Transcript logic-gates - Free Exam Papers

LOGIC GATES
A logic gate is an elementary
building block of a digital
circuit.
Most logic gates have two inputs
and one output terminals.
At any given moment, each
terminal is in one of the two
binary conditions low (0) or
high (1), represented by
different voltage levels
AND, OR and NOT gates
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There are three main
types of logic gate
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AND
NOT
OR.
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Two additional types
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In year 11 you also need to know about
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NAND – which is an AND + a NOT
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NOR – which is an OR + a NOT
Logic gates
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Logic gates are at the heart of digital electronics
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Video recorders, security lamps, alarm systems, and washing
machines are just some of the things controlled by electronic
switches called logic gates
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A logic gate contains a circuit with transistor
switches, but you do not need to know anything
about the circuit inside
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All you need to know is how the logic gate will
respond when it gets a signal – that is, what will its
output be.
Web sites
(copy to a word document to link to)

http://www.kpsec.freeuk.com/seriespa.htm#switchesseries
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http://isweb.redwoods.cc.ca.us/INSTRUCT/CalderwoodD/diglo
gic/and.htm
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http://en.wikipedia.org/wiki/Logic_gate#AND_gates
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http://isweb.redwoods.cc.ca.us/INSTRUCT/CalderwoodD/diglo
gic/index.htm
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http://www.bbc.co.uk/schools/gcsebitesize/design/electronics/co
ntrollogicrev2.shtml
The ideas behind logic gates
1.

Switches in Series
If two on-off switches are connected in series
they must all be closed (on) to complete the
circuit and light the lamp.
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if either A or B is OFF (open), the bulb is off
We say the output of the gate is 0
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If both A and B are ON (closed), the output is 1
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The AND gate symbol
One input (1 and 0) – no output so lamp is off (0)
(in the diagram, red means a signal)
Two inputs (1 and 1) – lamp is lit (output 1)
The truth table gives the results of all the possible switch
settings. It uses two logic numbers: 0 for OFF and 1 for ON
Input A
Input B
Output Q
0
0
0
0
1
0
1
0
0
1
1
1
The OR gate
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If two switches are connected in parallel then only one
needs to be closed (on) to complete the circuit.
One input – there is an output so
the lamp is on
the other input is on, so there is
an output
Two inputs – the lamp is on
No input – so no output and the
lamp is off
Truth table for the OR gate
Input A
Input B
Output Q
0
0
0
0
1
1
1
0
1
1
1
1
The NOT gate
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This is also called an INVERTER, because whatever
signal you put in, you get the opposite signal out.
‘signals’ mean a voltage, such as:
5v to mean 1 (on)
no voltage to mean 0 (off)
The NOT gate
No input, but there is an output
An input, but there is no output
Combining gates –
An inverter used to reverse a an output from a gate
The AND gate is giving an
output, but the inverter is not
Nothing from the AND gate, yet
the inverter gives a signal
A new truth table for this would be
A
0
0
1
1
B
0
1
0
1
C
0
1
1
1
Q
1
0
0
0
Tape recorder logic gate
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The following diagram shows one use for a logic gate.
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The cassette recorder will only start recording if the ‘record’ and
‘play’ buttons are pressed together.
Alarm system
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The next diagram shows how sensors and logic
gates can be used to control a security lamp.
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The sensors and gates have been connected so that
if it is dark and someone approaches, the lamp
comes on automatically.
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The last gate cannot provide enough power for the
lamp, so it switches on a relay instead. This switches
on a separate circuit with the lamp in it.
Logic gates working together to
make an alarm system
Checking the logic
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To check that the combination behaves as intended,
you can write a truth table for it.
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In this case, the light sensor’s output is LOW (0) in
the dark
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the body heat sensor’s output is HIGH (1) if
someone approaches
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the final output Q must be HIGH for the lamp to
come on.
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Using combinations of logic gates,
complex operations can be performed.
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In theory, there is no limit to the number
of gates that can be arrayed together in
a single device.
Two tasks for you to do
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Design a block diagram for a bank safe
that needs two keys to activate the lock.
The key sensors will send an output
when the correct key is used.
Design a logic system for street lights
that come on when it gets dark but only if
there are cars on the road.
For each one, write the truth table
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Arrays of logic gates are found in digital integrated
circuits (ICs) used in computers and all sorts of
electronic devices
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As technology advances, the required physical
volume for each individual logic gate decreases
and digital devices become capable of performing
ever-more-complicated operations at everincreasing speeds.
Sensors
the input devices for logic gates
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The role of sensors in control
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Sensors are used to measure physical quantities such as
temperature, light, pressure, sound, and humidity. They
send signals to the processor. For example:
A security alarm system may have an infra-red sensor
which sends a signal when the beam is broken.
A heat sensitive sensor in the corner of a room may
detect the presence of a person.
Temperature sensors could be used to control the
heating in a large building.
Pressure sensors in the road could detect traffic.
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Project task for this unit
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Using the web sites and other sources,
find out more about sensors and logic
gates.
Compile the information into a brief by
useful report. Direct downloads of
material is not accepted. The report
must show that you have organised the
information and presented is as a report.