1. For OCR GCSE Computing Unit 1 - Theory
Logic Gates and Binary
For OCR GCSE Computing Unit 1 - Theory

2. Symbols and truth tables
Logic Gates
AND, OR and NOT
Symbols and truth tables

3. Logic Gates
Why logic gates? They are used to manipulate the signals in the processor.
Logic gates are simple circuits which perform Boolean functions. In other words, a circuit which produces an output based on the input.
Both the input and the output must be 1’s and 0’s and therefore these gates are the basis of all logic circuits.
The 1’s and 0’s are actually where a small voltage is a 1 and a very small voltage is a 0

4. Logic Gates - NOT
The NOT gate is sometimes known as an INVERTER as it will output the opposite or inverse of the input.
We can show this in a truth table.
Input
Output 1

5. Logic Gates - AND
The AND gate takes two inputs and gives an output only if both inputs are true.
We can show this in a truth table. This time we have two inputs A and B.
Input A
Input B
Output 1

6. Logic Gates - OR
The OR gate takes two inputs and gives an output if either one input or the other input or both inputs are true.
We can show this in a truth table. Again we have two inputs A and B.
Input A
Input B
Output 1

7. Combining Logic Gates - NOT & AND - NAND
We can also combine two or more gates and create a composite truth table.
Input A
Input B
A AND B
NOT (A AND B) 1

8. Combining Logic Gates - NOT & OR - NOR
We can also combine two or more gates and create a composite truth table.
Input A
Input B
A OR B
NOT (A OR B) 1

9. Logic Gates - Worksheet
Complete the logic gates worksheet.
You can use to test your predictions.

10. Counting in binary - binary to denary.
Binary addition.

11. Binary and Denary Binary is a number system written in Base 2.
Denary is a number system written in Base this is the decimal system we use in everyday life.
Later we will also meet Hexadecimal - which is in Base 16.
Lets compare counting in denary to counting in binary.....

12. Complete the binary / denary conversions worksheet

13. We can also perform addition in Binary
_______

14. We can also perform addition in Binary
_______
Don't forget the carry bits!

15. In exams you often get asked to add two eight bit numbers
_____________
Don't forget the carry bits!

16. We call this an overflow error.
In exams you often get asked to add two eight bit numbers - and explain this error...
_____________
What has happened here?
We call this an overflow error.

17. Now complete the binary addition worksheet