8 - Logic Gates

• Computing
• Upper Secondary (Key Stage 4)
• GCSE

Logic Gates

Pens, pencils and paper needed

Can you remember the three main logic gates?

Key Objective: To gain undetstanding of boolean logic and Logic Gates.

All: Know the Main three logic gates, their symbols and truth tables.

Most: Understand some simple logic gate problems.

Some: Understand more complex problem solving with logic gates.

Anatomy of a logic gate

Truth Table: AND

Copy down the symbol and table

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

AND gate with two inputs.
Input A = 1, Input B = 0
What is the output?

1. 1
2. 0

AND gate with 3 inputs.
Input A = 1, Input B = 1, Input C = 0
What is the output?

1. 1
2. 0

Truth Table: OR

Copy down the symbol and table

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

OR gate with 2 inputs.
Input A = 0, Input B = 0
What is the output?

1. 1
2. 0

OR gate with 3 inputs.
Input A = 0, Input B = 0, Input C = 1
What is the output?

1. 1
2. 0

OR gate with 2 inputs.
Input A = 1, Input B = 1
What is the output?

1. 1
2. 0

Truth Table: NOT

Copy down the symbol and table

Input A	Output
0
1

Logic Gate Expressions

a AND (NOT b)

a	b	Output
0	0
1	0
0	1
1	1

Open Logic.ly here

Draw

(a OR b) AND (NOT c)

with logic gates:

(a OR b) AND (NOT c)

Now, draw the truth table.

(a OR b) AND (NOT c)

A	B	C	Output
0	0	0
0	0	1
0	1	0
1	0	0
0	1	1
1	0	1
1	1	0
1	1	1

Research task

Research with google:

NAND gates

NOR gates

XOR gates

Create with logic.ly a drawing using all three of these, and write the truth table for it.

Which Logic gate is this?

1. AND
2. OR
3. NOT

Which Logic gate is this?

1. AND
2. OR
3. NOT

Which Logic gate is this?

1. AND
2. OR
3. NOT

1. 1
2. 0