LOGIC GATES/CIRCUITS 1 - INTRODUCTION/DEFINITION, TYPES, SYMBOLS AND USES (JS3 NOTES)

 LOGIC GATES/CIRCUITS  1

INTRODUCTION/DEFINITION, TYPES, SYMBOLS AND USES

Objective: At the end of the lesson, the students should be able to:

1. Define logic gates

2. State the types of logic gate

3. Draw logic gates symbols

4.State the uses of logic gates

DEFINITION OF LOGIC GATES

A logic gate is an elementary building block of a digital circuit. Most logic gates have two inputs and one output. At any given moment, every terminal is in one of the two binary conditions low (0) or high (1), represented by different voltage levels.

Types of Standard Single Logic gate

There are three types of standard single logic gates. They are:
1. AND gate
2. OR gate
3. NOT gate

Alternative Logic Gates are:

4. NAND gate

5. NOR gate

6. EX -OR (Exclusive OR) gate

1.  AND gate: AND gate will give a high output only if all of the inputs are high. It means multiplication (x). For example if A = 0 and B= 1, A AND B = 0 X 1 = 0

AND GATE SYMBOL

2. OR gate : This is a digital logic gate that gives an output of 1 when any of its inputs are 1, otherwise 0. it means addition (+).For example if A = 1 and B=1, then A OR B = 1+1 = 1

3. NOT gate: This is often called an inverter. A NOT gate is slightly different because it has just one input. It will give a high output if the input is low and vice versa. For example if A = 0, then NOT A = 1 and if A = 1, then NOT  A = 0

4. NAND gate : This is the combination of AND gate and NOT gate. It is the inverse of AND gate.

5. NOR gate : This is the combination of OR gate and NOT gate. It is the inverse of OR gate.

6. EX -OR (Exclusive OR) gate : The output is false (0) if both inputs are "false" (0) or if both inputs are true (1) . Similarly, the output is 1 if the inputs are different but 0 if the inputs are the same.

                                                          XOR GATE SYMBOL

USES OF LOGIC GATES

1. They are used as building blocks of digital electronics hardware

2. They are used for decision making

3. It is used in constructing alarm system.

4. It is used in traffic light control system

5. It is used in light activated burglar system.

6. It is used in safety thermostat.

7. It is used in constructing switches.

8. It is used in car security systems.

CLASS ACTIVITY

1. Define logic gates

2. List the types of logic gates and draw them

3. State four (4) uses of logic gates

LOGIC GATES/CIRCUITS  3

TRUTH TABLE - MEANING AND CONSTRUCTION OF TRUTH TABLE

MEANING OF TRUTH TABLE

The truth table is the table that shows the possible combinations of variable values in the equation and the result (output) for each of the logic gates.

For a two-input truth table, there will be 4 (2²) possible combinations of variable inputs and generally for n-input truth table, there will be 2ⁿ possible combinations of the input variables.