Transcript sensors - MarsilingSecDNTresource

Basic Electricity &
Electronics
Let’s go on a
Learning Journey
Conductors and Insulators



Metals are good conductors because
current flows through them easily.
Wood and most plastics do not allow
current to flow and are electrical
insulators.
Some materials, such as silicon and
germanium, are called semi-conductors
because current flows through them, but
not as freely as through metals.
SWITCHES
• A switch with two connections is
called a single-pole, single-throw
(SPST) switch.
• A switch with three connections is
called a single-pole, double-throw
(SPDT) switch.
SPDT
SWITCHES
• A double-pole, double-throw
(DPDT) switch has six contacts.
DPDT can operate two independent
circuits at the same time.
DPDT
RESISTORS in series
• When resistors are connected in series, the combined
resistance is higher than the individual resistances
• Worked out by adding each individual value to the next.
R = R1 + R2
R1
R2
RESISTORS in parallel
• When resistors care connected in parallel, the combined
resistance is lower than the individual resistances
worked out by adding the reciprocal of each individual
value to the next.
1
R
=
1
R1
+
1
R2
R1
R2
Use of RESISTORS
• Resistors reduce the amount of
current flowing in a circuit and
are used for protecting
components from being
damaged by too much current.
• They are made in a range of
values, with higher values
providing a greater resistance.
• Resistors can be connected
either way round in a circuit.
RESISTORS
Resistors are found in many electronic
products
a) Draw the symbol of the resistor.
b) Describe the purpose of the
colour bands found on a resistor.
The colour bands are coded
to show the values of the
resistor
c) A gold or silver band is found
on the resistor. Explain the
purpose.
The band shows the
accuracy of the resisitor.
Gold band for +/- 5% and
silver for +/- 10%.
BIPOLAR TRANSISTOR
• A transistor is an automatic switch.
• Instead of pushing a button or sliding a lever to turn it on
or off, it has a contact, called the base, which is used to
activate it.
Types of SWITCHES
Rocker switch
Rotary switch
Slide switch
Push switch
Tilt switch
Microswitch
Reed switch
Key switch
Membrane
panel switch
OUTPUT COMPONENTS
• The filament bulb lights up when
current passes through.
• The current causes the filament to
become so hot that it glows.
OUTPUT COMPONENTS
• Buzzers make a noise when
connected to a battery and are
often used as warning devices.
• The wires are sometimes
coloured coded: red to connect to
the positive terminal and black to
connect to the negative terminal.
OUTPUT COMPONENTS
• Motors are used for turning wheels,
pulleys, gears or cams.
• D.C. brush motors having contacts for
the battery connection are the most
common type.
• The direction of the spindle rotation
can be reversed by swapping the
connections at the contacts.
• The larger the motor, the more current
it requires.
Stepper Motor
• A stepper motor has a permanent
magnet rotor that turns inside
fixed coils of wire.
• The coils are switched on and off in
a special sequence to turn the rotor.
• They are precise because the rotors
can be made to turn through an
exact number of steps.
Solenoid
• A solenoid also produces a linear
movement.
• It consists of a coil of wire with a
plunger in the centre. When the
current flows through the coil, it
creates a magnetic field, causing
the plunger to move through the
coil.
Relay
• A relay is a switch which is
operated by a small solenoid.
• The relay allows a low voltage
circuit called the primary circuit
to control a high voltage circuit
called the secondary circuit.
• It also allows high current devices
to be switched on and off by a
circuit which uses a much smaller
current.
NC
NO
OUTPUT COMPONENTS
• A light-emitting diode (LED) glows
when a current flows through it.
• LEDs are cheap and come in a
variety of colours.
• The leg next to the flat side of the
LED is to be connected to the
negative terminal of the battery.
anode
(+)
flat side
indicates
cathode
anode
cathode
(-)
cathode
OUTPUT COMPONENTS
• Diodes allow an electric current
to flow through in one direction
only.
• They are used in circuits for
controlling the flow of the
current.
• A standard semiconductor diode
does not light up when current
flows through it.
anode
(+)
cathode
(-)
anode
cathode
Components and Uses
Components
Application and Uses
Motor
Produces low cost rotary motion
that does not required speed
control. Used in children’s toys
Buzzer
For producing sound. Has built-in
signal generator and a predesigned tone. Used in door bells.
This lights up to give bright light.
It is used in torches
Bulb
Components and Uses
Components
Application/ Uses
Stepper Motor Rotary motion with accurate
angular position control,
accurate speed control by
controlling the input
frequency
Solenoid
Short stroke linear motion.
Used in electric door latch
Speaker
For producing sound.
Requires signal source
Components and Uses
Components
Cell/ Battery
Filament light
bulb
Variable
resisitor
Resistor
Application and Uses
Source of electricity (Direct
Current - DC). Used in toys
Lighting.
Used in torch light
Provides variable voltage in a
voltage divider circuit. Adjust the
gain of an amplified circuit
Controls or limits the current
flowing in a circuit
Components and Uses
Components
Lightdependent
resistor
Thermistor
Microphone
Application and Uses
High resistance in the dark,
low resistance when exposed
to light. Used in automatic
light switch circuit.
High resistance when cold,
low resistance when hot.
Used in temperature heater/
oven control circuit
For picking up sound wave
Components and Uses
Components
Capacitor
Application and Uses
Smoothens fluctuating
voltage from a rectifier and
delay or timing circuit. Used
with for electronic
differentiation and
integration
Light-emitting Indicator lights for
diode (LED)
instrument panel and home
appliances. Used Exit sign
and in traffic lights.
Components and Uses
Components
Thyristor
(diac)
Relay
Application and Uses
Switches a high power load
when triggered by a low
power signal. Remains on
even when trigger is
removed (use in Alarm
System)
Switches one or more high
power loads when operated
by a low signal source
Draw a simple electrical (light) circuit to
show: (a) its components and (b) its symbols
at its switch-off position

Include energy source Battery, light bulb
and a switch
(a)
(b)
How are components temporary
connected in a circuit
• Components can be
temporarily connected
using terminal blocks.
• The plastics covering
of the wires must be
removed from the
ends before they are
inserted in the
terminal blocks.
Bulbs
Circuit
Bulbsin
inaaParallel
Series Circuit
Components connected in series
and parallel circuit
A
C
B
What is a transistor?
•The transistor is an automatic
switch that can be operated by a
sensor and has three legs called the
base, collector and emitter.
•Sensor is connected to the base;
positive terminal of the battery
towards the collector; negative
terminal towards the emitter.
Draw the transistors and its symbol
Transistors
Symbol
Positive
negative
Explain how the input sensor
and transistor works?
• As the sensor detects
environmental changes, it
causes a voltage change
at the base of the
transistor.
• When the voltage rises
above 0.6 V, the
transistor switches on,
allowing current to flow
from the collector to the
emitter.
What is an open loop system?
•
•
In an automatic cooling system, when a
thermistor (input) senses a rise in the
temperature, a propellor blade (output)
connected to a motor is turned on by the
transistor. When the temperature falls, the
transistor switches off.
This is an open loop control system because the
fan does not actually cool the air.
INPUT
PROCESS
OUTPUT
thermistor
transistor
fan
What is a closed loop control
system?
• In an automatic water pump, when a moisture
sensor (input) detects water, a water pump
(output) operates when the transistor switches the
circuit on. When the water level is clear of the
sensor, the transistor switches off.
INPUT
PROCESS
OUTPUT
moisture
transistor
water pump
FEEDBACK
Explain how you can construct a simple
transistor circuit
to sensor
to
battery
variable
resistor
• The transistor circuit is
easily constructed using
self-adhesive copper
tape and a soldering
to output
iron.
component
• The tape may be stuck
transistor
to a small sheet of wood,
plastics or card and the
components are soldered
copper
directly to the tape.
tape
• The addition of a
variable resistor allows
the sensitivity of the
circuit to be adjusted
BIPOLAR TRANSISTOR
• A very small current at the base will switch on the transistor,
allowing a much larger current to flow between the collector
and emitter.
BIPOLAR TRANSISTOR
• The base of a transistor is usually connected to a sensor,
such as a light or temperature sensor, which determines the
amount of current flowing into the base.
• When the sensor allows enough current to flow, the
transistor switches on.
• A resistor is normally
connected to the base of a
transistor to prevent it from
being damaged by too much
current.
BIPOLAR TRANSISTOR
• In a P-N-P transistor, current is allowed to flow from the
emitter to the base and from the emitter to the collector.
SENSORS
• In dry conditions, there is a very high resistance
between the contacts of the sensor. No current is able to
flow into the base and the transistor is switched off.
In dry conditions,
no current flows
into the base of the
transistor so the
LED is off.
SENSORS
• In damp conditions, the resistance between the contacts is
much lower. A small current flows into the base and the
transistor switches on.
In damp conditions, a
small current flows
into the base of the
transistor so the LED
is on.
SENSORS
• INPUT-PROCESS-OUTPUT diagram for a moisturesensing circuit.
INPUT
PROCESS
OUTPUT
probes
transistor
LED/buzzer
SENSORS
• Light sensors use a component
called a light-dependent resistor
(LDR) to determine the amount of
current allowed through the
circuit when light falls on it.
• The LDR is a special kind of
resistor which changes its
resistance as the light level varies.
• The LDR is connected to the base
of the transistor.
SENSORS
• In darkness, the LDR has a high resistance (typically 1
M), so virtually no current flows through it. The
transistor is switched off.
In dark conditions,
no current flows
into the base of the
transistor so the
filament bulb is off.
SENSORS
• In bright light, the LDR has a low resistance (typically
270 ), allowing a larger current to flow. As the light
level increases, the flow of current through the LDR
increases, switching on the transistor.
In light conditions,
current flows into
the base of the
transistor so the
filament bulb is on.
SENSORS
• INPUT-PROCESS-OUTPUT diagram for a light-sensing
circuit.
INPUT
PROCESS
OUTPUT
lightdependent
resistor
transistor
bulb/LED/buzzer
SENSORS
• The amount of light needed to
switch on the transistor in the
light-sensing circuit can be
adjusted by adding a resistor
between the base and negative
side of the circuit.
• The lower the resistor value, the
greater the amount of light needed
to switch on the transistor.
• This arrangement of the LDR and
resistor is called a potential
divider.
SENSORS
• To switch on the transistor in
dark conditions and off in
light conditions, the LDR and
the resistor in the potential
divider can be reversed by
swapping positions.
SENSORS
• For extra sensitivity, two transistors may be used.
Darlington Pair for extra sensitivity
(very small current can pass through
SENSORS
• Temperature sensors use a
component called a thermistor to
determine the amount of current
allowed through the circuit when
temperature changes.
• The thermistor is a special kind of
resistor that changes its resistance
as the temperature varies.
• It is connected between the base
of the transistor and the positive
side of the circuit.
SENSORS
• Workings of the type of
thermistor that decreases its
resistance as the temperature
rises is described below and
in the next slide.
• In cold conditions, no current
flows through the thermistor
so the transistor is switched
off.
SENSORS
• As the temperature rises, the
flow of current through the
thermistor increases, switching
on the transistor.
SENSORS
• INPUT-PROCESS-OUTPUT diagram for a temperaturesensing circuit.
INPUT
PROCESS
OUTPUT
thermistor
transistor
bulb/LED/buzzer
SENSORS
• To adjust the sensitivity, a
potentiometer may be connected
between the base and the negative
track.
• The value printed on a potentiometer
states its resistance value between the
two outer contacts. By rotating the
shaft, the resistance between the
middle contact and either of the outer
contacts is adjusted.
SENSORS
• For extra sensitivity, two transistors may be used.
Electrolytic (polarised / +ve; -ve) Capacitor
• An electrolytic (polarised) capacitor must be connected so
that the leg marked with an arrow or ‘-’ sign is connected to
the negative side of the circuit.
non-electrolytic (non-polarised) capacitor
• A non-electrolytic capacitor (non-polarised) can be
connected either way around.