1. Lesson Objectives
Understand and produce simple logic diagrams using the operation NOT, AND and OR
Produce a truth table from a given logic diagram

2. Logic Gates
The digital computer uses an arrangement of tiny electronic switches (transistors) called logic gates connected together.
These logic gates are used to manipulate the signals within the processor.
The arrangement of the switches in the logic gate determines what it does to the input signal.
The logic gates are all connected to allow the processor to receive and pass on electronic signals

3. NOT There are three main logic gates. NOT:
If 0 is input then it outputs 1;
If 1 is input then it outputs 0.
INPUT
OUTPUT 1
INPUT
OUTPUT
OCR GCSE Computing © Hodder Education Slide 3

4. Test it out Go to http://logic.ly/ and click TRY ONLINE Add a swtich
Add a NOT gate
Add a light bulb

5. NOT A not gate only allows the data through if the signal is 0
When the switch is on = 0
When the switch is on = 1
The switch is on (1) therefore the output is off (0)
The switch is off (0) therefore the output is on (1)

6. AND: The AND gate outputs 1 only if both inputs are 1. INPUT A OUTPUT1
INPUT A
OUTPUT
INPUT B
OCR GCSE Computing © Hodder Education Slide 6

7. AND
The output will only be on (1) if both input are on (1)

8. OR: The OR gate outputs 1 if either of the inputs is 1. INPUT A OUTPUTB
OUTPUT 1
INPUT A
OUTPUT
INPUT B
OCR GCSE Computing © Hodder Education Slide 8

9. OR
The output will only be on (1) if EITHER or BOTH of the inputs are on (1)

10. Truth Tables
For every logic gate we can produce a truth table. This shows all of the possible inputs and the resulting outputs
What kind of logic gate does this diagram represent?
Inputs
Output
Good film?
Enough Money?
Go to cinema? No N
Yes

11. Worksheet 1
In the exam you may have to draw a logic diagram, or a truth table.
Complete the worksheet
Make sure that you label each diagram correctly with the different inputs and outputs

12. Logic Circuits
Computer systems are complicated things. Often, a number of logic diagrams are used together to create logic circuits.
AND
NOT

13. 1 1 1 AND NOT INPUT A INPUT B INPUT C OUTPUT Q INPUT A INPUT B INPUT C
INPUT A
INPUT B
INPUT C
OUTPUT Q 1
INPUT A
INPUT B
INPUT C
OUTPUT Q 1
We can now complete the output by looking at input C. As this is an NOT gate, the output is the opposite of the input
We can the fill the output of the AND gate. This output then becomes input C which is input into the NOT gate
We can first fill in the table with all of the different combinations of inputs from A and B
AND
NOT

14. Worksheet 2 Complete the work sheet on Logic Circuits
Everyone should attempt the first 2 questions.
Move on to questions 3 and 4 to stretch yourself