This document discusses logic signals and logic gates. It begins by defining logic signals as voltage signals with 0V corresponding to 0 and 5V/3V corresponding to 1. All digital information like text, images, sound and video are stored as patterns of binary values. The basic building blocks of digital circuits are logic gates, which perform operations on binary inputs. The three basic logic gates are AND, OR, and NOT. Truth tables are used to define the output of each gate given different combinations of binary inputs. More complex gates like NAND, NOR, XOR and XNOR are also discussed. Logic gates can have multiple inputs, and the document provides an example of a three input OR gate. Boolean algebra is used to represent the

1. Logic Signals
• A voltage signal with zero (0) corresponding to 0V
and one (1) corresponding to 5V or 3V.
• Switches, OPEN for "0" and CLOSED for "1".
• Modern digital computers store and process all kinds of
information as binary patterns.
• All the pictures, text. sound and video stored in this
computer are held as patterns of binary values.
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2. ICT1101/FND1120
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1 0 1 0 1 0 1 0
5v
0v

3. Logic Gates
• The Logic Gate is the basic building block of electronic circuits.
Central Processing Unit is made up of a collection of a large number
of logic gates.
• Basic logic gates perform logical operations of AND, OR and NOT on
binary numbers.
• Two voltage levels (High and Low), are represented in Boolean
Algebra. (in a standard truth tables it is denoted by binary digits
of “1” and “0”).
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Boolean Algebra Voltage State
1 High (5V/3V)
0 Low (0V)

4. How to develop the truth table
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• If there are 2 inputs, number of states = 22 = 4
• If there are 3 inputs, number of states = 23 = 8
• If there are 𝒏 inputs, number of states = 2n

5. Basic Logic Gates
• There are three types of basic logic gates. Those are,
1. AND gate
2. OR gate
3. NOT gate
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6. AND Gate
• When both switches 𝐴 and 𝐵 are closed only, the bulb
will be switched on.
• The bulb will not be switched on when either switch 𝐴
or 𝐵 is closed.
• When both switches are open, the bulb will not be
switched on.
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QQ

7. ICT1101/FND1120
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A
Q
B
• Relationships between inputs and outputs are given
below
Switch A Switch B Bulb Q
Open Open off
Open Closed off
Closed Open off
Closed Closed on
Switch A Switch B Bulb Q
0 0 0
0 1 0
1 0 0
1 1 1

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• Logic of AND gate is “A” AND“B”.
• According to Boolean expressions, it is represented as
𝐴. 𝐵

9. OR Gate
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10. • Relationships between inputs and outputs are given below
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A
QB
Switch A Switch B Bulb (Q)
Open Open off
Open Closed on
Closed Open on
Closed Closed on
Switch A Switch B Bulb (Q)
0 0 0
0 1 1
1 0 1
1 1 1

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• Logic of OR gate is “A” OR “B”.
• According to Boolean expressions, it is represented as
𝐴 + 𝐵

12. NOT Gate
• The task of the NOT gate is to provide the complement
as the output.
• Thus, complement of “0” is “1” and the complement of
“1” is “0”.
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• Relationships between inputs and outputs are given
below
A
Q
Switch A Bulb (Q)
Open on
Closed off
Switch A Bulb (Q)
0 1
1 0

14. • When the input is 𝐴 here, the complement of 𝐴 is the
output.
• According to Boolean expressions, it is represented as
𝐴
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15. Other Logic Gates
• NAND Gate
• NOR Gate
• XOR Gate
• XNOR Gate
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16. Logic Gates with three inputs
• AND , OR logic gates have minimum of two inputs. But there can be
more than one inputs.
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A
B
C
Q = A.B.C
The expanded view
A
B
C
A.B
Q =A.B.C

17. Exercise
1. How many states there can be for an OR gate
with three inputs?
2. Draw the expanded view of the above gate.
3. Develop a truth table for the OR gate mentioned
above
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18. Boolean expressions and Logical
circuits
Converting Boolean expressions to logic circuits.
• E.g.: Q = A + A.B
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A
B
A
A.B
Q = A + A.B

19. Converting Logic circuits to Boolean expressions.
E.g.:
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A
B
C
𝑄 = 𝐴 + 𝐵 . 𝐶

20. ICT1101/FND1120
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21. Thank You
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