1.1 Resistors
A resistor is an electrical component
that limits or regulates the flow of electrical current in an electronic
circuit. Resistors can also be used to provide a specific voltage for an active
device such as a transistor.
When a resistor is introduced to a
circuit the flow of current is reduced. The higher the value of the resistor is
the smaller/lower the flow of current. It produces a voltage across it’s
terminals that is proportional to the current passing through it in accordance
with Ohm’s law,
The symbol used in schematic and
electrical drawings for a resistor can either be a ‘zigzag’ type line (US,
Japan) or a rectangular box (Europe).
Figure 2.1:
Standard resistor symbols
2.1.1 Unit
Ohm (symbol: ?) is the SI unit of
electrical resistance, named after Georg Simon Ohm. Commonly used multiples and
submultiples in electrical and electronic usage are the milliohm (1 m? = 10-3
?), kilo-ohm (1 k? = 103 ?) and mega-ohm (1 M? = 106?).
2.1.2 Classification of Resistors
The primary characteristics of a
resistor are the resistance, tolerance, maximum working voltage and power
rating. Other characteristics include temperature coefficient, noise,
inductance and capacitance.
Although resistors come in various forms
they can be divided into two basic types —
a) Fixed resistors
b) Variable
resistors or Potentiometers
Fixed resistors: A fixed
resistor is a component with two wires which obeys Ohm’s law. Electronic engineers
and manufacturers have adopted some standards for fixed resistors.
Variable resistors: Variable
resistors are usually a stubby cylindrical shape with a rod poking out one end
and with three metal tags.
(a) Fixed
resistors (b)
Variable resistor
Figure 2.2: Two
basic types of resistors
All modern resistors can be classified
into four broad groups —
Carbon Composition Resistor: Made of carbon
dust or graphite paste, low wattage values.
Film or Cermet Resistor: Made from
conductive metal oxide paste, very low wattage values.
Wire-wound Resistor: Metallic bodies
for heat sink mounting, very high wattage ratings.
Semiconductor Resistor: High
frequency/precision surface mount thin film technology.
(a) Carbon Composite
Resistor (b) Film
or Cermet Resistor
(c) Wire-wound
Resistor (d) Semiconductor Resistor
Figure 2.3:
Different types of resistors
2.1.3 Function of Resistors
As we mentioned before the behavior of
an ideal resistor is dictated by the relationship specified in Ohm’s law:
Ohm’s law states that the voltage (V)
across a resistor is proportional to the current (I), where the constant of
proportionality is the resistance (R). Equivalently, Ohm’s law can be stated
as:
This formulation of Ohm’s law states
that, when a voltage (V)is maintained across a resistance (R), a current (I)
will flow through the resistance.
This formation is often used in
practice. For example, if V is 12 volts and R is 400 ohms, a current of
12/400 = 0.03 amperes will flow through the resistance R.
Resistors in a parallel configuration
have the same potential difference (voltage). To find their total equivalent
resistance (Req),
Figure 2.5: Parallel
Connection of Resistors
The parallel property can be represented
in equations by two vertical lines “||” (as in geometry) to simplify equations.
For two resistors,
The current through resistors in series
stays the same, but the voltage across each resistor can be different. The sum
of the potential differences is equal to the total voltage. To find their total
resistance (Req),
Figure 2.6:
Series connection of Resistors
A resistor network that is a combination
of parallel and series can be broken up into smaller parts that are either one
of the other. For instance,
Figure 2.7:
Combination of series and parallel connection
The practical application to resistors
is that a resistance of any non-standard value can be obtained by connecting
standard values in series or in parallel.
2.1.4 Power Dissipation
The power P dissipated by a resistor (or
the equivalent resistance of a resistor network) is calculated as:
The first form is a restatement of
Joule’s first law. Using Ohm’s law, the two other forms can be derived.
The total amount of heat energy released
over a period of time can be determined from the integral of the power over
that period of time:
If the average power dissipated by a
resistor is more than its power rating, damage to the resistor may occur,
permanently altering its resistance; this is distinct from the reversible
change in resistance due to its temperature coefficient when it warms.
Excessive power dissipation may raise the temperature of the resistor to a
point where it can burn the circuit board or adjacent components, or even cause
a fire. There are flameproof resistors that fail (open circuit) before they
overheat dangerously.
2.2 Capacitor
The capacitor is a passive electronic
component that holds a charge in the form of an electrostatic field. They are
often used in combination with transistors in DRAM, acting as storage cells to
hold bits.
Capacitors typically consist of
conducting plates separated by thin layers of dielectric material, such as dry
air or mica. The plates on opposite sides of the dielectric material are
oppositely charged and the electrical energy of the charged system is stored in
the polarized dielectric.
When a voltage is applied across the two
plates of a capacitor a concentrated field flux is created between them,
allowing a significant difference of free electrons (a charge) to develop
between the two plates.
Figure 2.8:
Different types of capacitors
The schematic symbol for a capacitor is
two short parallel lines (representing the plates) separated by a gap
(dielectric). At that two parallel lines (plates) are attached two pins for
connection to other components.
Figure 2.9:
Capacitor symbols
2.2.1 Unit
The unit of capacitor is farad (F).It is
named after Michael Faraday, a nineteenth century English chemist and
physicist.
A capacitor has a capacitance of 1 farad
if 1 coulomb of charge is deposited on the plates by a potential difference of
1 volt across the plates. The farad, however, is generally too large a measure
of capacitance for most practical applications. So microfarad (10-6 F)
or picofarad (10-12 F) is more commonly used.
2.2.2 Types of Capacitor
There are a very large variety of
different types of capacitor available in the market place and each one has its
own set of characteristics and applications from small delicate trimming
capacitors up to large power metal-can type capacitors used in high voltage
power correction and smoothing circuits. Like resistors, there are also
variable types of capacitors which allow us to vary their capacitance value for
use in radio or “frequency tuning” type circuits. Capacitor types are
—
Film Capacitor: Film Capacitors
are the most commonly available of all types of capacitors, consisting of a
relatively large family of capacitors with the difference being in their
dielectric properties. These include polyester (Mylar), polystyrene,
polypropylene, polycarbonate, metallized paper, Teflon etc. Film type
capacitors are available in capacitance ranges from as small as 5pF to as large
as 100uF depending upon the actual type of capacitor and its voltage rating.
Figure 2.10:
Film capacitors (Radial Lead type and Axial Lead type)
Ceramic Capacitors: Ceramic
Capacitors are made by coating two sides of a small porcelain or ceramic disc
with silver and are then stacked together to make a capacitor. Ceramic
capacitors have a high dielectric constant (High-K) and are available so that
relatively high capacitances can be obtained in a small physical size. Ceramic
capacitors have values ranging from a few picofarads to one or two microfarads
but their voltage ratings are generally quite low.
Electrolytic Capacitors: Electrolytic
capacitors are high voltage capacitors. The majority of electrolytic types of
capacitors are polarized. These are generally used in DC power supply circuits
due to their large capacitances and small size to help reduce the ripple
voltage or for coupling and decoupling applications. One main disadvantage of
electrolytic capacitors is their relatively low voltage rating.
Figure 2.11:
Electrolytic capacitor
Dielectric Capacitor: Dielectric Capacitors are usually of the variable
type were a continuous variation of capacitance is required for tuning
transmitters, receivers and transistor radios. Variable dielectric capacitors
are multi-plate air-spaced types that have a set of fixed plates (the stator
vanes) and a set of movable plates (the rotor vanes) which move in between the
fixed plates. The position of the moving plates with respect to the fixed
plates determines the overall capacitance value. The capacitance is generally
at maximum when the two sets of plates are fully meshed together. High voltage
type tuning capacitors have relatively large spacing or air-gaps between the
plates with breakdown voltages reaching many thousands of volts.
Figure 2.12:
Dielectric capacitor
2.2.3 Capacitor Characteristics
There are a bewildering array of
capacitor characteristics and specifications associated with the humble
capacitor, so here are just a few of the more important ones —
1. Working Voltage, (WV): The working
voltage is the maximum continuous voltage either DC or AC that can be applied
to the capacitor without failure during its working life. DC and AC voltage
values are usually not the same for a capacitor as the AC voltage value refers
to the R.M.S. value. Common working DC voltages are 10V, 16V, 25V, 35V, 50V,
63V, 100V, 160V, 250V, 400V and 1000V and are printed onto the body of the
capacitor.
2. Tolerance, (±%): As with
resistors, capacitors also have a tolerance rating expressed as a plus-or-minus
value either in picofarads (±pF) for low value capacitors, generally less than
100pF or as a percentage (±%) for higher value capacitors, generally higher
than 100pF. Capacitors are rated according to how near to their actual values
they are compared to the rated nominal capacitance with colored bands or
letters used to indicated their actual tolerance. The most common tolerance
variation for capacitors is 5% or 10% but some plastic capacitors are rated as
low as ±1%.
3. Leakage Current: The dielectric
used inside the capacitor to separate the conductive plates is not a perfect
insulator resulting in a very small current flowing or “leaking”
through the dielectric due to the influence of the powerful electric fields
built up by the charge on the plates when applied to a constant supply voltage.
This small DC current flow in the region of nano-amps (nA) is called the
capacitors Leakage Current. It is a result of electrons physically making their
way through the dielectric medium, around its edges or across its leads and
which will over time fully discharging the capacitor if the supply voltage is
removed. The film/foil type capacitor has extremely low leakage currents while
the leakage current of aluminium electrolytics increases with temperature.
4. Working Temperature, (T): Changes in
temperature around the capacitor affect the value of the capacitance because of
changes in the dielectric properties. If the air or surrounding temperature
becomes too hot or too cold the capacitance value of the capacitor may change
so much as to affect the correct operation of the circuit. The normal working
range for most capacitors is -30°C to +125°C with nominal voltage ratings given
for a Working Temperature of no more than +70°C. Generallyelectrolytics cannot be
used below about -10°C, as the electrolyte jelly freezes.
5. Temperature Coefficient, (TC): The Temperature
Coefficient of a capacitor is the maximum change in its capacitance over a
specified temperature range. It is generally expressed linearly as parts per
million per degree centigrade (PPM/°C), or as a percent change over a
particular range of temperatures. Some capacitors are nonlinear (Class 2
capacitors) and increase their value as the temperature rises giving them a
temperature coefficient that is expressed as a positive “P”. Some
capacitors decrease their value as the temperature rises giving them a
temperature coefficient that is expressed as a negative “N”. For example “P100” is +100 ppm/°C
or “N200”, which is -200 ppm/°C etc. However, some capacitors do not
change their value and remain constant over a certain temperature range; such
capacitors have a zero temperature coefficient or “NPO”.
6. Polarization: Polarization
generally refers to the electrolytic type capacitors but mainly the aluminiumelectrolytics,
with regards to their electrical connection. The majority are polarized types,
that are the voltage connected to the capacitor terminals must have the correct
polarity, i.e. positive to positive and negative to negative. Incorrect polarization can cause the oxide
layer inside the capacitor to break down resulting in very large currents flowing
through the device resulting in destruction.
7. Equivalent Series Resistance, (ESR): The Equivalent
Series Resistance, or ESR, of a capacitor is the AC impedance of the capacitor
when used at high frequencies and includes the resistance of the dielectric
material, the DC resistance of the terminal leads, the DC resistance of the
connections to the dielectric and the capacitor plate resistance all measured
at a particular frequency and temperature. The ESR of electrolytic capacitors
increases over time as their electrolyte dries out. Capacitors with very low
ESR ratings are available and are best suited when using the capacitor as a
filter.
2.2.4 Theory of Operation
The capacitor is a reasonably general
model for electric fields within electric circuits. An ideal capacitor is
wholly characterized by a constant capacitance C, defined as the ratio of
charge ±Q on each conductor to the voltage V between them,
Sometimes charge build-up affects the
capacitor mechanically, causing its capacitance to vary. In this case,
capacitance is defined in terms of incremental changes,
Work must be done by an external
influence to “move” charge between the conductors in a capacitor.
When the external influence is removed the charge separation persists in the
electric field and energy is stored to be released when the charge is allowed
to return to its equilibrium position. The work done in establishing the
electric field, and hence the amount of energy stored, is given by,
The current i(t) through any
component in an electric circuit is defined as the rate of flow of a charge q(t)
passing through it, but actual charges, electrons, cannot pass through the
dielectric layer of a capacitor, rather an electron accumulates on the negative
plate for each one that leaves the positive plate, resulting in an electron
depletion and consequent positive charge on one electrode that is equal and
opposite to the accumulated negative charge on the other. Thus the charge on
the electrodes is equal to the integral of the current as well as proportional
to the voltage as discussed above. As with any anti-derivative, a constant of
integration is added to represent the initial voltage v(t0).
This is the integral form of the capacitor equation,
Taking the derivative of this, and
multiplying by C, yields the derivative form,
Diode
Diodes are semiconductor devices which
might be described as passing current in one direction only. In electronics, a
diode is a two-terminal electronic component that conducts electric current in
only on direction.
Diodes can be used as voltage
regulators, tuning devices in rf tuned circuits, frequency multiplying devices
in rf circuits, mixing devices in rf circuits, switching applications or can be
used to make logic decisions in digital circuits.
2.3.1 Function of a Diodes
The main function of a diode is to block
the current in one direction and allow current to flow in the other direction.
Current flowing through the diode is called forward current.
2.3.2 Types of Diodes
There are several types of diodes. Such
as —
Rectifier diodes: These are the
most common type of diodes. They are mainly used to allow flow of current in
one direction and by doing that they can convert AC to DC.
Detector diodes: These are more
sensitive than normal rectifier diodes. They are used in radios and televisions
to convert radio signals to audio or television signals.
Zenner diodes: These diodes
are the opposite of the normal diodes, because they are designed to conduct
current in the backwards direction but only at a very precise voltage.
Capacitance diodes: it act as
tunable capacitance and are also used in radios and TVs to allow electronic
automatic tuning.
2.3.3 Current-voltage Characteristics
The graph below shows the electrical
characteristics of a typical diode —
Figure 2.14:
Diode characteristic graph
When a small voltage is applied to the
diode in the forward direction, current flows easily. Because the diode has a
certain amount of resistance, the voltage will drop slightly as current flows
through the diode. A typical diode causes a voltage drop of about 0.6 – 1V (VF)
(In the case of silicon diode, almost 0.6V). This voltage drop needs to be
taken into consideration in a circuit which uses many diodes in series. Also,
the amount of current passing through the diodes must be considered. When
voltage is applied in the reverse direction through a diode, the diode will
have a great resistance to current flow. Different diodes have different
characteristics when reverse-biased. A given diode should be selected depending
on how it will be used in the circuit. The current that will flow through a
diode biased in the reverse direction will vary from several mA to just µA,
which is very small.
2.3.4 Application of Diodes
Some ways in which the diode can be used
are listed here.
·
A
diode can be used as a rectifier that converts AC (Alternating Current) to DC
(Direct Current) for a power supply device.
·
Diodes
can be used to separate the signal from radio frequencies.
·
Diodes
can be used as an on/off switch that controls current.
Full Wave Rectifier
In a full wave
rectifier circuit two diodes are used, one for each half of the cycle. A
transformer is used whose secondary winding is split equally into two halves
with a common center tapped connection, (C). This configuration results in each
diode conducting in turn when its anode terminal is positive with respect to
the transformer center point C producing an output during both half-cycles.
The full wave rectifier circuit consists
of two power diodes connected to a single loadresistance (RL) with
each diode taking it in turn to supply current to the load. When point A of the
transformer is positive with respect to point C, diode D1 conducts
in the forward direction as indicated by the arrows. When point B is positive
(in the negative half of the cycle) with respect to point C, diode D2
conducts in the forward direction and the current flowing through resistor R is
in the same direction for both half-cycles. As the output voltage across the
resistor R is the phasor sum of the two waveforms combined, this type of full
wave rectifier circuit is also known as a “bi-phase” circuit.
As the spaces between each half-wave
developed by each diode is now being filled in by the other diode the average
DC output voltage across the load resistor is now double that of the single
half-wave rectifier circuit and is about
0.637Vmax of the peak
voltage, assuming no losses.
Where: Vmax is the maximum
peak value in one half of the secondary winding and VRMS is the rms
value.
The peak voltage of the output waveform
is the same as before for the half-wave rectifier provided each half of the
transformer windings have the same rms voltage value. To obtain a different DC
voltage output different transformer ratios can be used.
2.4 Voltage Transformer
A transformer is a device that transfers
electrical energy from one circuit to another through inductively coupled
conductors (the transformer’s coils). The main purpose of a voltage transformer
is to transfer electrical power between different circuits by converting one AC
voltage source into another AC voltage at the same frequency. Transformers are
basically mechanical devices that consist of one or more coil(s) of wire
wrapped around a common ferromagnetic laminated core. These coils are usually
not electrically connected together however, they are connected magnetically
through the common magnetic flux Qm confined to a central core.
|
Figure 2.16: Typical |
Voltage
transformers work on Faraday’s principal of electromagnetic induction. When a
current flows through a coil, a magnetic flux (?) is produced around the coil.
If we now place a second similar coil next to the first so that this magnetic
flux cuts the second coil of wire, an e.m.f. voltage will be induced in the
second coil. This effect is known as “mutual induction” and is the basic
operation principal of voltage transformers.
The value of the induced e.m.f in the
second coil is proportional to the number of turns and to the rate of change of
magnetic flux. In a voltage transformer these two coils known as the primary
and secondary windings are tightly wrapped around a single core material such
as steel or iron which improves the magnetic coupling between these two coils.
Therefore, each coil has the same number of volts per turn in it producing two
different voltages that are proportional to each other.
2.4.1 Single Phase Voltage Transformer
Figure 2.17: Single phase voltage transformer
The single-phase voltage transformer has
two coils or windings, a primary winding and a secondary winding that are not
in electrical contact with each other. When an electric current passed through
the primary winding, a magnetic field is developed which induces a voltage into
the secondary winding.
Voltage Transformers alter both voltage
and current of AC waveforms. The voltage induced in the secondary winding can
be greater or lower than the voltage in primary winding. As each winding has the
same number of volts per turn, the volt-ampere (VI) product in each winding
will also be the same assuming no power losses. Then the power consumed by the
secondary connected load will be equal the power supplied by the primary
winding (Pin = Pout) and is given as:
Where,
Vp
is the primary voltage, Ip is the primary current
Vs
is the secondary voltage, Is is the secondary current
cos?
is the power factor of the load
2.4.2 Principal of Transformer Action
The frequency of the secondary waveform will
be “in-phase” with the frequency of the primary waveform then the cos? term
cancels out on both sides of the above equation. We now know that the output
voltage from the secondary winding is directly proportional to the number of
turns of wire in the secondary coil. If we increase or decrease this number of
turns, a larger or smaller voltage will be induced into the secondary winding.
Then we can define the “Turns Ratio” as the number of windings of the primary
coil (Np) divided by number of windings of the secondary coil (Ns).
Then the ratio of the primary and secondary voltages is the same as the ratio
of the number of turns in each winding. This ratio is known commonly as the
Transformation Ratio and is presented as Np:Ns (no units
as it is a ratio).
Then voltage transformers are all about
ratios, that is the ratio of windings called the “turns ratio” to the ratio of
the voltage called the “voltage ratio” and this is given as:
If this turns ratio is equal to one
(unity) that is the number of secondary turns equals the number of primary
turns giving a turns ratio of 1:1, the secondary voltage will be of the same
value as the primary voltage producing an isolation transformer. However, if
the ratio is greater than unity and Vs is greater than Vp,
this produces a step up transformer. Likewise, if the ratio is less than unity,
and Vs is less than Vp, this produces a step down
transformer.
Transformers do not change power they
transfer the same amount of power (assumes ideal transformer) from the primary
side to the secondary side. As electrical power is the product of volts x amps,
if the voltage changes then the current must change to maintain the same amount
of power. That is the current changes opposite to the voltage change and if one
goes up, the other goes down. So if the secondary voltage is greater than the
primary voltage then the secondary current is less than the primary current. If
the secondary voltage is less than the primary voltage then the secondary
current is greater than the primary current. This is called the “current ratio”
and is opposite to the previous voltage and turns ratios.
The ratios of voltage, current and turns
for a voltage transformer can be defined as:
2.5 Light Emitting Diode (LED)
A light-emitting diode or LED is a semiconductor
light source. LEDs are used as indicator lamps in many devices and are
increasingly used for other lighting.
When a light-emitting diode is forward
biased (switched on), electrons are able to recombine with electron holes
within the device, releasing energy in the form of photons. This effect is
called electroluminescence and the color of the light (corresponding to the
energy of the photon) is determined by the energy gap of the semiconductor.
LEDs are often small in area (less than 1 mm2), and integrated optical
components may be used to shape its radiation pattern.
Figure 2.18: Different types of LEDs
2.5.1 Types of LEDs
Miniature LEDs:These are mostly
single-die LEDs used as indicators, and they come in various-sizes from
2 mm to 8 mm, through-hole and surface mount packages. They usually
don’t use a separate heat sink.A typical current rating ranges from around 1 mA
to above 20 mA. The small size sets a natural upper boundary on power
consumption due to heat caused by the high current density and need for heat
sinking.
Mid-range LEDs:Medium power
LEDs are often through-hole mounted and used when an output of a few lumens is
needed. They sometimes have the diode mounted to four leads (two cathode leads,
two anode leads) for better heat conduction and carry an integrated lens. An
example of this is the Superflux package, from Philips Lumileds. These LEDs are
most commonly used in light panels, emergency lighting and automotive
tail-lights. Due to the larger amount of metal in the LED, they are able to
handle higher currents (around 100 mA). The higher current allows for the
higher light output required for tail-lights and emergency lighting.
High power LEDs:High power LEDs
(HPLED) can be driven at currents from hundreds of mA to more than an ampere,
compared with the tens of mA for other LEDs. Some can emit over a thousand
lumens. Since overheating is destructive, the HPLEDs must be mounted on a heat
sink to allow for heat dissipation. If the heat from a HPLED is not removed,
the device will fail in seconds. One HPLED can often replace an incandescent
bulb in a torch, or be set in an array to form a powerful LED lamp.
2.5.2 Colors of LEDs
LEDs are available in red, orange,
amber, yellow, green, blue and white. The color of an LED is determined by the
semiconductor material, not by the coloring of the package (the plastic body).
LEDs of all colors are available in uncolored packages which may be diffused
(milky) or clear (often described as ‘water clear’). The colored packages are
also available as diffused (the standard type) or transparent.
Tri-color LEDs: The most
popular type of tri-color LED has a red and a green LED combined in one package
with three leads. They are called tri-color because mixed red and green light
appears to be yellow and this is produced when both the red and green LEDs are
on.
Bi-color LEDs:A bi-color 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-color LEDs described above.
2.5.3 Sizes, Shapes and Viewing Angles
of LEDs
LEDs are available in a wide variety of
sizes and shapes. The standard LED has a round cross-section of 5mm diameter
and this is probably the best type for general use, but 3mm round LEDs are also
popular.
Round cross-section LEDs are frequently
used and they are very easy to install on boxes by drilling a hole of the LED
diameter, adding a spot of glue will help to hold the LED if necessary. LED
clips are also available to secure LEDs in holes. Other cross-section shapes
include square, rectangular and triangular.
As well as a variety of colors, sizes
and shapes, LEDs also vary in their viewing angle. This tells you how much the
beam of light spreads out. Standard LEDs have a viewing angle of 60° but others
have a narrow beam of 30° or less.
2.5.4 Considerations for Use
Power sources: The
current/voltage characteristic of an LED is similar to other diodes, in that
the current is dependent exponentially on the voltage. This means that a small
change in voltage can cause a large change in current. If the maximum voltage
rating is exceeded by a small amount, the current rating may be exceeded by a
large amount, potentially damaging or destroying the LED. The typical solution
is to use constant current power supplies, or driving the LED at a voltage much
below the maximum rating. Since most common power sources (batteries, mains)
are not constant current sources, most LED fixtures must include a power
converter. However, the I/V curve of nitride-based LEDs is quite steep above
the knee and gives an If of a few mill amperes at a Vf of 3V, making
it possible to power a nitride-based LED from a 3V battery such as a coin cell
without the need for a current limiting resistor.
Electrical polarity: As with all
diodes, current flows easily from p-type to n-type material. However, no
current flows and no light is emitted if a small voltage is applied in the
reverse direction. If the reverse voltage grows large enough to exceed the
breakdown voltage, a large current flows and the LED may be damaged. If the
reverse current is sufficiently limited to avoid damage, the reverse-conducting
LED is a useful noise diode.
Safety: The vast
majority of devices containing LEDs are “safe under all conditions of
normal use”, and so are classified as “Class 1 LED product”. At
present, only a few LEDs—extremely bright LEDs that also have a tightly focused
viewing angle of 8° or less—could, in theory, cause temporary blindness, and so
are classified as “Class 2”. In general, laser safety regulations—and
the “Class 1”, “Class 2”, etc. system—also apply to LEDs.
2.5.5 Advantages
Efficiency: LEDs emit more
light per watt than incandescent light bulbs. Their efficiency is not affected
by shape and size, unlike fluorescent light bulbs or tubes.
Color: LEDs can emit light of an
intended color without using any color filters as traditional lighting methods
need. This is more efficient and can lower initial costs.
Size: LEDs can be very small (smaller than
2 mm2) and are easily populated onto printed circuit boards.
On/Off time: LEDs light up
very quickly. A typical red indicator LED will achieve full brightness in under
a microsecond. LEDs used in communications devices can have even faster
response times.
Cycling: LEDs are ideal
for uses subject to frequent on-off cycling, unlike fluorescent lamps that fail
faster when cycled often, or HID lamps that require a long time before
restarting.
Dimming: LEDs can very
easily be dimmed either by pulse-width modulation or lowering the forward
current.
Cool light: In contrast to
most light sources, LEDs radiate very little heat in the form of IR that can
cause damage to sensitive objects or fabrics. Wasted energy is dispersed as
heat through the base of the LED.
Slow failure: LEDs mostly
fail by dimming over time, rather than the abrupt failure of incandescent
bulbs.
Lifetime: LEDs can have a
relatively long useful life. One report estimates 35,000 to 50,000 hours of
useful life, though time to complete failure may be longer. Fluorescent tubes
typically are rated at about 10,000 to 15,000 hours, depending partly on the
conditions of use, and incandescent light bulbs at 1,000–2,000 hours.
Shock resistance: LEDs, being
solid state components, are difficult to damage with external shock, unlike
fluorescent and incandescent bulbs which are fragile.
Focus: The solid package of the LED can
be designed to focus its light. Incandescent and fluorescent sources often
require an external reflector to collect light and direct it in a usable
manner.
Voltage Regulator
A voltage regulator is an electrical
regulator designed to automatically maintain a constant voltage level. A
voltage regulator may be a simple “feed-forward” design or may
include negative feedback control loops. It may use an electromechanical
mechanism, or electronic components. Depending on the design, it may be used to
regulate one or more AC or DC voltages.
2.6.1 Measures of Regulator Quality
The output voltage can only be held
roughly constant; the regulation is specified by two measurements:
Load regulation is the change
in output voltage for a given change in load current (for example:
“typically 15mV, maximum 100mV for load currents between 5mA and 1.4A, at
some specified temperature and input voltage”).
Line regulation or input
regulation is the degree to which output voltage changes with input (supply)
voltage changes – as a ratio of output to input change (for example
“typically 13mV/V”), or the output voltage change over the entire
specified input voltage range (for example “plus or minus 2% for input
voltages between 90V and 260V, 50-60Hz”).
Other important parameters are:
Temperature coefficient of the output
voltage is the change in output voltage with temperature (perhaps averaged over
a given temperature range), while…
Initial accuracy of a voltage
regulator (or simply “the voltage accuracy”) reflects the error in
output voltage for a fixed regulator without taking into account temperature or
aging effects on output accuracy.
Dropout voltage is the minimum
difference between input voltage and output voltage for which the regulator can
still supply the specified current. A Low Drop-Out (LDO) regulator is designed
to work well even with an input supply only a volt or so above the output
voltage. The input-output differential at which the voltage regulator will no
longer maintain regulation. Further reduction in input voltage will result in
reduced output voltage. This value is dependent on load current and junction
temperature.
Absolute maximum ratings are defined for
regulator components, specifying the continuous and peak output currents that
may be used (sometimes internally limited), the maximum input voltage, maximum
power dissipation at a given temperature, etc.
Output noise (thermal white
noise) and output dynamic impedance may be specified as graphs versus
frequency, while output ripple noise (mains “hum” or switch-mode
“hash” noise) may be given as peak-to-peak or RMS voltages, or in
terms of their spectra.
Quiescent current in a regulator
circuit is the current drawn internally, not available to the load, normally measured
as the input current while no load is connected (and hence a source of
inefficiency; some linear regulators are, surprisingly, more efficient at very
low current loads than switch-mode designs because of this).
Transient response is the reaction
of a regulator when a (sudden) change of the load current (called the load
transient) or input voltage (called the line transient) occurs. Some regulators
will tend to oscillate or have a slow response time which in some cases might
lead to undesired results. This value is different from the regulation
parameters, as that is the stable situation definition. The transient response
shows the behavior of the regulator on a change. This data is usually provided
in the technical documentation of a regulator and is also dependent on output
capacitance.
2.6.2 DC voltage Stabilizers
Many simple DC power supplies regulate
the voltage using a shunt regulator such as a zener diode, avalanche breakdown
diode, or voltage regulator tube. Each of these devices begins conducting at a
specified voltage and will conduct as much current as required to hold its
terminal voltage to that specified voltage. The power supply is designed to
only supply a maximum amount of current that is within the safe operating
capability of the shunt regulating device (commonly, by using a series
resistor).
2.7 Relay
A relay is an electrically operated
switch. Many relays use an electromagnet to operate a switching mechanism
mechanically, but other operating principles are also used. Relays are used
where it is necessary to control a circuit by a low-power signal (with complete
electrical isolation between control and controlled circuits), or where several
circuits must be controlled by one signal. The first relays were used in long
distance telegraph circuits, repeating the signal coming in from one circuit
and re-transmitting it to another. Relays were used extensively in telephone
exchanges and early computers to perform logical operations.
2.7.1 Electromechanical Relay
Construction
Relays may be “Normally Open”,
or “Normally Closed”. One pair of contacts is classed as Normally
Open, (NO) or make contacts and another set which are classed as Normally
Closed, (NC) or break contacts. In the normally open position, the contacts are
closed only when the field current is “ON” and the switch contacts
are pulled towards the inductive coil. In the normally closed position, the
contacts are permanently closed when the field current is “OFF” as
the switch contacts return to their normal position. These terms Normally Open,
Normally Closed or Make and Break Contacts refer to the state of the electrical
contacts when the relay coil is “de-energized”, i.e, no supply
voltage connected to the inductive coil.
Figure 2.20:
Electromechanical Relay Construction
The relays contacts are electrically
conductive pieces of metal which touch together completing a circuit and allow
the circuit current to flow, just like a switch. When the contacts are open the
resistance between the contacts is very high in the Mega-Ohms, producing an
open circuit condition and no circuit current flows. When the contacts are
closed the contact resistance should be zero, a short circuit, but this is not
always the case. All relay contacts have a certain amount of “contact
resistance” when they are closed and this is called the
“On-Resistance”. With a new relay and contacts this ON-resistance
will be very small, generally less than 0.2?’s because the tips are new and
clean.
As the contact tips begin to wear, and
if they are not properly protected from high inductive or capacitive loads,
they will start to show signs of arcing damage as the circuit current still
wants to flow as the contacts begin to open when the relay coil is
de-energized. This arcing or sparking will cause the contact resistance of the
tips to increase further as the contact tips become damaged. If allowed to
continue the contact tips may become so burnt and damaged to the point where
they are physically closed but do not pass any or very little current. To
reduce the effects of contact arcing and high “On-resistances”,
modern contact tips are made of, or coated with, a variety of silver based
alloys to extend their life span as given in the following table.
2.7.2 Relay Contact Types
As well as the standard descriptions of
Normally Open, (NO) and Normally Closed, (NC) used to describe how the relays
contacts are connected, relay contact arrangements can also be classed by their
actions. Electrical relays can be made up of one or more individual switch
contacts with each “contact” being referred to as a “pole”.
Each one of these contacts or poles can be connected or “thrown”
together by energizing the relays coil and this gives rise to the description
of the contact types as being:
·
SPST
– Single Pole Single Throw
·
SPDT
– Single Pole Double Throw
·
DPST
– Double Pole Single Throw
·
DPDT
– Double Pole Double Throw
with the action of the contacts being
described as “Make” (M) or “Break” (B). Then a simple relay
with one set of contacts as shown above can have a contact description of:
Figure 2.21:
Relay Contact Configurations
2.8 MOSFET
The Metal Oxide Semiconductor Field
Effect Transistor or MOSFET is a transistor used for amplifying or switching
electronic signals. In MOSFETs, a voltage on the oxide-insulated gate electrode
can induce a conducting channel between the two other contacts called source
and drain. The channel can be of n-type or p-type, and is accordingly called an
nMOSFET or a pMOSFET (also commonly known as nMOS, pMOS). It is by far the most
common transistor in both digital and analog circuits.
2.8.1 Basic Structure
MOSFETs use an electrical field produced
by a gate voltage to alter the flow of charge carriers, electrons for N-channel
or holes for P-channel, through the semi conductive drain-source channel. The
gate electrode is placed on top of a very thin insulating layer and there are a
pair of small N-type regions just under the drain and source electrodes.
it is possible to bias the gate of a
MOSFET in either polarity, +ve or -ve. This makes MOSFETs especially valuable
as electronic switches or to make logic gates because with no bias they are
normally non-conducting and this high gate input resistance means that very
little or no control current is needed as MOSFETs are voltage controlled
devices.
Figure 2.23:
Basic MOSFET structure
2.8.2 Types of MOSFET
Both the P-channel and the N-channel
MOSFETs are available in two basic forms, the Enhancement type and the Depletion
type.
Depletion type MOSFET:The
Depletion-mode MOSFET, which is less common than the enhancement types is
normally switched “ON” without the application of a gate bias voltage
making it a “normally-closed” device. However, a gate to source
voltage (VGS) will switch the device “OFF”. For an N-channel
MOSFET, a “positive” gate voltage widens the channel, increasing the
flow of the drain current and decreasing the drain current as the gate voltage
goes more negative. In other words, for an N-channel depletion mode MOSFET: +VGS
means more electrons and more current. While a -VGS means less electrons and
less current. The opposite is also true for the P-channel types. Then the
depletion mode MOSFET is equivalent to a “normally-closed” switch.
Figure 2.24:
Characteristics graph and symbol of Depletion type MOSFET
Enhancement type MOSFET:The more common
Enhancement-mode MOSFET is the reverse of the depletion-mode type. Here the
conducting channel is lightly doped or even undoped making it non-conductive.
This results in the device being normally “OFF” when the gate bias
voltage is equal to zero.A drain current will only flow when a gate voltage (VGS)
is applied to the gate terminal greater than the threshold voltage (VTH)
level in which conductance takes place making it a transconductive device. This
positive +ve gate voltage pushes away the holes within the channel attracting
electrons towards the oxide layer and thereby increasing the thickness of the
channel allowing current to flow. This is why this kind of transistor is called
an enhancement mode device as the gate voltage enhances the channel.
Increasing this positive gate voltage
will cause the channel resistance to decrease further causing an increase in
the drain current, ID through the channel. In other words, for an
N-channel enhancement mode MOSFET: +VGS turns the transistor
“ON”, while a zero or -VGS turns the transistor
“OFF”. Then, the enhancement-mode MOSFET is equivalent to a
“normally-open” switch.
Figure 2.25:
Characteristics graph and symbol of Enhancement type MOSFET
Enhancement-mode MOSFETs make excellent
electronics switches due to their low “ON” resistance and extremely
high “OFF” resistance as well as their infinitely high gate
resistance.
2.8.3 Modes of Operation
The operation of a MOSFET can be
separated into three different modes, depending on the voltages at the
terminals.For an enhancement-mode, n-channel MOSFET, the three operational
modes are:
Cutoff, subthreshold, or weak-inversion
mode (When
basic threshold model, the transistor is turned off, and there is no conduction
between drain and source. In reality, the Boltzmann distribution of electron
energies allows some of the more energetic electrons at the source to enter the
channel and flow to the drain, resulting in a subthreshold current that is an
exponential function of gate–source voltage. While the current between drain
and source should ideally be zero when the transistor is being used as a
turned-off switch, there is a weak-inversion current, sometimes called
subthreshold leakage.In weak inversion the current varies exponentially with
gate-to-source bias VGS as given approximately by:
Where
voltage
With
Triode mode or linear region (When
is turned on, and a channel has been created which allows current to flow
between the drain and the source. The MOSFET operates like a resistor,
controlled by the gate voltage relative to both the source and drain voltages.
The current from drain to source is modeled as:
Where
The transition from the exponential subthreshold region to the triode region is
not as sharp as the equations suggest.
Saturation or active mode (When
turned on, and a channel has been created, which allows current to flow between
the drain and source. Since the drain voltage is higher than the gate voltage,
the electrons spread out, and conduction is not through a narrow channel but
through a broader, two- or three-dimensional current distribution extending
away from the interface and deeper in the substrate. The onset of this region
is also known as pinch-off to indicate the lack of channel region near the
drain. The drain current is now weakly dependent upon drain voltage and
controlled primarily by the gate–source voltage, and modeled approximately as:
The additional factor involving ?, the
channel-length modulation parameter, models current dependence on drain voltage
due to the early effect, or channel length modulation. According to this
equation, a key design parameter, the MOSFET transconductance is:
where the combination
between the triode and saturation regions.
Another key design parameter is the
MOSFET output resistance
2.9 Buzzer
A buzzer or beeper is an audio signaling
device, which may be mechanical, electromechanical, or piezoelectric. Typical
uses of buzzers and beepers include alarm devices, timers and confirmation of
user input such as a mouse click or keystroke.
A buzzer Symbol
Figure 2.26: An
electronic buzzer
2.9.1 Types of Buzzers
Mechanical:
A joy buzzer is an example of a purely
mechanical buzzer.
Electromechanical:
Early devices were based on an
electromechanical system identical to an electric bell without the metal gong.
Similarly, a relay may be connected to interrupt its own actuating current,
causing the contacts to buzz. Often these units were anchored to a wall or
ceiling to use it as a sounding board. The word “buzzer” comes from the
rasping noise that electromechanical buzzers made.
Piezoelectric:
A piezoelectric element may be driven by
an oscillating electronic circuit or other audio signal source, driven with a
piezoelectric audio amplifier. Sounds commonly used to indicate that a button
has been pressed are a click, a ring or a beep.
2.9.2 Working Principle of Passive
Electromagnetic Buzzer
Ac signal through the bypass line in the
stent in the stent core package produce a column of alternating magnetic flux,
the alternating magnetic flux and magnetic flux for a constant stack, so that
films of molybdenum to a given exchange of signals with the frequency of
vibration and sound resonator. Products of the frequency response and sound
pressure curve and the value gap, molybdenum films inherent vibration frequency
(which can be rough refraction.
For small film thickness of molybdenum),
Shell (Tune Helmholtz resonance) frequency of the magnetometer magnetic wire is
directly related to the diameter composed of electromagnetic buzzer
by electromagnetic oscillator, the electromagnetic coil, magnet, vibration,
such as the composition of membrane and shell.
Access to power, the audio signal
generated by oscillator current through the electromagnetic coil to generate
magnetic fields of electromagnetic coils. Membrane vibration in the
electromagnetic coil and magnet interaction, the buzzer sound vibration
periodically.
2.9.3 Application
·
Annunciator
panels
·
Electronic
metronomes
·
Game
shows
·
Microwave
ovens and other household appliances
·
Sporting
events such as basketball games
2.10 Close Circuit Camera
Closed-circuit television (CCTV) cameras
can produce images or recordings for surveillance purposes, and can be either
video cameras, or digital stills cameras.
Figure 2.27:
Different types of CC camera
2.10.1 Video Cameras
Video cameras are either analogue or
digital, which means that they work on the basis of sending analogue or digital
signals to a storage device such as a video tape recorder or desktop computer
or laptop computer.
Analogue: Can record straight to a video
tape recorder which is able to record analogue signals as pictures. If the
analogue signals are recorded to tape, then the tape must run at a very slow
speed in order to operate continuously. This is because in order to allow a 3
hour tape to run for 24 hours, it must be set to run on a time lapse basis
which is usually about 4 frames a second. In one second, the camera scene can
change dramatically. A person for example can have walked a distance of 1
meter, and therefore if the distance is divided into 4 parts i.e. 4 frames or
‘snapshots’ in time, then each frame invariably looks like a blur, unless the
subject keeps relatively still.
Digital: These cameras do not require a
video capture card because they work using a digital signal which can be saved
directly to a computer. The signal is compressed 5:1, but DVD quality can be
achieved with more compression (MPEG-2 is standard for DVD-video, and has a
higher compression ratio than 5:1, with a slightly lower video quality than 5:1
at best, and is adjustable for the amount of space to be taken up versus the
quality of picture needed or desired). The highest picture quality of DVD is
only slightly lower than the quality of basic 5:1-compression DV.
Network: IP cameras or network cameras
are analogue or digital video cameras, plus an embedded video server having an
IP address, capable of streaming the video (and sometimes, even audio).Because
network cameras are embedded devices, and do not need to output an analogue
signal, resolutions higher than CCTV analogue cameras are possible. A typical
analogue CCTV camera has a PAL (768×576 pixels) or NTSC (720×480 pixels),
whereas network cameras may have VGA (640×480 pixels), SVGA (800×600 pixels) or
quad-VGA (1280×960 pixels, also referred to as ‘megapixel’) resolutions.
2.10.2 Digital Still Cameras
These cameras can be purchased in any
high street shop and can take excellent pictures in most situations. The pixel
resolution of the current models has easily reached 7 million pixels (7-mega
pixels). Some point and shoot models like those produced by Canon or Nikon
boast resolutions in excess of 10 million pixels. At these resolutions, and
with high shutter speeds like 1/125th of a second, it is possible to take jpg
pictures on a continuous or motion detection basis that will capture not only
anyone running past the camera scene, but even the faces of those driving past.
These cameras can be plugged into the USB port of any computer (most of them
now have USB capability) and pictures can be taken of any camera scene. All
that is necessary is for the camera to be mounted on a wall bracket and pointed
in the desired direction.
2.10.3 Wireless Security Cameras
Many consumers are turning to wireless
security cameras for home surveillance also. Wireless cameras do not require a
video cable for video/audio transmission, simply a cable for power. Wireless
cameras are also easy and inexpensive to install. Previous generations of
wireless security cameras relied on analog technology; modern wireless cameras
use digital technology which delivers crisper audio, sharper video, and a
secure and interference-free signal.
Sensor
and Counter
3.1 Sensor
A sensor is a device that measures a
physical quantity and converts it into a signal which can be read by an observer
or by an instrument.
3.1.1 Types of Sensors
Input type transducers or sensors,
produce a proportional output voltage or signal in response to changes in the
quantity that they are measuring (the stimulus) and the type or amount of the
output signal depends upon the type of sensor being used. Generally, all types
of sensors can be classed as two kinds, passive and active.
Active: Active sensors require some form
of external power to operate, called an excitation signal which is used by the
sensor to produce the output signal. Active sensors are self-generating devices
because their own properties change in response to an external effect and
produce an output voltage, for example, 1 to 10v DC or an output current such
as 4 to 20mA DC. For example, a strain gauge is a pressure-sensitive resistor.
It does not generate any electrical signal, but by passing a current through it
(excitation signal), its resistance can be measured by detecting variations in
the current and/or voltage across it relating these changes to the amount of
strain or force.
Passive: Unlike the active sensor, a
passive sensor does not need any additional energy source and directly
generates an electric signal in response to an external stimulus. For example,
a thermocouple or photodiode. Passive sensors are direct sensors which change
their physical properties, such as resistance, capacitance or inductance etc.As
well as analogue sensors; Digital Sensors produce a discrete output
representing a binary number or digit such as a logic level “0” or a
logic level “1”.
3.1.2 Passive Infrared (PIR) Sensor
A Passive Infrared sensor (PIR sensor)
is an electronic device that measures infrared (IR) light radiating from
objects in its field of view. PIR sensors are often used in the construction of
PIR-based motion detectors. Apparent motion is detected when an infrared source
with one temperature, such as a human, passes in front of an infrared source
with another temperature, such as a wall. This is not to say that the sensor
detects the heat from the object passing in front of it but that the object
breaks the field which the sensor has determined as the “normal”
state. Any object, even one exactly the same temperature as the surrounding
objects will cause the PIR to activate if it moves in the field of the sensors.
All objects above absolute zero emit
energy in the form of radiation. Usually infrared radiation is invisible to the
human eye but can be detected by electronic devices designed for such a
purpose. The term passive in this instance means that the PIR device does not
emit an infrared beam but merely passively accepts incoming infrared radiation.
“Infra” meaning below our ability to detect it visually, and “Red” because this
color represents the lowest energy level that our eyes can sense before it
becomes invisible. Thus, infrared means below the energy level of the color
red, and applies to many sources of invisible energy.
Figure 3.1: A
PIR Sensor
General description: The PIR
(Passive Infra-Red) Sensor is a pyroelectric device that detects motion by
measuring changes in the infrared levels emitted by surrounding objects. This motion can be detected by checking for a
high signal on a single I/O pin.
In a PIR-based motion detector (usually
called a PID, for Passive Infrared Detector), the PIR sensor is typically
mounted on a printed circuit board containing the necessary electronics
required to interpret the signals from the pyroelectric sensor chip. The
complete assembly is contained within a housing mounted in a location where the
sensor can view the area to be monitored. Infrared energy is able to reach the
pyroelectric sensor through the window because the plastic used is transparent
to infrared radiation (but only translucent to visible light). This plastic
sheet also prevents the intrusion of dust and/or insects from obscuring the
sensor’s field of view, and in the case of insects, from generating false
alarms.
A few mechanisms have been used to focus
the distant infrared energy onto the sensor surface. The window may have
multiple Fresnel lenses molded into it. Alternatively, some PIDs are
manufactured with internal plastic, segmented parabolic mirrors to focus the
infrared energy. Where mirrors are used, the plastic window cover has no
Fresnel lenses molded into it. This filtering window may be used to limit the
wavelengths to 8-14 micrometers which is closest to the infrared radiation
emitted by humans (9.4 micrometers being the strongest).
Feature:
·
Single
bit output
·
Small
size makes it easy to conceal
·
Compatible
with all Parallax microcontrollers
·
3.3V & 5V operation with <100uA current
draw
Theory of Operation:
Pyroelectric devices, such as the PIR
sensor, have elements made of a crystalline material that generates an electric
charge when exposed to infrared radiation.
The changes in the amount of infrared striking the element change
the voltages generated,
which are measured
by an on-board
amplifier. The device contains
a special filter called a Fresnel lens, which focuses the infrared signals onto
the element. As the ambient infrared signals change rapidly, the on-board
amplifier trips the output to indicate motion.
A person entering a monitored area is
detected when the infrared energy emitted from the intruder’s body is focused
by a Fresnel lens or a mirror segment and overlaps a section on the chip that
had previously been looking at some much cooler part of the protected area.
That portion of the chip is now much warmer than when the intruder wasn’t
there. As the intruder moves, so does the hot spot on the surface of the chip.
This moving hot spot causes the electronics connected to the chip to
de-energize the relay, operating its contacts, thereby activating the detection
input on the alarm control panel. Conversely, if an intruder were to try to
defeat a PID, perhaps by holding some sort of thermal shield between himself
and the PID, a corresponding ‘cold’ spot moving across the face of the chip
will also cause the relay to de-energize — unless the thermal shield has the
same temperature as the objects behind it.
Calibration: The PIR Sensor
requires a ‘warm-up’ time in order to function properly. This is due to the settling time involved in
‘learning’ its environment. This could
be anywhere from 10-60 seconds. During
this time there should be as little motion as possible in the sensors field of
view.
Sensitivity: The PIR Sensor
has a range of approximately 20 feet.
This can vary with environmental conditions. The sensor
is designed to
adjust to slowly
changing conditions that
would happen normally
as the day progresses and
the environmental conditions
change, but responds
by making its
output high when sudden changes occur, such as when there
is motion.
Application: PIDs come in
many configurations for a wide variety of applications. The most common, used
in home security systems, have numerous Fresnel lenses or mirror segments and
an effective range of about thirty feet. Some larger PIDs are made with single
segment mirrors and can sense changes in infrared energy over one hundred feet
away from the PID. There are also PIDs designed with reversible orientation
mirrors which allow either broad coverage (110° wide) or very narrow ‘curtain’
coverage.
PIDs can have more than one internal
sensing element so that, with the appropriate electronics and Fresnel lens, it
can detect direction. Left to right, right to left, up or down and provide an
appropriate output signal.
3.2 Counter
A counter is a device which stores (and
sometimes displays) the number of times a particular event or process has
occurred, often in relationship to a clock signal. The counter we used in this
project has two major components —
·
Analog
to Digital Converter, and
·
Seven-segment
Display
3.2.1 Analog to Digital Converter
An Analog to Digital or ADC is an
electronic device that converts an input analog voltage or current to a digital
number proportional to the magnitude of the voltage or current. However, some
non-electronic or only partially electronic devices, such as rotary encoders,
can also be considered ADCs.
The digital output may use different
coding schemes. Typically the digital output will be a two’s complement binary
number that is proportional to the input, but there are other possibilities. An
encoder, for example, might output a Gray code.
Figure3.2:
Analog to Digital Converter
Resolution:
The resolution of the converter
indicates the number of discrete values it can produce over the range of analog
values. The values are usually stored electronically in binary form, so the
resolution is usually expressed in bits. In consequence, the number of discrete
values available, or “levels”, is a power of two. For example, an ADC
with a resolution of 8 bits can encode an analog input to one in 256 different
levels, since 28 = 256. The values can represent the ranges from 0 to
255 (i.e. unsigned integer) or from ?128 to 127 (i.e. signed integer),
depending on the application.
Response type:
Most ADCs are linear types. The term
linear implies that the range of input values has a linear relationship with
the output value.
Accuracy:
An ADC has several sources of errors.
Quantization error and (assuming the ADC is intended to be linear)
non-linearity are intrinsic to any analog-to-digital conversion. There is also
a so-called aperture error which is due to a clock jitter and is revealed when
digitizing a time-variant signal (not a constant value).These errors are
measured in a unit called the least significant bit (LSB). In the above example
of an eight-bit ADC, an error of one LSB is 1/256 of the full signal range, or
about 0.4%.
Sampling rate:
The analog signal is continuous in time
and it is necessary to convert this to a flow of digital values. It is
therefore required to define the rate at which new digital values are sampled
from the analog signal. The rate of new values is called the sampling rate or
sampling frequency of the converter.
A continuously varying band limited
signal can be sampled (that is, the signal values at intervals of time T, the sampling
time, are measured and stored) and then the original signal can be exactly
reproduced from the discrete-time values by an interpolation formula. The
accuracy is limited by quantization error. However, this faithful reproduction
is only possible if the sampling rate is higher than twice the highest
frequency of the signal. This is essentially what is embodied in the Shannon-SyQuest
sampling theorem.
Since a practical ADC cannot make an
instantaneous conversion, the input value must necessarily be held constant
during the time that the converter performs a conversion (called the conversion
time). An input circuit called a sample and hold performs this task—in most
cases by using a capacitor to store the analog voltage at the input, and using
an electronic switch or gate to disconnect the capacitor from the input. Many
ADC integrated circuits include the sample and hold subsystem internally.
Aliasing:
All ADCs work by sampling their input at
discrete intervals of time. Their output is therefore an incomplete picture of
the behavior of the input. There is no way of knowing, by looking at the
output, what the input was doing between one sampling instant and the next. If
the input is known to be changing slowly compared to the sampling rate, then it
can be assumed that the value of the signal between two sample instants was
somewhere between the two sampled values. If, however, the input signal is
changing rapidly compared to the sample rate, then this assumption is not
valid.
If the digital values produced by the
ADC are, at some later stage in the system, converted back to analog values by
a digital to analog converter or DAC, it is desirable that the output of the
DAC be a faithful representation of the original signal. If the input signal is
changing much faster than the sample rate, then this will not be the case, and
spurious signals called aliases will be produced at the output of the DAC. The
frequency of the aliased signal is the difference between the signal frequency
and the sampling rate. For example, a 2 kHz sine wave being sampled at
1.5 kHz would be reconstructed as a 500 Hz sine wave. This problem is
called aliasing.
Dither:
In A-to-D converters, performance can
usually be improved using dither. This is a very small amount of random noise
(white noise), which is added to the input before conversion. Its effect is to
cause the state of the LSB to randomly oscillate between 0 and 1 in the
presence of very low levels of input, rather than sticking at a fixed value.
Rather than the signal simply getting cut off altogether at this low level
(which is only being quantized to a resolution of 1 bit), it extends the
effective range of signals that the A-to-D converter can convert, at the
expense of a slight increase in noise – effectively the quantization error is
diffused across a series of noise values which is far less objectionable than a
hard cutoff. The result is an accurate representation of the signal over time.
A suitable filter at the output of the system can thus recover this small
signal variation.
3.2.2 Seven-segment Display
A seven-segment display (SSD), or
seven-segment indicator, is a form of electronic display device for displaying
decimal numerals that is an alternative to the more complex dot-matrix
displays. Seven-segment displays are widely used in digital clocks, electronic
meters, and other electronic devices for displaying numerical information.
Figure 3.3: A
typical seven-segment LED display
Concept and Visual Structure:
A seven segment display, as its name
indicates, is composed of seven elements. Individually on or off, they can be
combined to produce simplified representations of the Arabic numerals. Often
the seven segments are arranged in an oblique (slanted) arrangement, which aids
readability. In most applications, the seven segments are of nearly uniform
shape and size (usually elongated hexagons, though trapezoids and rectangles
can also be used), though in the case of adding machines, the vertical segments
are longer and more oddly shaped at the ends in an effort to further enhance readability.
Each of the numbers 0, 6, 7 and 9 may be represented by two or more different
glyphs on seven-segment displays.
The seven segments are arranged as a
rectangle of two vertical segments on each side with one horizontal segment on
the top, middle, and bottom. Additionally, the seventh segment bisects the
rectangle horizontally. There are also fourteen-segment displays and
sixteen-segment displays (for full alphanumeric); however, these have mostly
been replaced by dot-matrix displays.
Figure 3.4:
Schematic diagram of seven-segment display
Implementation:
In a simple LED package, typically all
of the cathodes (negative terminals) or all of the anodes (positive terminals)
of the segment LEDs are connected together and brought out to a common pin;
this is referred to as a “common cathode” or “common anode”
device. Hence a 7 segments plus decimal point package will only require nine
pins (though commercial products typically contain more pins, and/or spaces
where pins would go, in order to match industry standard pinouts).
Numbers to 7-segment-code:
A single byte can encode the full state
of a 7-segment-display. The most popular bit encodings are gfedcba and abcdefg
– both usually assume 0 is off and 1 is on. The following table gives the hexadecimal
encodings for displaying the digits 0 to F:
Circuit
Operation
4.1 Introduction
Security monitoring system is used all
over the world to ensure the security of our family and valuables. Still now
security assurance is a curse for our country. Available security systems are
not user friendly and affordable enough for every class to maintain. Hence we
designed a security monitoring system based on PIR sensor which is affordable
and at the same time flexible enough to purchase.
4.2 Component Used
·
AC
source
·
Fuse
·
Fixed
resistor, variable resistor & capacitor
·
Voltage
transformer
·
Full
wave rectifier
·
Voltage
regulator
·
Breadboard
·
Relay
·
PIR
sensor
·
MOSFET
·
LED
·
Analog-to-Digital
converter
·
Seven-segment
display
·
Buzzer
·
Close-circuit
camera
·
Display
monitor
4.3 Operation
·
An
AC voltage source supplies the main power.
·
A
step down transformer takes the source voltage on its primary side and it
divides the voltage into two portions on its secondary side.
·
Both
portion of the AC voltage are taken by two rectifiers and converted into DC
voltage.
·
One
of the DC voltages is passed through a voltage regulator to drive the sensor.
Another DC voltage is used to power up a relay.
·
The
sensor output is connected to an analog-to-digital converter (ADC). The ADC
converts the analog output received from the sensor to a digital pulse and
passes it to a seven-segment display through a driver IC to show the counting.
·
The
sensor output is also connected to the gate of an enhancement type MOSFET which
conducts current from drain to source when it gets a signal from the sensor.
·
One
terminal of the relay is connected to the drain of the MOSFET and when the
MOSFET switches on the relay powers up the buzzer circuit as well as the camera
controller.
·
The
buzzer rings a bell, the LEDs lit on and the camera starts recording
surrounding activities. The output of the camera is shown on a display monitor.
Figure 4.1:
Schematic diagram of the system
Chapter 5
Overall
Conclusion
5.1 Discussion
It’s not always very easy task to
establish a project with an arrant success. Different environment brings
different types there are many obstacles that have to be considered. Different
environment arise different types of problems. Besides, the use of the
components must be precisely valued.
The PIR sensor has a definite rating of
power. The power consumed by the sensor is passed through a voltage regulator.
So we had to limit the current flow to an accurate value by calculating
resistor values. We came to understand this fact after damaging a sensor for
over dissipation of power in the circuit.
5.2 Limitation
Almost every project comes out with some
limitations or disadvantages. Our project is not an exception of that. The
circuit in our project has some constrains. The perfect execution of a wireless
system depends on both the transmitter and the receiver. The PIR sensor we used
is quite sensitive and detects every possible movement at its range. We had to
lessen the range by controlling its triggering voltage.
5.3 Future Plans
A perfect security system is hard to
build. Our monitoring system will do a good job to provide a medium-level
security but when it comes to security there are always scopes for improvement.
We may try to use Global Positioning System (GPS) and microcontroller to
automate the system even more effectively. If we can utilize GPS technology
then length coverage problem will be removed.
5.4 Conclusion
The main objective of our project was to
design and implement an indoor security system using general electronic
components which are easy to use and cost effective. Our system is very
reliable that can be afforded by every class of our society. So if anyone
intends to have a secured system, the one that we designed is one of the most
efficient one, with prompt response without any delay.
Appendices
Appendix A
Data sheet for Series Voltage Regulator
7812
Appendix B
Data sheet for ADC 0804
Reference
1. Muhammad H.
Rashid, “Power Electronics”, Prentice-Hall of India Private Limited, New
Delhi-110 001, pp-631.
2. B.L.
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& DC Mechanics 2004, S. Chand & Company Ltd. Pp-1028, 1035.
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R.C. Poon, “An Integral Charge Control Model of Bipolar Transistor”, Bell Syst.
Tech. J., vol 49, pp. 827-852, May-June 1970.
4. R. S. Muller,
Kamins TI & Chan M (2003). Device Electronic (Third Edition ed.). New York:
Wiley. P. 280 ff.
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K.C. Smith (2004), “Microelectronic Circuit” (5thed.). New York:
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7. http://www.semico.com
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Communications Simplified” by A.K. Maini, 9th Ed.
10. “Home Security:
Alarms, Sensors and Systems” by Vivian Capel, Newnes, 1997.
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12. http://extremeelectronics.co.in
13. http://www.electronics-tutorials.ws
14. http://www.datasheetcatalog.com
15. http://www.datasheet.org.uk