A liquid crystal display is a type of electrically generated image on a thin flat panel. The first LCDs that came out in the 1970s were tiny screens used mainly in calculators and digital clocks displaying black numbers on a white background. LCD displays can be found everywhere in home electronics systems, mobile phones, cameras and computer monitors, as well as in watches and televisions. Today, state-of-the-art LCD flat panel televisions have largely replaced traditional bulky cathode ray tubes and can create high-definition color images up to 108 inches across the screen.
History of liquid crystals
Liquid crystals were discovered by chance in 1888 by the botanist F. Reinitzzer 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. To find an explanation for this phenomenon, he transferred his samples to the physicist Otto Lehmann. Using a microscope equipped with step heating, Lehman showed that the substance has optical properties characteristic of some crystals, but it is still a liquid, and therefore the term “liquid crystal” appeared.
During the 1920s and 1930s, researchers studied the effect of electromagnetic fields on liquid crystals. In 1929, Russian physicist Vsevolod Fredericks 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 a modern voltage LCD. The pace of technological development since the early 1990s has been fast and continues to grow.
LCD evolution technology has gone from black and white for simple watches and calculators to multi-color for mobile phones, computer monitors and televisions. The global LCD market is currently approaching $ 100 billion a year, up from $ 60 billion in 2005 and $ 24 billion in 2003, respectively. LCD production is globally concentrated in the Far East and is 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.
Crystallization Physics
Most liquid crystals, such as cholesteryl benzoate, are composed of molecules with long, rod-like structures. This special structure of liquid crystal molecules between two polarizing filters can be disrupted by applying voltage to the electrodes, the element of the liquid crystal indicator becomes opaque and remains dark. Thus, the various display elements can either be switched to light or dark colors, thereby displaying numbers or signs.
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 constantly keep the molecules in place. 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 principle of operation of the liquid crystal indicator is based on the properties of electrosensitive 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 the liquid state they can freely move in random order.
A liquid crystal display device consists of molecules, often rod-shaped, that organize in one direction but can still move. Molecules of a liquid crystal react to an electrical voltage that changes their orientation and changes the optical characteristics of the material. It is this property that is used on LCDs.
On average, such a panel consists of thousands of image elements (“pixels”) that are individually energized. They are thinner, lighter and have a lower 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 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 performed by logic and drivers separately from the digital liquid crystal display. 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 controlled at regular intervals.
Passive ones 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, therefore, the management of LCD indicators is very simple.
Passive ones are widely used with segmented numbers and symbols for small readout in devices such as calculators, printers, and remote controls, many of which are monochrome or have only a few colors. Passive monochrome and color graphic displays were used in the first laptops, and they are still used as an alternative to the active matrix.
Active TFT Displays
In active matrix displays, each of them uses a single transistor to control and, as a charge storage, a capacitor. In IPS (In Plane Switching) technology, the principle of operation of the liquid crystal display uses a design when the electrodes are not folded, but are located next to each other in the same plane on a glass substrate. An electric field penetrates the LC molecules horizontally.
They are aligned parallel to the screen surface, which significantly increases the viewing angle. The disadvantage of IPS is that for each cell, two transistors are needed. 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 the molecules, even those located on the edges of the substrate, are uniformly vertically aligned, the resulting black value is very large in all angles. Unlike passive matrix liquid crystal displays, active matrix displays have a transistor in each red, green and blue subpixels, which keeps them at the desired intensity until this line is addressed in the next frame.
Cell switching time
Display response times have always been a big problem. Due to the relatively high viscosity of the liquid crystal, the LCD cells switch rather slowly. Due to the fast movements in the image, this leads to the formation of stripes. Low viscous liquid crystal and modified control of liquid crystal cells (overdrive) usually solve these problems.
The reaction time of modern LCD displays is currently about 8 ms (the fastest reaction time is 1 ms), the brightness of the image area changes from 10% to 90%, where 0% and 100% are the brightness of the stationary state, 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 overvoltage of the power supply of the liquid crystal indicator, inert liquid crystals literally break out of their position and align much faster. For this process level, the image must be cached. Together with the display voltage specifically designed for the respective display correction values, the corresponding voltage height depends on the gamma and is controlled by look-up tables from the signal processor for each pixel, and the exact time of the image information is calculated.
The main components of indicators
The rotation in the polarization of light created by the liquid crystal is the basis of the LCD. There are basically two types of LCDs, Transmissive and Reflective:
- Transmissive.
- Transmission.
Transmission LCD 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. Then this light enters the liquid crystal and will twist the polarization, if it is turned on. Therefore, when vertically polarized light passes through the ON liquid crystal segment, it becomes horizontally polarized.
Next, the frontal 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 light, so it will remain vertically polarized. Thus, the front polarizer transmits this light. These displays, commonly called backlit LCDs, use ambient light as a source:
- Clock.
- Reflective LCD.
- Typically, 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 rear will be a horizontal polarizer. Thus, OFF and background will transmit light, while ON will block. These displays are typically used in devices in which ambient light is present.
It is also capable of creating semiconductor and liquid crystal indicators 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.
Thus, the OFF segments and the background block the light, and the ON segments transmit light, creating a light display against a dark background. Backlit LCDs typically use this view, which is used where ambient light is weak. It is also capable of creating different background colors.
RAM display memory
DD is the memory in which characters displayed on the screen are stored. 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 ordinary characters stored in ROM. CG RAM uses 8-bit wide words, but only the 5 least significant bits appear on the LCD.
Thus, D4 is the leftmost point, and D0 is the pole to the right. For example, loading the CG RAM byte at 1Fh calls all the points on this line.
Bit mode control
Two display modes are 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 should also be set to 0 to indicate the display to be recorded. 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 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 a high line condition.
4-bit mode. In some cases, it may be necessary to reduce the number of wires used to control the display, for example, when there are very few I / O pins on the microcontroller. In this case, you can use the four-bit LCD mode. In this mode, only the 4 most significant bits (from D4 to D7) of the display are used to transmit data and read them.
The 4 significant bits (D0 to D3) are then connected to the ground. Data is then written or read by sequentially sending the four most significant bits, followed by the four least significant bits. A positive pulse of at least 450 ns must be sent on line E to verify each nibble.
In both modes, after each action on the display, you can verify that it can process the following information. To do this, request read in command mode and check the Busy BF flag. When BF = 0, the display is ready to accept a 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 to the right or almost at a right angle, these paths are not visible. However, at certain viewing angles, they become visible.
Schematic diagram.
The tester described here consists of a rectangular generator that generates an absolutely symmetrical alternating voltage without any DC component. Most logic generators are not able to generate a rectangular signal; they generate rectangular waveforms, whose duty cycle fluctuates around 50%. The 4047 used in the tester has a binary scalar output that guarantees symmetry. The oscillator frequency is about 1 kHz.
It can be powered by a source of 3–9 V. It will usually be a battery, but a variable power supply has its advantages. It shows at what voltage the liquid crystal voltage indicator works satisfactorily, and there is also a clear relationship between the voltage level and the angle at which the display is clearly distinguishable. The tester consumes a current not exceeding 1 mA.
The test voltage must always be connected between a common terminal, i.e. the back plane, and one of the segments. If it is not known which of the terminals is the rear plane, then connect one tester probe to the segment, and the other to all other terminals until the segment becomes visible.