1.5 LOGIC GATES
YOU WILL LEARN:
 HOW TO DESCRIBE THE AND, OR AND NOT LOGIC GATES USED IN A
COMPUTER
 HOW TO DRAW TRUTH TABLES AND AND, OR AND NOT GATES
 HOW COMPUTER LOGIC GATES COMPARE TO LOGIC IN THE REAL WORLD
THE AND LOGIC GATE
• The AND gate has two inputs.
• They are called A and B.
• It’s inside the CPU – it can only understand binary
• The value of each input can be 0 or 1
• The output of an AND gate is 1 if both input A and input B are 1.
• Otherwise, it is 0.
A
B
Output
0
0
0
0
1
0
1
0
0
1
1
1
THE AND
GATE
THE OR GATE
• It has two inputs (A and B)
• It has one output
• The output of an OR gate is 1 if either input A or input B is 1
THE OR GATE
A
B
Output
0
0
0
0
1
1
1
0
1
1
1
1
THE NOT GATE
• It has only one input and one output
• A NOT gate reverses the input
• If the input is 1 the output is 0
• If the input is 0 the output is 1
Input
Output
0
1
1
0
CIRCUITS
• Gates become more powerful and more useful when they are joined
together.
• When they are joined together, they form a circuit
CIRCUITS
A
B
C
Output
0
0
0
1
0
1
0
1
1
0
0
1
1
1
1
0
YOUR TURN
A
B
C
Output
0
0
1
0
0
1
1
1
1
0
0
0
1
1
0
0