basic electronics

11
A light emitting diode (LED) is essentially a PN junction
opto-semiconductor that emits a monochromatic (single color) light
when operated in a forward biased direction.
LEDs convert electrical energy into light energy. They are
frequently used as "pilot" lights in electronic appliances to indicate
whether the circuit is closed or not.

12
About LEDs (1/2)
The most important part of a light emitting diode (LED) is the
semi-conductor chip located in the center of the bulb as shown at the
right. The chip has two regions separated by a junction. The p region
is dominated by positive electric charges, and the n region is
dominated by negative electric charges. The junction acts as a barrier
to the flow of electrons between the p and the n regions. Only when
sufficient voltage is applied to the semi-conductor chip, can the
current flow, and the electrons cross the junction into the p region.

13
How Does A LED Work? (1/2)
When sufficient voltage is applied to the
chip across the leads of the LED, electrons can
move easily in only one direction across the junction
between the p and n regions.
In the p region there are many more
positive than negative charges.
When a voltage is applied and the current
starts to flow, electrons in the n region have
sufficient energy to move across the junction into
the p region.

14
How Does A LED Work? (2/2)
Each time an electron recombines
with a positive charge, electric potential
energy is converted into electromagnetic
energy.
For each recombination of a negative
and a positive charge, a quantum of
electromagnetic energy is emitted in the
form of a photon of light with a frequency
characteristic of the semi-conductor material
(usually a combination of the chemical
elements gallium, arsenic and phosphorus)..

15
Testing LEDs
Never connect an LED directly to a
battery or power supply! It will be destroyed
almost instantly because too much current will
pass through and burn it out.
LEDs must have a resistor in series to
limit the current to a safe value, for quick
testing purposes a 1k resistor is suitable for
most LEDs if your supply voltage is 12V or less.
Remember to connect the LED the
correct way round!

16
Applications
• Sensor Applications
• Mobile Applications
• Sign Applications
• Automative Uses
• LED Signals
• Illuminations
• Indicators

17
Sensor Applications
 Medical Instrumentation
 Bar Code Readers
 Color & Money Sensors
 Encoders
 Optical Switches
 Fiber Optic Communication

18
Mobile Applications
 Mobile Phone
 PDA's
 Digital Cameras
 Lap Tops
 General Backlighting

19
Sign Applications
 Full Color Video
 Monochrome Message Boards
 Traffic/VMS
 Transportation - Passenger Information

20
Automative Applications
 Interior Lighting - Instrument Panels & Switches, Courtesy Lighting
 Exterior Lighting - CHMSL, Rear Stop/Turn/Tail
 Truck/Bus Lighting - Retrofits, New Turn/Tail/Marker Lights

21
Signal Appications
 Traffic
 Rail
 Aviation
 Tower Lights
 Runway Lights
 Emergency/Police Vehicle Lighting
LEDs offer enormous benefits over traditional incandescent lamps
including:
 Energy savings (up to 85% less power than incandescent)
 Reduction in maintenance costs
 Increased visibility in daylight and adverse weather conditions

22
Illumination (1/2)
Architectural Lighting
Signage (Channel Letters)
Machine Vision
Retail Displays
Emergency Lighting (Exit Signs)
Neon Replacement
Bulb Replacements
Flashlights
Outdoor Accent Lighting - Pathway, Marker Lights

23
Illumination (2/2)
LEDs not only consume far less electricity than traditional
forms of illumination, resulting in reduced energy costs, but require
less maintenance and repair. Studies have shown that the use of LEDs
in illumination applications can offer:
Greater visual appeal
Reduced energy costs
Increased attention capture
Savings in maintenance and lighting replacements
As white LED technology continues to improve, the use of
LEDs for general illumination applications will become more prevalent
in the industry.

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Indication
 Household appliances
 VCR/ DVD/ Stereo and other audio and video devices
 Toys/Games
 Instrumentation
 Security Equipment
 Switches

25
Driving LEDs
 Analog LED Drive Circuits
 Digital LED Drive Circuits

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Colours of LEDs (1/3)
LEDs are available in red, orange, amber, yellow, green, blue and
white. Blue and white LEDs are much more expensive than the other
colours. The colour of an LED is determined by the semiconductor
material, not by the colouring of the 'package' (the plastic body). LEDs of
all colours are available in uncoloured packages which may be diffused
(milky) or clear (often described as 'water clear'). The coloured packages
are also available as diffused (the standard type) or transparent.
LEDs are made from gallium-based
crystals that contain one or more additional
materials such as phosphorous to produce a
distinct color. Different LED chip
technologies emit light in specific regions
of the visible light spectrum and produce
different intensity levels.

27
Tri-colour LEDs
The most popular type of tri-colour LED has a red and a green
LED combined in one package with three leads. They are called tri-
colour because mixed red and green light appears to be yellow and this
is produced when both the red and green LEDs are on.

28
Bi-colour LEDs
A bi-colour LED has two LEDs wired in
'inverse parallel' (one forwards, one
backwards) combined in one package with two
leads. Only one of the LEDs can be lit at one
time and they are less useful than the tri-
colour LEDs described above.

29
LED Performance (1/8)
 Color
 White light
 Intensity
 Eye safety information
 Visibility
 Operating Life
 Voltage/Design Current
LED performance is based on a few primary characteristics:

30
Colour
Peak wavelength is a function of the LED chip material.
Although process variations are ±10 NM, the 565 to 600 NM
wavelength spectral region is where the sensitivity level of the
human eye is highest. Therefore, it is easier to perceive color
variations in yellow and amber LEDs than other colors.

31
White Light
When light from all parts of the visible spectrum overlap one
another, the additive mixture of colors appears white. However, the
eye does not require a mixture of all the colors of the spectrum to
perceive white light. Primary colors from the upper, middle, and
lower parts of the spectrum (red, green, and blue), when combined,
appear white.

32
Intensity
LED light output varies with the type of chip, encapsulation,
efficiency of individual wafer lots and other variables. Several LED
manufacturers use terms such as "super-bright," and "ultra-bright“ to
describe LED intensity. Such terminology is entirely subjective, as
there is no industry standard for LED brightness.

33
Eye Safety
The need to place eye safety labeling on LED products is
dependent upon the product design and the application. Only a few
LEDs produce sufficient intensity to require eye safety labeling.
However, for eye safety, do not stare into the light beam of any LED
at close range

34
Visibility
Luminous intensity (Iv) does not represent the total light
output from an LED. Both the luminous intensity and the spatial
radiation pattern (viewing angle) must be taken into account. If two
LEDs have the same luminous intensity value, the lamp with the larger
viewing angle will have the higher total light output.

35
Operating Life
Because LEDs are solid-state devices they are not subject to
catastrophic failure when operated within design parameters. DDP®
LEDs are designed to operate upwards of 100,000 hours at 25°C
ambient temperature. Operating life is characterized by the
degradation of LED intensity over time. When the LED degrades to half
of its original intensity after 100,000 hours it is at the end of its useful
life although the LED will continue to operate as output diminishes.
Unlike standard incandescent bulbs, DDP® LEDs resist shock and
vibration and can be cycled on and off without excessive degradation.

36
Voltage/Design Current
LEDs are current-driven devices, not voltage driven. Although
drive current and light output are directly related, exceeding the
maximum current rating will produce excessive heat within the LED chip
due to excessive power dissipation. The result will be reduced light
output and reduced operating life.
LEDs that are designed to operate at a specific voltage contain a
built-in current-limiting resistor. Additional circuitry may include a
protection diode for AC operation or full-bridge rectifier for bipolar
operation. The operating current for a particular voltage is designed to
maintain LED reliability over its operating life.

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Bargraph 7-segment Starburst Dot matrix
Some Types of LEDs

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1. What’s Liquid Crystals (LC)
 intermediary substance between a liquid and solid
state of matter.
e.g. soapy water
 light passes through liquid crystal changes when it is
stimulated by an electrical charge.

41
2. Introduction to Liquid Crystal Displays
 Consists of an array of tiny segments (called
pixels) that can be manipulated to present
information.
 Using polarization of lights to display objects.
 Use only ambient light to illuminate the
display.
 Common wrist watch and pocket calculator to
an advanced VGA computer screen

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Different types of LCDs
Passive Matrix LCDs (AMLCD) and
Active Matrix LCDs (AMLCD)
 Passive Twisted Nematic Displays (TNLCD)
 Super Twisted nematic LCD (STNLCD)
 Thin Film Transistor LCD (TFT LCD)
 Reflective LCD
 Rear Projection LCD

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3. Operating Principle
 The parallel arrangement of liquid crystal molecules
along grooves
 When coming into contact with grooved surface in a
fixed direction, liquid crystal molecules line up parallel
along the grooves.

44
Molecules movement
Offline (no voltage is applied)
 Along the upper plate : Point
in direction 'a'
 Along the lower plate : Point
in direction 'b‘
 Forcing the liquid crystals into
a twisted structural
arrangement. (Resultant
force)

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Light movement
Offline (no voltage is applied)
 Light travels through the
spacing of the molecular
arrangement.
 The light also "twists" as it
passes through the twisted
liquid crystals.
 Light bends 90 degrees as it
follows the twist of the
molecules.
 Polarized light pass through
the analyzer (lower polarizer).

46
Molecules movement
Online (voltage is applied)
 Liquid crystal molecules
straighten out of their helix
pattern
 Molecules rearrange
themselves vertically (Along
with the electric field)
 No twisting thoughout the
movement
 Forcing the liquid crystals into
a straight structural
arrangement. (Electric force)

47
Light movement
Online (voltage is applied)
 Twisted light passes straight
through.
 Light passes straight through
along the arrangement of
molecules.
 Polarized light cannot pass
through the lower analyzer
(lower polarizer).
 Screen darkens.

48
Offline Online
Sequences of offline and
online mode
Offline
1. Surrounding light is
polarized on the upper
plate.
2. Light moves along with
liquid crystals and twisted
at right angle.
3. Molecules and lights are
parallel to the lower
analyzer.
4. Light passes through the
plate.
5. Screen appear
transparent.

49
Sequences of offline and
online mode
Online
1. Surrounding light is
polarized on the upper
plate.
2. Light moves along with
liquid crystals which moves
straight along the electric
field.
3. Molecules and lights are
perpendicular to the lower
analyzer.
4. Light cannot pass through
the plate.
5. Screen appear dark.
Offline Online

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Polarization of light
 When unpolarized light passes through a polarizing filter, only
one plane of polarization is transmitted. Two polarizing
filters used together transmit light differently depending on
their relative orientation.
Online Offline

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Construction of
Liquid Crystal Display
 Two bounding plates (usually glass
slides), each with a transparent
conductive coating (such as
indium tin oxide) that acts as an
electrode;
 A polymer alignment layer :
undergoes a rubbing process as
grooves.
 Spacers to control the cell gap
precisely;
 Two crossed polarizers (the
polarizer and the analyzer);
 Polarizers are usually perpendicular
to each other.

52
Properties of LCD Display
 Small footprint (approx 1/6 of CRT)
 Light weight (typ. 1/5 of CRT)
 power consumption (typ. 1/4 of CRT)
 Completely flat screen - no geometrical
errors
 Crisp pictures - digital and uniform colors
 No electromagnetic emission
 Fully digital signal processing possible
 Large screens (>20 inch) on desktops
 High price (presently 3x CRT)
 Poor viewing angle (typ. 50 degrees)
 Low contrast and luminance (typ. 1:100)
 Low luminance (typ. 200 cd/m2)
Maximum luminosity : 50%
of CRT as 50% of light is
blocked by the upper
polarizer.

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Advantage of LCD over CRT
 Smaller size—AMLCDs occupy approximately 60
percent less space than CRT displays—an
important feature when office space is limited.
 Lower power consumption—AMLCDs typically
consume about half the power and emit much
less heat than CRT displays.
 Lighter weight—AMLCDs weigh approximately
70 percent less than CRT displays of comparable
size.
 No electromagnetic fields—AMLCDs do not
emit electromagnetic fields and are not
susceptible to them. Thus, they are suitable for
use in areas where CRTs cannot be used.
 Longer life—AMLCDs have a longer useful life
than CRTs; however, they may require
replacement of the backlight.
Maximum luminosity : 50%
as 50% of light is blocked by
the upper polarizer.

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4. Display Addressing
 Addressing is the process by which pixels are turned on and off
in order to create an image.
 There are two main types of addressing, direct and
multiplexing.
 Direct addressing is convenient for displays where there are
only a few elements that have to be activated. With direct
addressing, each pixel in the display has its own drive
circuit. A microprocessor must individually apply a voltage to
each element. A common application of direct addressing is the
traditional seven segment liquid crystal display, found in
wristwatches and similar devices.

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 In multiplex addressing, a larger number of pixels are
involved. When the elements are in a regular order, they can
be addressed by their row and column instead of each
element being driven separately. This reduces the complexity
of the circuitry because each pixel no longer needs its own
driver circuit.
 If you have a 10x10 matrix of pixels, with direct addressing, you
need 100 individual drivers. However, if you use multiplex
addressing, you only need 20 drivers, one for each row and
one for each column.
 This is a tremendous advantage, especially as displays
become larger and larger.

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Optical Response
 twisted nematic displays can switch between light and dark states,
or somewhere in between (grayscale).
 Electro-distortional curve is shown as follows :
 the electro-distortional response determines the transmission of
light through the cell.
 Different light intensity of an image projected on the screen is
determined by different voltage supply. Thus the level of blocking
of light may vary.

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5. Applications
A) Thin Film Transistor (TFT)
 Constructed on a glass surface using a photolithographic
process.
 The source and gate are the control electrodes. The drain
electrode connects to the liquid crystal pixel. The thin layer
of amorphous silicon is the semiconducting material that
allows the TFT to function. The capacitor is attached to the
pixel electrode, but is not an integral part of the TFT.

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5. Applications
B) Alpha-numeric display
 Digital letters can be displayed by blocking the lights in
different plates we place.
 For applications such as digital watches and calculators,
a mirror is used under the bottom polarizer. With no
voltage applied, ambient light passes through the cell,
reflects off the mirror, reverses its path, and re-emerges
from the top of the cell, giving it a silvery appearance.
 When the electric field is on, the aligned LC molecules do
not affect the polarization of the light. The analyzer
prevents the incident light from reaching the mirror and
no light is reflected, causing the cell to be dark. When the
electrodes are shaped in the form of segments of
numbers and letters they can be turned on and off to form
an alpha-numeric display.

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5. Applications
C) Back lighting systems
 Alpha-numeric displays are not very bright because the
light must pass through multiple polarizers which
severely cut down on the intensity of the light, in
addition to the various layers of the display which are only
semi-transparent. Therefore a more intense source is
employed in the form of a back lighting system.
For brighter displays
 Light bulbs mounted behind
 At the edges of the display replace the reflected ambient
light.
 Disadvantage : very power intensive. Back lighting
systems are used in more complex displays such as laptop
computer screens, monitors, LCD projectors, pda,
digital devices such as digital camera and DV.

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