LOGIC GATES

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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

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AND, OR and NOT gates

• There are three main types of logic gate
• AND
• NOT
• OR.

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Two additional types

• In year 11 you also need to know about
• NAND which is an AND a NOT
• NOR which is an OR a NOT

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Logic gates

• Logic gates are at the heart of digital
  electronics
• Video recorders, security lamps, alarm systems,
  and washing machines are just some of the things
  controlled by electronic switches called logic
  gates
• A logic gate contains a circuit with transistor
  switches, but you do not need to know anything
  about the circuit inside
• All you need to know is how the logic gate will
  respond when it gets a signal that is, what
  will its output be.

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Web sites (copy to a word document to link to)

• http//www.kpsec.freeuk.com/seriespa.htmswitchess
  eries
• http//isweb.redwoods.cc.ca.us/INSTRUCT/Calderwood
  D/diglogic/and.htm
• http//en.wikipedia.org/wiki/Logic_gateAND_gates
• http//isweb.redwoods.cc.ca.us/INSTRUCT/Calderwood
  D/diglogic/index.htm
• http//www.bbc.co.uk/schools/gcsebitesize/design/e
  lectronics/controllogicrev2.shtml

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1. Switches in Series
The ideas behind logic gates

• 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
• If both A and B are ON (closed), the output is 1

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The AND gate symbol
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One input (1 and 0) no output so lamp is off
(0) (in the diagram, red means a signal)
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Two inputs (1 and 1) lamp is lit (output 1)
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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
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The OR gate

• If two switches are connected in parallel then
  only one needs to be closed (on) to complete the
  circuit.

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One input there is an output so the lamp is on
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the other input is on, so there is an output
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Two inputs the lamp is on
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No input so no output and the lamp is off
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Truth table for the OR gate
Input A Input B Output Q
0 0 0
0 1 1
1 0 1
1 1 1
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The NOT gate

• 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)

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The NOT gate
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No input, but there is an output
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An input, but there is no output
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Combining gates An inverter used to reverse a
an output from a gate
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The AND gate is giving an output, but the
inverter is not
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Nothing from the AND gate, yet the inverter gives
a signal
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A new truth table for this would be

• A B C Q
• 0 0 0 1
• 0 1 1 0
• 1 0 1 0
• 1 1 1 0

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Tape recorder logic gate

• The following diagram shows one use for a logic
  gate.
• The cassette recorder will only start recording
  if the record and play buttons are pressed
  together.

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Alarm system

• The next diagram shows how sensors and logic
  gates can be used to control a security lamp.
• The sensors and gates have been connected so that
  if it is dark and someone approaches, the lamp
  comes on automatically.
• 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.

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Logic gates working together to make an alarm
system
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Checking the logic

• To check that the combination behaves as
  intended, you can write a truth table for it.
• In this case, the light sensors output is LOW
  (0) in the dark
• the body heat sensors output is HIGH (1) if
  someone approaches
• 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.
• In theory, there is no limit to the number of
  gates that can be arrayed together in a single
  device.

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Two tasks for you to do

• 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
• 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
  ever-increasing speeds.

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Sensorsthe input devices for logic gates

• The role of sensors in control
• 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

• 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.