. to represent the AND gate + to represent the OR gate ⊕ to represent the…

The idea

Boolean logic notation uses agreed symbols so a circuit can be written as an expression. A dot (often written as a centred dot) stands for the AND gate, so A AND B is written with the dot between the two inputs. A plus sign stands for the OR gate, so A OR B uses the plus between inputs. A circled plus symbol stands for the XOR gate, so A XOR B uses that symbol between inputs. A bar drawn over a term, called an overbar, stands for the NOT gate, so NOT A is written as A with a bar above it. When you read or write an expression, match each symbol to its gate, keep the inputs in order and use brackets where grouping matters.

For example: The AND of A and B combined by OR with the NOT of C is written with a dot between A and B, a plus joining that group to C carrying an overbar.

What earns the marks

• The dot symbol represents the AND gate and is placed between its two inputs.
• The plus sign represents the OR gate and is placed between its two inputs.
• The circled plus symbol represents the XOR gate and is placed between its two inputs.
• An overbar above a term represents the NOT gate applied to that term.
• Correct reading of an expression requires matching each symbol to its gate and respecting brackets and grouping.

Common mistakes and how to put them right

• Confusing the plus sign with ordinary addition rather than the OR gate.

  Put it right: Read plus as OR in Boolean expressions.

• Treating the circled plus as the same as the plain plus.

  Put it right: The circled plus is XOR while the plain plus is OR.

• Writing NOT as a minus sign or the word placed after the term.

  Put it right: NOT is shown by an overbar above the term.

• Ignoring brackets and evaluating symbols in the wrong order.

  Put it right: Apply brackets first so the grouping is preserved.

Exam tips

• Learn each symbol with its gate name as a paired fact so recognition is instant.
• When reading an expression, name each gate aloud in turn before simplifying.
• Check brackets before applying any symbol, because grouping changes the result.