Computer Science 9618

3.2 Logic gates & Logic circuits with Majid Tahir

Syllabus Content:
1.3.3 Logic gates and logic circuits
use the following logic gate symbols

understand and defi ne the functions of NOT, AND, OR, NAND, NOR and XOR
(EOR) gates including the binary output produced from all the possible binary
inputs (all gates, except the NOT gate, will have two inputs only)
construct the truth table for each of the logic gates above
construct a logic circuit from either:
o – a problem statement
o – a logic expression
o -- a truth table
construct a truth table from either:
o – a logic circuit
o – a problem statement
o – a logic expression
Construct a logic expression from:
o – a logic circuit
o – a problem statement
o – a truth table

Logic gates and logic circuits

Introduction
Electronic circuits in computers, many new memories and controlling devices are made
up of thousands of LOGIC GATES. Logic gates take binary inputs and produce a binary
output.

Several logic gates combined together form a LOGIC CIRCUIT and these circuits are
designed to carry out a specific function. The checking of the output from a logic gate or
logic circuit is done using a TRUTH TABLE.

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3.2 Logic gates & Logic circuits with Majid Tahir

This chapter will consider the function and role of logic gates, logic circuits and truth
tables. Also a number of possible applications of logic circuits will be considered.
A reference to BOOLEAN ALGEBRA will be made throughout the chapter.

Truth tables:
Truth tables are used to trace the output from a logic gate or logic circuit.
The NOT gate is the only logic gate with one input; the other five gates have two inputs.
When constructing truth tables, all possible combinations of 1s and 0s which can be
input are considered. For the NOT gate (one input) there are only 21 (2) possible binary
combinations.
For all other gates (two inputs), there are 22 (4) possible binary combinations.

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3.2 Logic gates & Logic circuits with Majid Tahir

For logic circuits, the number of inputs can be more than 2;

For three inputs give a possible 23 (8) binary combinations.

And for four inputs, the number of possible binary combinations is 24 (16). It is clear
that the number of possible binary combinations is a multiple of the number 2 in every
case.

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3.2 Logic gates & Logic circuits with Majid Tahir

Logic Gates, Boolean Algebra and Truth Tables

Boolean Algebra is the mathematical foundation of digital circuits. Boolean Algebra specifies the
relationship between Boolean variables which is used to design combinational logic
circuits using Logic Gates. The truth table shows a logic circuit's output response to all of the
input combinations.

 Boolean Algebra
A Boolean Variable takes the value of either 0 (False) or 1 (True).
Symbols are used to represent Boolean variables e.g. A, B, C, X, Y, Z
There are three basic logic operations AND, OR, NOT
The Boolean Operators are • + ‾
 A + B means A OR B
 A • B means A AND B
 A means NOT A
Nodes in a circuit are represented by Boolean Variables

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3.2 Logic gates & Logic circuits with Majid Tahir

The function of the logic gates

NOT gate
A X

X = NOT A

X=A

AND gate
X

Input A Input B Output X

0 0 0
0 1 0
1 0 0
1 1 1

X = A AND B (logic notation)

X = A · B (Boolean algebra)

NAND gate
X

Input A Input B Output X

0 0 1
0 1 1
1 0 1
1 1 0

X = A AND B (logic notation)

X= A · B (Boolean algebra)

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3.2 Logic gates & Logic circuits with Majid Tahir

OR gate
X

Input A Input B Output X

0 0 0
0 1 1
1 0 1
1 1 1

X = A OR B (logic notation)
X = A + B (Boolean algebra)
NOR gate

X
Input A Input B Output X
0 0 1
0 1 0
1 0 0
1 1 0

X = A NOR B (logic notation)

X = A + B (Boolean algebra)
XOR gate

X
Input A Input B Output X
0 0 0
0 1 1
1 0 1
1 1 0
X = A XOR B (logic notation)

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3.2 Logic gates & Logic circuits with Majid Tahir

X = A . B + A . B (Boolean algebra)

Logic circuits:
When logic gates are combined together to carry out a particular function, such as
controlling a robot, they form a logic circuit. The output from the logic circuit is checked
using a truth table.
There now follows three examples which show:
how to produce a truth table
how to design a logic circuit from a given logic statement/Boolean algebra
how to design a logic circuit to carry out an actual safety function.

Example 1
Produce a truth table for the following logic circuit (note the use of • at junctions):

There are three inputs to this logic circuit, therefore there will be eight possible binary
values which can be input.
To show step-wise how the truth table is produced, the logic circuit has been split up
into three parts and intermediate values are shown as P, Q and R.

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3.2 Logic gates & Logic circuits with Majid Tahir

The truth table for the logic circuit will look like this:

Example 2:
A safety system uses three inputs to a logic circuit. An alarm, X, sounds if input A
represents ON and input B represents OFF; or if input B represents ON and input C
represents OFF.

Produce a logic circuit and truth table to show the conditions which cause the output X
to be 1.

The first thing to do is to write down the logic statement representing the scenario in this
example. To do this, it is necessary to recall that ON = 1 and OFF = 0 and also that 0 is
usually considered to be NOT 1. So we get the following logic statement:

Note: this statement can also be written in Boolean algebra as:

The logic circuit is made up of three parts as shown in the logic statement.

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We will produce the logic gate for the first part and the third part. Then join both parts
together with the OR gate.

Now combining both parts with the OR gate gives us:

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3.2 Logic gates & Logic circuits with Majid Tahir

Example 3:
A wind turbine has a safety system which uses three inputs to a logic circuit. A certain
combination of conditions results in an output, X, from the logic circuit being equal to 1.
When the value of X = 1 then the wind turbine is shut down.

The following table shows which parameters are being monitored and form the three
inputs to the logic circuit.

The output, X, will have a value of 1 if any of the following combination of conditions
occur:

either turbine speed <= 1000 rpm and bearing temperature > 80°C
or turbine speed > 1000 rpm and wind velocity > 120 kph
or bearing temperature <= 80°C and wind velocity > 120 kph.

Design the logic circuit and complete the truth table to produce a value of X =1 when
any of the three conditions above occur.

This is a different type of problem to those covered in Examples 1 and 2. This time a
real situation is given and it is necessary to convert the information into a logic
statement and then produce the logic circuit and truth table.

Stage 1:
The first thing to do is to convert each of the three statements into logic statements. Use
the information given in the table and the three condition statements to find how the
three parameters, S, T and W, are linked.

We usually look for the key words AND, OR and NOT when converting actual
statements into logic. We end up with the following three logic statements:

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3.2 Logic gates & Logic circuits with Majid Tahir

1. turbine speed <= 1000 rpm and bearing temperature > 80°C
logic statement: (S = NOT 1 AND T = 1)
2. turbine speed > 1000 rpm and wind velocity > 120 kph
logic statement: (S = 1 AND W = 1)
3. bearing temperature <= 80°C and wind velocity > 120 kph
logic statement: (T = NOT 1 AND W = 1)

We will start by joining (1) and (2) together using an OR gate:

Finally, we connect the logic circuit in 1,2 to 3 to obtain the answer:

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3.2 Logic gates & Logic circuits with Majid Tahir

The final part is to produce the truth table. We will do this using the original logic
statement. This method has the bonus of allowing an extra check to be made on the
logic circuit to see whether or not it is correct.
It is possible, however, to produce the truth table straight from the logic circuit. There
were three parts to the problem, so the truth table will first evaluate each part.

Then, by applying OR gates, as shown below, the final value, X, is obtained:

i. (S = NOT 1 AND T = 1)
ii. (S = 1 AND W = 1)
iii. (T = NOT 1 AND W = 1)

We find the outputs from parts (i) and (ii) and then OR these two outputs together to
obtain a new intermediate, which we will label part (iv). We then OR parts (iii) and (iv)
together to get the value of X.

Electronics companies need to consider the cost of components, ease of fabrication and
time constraints when designing and building logic circuits.

We will mention two possible ways electronics companies can review logic circuit
design:

One method is to use ‘off-the-shelf’ logic units and build up the logic circuit as a
number of ‘building blocks’.
Another method involves simplifying the logic circuit as far as possible; this may
be necessary where room is at a premium (e.g. in building circuit boards for use
in satellites to allow space exploration).

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3.2 Logic gates & Logic circuits with Majid Tahir

Using logic ‘building blocks’

One very common ‘building block’ is the NAND gate. It is possible to build up any logic
gate, and therefore any logic circuit, by simply linking together a number of NAND
gates.

For example, the AND, OR, NOT and XOR gates can be built from these gates as
shown below:

The AND gate using NAND Gates:

The OR gate using NAND Gates only:

The NOT gate using NAND Gate:

The XOR gate using NAND Gates:

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Exam questions:
Q.1 A system is monitored using sensors. The sensors output binary values corresponding to
physical conditions, as shown in the table:

The outputs of the sensors form the inputs to a logic circuit. The output from the circuit, X, is 1 if
any of the following three conditions occur:
either oil pressure >= 3 bar and temperature >= 200°C
or oil pressure < 3 bar and rotation > 1000 rpm
or temperature >= 200°C and rotation > 1000 rpm

(a) Draw a logic circuit to represent the above system.

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9608/13/M/J/18
Q.5 (a) A student needs to design a logic circuit to model the requirements for membership of a
snooker club. Membership (X) depends on four criteria, as shown in the table:

Membership is approved (X = 1) if the person:

 is over the age of 18 and has been recommended by a pre-existing member and
 either is working full-time or is retired, but not both.

Draw a logic circuit to represent the membership requirements.

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3.2 Logic gates & Logic circuits with Majid Tahir

9608/11/M/J/18
Q4 (a) An alarm system (X) is enabled and disabled using either a switch (A) or a
remote control (B). There are two infra-red sensors (C, D) and one door pressure
sensor (E).

The alarm sounds (X = 1) if the alarm is enabled and any one or more of the sensors is
activated. Draw a logic circuit to represent the alarm system.

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3.2 Logic gates & Logic circuits with Majid Tahir

Answers on the next page

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Answer:
9608/13/M/J/18
Q.5.

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3.2 Logic gates & Logic circuits with Majid Tahir

9608/11/M/J/18
Q5

References:
IGCSE Computer Science by Hodder Education.
Past papers.
http://electronics-course.com/logic-gates

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