Download Electricity – Electronic Control

KS4 Physics
Electronic Control
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Contents
Electronic Control
Controlling current
Electronic control systems
Logic gates
Bistable latch circuits
Summary activities
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Switches
A switch is a simple device
which makes it possible to
control the current flowing
in a circuit.
When the switch is open, it
creates a gap in the circuit
which stops current flowing.
The switch in this circuit
controls the lamp.
When the switch is closed,
it closes the gap and allows
current to flow.
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Relays
A relay is an electromagnetic switch which allows one circuit
to control another circuit by turning it on and off.
A relay is used when it is necessary to operate a high power
circuit with a small switch and thin wire.
The starter motor in a car
uses a current of several
hundred amps.
A switch to control that much
current would have to be
very large, so a relay is used
instead.
In this way, a small switch
can be used to start the car.
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Relays
A relay uses an electromagnet to operate a switch.
coil
steel
switch
M
input
output
As electricity flows in the input
circuit, the coil becomes an
electromagnet.
The coil attracts the steel
switch in the output circuit and
completes the second circuit.
Electricity flows in the output
circuit and so the motor works.
Circuit symbol
for a relay
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Parts of a relay
spring
coil of
wire
input
connections
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soft iron
armature
switch
contacts
output
connections
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Combining switches
More than one switch can be added to a circuit to give
more control over the output.
If there are alternative routes that the current can take,
a combination of switches is used to control these.
Switches at the top and
bottom of a flight of stairs,
which control the same
light, are an example of
this type of circuit.
+ -
If the light is on, operating either of the switches will turn it off.
If the light is off, operating either of the switches will turn it on.
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Combining switches
Car doors are fitted with
switches that operate the
interior light.
In this circuit, opening either
door or both doors will turn on
the light. The light will only go
out when both doors are shut.
The table shows the state of
the light for all the
combinations of the doors
being open or closed.
A closed door (open switch) is
denoted by a 0 and an open
door (closed switch) by a 1.
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door A
switch
door B
switch
door A door B
light
0
0
off
0
1
on
1
0
on
1
1
on
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Contents
Electronic control
Controlling current
Electronic control systems
Logic gates
Bistable latch circuits
Summary activities
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Parts of an electronic control system
A control system has three parts:
input sensor
processor
output device
This detects
changes that
the system
responds to.
This processes
information from
the input sensor
and operates the
output device.
This device is
controlled by
the processor.
How many different input sensors and the output devices
can you think of?
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Input sensors – switches
Switches that are activated by pressure, tilt, magnetic fields
or moisture may be used as input sensors.
A reed switch consists of
two springy metal strips in a
sealed glass tube. A magnetic
field near the switch makes
the strips come together.
A mercury switch consists of
some mercury and electrical
contacts in a sealed glass tube.
When the switch is tilted in a
certain direction, the liquid
mercury touches the contacts,
and completes the circuit.
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Input sensors – resistors
The resistance of a light
dependent resistor (LDR)
depends on the intensity of
the light shining on it.
This makes an LDR useful in
circuits which are controlled
by light intensity.
The resistance of a thermistor
depends on its temperature.
This makes it useful in circuits
which monitor and control
temperatures.
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Potential dividers
In a potential divider, the
input sensor is connected
in series with a resistor.
This type of device turns a
change in resistance in the
input device into a change
in voltage.
This change in voltage
may then be used to input
information into the
processor.
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Vin
R1
Vout
R2
0V
0V
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Potential dividers
A processor treats all
voltages below its threshold
voltage as LOW signals and
above its threshold voltage
as HIGH signals.
Vin
By using a variable resistor,
the level at which the input
sensor causes Vout to reach
the threshold voltage can be
adjusted.
This is useful in a thermostat
where the temperature that
triggers a heater can be
adjusted.
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Vout
0V
0V
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Output devices – light
A filament lamp is a common
output device which produces
light. Filament lamps are capable
of producing large amounts of
light but are not very efficient.
A light-emitting diode (LED)
is a type of diode which gives
out light when current is
passed through it.
LEDs require less current and
do not produce as much heat
as filament lamps. They are
often used as indicators and
in displays.
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Circuit symbol
for an LED
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Output devices – movement
A motor is a device that is
used to produce movement.
Circuit symbol
for a motor
M
Motors, especially those that do
a lot of work, require high
currents and voltages to work.
If a motor is being controlled by
a processor, the processor can
be damaged by the high current
used to power the motor.
A relay is used to prevent high
current damaging the processor.
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Output devices – sound and heat
Electric buzzers or loudspeakers
are output devices that can be
used to produce sound.
Circuit symbol
for a buzzer
Electric heaters are
output devices that can
be used to produce heat.
Circuit symbol
for a heater
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Components of control systems
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Contents
Electronic control
Controlling current
Electronic control systems
Logic gates
Bistable latch circuits
Summary activities
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What are logic gates?
Logic gates are electronic
switches that process
information.
They are called gates
because they open and
produce an output signal
This chip contains 4 NOT gates.
based on the correct
combination of input signals.
Logic gates are used in a
wide range of devices
including computers.
A large number of logic
gates can be incorporated
in one electronic chip.
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What is an AND gate?
An AND gate has two inputs,
input A input B output
A and B, and one output.
The output is only high when
A AND B are high.
0
0
0
0
1
0
1
0
0
1
1
1
Truth table for
an AND gate
An AND logic circuit
Circuit symbol
for an AND gate
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A
B
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What is an OR gate?
An OR gate has two inputs,
A and B, and one output.
The output is high when
either A OR B is high.
input A input B output
0
0
0
0
1
1
1
0
1
1
1
1
Truth table for
an OR gate
An OR logic circuit
Circuit symbol
for an OR gate
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A
B
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What is a NOT gate?
A NOT gate has only one input
and one output.
input
output
The output is always the
opposite of its input, so a
NOT gate is called an inverter.
0
1
1
0
NOT gates are often used in
combination with other logic
gates.
Truth table for
a NOT gate
Circuit symbol
for a NOT gate
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What is a NAND gate?
A NAND gate is a NOT gate
and a AND gate combined.
A NAND gate has two inputs
and one output.
The outputs are the opposite
to those that would be
obtained with an AND gate.
input A input B output
0
0
1
0
1
1
1
0
1
1
1
0
Truth table for
a NAND gate
Circuit symbol
for a NAND gate
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A
B
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What is a NOR gate?
A NOR gate is a NOT gate
and a OR gate combined.
A NOR gate has two inputs
and one output.
The outputs are the opposite
to those that would be
obtained with an AND gate.
input A input B output
0
0
1
0
1
0
1
0
0
1
1
0
Truth table for
a NOR gate
Circuit symbol
for a NOR gate
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A
B
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The alternative guide to logic gates!
What are the names of the different logic gates in this scene?
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Combining logic gates
Logic gates are normally used
in combination.
Consider a system in which
a heater is turned when it is
cold (A) and light (B).
The system must also have
a master switch (C) which
allows the system to be
switched on.
Next
inputs
Athe
and
B into
the
Firstly,
all
the
combinations
To
Lastly
work
inputs
out
C and
truth
Dtable
into
AND
toto
of
A,
B gate
and
areused
entered
the
the
output
OR
gate
ofCare
the
are
AND
used
gate
workout
values
of E.
D.
on
thethe
truth
table.
(D)
workout
must
the
be
calculated.
value
of
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A
D
B
E
C
A
0
0
0
0
B
0
0
1
C
0
D
0
E
1
0
0
0
1
0
1
1
1
0
0
1
1
0
0
1
0
1
0
0
1
1
1
1
1
1
1
1
0
1
0
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Combining logic gates problem
Consider a system in which an alarm sounds if it is cold or
raining. The temperature sensor gives a low signal if it is cold
and the rain sensor gives a high signal if it is raining. There
should also be a switch which turns on the alarm.
How could logic gates be connected to control this alarm?
There are two possible circuits…
temperature
rain
switch
temperature
switch
rain
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Contents
Electronic control
Controlling current
Electronic control systems
Logic gates
Bistable latch circuits
Summary activities
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What is a bistable latch circuit?
A bistable latch is a simple
memory circuit. It consists
of two NOR gates that are
connected together.
The circuit has two inputs
S (set) and R (re-set).
S=1
1
0
R=0
There are two stable states S = 0
which are set by a high
0
input on S or R.
If the high input stops, the
bistable stays (is latched)
in that state.
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0
output = 1
1
1
R=1
output = 0
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A bistable latch circuit – two NOR gates
S=0
1
A
0
1
1
0
10
B
R=0
output = 1
0
Imagine the bistable is in the state shown above. Both the
S and R inputs are low and so the output signal is low.
A high signal on R would have no effect on the circuit.
However, a high signal on S would cause a change to the
output of NOR gate A. This, in turn, would cause a change
to the input to NOR gate B and change its output.
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A bistable latch circuit – two NOR gates
S=1
0
A
10
0
1
0
1
B
R=1
0
1
output = 0
If input S now changes to a low signal, this will have no effect
on NOR gate A and the bistable will not change state and the
output will remain high.
The bistable can only change state if the reset input (R) is
high. This causes NOR gate B to have a low output, which
causes NOR gate A to have a high output. The bistable has
changed state and now has a low output.
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A bistable latch circuit – two NOR gates
S=0
A
0
1
1
B
R=1
output = 0
If the reset input (R) now reverted to low, this would have
no effect on the state of the bistable circuit.
In effect the bistable circuit
‘remembers’ which of the
inputs S or R was last in the
high state. This makes it
64 Mb of RAM has over
useful in electronic memory. 536 million bistable circuits!
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A bistable latch circuit – two NAND gates
R=1
A
0
1
10
1
0
B
S=0
1
output = 1
0
A bistable circuit can also be made using NAND gates
instead of NOR gates.
A bistable circuit using NAND gates changes state when
one of the inputs switches from high to low.
If S switches from high to low this will cause the output
from NAND gate B to switch to high.
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A bistable latch circuit – two NAND gates
R=0
1
A
0
1
0
1
1
0
B
S=1
0
output = 0
1
Switching S back to high would have no effect on the circuit.
Switching R to low would change the output from NAND gate
A which would switch NAND gate B and change the state of
the bistable circuit.
Switching R back to high would not effect the circuit and the
bistable is reset.
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Contents
Electronic control
Controlling current
Electronic control systems
Logic gates
Bistable latch circuits
Summary activities
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Glossary
 bistable latch circuit – A simple memory circuit which
consists of two NOR gates.
 input sensor – The part of a control system which detects
changes that the system responds to.
 logic gates – Electronic switches that produce an output
signal based on the correct combination of input signals.
 output device – The part of a control system which is
controlled by the processor.
 potential divider – A device which converts a change in
resistance in the input device into a change in voltage.
 processor – The part of a control system which processes
information from the input sensors.
 relay – A switch operated by an electromagnet, which
allows one circuit to control another circuit.
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Anagrams
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Multiple choice-quiz
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