Logic gates, truth tables and Boolean expressions

Overview

NOT, AND, OR and XOR gates and their symbols, writing and evaluating Boolean expressions, truth tables for three inputs, and building circuits from expressions.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   What does a NOT gate do?

   Model answer

   A NOT gate outputs the opposite of its single input. An input of 1 gives an output of 0, and an input of 0 gives 1.

2. 2

   When does an AND gate output 1?

   Model answer

   An AND gate outputs 1 only when both of its inputs are 1. Any other combination of inputs gives 0.

3. 3

   When does an OR gate output 1?

   Model answer

   An OR gate outputs 1 when at least one of its inputs is 1. The output is 0 only when both inputs are 0.

4. 4

   When does an XOR gate output 1?

   Model answer

   An XOR gate outputs 1 when exactly one of its inputs is 1. The output is 0 when both inputs are the same.

5. 5

   How does an XOR gate differ from an OR gate?

   Model answer

   An OR gate outputs 1 when both inputs are 1. An XOR gate outputs 0 when both inputs are 1, because its inputs must differ.

6. 6

   Which symbols represent AND, OR, XOR and NOT in a Boolean expression?

   Model answer

   A dot (written . or ·) represents AND and a plus sign represents OR. The symbol ⊕ represents XOR, and a bar over a term represents NOT.

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