Types of LCD
LCD element form the pixels in LCD screen.
The LCD elements are switched on and off by different techniques to
display an image on the screen.
There are two types of LCD screens: passive-matrix and active-matrix.
Passive Matrix LCD
Each LCD element in placed between two electrodes
and arranged in rows and columns in X-Y matrix
The bottom electrodes of each row elements are
connected together and the top electrodes of each
column elements are connected together.
The elements in a row are controlled by a row-driver
and the elements in a column by a column-driver.
Activating a row-driver and a column driver results in
a current through a specific LCD element.
Passive Matrix LCD (continue)
An image is created on the screen by activating a row
driver and activating the column drivers in sequence like
scanning a line.
The drawback of this simple design is that if a current
through a cell is large, then adjacent elements, often those
in the same column, also get affected and this results in
If the current through an element is too weak, the
elements don’t switch on and off very quickly which results
in poor contrast and losing detail in moving images.
The dual scan technique improves the performance.
Active matrix LCD
AMLCDs use one transistor driver for each LCD element.
The transistors are arranged in the matrix format.
It allows the current through individual elements to be switched on
and off quickly with sufficient current to get good contrast.
The electrodes for active matrix panels are also made of the
The transistor drivers are fabricated right on the panel substrate,
using thin film technology.
As the transistors are able to switch the elements more rapidly than
the passive matrix scheme and adjacent elements are not affected,
large currents can be used without ghosting.
The higher currents provide higher contrast and sharper and
Techniques for solving
Viewing Angle Problem
The simplest technique uses a diffusing film on the top layer of the
It causes the light traveling in specific direction to divert in different directions.
In the second technique, instead of passing the current from top
through the liquid crystal to bottom
The electrodes are arranged in such a way that the current flows from side to side.
It causes the molecules to align in horizontal direction, which results in wider viewing
In the third technique, each liquid crystal element is divided into
The substrate is pre-tilt in different directions so that the molecules align in different
It causes the light to emerge different directions, which results in wider viewing angles.
Printers are the primary output devices used to prepare
permanent documents for human use.
Printing output is called hard copy output.
There are two types of printers −
Three different types of printers are in use today. They are-
Ink-Jet printer and
Impact printers print the characters by striking them on
the ribbon, which is then pressed on the paper.
These printers are of two types-
Character printers (Dot Matrix Printer(DMP), Daisy Wheel)
Line printers (Drum Printer, Chain Printer)
Characteristics of Impact Printers-
Very low consumable costs
Useful for bulk printing due to low cost
There is physical contact with the paper to produce an image
Non-impact printers print the characters without using
the ribbon. These printers print a complete page at a time,
thus they are also called as Page Printers.
These printers are of two types-
Characteristics of Non-impact Printers-
Faster than impact printers
They are not noisy
A dot matrix printer has a printer head, which contains an array
of 7, 9, 14, 18 or 24 pins arranged in a vertical column.
While printing, the pins forming one column of dot pattern of
the character matrix project from the print head and press the
inked ribbon against a plain paper.
Only one column of dot pattern of character matrix is printed at
As the print head moves across the line, series of vertical dot
patterns representing the character are retrieved from the
printer’s internal memory and printed.
Dot-Matrix Printer (continue)
The print head is mounted on a carriage and
moved back and forth along a rail by a belt.
The dot-matrix printers receive stream of
character from the computer, convert to series of
column of dot patterns and print.
The print quality depends on the number of pins
provided on the print head.
24-pin printers produce high quality printouts.
The operation of the ink-jet printer is similar to that of a
Ink-jet printers have tiny nozzles on the print head that
spray ionized ink onto a plain paper in the pattern to form
the characters and images.
The print head cartridge has 50 or more nozzles and is
mounted on a carriage that is carried back and forth along
a rail by a belt that is driven by a motor.
The print head is connected to the electronic control unit
through a long, flat, flexible print head cable.
Ink-Jet Printer (continue)
When the PC sends a character to be printed, a series of vertical dot patterns representing
the character is retrieved from the printer’s internal memory and sent through a series of
print nozzle driver circuits.
The driver amplifies the signals into the fast, high-energy pulses needed to fire minute
droplets of ink from each nozzle.
As the pulse reaches the nozzle, it creates an intense surge of heat that literally creates a
The nozzle cools down in a fraction of a second and becomes ready for another pulse.
The print quality of ink-jet printer depends on the quality of paper also.
They require periodic maintenance, as they use liquid ink.
The ink may dry in the nozzles and clog them.
Laser-jet printers function like a plain paper copier.
While a bright light source is used in a plain paper copier
for generating the images, a laser is used in laser printer.
The laser printer creates an electrostatic image of a text or
graphics page on a photosensitive drum using the laser
When the drum is moved across the toner, it attracts the
toner particles and deposits on the electrostatic image.
Laser-jet printer (continue)
The drum is then pressed against a plain paper to transfer the
image on to the paper.
The paper is then passed between hot rollers to fuse the toner
particles into the paper.
The area of the drum is the same as that of the paper onto which
the image is transferred.
Every point on the drum corresponds to a point on the paper.
Laser printers are very popular today because of their print
quality and relatively low running cost.
The print quality is less sensitive to paper quality.
Laser-jet printer (continue)
Laser printers get all the information about
the page to be printed and keep them in their
internal memory before starting the printing.
The amount of memory present in a printer
determines its capability.
Daisy Wheel Printer
Head is lying on a wheel and pins corresponding to characters are like petals
of Daisy (flower) which is why it is called Daisy Wheel Printer.
These printers are generally used for word-processing in offices that require
a few letters to be sent here and there with very nice quality.
More reliable than Dot Matrix Printer (DMP)
Fonts of character can be easily changed
Slower than DMP
More expensive than DMP
The line printer is an impact printer that prints one
entire line of text at a time.
It is mostly associated with unit record equipment and
the early days of digital computing, but the technology is
still in use.
Line printers are of two types −
This printer is like a drum in shape hence it is called drum printer. The
surface of the drum is divided into a number of tracks. Total tracks are equal
to the size of the paper, i.e. for a paper width of 132 characters, drum will have
Different character sets available in the market are 48 character set, 64 and
96 characters set.
Drum printers are fast in speed and can print 300 to 2000 lines per minute.
Very high speed
Characters fonts cannot be changed
In this printer, a chain of character sets is used,
hence it is called Chain Printer. A standard
character set may have 48, 64, or 96 characters.
Character fonts can easily be changed.
Different languages can be used with the same printer.