Boolean logic: Create a logic circuit from a simple Boolean expression.

The idea

A Boolean expression such as Q = (A AND B) OR (NOT C) is a written rule for an output. To turn it into a circuit, work from the inside out. Identify each input (A, B, C) and draw it as a labelled line. Build the bracketed group first: an AND gate takes A and B and produces an intermediate signal. Separately pass C through a NOT gate to invert it. Then feed the AND output and the NOT output into an OR gate, whose output is Q. Each gate has a fixed shape and symbol, and every connection must be drawn so the route from input to output is unambiguous. Check the finished diagram by tracing one combination of inputs, for example A = 1, B = 1, C = 0, and confirming the gates give the value the expression predicts.

What earns the marks

• Recognises that each named variable in the expression becomes a labelled input line to the circuit.
• Selects the correct gate for each operator: AND for conjunction, OR for disjunction and NOT for negation.
• Builds sub-expressions in the correct order, using intermediate signals where brackets group terms.
• Connects gate outputs to the inputs of later gates so the final output matches the whole expression.
• Verifies the circuit by tracing at least one input combination and comparing with the expression's predicted output.

Common mistakes and how to put them right

• Drawing gates in the order the symbols appear left to right without respecting brackets.

  Put it right: Evaluate the bracketed sub-expression first and feed its output onward.

• Using an OR gate where the expression uses AND (or vice versa).

  Put it right: Match each operator to its own gate symbol before drawing.

• Leaving inputs unlabelled or joining lines at a point without a gate.

  Put it right: Label every input and ensure every junction is a defined gate output.

Exam tips

• Underline each operator and bracket in the expression before drawing anything, so the build order is clear.
• Draw intermediate signal lines and label them, which makes a multi-gate circuit easier to mark and to check.
• Finish by tracing one or two input combinations through your diagram to confirm the output matches the expression.