Logic Circuits
Master Logic Circuits for GCSE Computer Science with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA GCSE Computer Science (8525). Logic circuits combine gates to carry out more complex Boolean operations.
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These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA specification. Every worksheet comes with a full mark scheme.
Topic overview
A logic circuit combines gates so that the output depends on several inputs, and it is how a processor implements decisions.
To work out the output, trace signals from the inputs on the left through each gate to the output on the right. Evaluate each gate in turn, using the outputs of earlier gates as the inputs to later ones.
A Boolean expression describes the same circuit in writing. A circuit where A and B feed an AND gate, whose output feeds a NOT gate, is written as NOT(A·B). Brackets show which operation happens first, exactly as in arithmetic — and getting the bracket placement wrong changes the circuit entirely.
Revision notes
Tracing a circuit
Work from the inputs on the left to the output on the right.
Evaluate each gate in turn, using earlier gates' outputs as inputs to later ones. Never try to evaluate the final output directly without working through the intermediate stages.
Writing the expression
Each gate contributes to the expression: AND as a dot, OR as a plus, NOT as a bar or as NOT(...).
A circuit where A and B feed an AND gate, whose output feeds a NOT gate, is NOT(A·B). Brackets show which operation happens first.
Bracket placement
NOT(A·B) and NOT(A)·B are completely different circuits.
The first inverts the result of the AND; the second inverts A before the AND. Reading the circuit carefully to see which gate comes first determines where the brackets go.
Key points
- A logic circuit combines several gates.
- Trace signals from inputs to output.
- Evaluate each gate in turn.
- Earlier outputs become later inputs.
- Brackets show which operation happens first.
- Bracket placement changes the circuit entirely.
Worked examples
Example 1
A circuit has inputs A and B feeding an AND gate, whose output feeds a NOT gate. Write the Boolean expression. [2 marks]
Working
Example 2
Inputs A = 1 and B = 0 feed an AND gate followed by a NOT gate. Work out the output. [3 marks]
Working
Example 3
Explain why NOT(A·B) and NOT(A)·B represent different circuits. [2 marks]
Working
Common mistakes
Evaluating the final gate first.
Work from inputs to output, gate by gate.
Misplacing brackets in the expression.
They determine which gate acts first.
Forgetting intermediate values.
Write each gate's output down before continuing.
Reading the circuit right to left.
Signals flow left to right.
Exam tips
- Trace circuits left to right, one gate at a time.
- Write down each intermediate output.
- Place brackets to match the gate order.
- Check your expression against the diagram.
Key terms
- Logic circuit
- Several gates combined to produce an output.
- Boolean expression
- A written description of a logic circuit.
- Intermediate output
- The output of one gate feeding another.
- Bracket
- A symbol showing which operation happens first.
Related topics
Written and reviewed against the current AQA specification. Spotted an error? Let us know.