Logic Circuits

A logic circuit (also called a logic diagram or combinational logic circuit) is a diagram showing how logic gates are connected together to process inputs and produce outputs. In your GCSE exam, you must be able to read logic circuit diagrams, trace values through them, write their Boolean expressions and construct their truth tables.

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What Is a Logic Circuit?

A logic circuit is a visual representation of a Boolean expression using standard logic gate symbols. It shows:

• The inputs on the left (labelled with letters such as A, B, C).
• The logic gates in the middle, connected by wires.
• The output on the right (usually labelled Q or Z).

Logic circuits are read from left to right — inputs enter on the left, pass through one or more gates, and the final output exits on the right.

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Standard Gate Symbols Reference

Before working with circuits, make sure you can recognise each gate symbol:

NOT gate:          AND gate:           OR gate:
  +-----\          +------\           +------\
--|      >o--    --|       )--       --|       \
  +-----/        --|       )         --|        )--
                   +------/            +------/