Logic Gates
Logic Gates
• Logic gates take binary inputs and produce a binary output.
• The values it deals with are 0 and 1 or on and off.
• The binary states are the electrical voltage states, i.e. 0 represents an ON state and 1 represents an OFF state.
• Multiple logic gates are used to create a logic circuit.
• Logic gates are a crucial element of a logic circuit.
• Different logic gates have different characteristics.
• Truth Tables are used to verify the outputs of the logic gates.
Six (6) Types of Logic Gates:
• This figure illustrates the symbols of different logic gates.
1. NOT Gate:
• NOT Gate is the most simple logic gate.
• It takes a single input and inverts the value.
• If an input is 0, NOT will make it a 1 and vice versa.
• It is represented by 
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Truth Table:
2. AND Gate:
• AND Gate produces ‘1’ as an output only and only if all inputs are ‘1’.
• If anyone input is '0' then the result will be '0'.
• It is represented by A.B.
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Truth Table:
3. OR Gate:
• OR Gate produces ‘1’ in output if a single input is 1.
• It produces a ‘0’ in output only if both inputs are ‘0’.
• It is represented by a ‘+” sign.
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Truth Table:
4. NAND Gate:
• It is also known as the NOT AND Gate.
• It is the reverse of AND Gate.
• It gives ‘1’ in output if and only if A and B both are NOT ‘1’.
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Truth Table:
5. NOR Gate:
• It is also known as NOT OR Gate.
• It is the reverse of OR Gate.
• It produces ‘1’ in output if NOT a single input is 1.
Logic Notation: X = A NOR B |
Truth Table:
6. XOR Gate:
• It is also known as Exclusive OR Gate.• It produces ‘1’ in output if, (A is ‘1’ AND B is ‘0’) OR (A is ‘0’ AND B is ‘1’).
Logic Notation: X = A XOR BBoolean Algebra X = |
Truth Table:
Logic Circuits:
• Logic circuits are created by connecting logic gates together.• These circuits are created to perform a particular function.
• A logic circuit truth table is required to understand its function’s nature.
How to Create a Logic Circuit Truth Table:
• It is considerably complicated to create a logic circuit truth table.
• But if you follow the steps defined below, it will become easy for you to make it yourself.
• Don’t overdo yourself by trying to do everything in your head.
Steps:
1. First, add temporary output letters after each logic gate in a circuit as shown below:
2. Now find out the values of these new letters first i.e. K,L & M.
3. As you can see from the figure, that K is NOT A.
4. L is the output of A OR B
5. M is the output of K AND L.
6. And the final output will be M AND L.
7. This can also be written in the equation as X = 
8. Therefore, it is advisable to solve in the same sequence and you will get your truth table easily.
Note: Just as you can make a boolean equation by seeing the logic circuit you can also create a logic circuit from the equation.
Practice Exercise: Create a logic circuit and truth table for the following:
