primary colours) together to obtain white in the overlap of all three. Conversely, white light can be split into colours (e.g. prisms, rainbows). You get cyan, magenta and yellow in the other overlaps. These are additive secondary colours. Black is the absence of light when dealing with additive colours. The document shows the following diagram Description: The color diagram displays the additive color with Red Green and Blue. Red and Blue makes magenta, Red and Green create Yellow, and Blue and Green make Cyan. Red, green and blue stripes are arranged in a regular matrix. The gaps between pixels (the shadows) are for the drive circuits and wires. The black lines are significant in that they cannot have their colour changed, yet they take up space. Getting as close as possible to the right colours in the right places is what image display quality is all about. Note how sub-pixels (magnified) are controlled to obtain colour and clarity. The document contains the following image to show the pixels on the screen Description: This colored diagram provides a close up of three pixel stripes arranged on a regular matrix. #### Colour LCDS Colour displays use an enormous number of transistors. For example, a computer that supports resolutions up to 1024 × 768 has 2,359,296 transistors etched onto the glass (multiply 1024 columns by 768 rows by 3 sub-pixels). If there is a problem with any of these transistors, it creates a 'bad pixel' on the display. Bad pixels may appear black (no operation of any colour sub-pixel) or bright (where one or all of the colours are fully on), producing a bright single primary colour or bright white pixel. The document also contains the following close up of the pixels magnified Description: Image displays a microscopic look into a display with R, G, B labels. ### Cathode Ray Tube #### Thermionic Emission – Edison Effect Thomas Edison discovered the principle of thermionic emission as he looked for ways to keep soot from clouding his incandescent light bulb. Edison placed a metal plate inside his evacuated bulb along with the normal filament. He left a gap, a space, between the filament and the plate. He then placed a battery in series between the plate and the filament, with the positive side towards the plate and the negative side towards the filament. When Edison connected the filament battery and allowed the filament to heat until it glowed, he discovered that the ammeter in the filament-plate circuit had deflected and remained deflected. He reasoned that an electrical current must be flowing in the circuit - even across the gap between the filament and plate. In the following diagram the heated filament causes electrons to boil from its surface. The battery in the filament-plate circuit places a positive charge on the plate (because the plate is connected to the positive side of the battery). The electrons (negative charge) that boil from the filament are attracted to the positively charged plate. They continue through the ammeter and the battery, and back to the filament. The electron flow across th