August 31, 2017 | Author: Senthil Murugan | Category: Liquid Crystal, Printer (Computing), Liquid Crystal Display, Cathode, Electron
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INTRODUCTION • Display devices which are helpful to the experimenter to examine the parameters of an electronic circuit • Examples- voltmeter, ammeter • Further development in the field of display devices lead to the discovery of recording devices



MAGNETIC TAPE RECORDERS • It is used for recording the data by use of magnetic heads in electrical form • Components Recording head Magnetic tap Reproducing head Tape transport mechanism Conditioning devices

RECORDING HEAD • Recording head produces a magnetic pattern in a magnetisable medium • Construction is similar to that of a transformer

MAGNETIC TAPE • This tape is made of magnetic iron oxide(Fe2O3 ) on a plastic ribbon • Dimensions 12.7 mm wide and 25.4μm thick

REPRODUCING HEAD • Used to detect magnetic pattern and convert it back to electrical signal • Similar to recording head

TAPE TRANSPORT MECHANISM • Its function is move the tape along the recording or reproducing head • It is used to guide the tape pass the magnetic head with great precision, maintain proper tension • Also having the arrangement fast winding and reversing

CONDITIONING DEVICES • It consist of amplifiers and filters • Modifying signal to a format that can be properly recorded on a tape

METHODS OF RECORDING • Direct recording • FM recording • PDM recording


DIRECT RECORDING • The signal to be recorded modulates the current in the recording head • When the tape is moved under the recording head, the magnetic particles retain a state of permanent magnetisation • The input signal is converted to a spatial variation of the magnetisation of the particles on the tape.

DIRECT RECORDING • Reproducing head detect the changes as changes in the reluctance • Induced voltage is proportion to change in flux • Disadvantages  cant be used in dc  lower limit around 100 Hz and upper limit around 2MHz


FM RECORDING • In this system, fc is modulated by the level of the input signal • When there is no input signal the modulation is at a fc • “+” signal is applied, the frequency deviates from the centre frequency in some direction • “-”signal is applied, the frequency deviates from the centre frequency in opposite direction

FM RECORDING • • • • • •

Speed range selector is provided. S/N is 40-50 db, accuracy of less than 1% Max speed range is 80 kHz at 120 in/s Output of the modulation is fed in to the tape It uses both ac & dc inputs Output of the reproducing head is demodulated and fed through LPF

FM RECORDING • Advantages  used for both dc & ac

 wide range of frequency

• Disadvantages  less sensitive

 require high tape speed


PDM RECORDING • It allows simultaneous recording • The input signal is converted at the instant of sample to a pulse • The no.of channels per track is 86 • Input frequencies are less than 1.5

PDM RECORDING • Advantages  capable to simultaneous record

 high S/N ratio  high accuracy and calibrated itself • Disadvantages limited frequency response complex electronic circuit used for special application only



Return to zero method Non-Return to zero method NRZ –L method Biphase –L method


Non-Return to zero method

NRZ –L method head


X-Y RECORDER • Used plot the instantaneous relationship between two variables[Y=F(x)], rather than to plot each variable separately as a fn of time • Writing head move both x & y axis • Graph paper is held fixed by electrostatic attraction or by vacuum • It is also used to plot the physical qty

X-Y RECORDER • Accuracy of less than 0.3% of full scale • Zero offset adjustment are also provided • Application  speed torque characteristics of motor  regulation curve of power supply


DIGITAL PLOTTER • Analog x-y recorder are replaced by digital x-y recorders. It is known as digital plotters • A/D conversion .(input is analog; output is digital) • Digital value is stored in memory and indicated the time varying analog signal .The data in the memory can be scanned and going to any recorder or any display devices • ADVANTAGE:• Can plot or draw grids, axis, storage data. • DISADVANTAGE:• Cost is high

PRINTERS 2 types • Impact printers 1. Dot matrix 2. Drum printer 3. Chain /band printers • Non-impact printers 1. Ink-jet printer 2. Laser printer

IMPACT PRINTERS • A printer in which printing is the result of mechanically striking the printer medium Example • Type writer key striking a ribbon against the paper


DOT MATRIX PRINTERS • Hamming the pins on the inked ribbon Serial printer • These printers are character printers which print one character at a time • Can move horizontally (left to right and right to left)across the paper. • The printer head contains an array of pins which can be activated independent of each other to extend and strike against an inked ribbon to form a pattern of dots on the paper • ESPON ,CH ,SAMSUNG :5*7,7*9,9*13

• DISADVANTAGE:• Slow with speed • Quality of printer is poor



Another name is line printer Which print one line at a time Character embraced Drum character with 132 character /line and supporting a character set of 96 character hence 132*96 character embossed on its surface • Paper can be placed between hammer and drum • Drum would have to complete one full revolution to print line of output .This means that all character on a line are not printed at exactly same time DISADVANTAGE: • Noisy operation • Expensive

CHAIN/BAND PRINTER(line printer) • A set of 42 character(64*4=256 character embossed on it) DRAWBACKS: • Drum printer ,chain/band printer can only print pre-defined set only. It do not have the ability to print any shape character • Noisy operation

Non-impact printers • Printer do not strike characters against ribbon or paper when they print are non-impact printers

Ink-jet printer • High resolution • appearance where characters are represented by sharp continuous lines • Print head consist of small holes or nozzles . Individual holes can be heated very rapidly(in few micro seconds)by an integrated circuit resistor • Tiny dots • Fair complex electronic system selects the holes to be heated

Laser printer • LASER PRINTER- Page printer • Which prints one page at a time • Laser beam source ,a multi sided motor ,photo conducive drum and tones • Laser beam is focused on the electro statically charged drum by spinning multisided mirror .the mirror focuses the laser beam on the surface of the drum in a manner to create the pattern of character/image to be printed on the page ADVANTAGE:• Very high quality(high resolution) • Very very tiny holes • 4 to 12 page per minute (very high speed) DISADVANTAGE:• Costlier than other printer




The interior of the tube is a very good vacuum, with a pressure of around 0.01 Pa (10−7 atm) or less. At any greater pressure, collisions of electrons with air molecules would scatter the electron beam excessively.

The cathode, at the left end in the figure, is raised to a high temperature by the heater, and electrons evaporate from the surface of the cathode.

The accelerating anode, with a small hole at its center, is maintained at a high positive potential V1, of the order of 1 to 20 kV, relative to the cathode. This potential difference gives rise to an electric field directed from right to left in the region between the accelerating anode and the cathode.

Electrons passing through the hole in the anode form a narrow beam and travel with constant horizontal velocity from the anode to the fluorescent screen.

The area where the electrons strike the screen glows brightly.

The control grid regulates the number of electrons that reach the anode and hence the brightness of the spot on the screen.

The focusing anode ensures that electrons leaving the cathode in slightly different directions are focused down to a narrow beam and all arrive at the same spot on the screen.

The assembly of cathode, control grid, focusing anode, and accelerating electrode is called the electron gun.

The beam of electrons passes between two pairs of deflecting plates. An electric field between the first pair of plates deflects the electrons horizontally, and an electric field between the second pair deflects them vertically.

If no deflecting fields are present, the electrons travel in a straight line from the hole in the accelerating anode to the center of the screen, where they produce a bright spot.


DIGITAL STORAGE OSCILLOSCOPE • The input signal is digitized and stored in memory in digital form. DRAW BACKS IN CRT • Storage occurs in electronic digital memory • To view the display on CRT the data from memory is reconstructed in analog form


DIGITAL STORAGE OSCILLOSCOPE Working • Input is amplified and attenuated with input amplifier • S/H snaps the picture(vertical and horizontal) • CRT accept only analog signal • So convert A/D



LIGHT EMITTING DIODE • LED are semiconductor p-n junctions that under forward bias conditions can emit radiation by electroluminescence in the UV, visible or infrared regions of the electromagnetic spectrum. • The quanta of light energy released is approximately proportional to the band gap of the semiconductor.

Advantages of Light Emitting Diodes (LEDs) • Low power consumption • Long life, Small size

LIQUID CRYSTAL DISPLAY • Lcd is a thin flat panel used for electronically displaying information such as text, images and moving pictures. • These compounds have a crystalline arrangement of molecules. • Liquid crystal displays do not emit or generate light but rather alter externally generated illumination.

LIQUID CRYSTAL DISPLAY  Viewing angle: ( more specifically known as viewing direction).  Color support: How many types of colors are supported ( more specifically known as color gamut).  Brightness: The amount of light emitted from the display (more specifically known as luminance).  Contrast ratio: The ratio of the intensity of the brightest bright to the darkest dark.  Aspect ratio: The ratio of the width to the height (for example, 4:3, 5:4, 16:9 or 16:10).

CLASSIFICATIONS OF LIQUID CRYSTALS • Smectic • Nematic • cholesteric

SMECTIC • Smectic is a layered structure with the molecules oriented parallel or tilted relative to the layer normal. • They are characterized by absence of positioned order with in the layers. • Both orientation order and positional order is present in the smectic crystals.

NEMATIC •One of the most common LC phases is the nematic ,where the molecules have no positional order, but they have long range orientation order. •Thus, the molecules flow and their centre of mass positions are randomly distributed as in a liquid, but they all point in the same direction. •Have fluidity similar to that of ordinary(isotropic) liquids but they can be easily aligned by an external magnetic or electric field.

CHOLESTERIC • In these crystals the rod like molecules in each layer are oriented at different angle with in each layer. • This phase exhibits a twisting of the molecules perpendicular to the direction with the molecular axis parallel to the director. • The optical activity of the crystal depends upon the orientation and the twist of the molecules as one goes from one layer to another.

TYPES OF LCD’S • Dynamic Scattering Displays • Field Effect Displays

Dynamic Scattering Display

• In the case of dynamic scattering display in the absence of an applied voltage the dives is transparent. But, when the voltage is applied, the liquid crystal becomes an efficient scatter of white light.

Construction • It consists of a pure nematic liquid crystal in which impurities are added to increase its conductivity, due to ions in them. • The liquid crystal is filled in between two glass plates which are coated with transparent tin oxide and act as electrodes. • The spacer and scalar help in filling of liquid and to change in liquid level.


• In the normal State, without application of electric field, all the molecules in the nematic liquid crystal are parallel to the glass plate. The materials is transparent. • When the electric field is applied, the dipoles of the molecules are rotated into alignment in the direction, exactly perpendicular to filed direction. • Now, the ions are pukked by the electrodes and hence the positive irons will go towards negative potential and vice versa.

Working • Therefore these ions (impurity) distrupt the molecules which are orderly arranged and create a small turbulence. Due to turbulence the negative charges are gathered as crests and positive charge are gathered as through, with respect to the field direction. • When the applied voltage is at critical value, the turbulence becomes more, which results in the variation of refractive index of the medium of the crystal. So, the light falling to the crystal is scattered and hence liquid appears dark in the white background. • When the field is switched OFF, the molecules are locally rearranged and material becomes transparent and the display is erased.


TYPICAL LIQUID CRYSTAL • In the absence of the electrical signal ,orientation order is maintained in the crystal allowing light to transmit. • This makes LCD displays clear. • The random orientation results scattering of light which lights display segment on a dark background.

Field effect LCD/The Twisted Nematic Crystal Display Construction •In this nematic phase, liquid crystal molecules are kept between two transparent glass plates. The top glass plate and the bottom glass plates are coated with a transparent electrode. The polarizer and analyzer are arranged in the crossed position, above and below the glass plates respectively.


• The top glass plate is rotated through 90 and hence the molecules are also twisted through 90. Without Field • The lights passed through the polarizer, then through the molecules and finally through the analyzer. • Here, the field of view appears bright, because the plane of polarization is rotated through 90 by the crossed position of polarizer and analyzer and another 90 by twister nematic molecules.

With field

With field • Now, then the field is applied, the twisted molecules come to normal position and there is only 90 rotation of plane of polarization between polarizer and analyzer. Therefore, there is no transmission of light and the display appears black in white back ground. • When the electric field is switched off, once again the molecules return back to their twisted position and so display is bright and erased.

APPLICATIONS Application of LCD  Used as numerical counters for counting production items.  Analog quantities like voltage, current etc, can also be displayed as a number on a suitable device. (e.g) Digital Multimeter.  Used for solid state video displays.  Used for image sensing circuits.  Used for numerical display in pocket calculators. Application of Display Devices • They are use in wrist-watches, clocks. • They are used in calculators, measuring instruments like digital multimeter. • Digital Balance also has application of display devices. • They are used in measuring the temperature.(Digital Thermometers) • They are used in Digital Gauss Meters. • They are used in large display units. • They are used in optical switching devices. • LEDs are used in dot matrix display units.

SEGMENTAL AND DOT MATRIX DISPLAY Two types:• Segmental gas discharge display • Segmental LCD and LED display

Segmental gas discharge display • • • •

Works on the principle of gas discharge glow Anode (common) Cathode(separate) Space between the anode and cathode contains the gas Working:• If the gas struck between cathode and anode of a chosen segment so that the cathode glow provides the illumination. All the numeric character can be displayed by activating the appropriate segments

Segmental gas discharge display Advantage:• Power consumption is low • Back electrode of thick film type series as cathode • Front electrode of thin film type series as anode. Disadvantage:• Required high voltage • Gas –neon

Segmental LCD and LED display • Employ a single LED for each segment • Common anode connection or common cathode connection • It is also possible to generate hexadecimal numeric characters and conventional using 7 segments or 14 or 16 segments LED display with a proper code converter


DOT MATRIX DISPLAY • 5*7,5*8,7*9 (R*C) • x-y array connection(economical and can be extended vertically or horizontally using a minimum number of wires


DATA LOGGERS Detailed operation :• Input signal • High ,low ,Ac signals • Pneumatic ,on/ off signal from switches relay etc Input scanner • The rate of scanning has to be matched with the rate of change of input data and time required by the recorder Signal conditioning • Amplify low level signals ,isolated filter to provide high levels Analog to digital converter:• Converts analog to digital signal

DATA LOGGERS Recorder • May be type writer ,digital recorder (CRO) etc. Programmer:• It is an automatic sequence switch which controls the operation of all other units of data loggers Operation Taken By Programmer • Set amplifier gain set high and low alarm limit .Initiate alarm for abnormal condition start A/D conversion ,display recording ,reset loggers

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