A liquid crystal display is a type of electrically generated image on a thin flat panel. The first LCDs, which came out in the 1970s, were tiny screens used primarily in calculators and digital watches that displayed black numbers on a white background. LCDs can be found everywhere in home electronics systems, mobile phones, cameras and computer monitors, as well as watches and televisions. Today's state-of-the-art LCD flat panel TVs have largely replaced traditional bulky CRTs in televisions and can produce high-definition color images up to 108 inches diagonally across the screen.
History of liquid crystals
Liquid crystals were discovered by accident in 1888 by the botanist F. Reinitzer from Austria. He found that cholesteryl benzoate has two melting points, turning into a cloudy liquid at 145 ° C, and at temperatures above 178.5 ° C, the liquid becomes transparent. Tofind an explanation for this phenomenon, he gave his samples to the physicist Otto Lehmann. Using a microscope equipped with stepped heating, Lehman showed that the substance has optical properties characteristic of some crystals, but is still a liquid, and hence the term “liquid crystal” was coined.
During the 1920s and 1930s, researchers studied the effects of electromagnetic fields on liquid crystals. In 1929, Russian physicist Vsevolod Frederiks showed that their molecules in a thin film sandwiched between two plates changed their alignment when a magnetic field was applied. It was the forerunner of the modern voltage liquid crystal display. The pace of technological development since the early 1990s has been rapid and continues to grow.
LCD technology has evolved from black and white for simple watches and calculators to multicolor for mobile phones, computer monitors and televisions. The global LCD market is now approaching $100 billion a year, up from $60 billion in 2005 and $24 billion in 2003, respectively. LCD manufacturing is globally concentrated in the Far East and growing in Central and Eastern Europe. American firms lead the way in manufacturing technology. Their displays now dominate the market and this is unlikely to change in the near future.
Physics of the crystallization process
Most liquid crystals, such as cholesteryl benzoate, are made up of molecules with long rod-like structures. This special structure of liquid moleculescrystals between two polarizing filters can be broken by applying voltage to the electrodes, the LCD element becomes opaque and remains dark. In this way, various display elements can be either switched to light or dark colors, thereby displaying numbers or characters.
This combination of attractive forces existing between all molecules associated with a rod-like structure causes the formation of a liquid crystal phase. However, this interaction is not strong enough to keep the molecules in place permanently. Since then, many different types of liquid crystal structures have been discovered. Some of them are arranged in layers, others in the form of a disk or form columns.
LCD technology
The working principle of a liquid crystal display is based on the properties of electrically sensitive materials called liquid crystals, which flow like liquids but have a crystalline structure. In crystalline solids, the constituent particles - atoms or molecules - are in geometric arrays, while in a liquid state they are free to move around randomly.
The liquid crystal display device consists of molecules, often rod-shaped, that organize in one direction but can still move. Liquid crystal molecules react toan electrical voltage that changes their orientation and changes the optical characteristics of the material. This property is used on LCDs.
On average, such a panel consists of thousands of image elements (“pixels”), which are individually powered by voltage. They are thinner, lighter and have a lower operating voltage than other display technologies and are ideal for battery powered devices.
Passive Matrix
There are two types of displays: passive and active matrix. Passive ones are controlled by only two electrodes. They are strips of transparent ITO that rotate 90 to each other. This creates a cross matrix that controls each LC cell individually. Addressing is done by logic and drivers separate from the digital LCD. Since there is no charge in the LC cell in this type of control, the liquid crystal molecules gradually return to their original state. Therefore, each cell must be monitored at regular intervals.
Passives have a relatively long response time and are not suitable for television applications. Preferably, no drivers or switching components such as transistors are mounted on the glass substrate. Loss of brightness due to shading by these elements does not occur, so the operation of the LCDs is very simple.
Passive are widely used with segmented digits and symbols for small reading in devices such ascalculators, printers, and remote controls, many of which are monochrome or have only a few colors. Passive monochrome and color graphic displays were used in early laptops and are still used as an alternative to active matrix.
Active TFT displays
Active matrix displays each use one transistor to drive and a capacitor to store charge. In IPS (In Plane Switching) technology, the principle of operation of a liquid crystal indicator uses a design where the electrodes do not stack, but are located next to each other in the same plane on a glass substrate. The electric field penetrates the LC molecules horizontally.
They are aligned parallel to the screen surface, which greatly increases the viewing angle. The disadvantage of IPS is that each cell needs two transistors. This reduces the transparent area and requires a brighter backlight. VA (Vertical Alignment) and MVA (Multi-Domain Vertical Alignment) use advanced liquid crystals that align vertically without an electric field, that is, perpendicular to the screen surface.
Polarized light can pass through but is blocked by the front polarizer. Thus, a cell without activation is black. Since all molecules, even those located at the edges of the substrate, are uniformly vertically aligned, the resulting black value is thus very large at all corners. Unlike passive matrixliquid crystal displays, active matrix displays have a transistor in each red, green, and blue sub-pixel that keeps them at the desired intensity until that row is addressed in the next frame.
Cell switching time
The response time of displays has always been a big problem. Due to the relatively high viscosity of the liquid crystal, LCD cells switch rather slowly. Due to the rapid movements in the image, this leads to the formation of stripes. Low viscosity liquid crystal and modified liquid crystal cell control (overdrive) usually solve these problems.
The response time of modern LCDs is currently about 8ms (the fastest response time is 1ms) changing the brightness of an image area from 10% to 90%, where 0% and 100% are steady state brightness, ISO 13406-2 is the sum of the switching time from bright to dark (or vice versa) and vice versa. However, due to the asymptotic switching process, a switching time of <3 ms is required to avoid visible bands.
Overdrive technology reduces the switching time of liquid crystal cells. For this purpose, a higher voltage is temporarily applied to the LCD cell than is necessary for the actual brightness value. Due to the short voltage surge of the liquid crystal display, the inert liquid crystals literally break out of their position and level out much faster. For this process level, the image must be cached. Together with specially designed for the corresponding valuesdisplay correction, the corresponding voltage height depends on the gamma and is controlled by lookup tables from the signal processor for each pixel, and calculate the exact time of the image information.
Main components of indicators
The rotation in the polarization of light produced by liquid crystal is the basis for how an LCD works. There are basically two types of LCDs, Transmissive and Reflective:
- Transmissive.
- Transmission.
Transmission LCD display operation. On the left side, the LCD backlight emits unpolarized light. When it passes through the rear polarizer (vertical polarizer), the light will become vertically polarized. This light then hits the liquid crystal and will twist the polarization if turned on. Therefore, when vertically polarized light passes through the ON liquid crystal segment, it becomes horizontally polarized.
Next - the front polarizer will block horizontally polarized light. Thus, this segment will appear dark to the observer. If the liquid crystal segment is turned off, it will not change the polarization of the light, so it will remain vertically polarized. So the front polarizer transmits this light. These displays, commonly referred to as backlit LCDs, use ambient light as their source:
- Clock.
- Reflective LCD.
- Usually calculators use this type of display.
Positive and negative segments
A positive image is created by dark pixels or segments on a white background. In them, the polarizers are perpendicular to each other. This means that if the front polarizer is vertical, then the back polarizer will be horizontal. So OFF and the background will let the light through, and ON will block it. These displays are typically used in applications where ambient light is present.
It is also capable of creating solid state and liquid crystal displays with different background colors. A negative image is created by light pixels or segments on a dark background. In them, the front and rear polarizers are combined. This means that if the front polarizer is vertical, the rear will also be vertical and vice versa.
So the OFF segments and the background block the light, and the ON segments let the light through, creating a light display against a dark background. Backlit LCDs typically use this kind, which is used where ambient light is weak. It is also capable of creating different background colors.
Display memory RAM
DD is the memory that stores the characters displayed on the screen. To display 2 lines of 16 characters, addresses are defined as follows:
| Line | Visible | Invisible |
| Top | 00H 0FH | 10H 27H |
| Low | 40H - 4FH | 50H 67H |
It allows you to create a maximum of 8 characters or 5x7 characters. Once new characters are loaded into memory, they can be accessed as if they were normal characters stored in ROM. CG RAM uses 8-bit wide words, but only the 5 least significant bits appear on the LCD.
So D4 is the leftmost point and D0 is the pole on the right. For example, loading a RAM byte CG at 1Fh calls all the dots of this line.
Bit mode control
There are two display modes available: 4-bit and 8-bit. In 8-bit mode, data is sent to the display by pins D0 to D7. The RS string is set to 0 or 1, depending on whether you want to send a command or data. The R/W line must also be set to 0 to indicate the display to be written. It remains to send a pulse of at least 450 ns to input E to indicate that valid data is present on pins D0 to D7.
The display will read data on the falling edge of this input. If a read is required, the procedure is identical, but this time the R/W line is set to 1 to request a read. The data will be valid on lines D0-D7 on the high line state.
4-bit mode. In some cases, it may be necessary to reduce the number of wires used to drive the display, such as when the microcontroller has very few I/O pins. In this case, the 4-bit LCD mode can be used. In this mode, to transmitdata and reading them, only the 4 most significant bits (D4 to D7) of the display are used.
4 significant bits (D0 to D3) are then connected to ground. Data is then written or read by sending the four most significant bits in sequence, followed by the four least significant bits. A positive pulse of at least 450 ns must be sent on line E to test each nibble.
In both modes, after each action on the display, you can make sure that it can process the following information. To do this, you need to request a read in command mode and check the Busy BF flag. When BF=0, the display is ready to accept new command or data.
Digital voltage devices
Digital liquid crystal indicators for testers consist of two thin sheets of glass, on the facing surfaces of which thin conductive tracks were applied. When the glass is viewed from the right, or almost at a right angle, these tracks are not visible. However, at certain viewing angles, they become visible.
Electrical circuit diagram.
The tester described here consists of a rectangular oscillator that generates a perfectly symmetrical AC voltage without any DC component. Most logic generators are not capable of generating a square wave, they generate square waveforms whose duty cycle fluctuates around 50%. The 4047 used in the tester has a binary scalar output that guarantees symmetry. Frequencyoscillator is about 1 kHz.
It can be powered by a 3-9V supply. Usually it will be a battery, but a variable power supply has its advantages. It shows at what voltage the voltage indicator liquid crystal works satisfactorily, and there is also a clear relationship between the voltage level and the angle at which the display is clearly visible. The tester draws no more than 1 mA.
The test voltage must always be connected between the common terminal, i.e. the rear plane, and one of the segments. If it is not known which terminal is the back plane, then connect one probe of the tester to the segment and the other to all other terminals until the segment is visible.
