lesson objectives understand and produce simple logic diagrams using the operation not, and and or...

Lesson Objectives

• Understand and produce simple logic diagrams using the operation NOT, AND and OR

• Produce a truth table from a given logic diagram

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

OCR GCSE Computing © Hodder Education 2013 Slide 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

INPUT OUTPUT

0 1

1 0

Test it out

Go to http://logic.ly/ and click TRY ONLINE

1. Add a swtich

2. Add a NOT gate

3. Add a light bulb

NOTWhen the switch is on = 0When the switch is on = 1

A not gate only allows the data through if the signal is 0

The switch is on (1) therefore the output is off (0)

The switch is off (0) therefore the output is on (1)

OCR GCSE Computing © Hodder Education 2013 Slide 6

• AND: – The AND gate outputs 1 only if both inputs are 1.

INPUT A

OUTPUT

INPUT B

INPUTA

INPUTB

OUTPUT

0 0 0

0 1 0

1 0 0

1 1 1

AND

• The output will only be on (1) if both input are on (1)

OCR GCSE Computing © Hodder Education 2013 Slide 8

• OR: – The OR gate outputs 1 if either of the inputs is 1.

INPUT A

OUTPUT

INPUT B

INPUTA

INPUTB

OUTPUT

0 0 0

0 1 1

1 0 1

1 1 1

OR

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

Truth Tables

Inputs Output

Good film? Enough Money? Go to cinema?

No No N

Yes No No

No Yes No

Yes Yes Yes

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?

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

Logic Circuits

• Computer systems are complicated things. Often, a number of logic diagrams are used together to create logic circuits.

AND NOT

INPUT A INPUT B INPUT C OUTPUT Q

1 1

1 0

0 1

1 0

AND NOT

INPUT A INPUT B INPUT C OUTPUT Q

1 1 1 0

1 0 0 1

0 1 0 1

1 1 0 1

INPUT A INPUT B INPUT C OUTPUT Q

1 1 1 0

1 0 0 1

0 1 0 1

1 1 0 1

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

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