This document discusses logic circuits and their application in aircraft systems. It introduces common logic gates like AND, OR, NOT, NAND and NOR gates. Their truth tables, symbol representations and equivalent circuits are defined. The document then discusses Boolean algebra and theorems. It provides examples of logic gate applications to aircraft systems for take-off configuration warning. It includes tutorial questions asking the reader to analyze logic gate arrangements and derive their Boolean expressions.

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DIGITAL TECH (MECH)
AKD 21102
CHAPTER 2
LOGIC CIRCUIT
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INTRODUCTION
• An ability to make decisions based on a variety
of different factors is crucial to the safe operation
of a modern aircraft.
• It is therefore not surprising that logic systems
are widely used in aircraft today
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CHAPTER CONTENT
1. Identification of common logic gate symbol,
tables, equivalent circuits and Boolean
expression
2. Introduction to :
• Application used for aircraft systems
• Interpretation of schematic diagrams
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BOOLEAN CONSTANT AND VARIABLE
• Allowed 2 binary variables only : 1 & 0
• Represent voltages or logic level
• May represent (in circuit) : ON,CLOSED (logic 1) and
OFF,OPEN (logic 0)
• Only 3 operation in Boolean algebra
• Logical addition (+), Logical Multiplication (•) and
Complementation ( ‾ )
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• Logic symbol:
• Input : A and B
• Output : X
• Truth table:
AND GATE
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• Simplified equivalent AND gate circuits
• Boolean Expression : X = A • B
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AND GATE

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• Logic symbol: • Boolean equation:
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X = A + B
• Truth table: • Equivalent circuit
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OR GATE

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• Logic symbol: • Boolean equation:
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X = Ā
• Truth table: • Equivalent circuit
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NOT GATE (Inverter)

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• Logic symbol: • Boolean equation:
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X = 퐴퐵
• Truth table: • Equivalent circuit
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NAND GATE

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• Logic symbol: • Boolean equation:
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X = 퐴 + 퐵
• Truth table: • Equivalent circuit
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NOR GATE

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Exclusive-OR (XOR) GATE
• Logic symbol: • Boolean equation:
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X =퐴 퐵 + 퐴퐵
• Truth table: • Equivalent circuit
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Exclusive-NOR (XNOR) GATE
• Logic symbol: • Boolean equation:
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X =A퐵 + 퐴 퐵
• Truth table: • Equivalent circuit
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BOOLEAN THEOREM
• Single variable theorem
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BOOLEAN THEOREM
• Multi variable theorem
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APPLICATION
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APPLICATION
• Consist of 2 OR gates and an AND gate, with
logic states received from 7 parameters
• When either throttle lever is pushed forward, the
switch at that position is made (advance) and
there is a logic state 1 to OR gate1.
• 5 other parameters are sensed and logic states
sent to OR gate2.
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APPLICATION
• If all the other parameters are in the take-off position, i.e.
1. Slats not fully extended
2. Flaps in take-off position i.e. less than 25°
3. Spoiler handle in retract position
4. Horizontal stabilizer in the green band (correct angle
of incidence for take-off).Then the inputs to OR gate2 are
all logic 0 and it’s output to the AND gate is logic state 0
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APPLICATION
• The aircraft on ground (weight switch) gives
another logic 1 to the AND gate, which has at
this time 2 logic 1’s and a logic 0
• If either of 4 inputs go out of take-off position eg
flaps >25°,then the flap input signal to logic
gate2 is logic 1 which makes the input to the
AND gate logic 1
• The AND gate now has 3 logic 1’s which now
gives an output to warning circuits (CONFIG
light and aural warning)
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TUTORIAL
1. Draw the truth table and state the Boolean expression for the logic
gate arrangement shown in Fig. 1.
Figure 1
2. State the Boolean logic expression for the logic gate arrangement
shown in Fig. 2.
» Figure 2
3. Devise a logic gate arrangement that provides
the output described by the truth table shown
in Fig. 3.
Figure 3
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