AQA · GCSE · Computer Science
Course version: 8525 · Version from Sept 2020
Boolean logic: Create, modify and interpret simple logic circuit diagrams.
A logic circuit diagram shows how gates are wired between inputs and an output.
Checked against the AQA 8525 specification (opens in a new tab) on .
- Point 8 of 16
Study notes
From the checked teaching notes for this specification point.
The idea
A logic circuit diagram shows how gates are wired between inputs and an output. To create one, name the inputs (for example A, B and C), place the gate that handles the first operation, then feed its output into the next gate until a single output is reached. To modify one, swap a gate for another (for example replace an AND gate with an OR gate) or move an input to a different gate, then re-trace the path. To interpret one, work left to right: write the Boolean expression for each gate output, substitute 0 or 1 for the inputs, and evaluate gate by gate until the final output is found. Always check that every gate output connects onward and that no input is left unconnected.
What earns the marks
- Identify the input variables and the order in which gates are applied when reading a diagram from left to right.
- Write the Boolean expression produced by each gate before combining them into the final output expression.
- Substitute given input values (0 or 1) and evaluate each gate in turn to find the output.
- Recognise that changing a gate type or rewiring an input changes the Boolean expression and therefore the output.
- Check that each gate output is connected onward and that the circuit ends in a single output.
Common mistakes and how to put them right
Reading the diagram right to left and starting from the output.
Put it right: Trace from the inputs on the left through each gate to the output on the right.
Evaluating all gates at once and losing track of intermediate values.
Put it right: Record the output of each gate separately before moving to the next.
Assuming an AND gate behaves like an OR gate.
Put it right: AND outputs 1 only when all its inputs are 1, whereas OR outputs 1 when at least one input is 1.
Exam tips
- Label each gate output with a letter or expression as you work through the diagram so the final step is clear.
- When asked to interpret, substitute the given 0/1 values into the expression and show each intermediate result.
- When asked to modify, state exactly which gate or connection changes and how that alters the Boolean expression.
Try the lesson
The start of the Boolean logic: Create, modify and interpret simple logic circuit diagrams. lesson, made from checked teaching notes for this specification point.
Have a go first
A circuit takes two inputs, A and B, and feeds them into an AND gate. The output of that AND gate is then fed into a NOT gate. What is the output when A = 1 and B = 0?
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