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OCR H446 1.4.3 Logic gates and signal tracing

Part 2 of 11 · H446 1.4.3 · Boolean algebra

Reading a circuit is a separate skill from knowing what a gate does, and OCR H446 1.4.3 examines both. This worksheet covers the four required gate symbols and the discipline of tracing one signal at a time, including why a flowchart decision symbol never answers a logic-circuit question.

Students will:

  • recognise the OCR symbols for AND, OR, NOT and XOR gates
  • name the intermediate signals between gates instead of jumping to the output
  • work out a circuit output for a given set of input values
  • explain what a small circle on a gate output indicates
  • predict how a circuit behaves when one of its gates is replaced

Inside: 5 explanation cells, 3 multiple-choice questions, 2 fill-in-the-blanks cells and 1 written answer. 15 marks, about 40 to 50 minutes.

Series: H446 1.4.3 · Boolean algebra, part 2 of 11.

Shared by Coding PathwayVerified teacher

  • 11 cells
  • About 45 minutes
  • CC BY-SA 4.0
  • Shared 31 Aug 2026
  • Updated 3 Sept 2026

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The whole resource, exactly as a class sees it. Answers and marking are held back.

Logic gates and signal tracing

A logic circuit is a diagram of connected gates. You will learn to recognise the four required gate symbols and trace one signal at a time.

You should already know: the truth conditions for AND, OR, NOT and XOR.

Worked model: read shape, then read connection

OCR logic gate symbolsNOT¬AANDA ∧ BORA ∨ BXORA ⊕ BA bubble means inversion. XOR has the extra curved input line.

A NOT gate has one input and a small inversion bubble. AND has a flat left edge and curved right edge. OR has a curved input edge. XOR looks like OR with one extra curved line at the inputs.

A logic circuit is not a flowchart: wires carry Boolean signals and gate shapes perform Boolean operations.

Multiple choice1 mark

A student draws a decision diamond and labels it AND. Why is this not a valid answer to a logic-circuit question?

  • AAND gates must have three inputs
  • BLogic circuits cannot contain labels
  • CAND must always follow OR
  • DA decision diamond is flowchart notation, not a logic-gate symbol

Worked trace: name intermediate signals

Circuit for P equals open bracket A OR B close bracket AND NOT CABCDEP

For the circuit above:

  1. D = A ∨ B
  2. E = ¬C
  3. P = D ∧ E

If A=0, B=1 and C=0, then D=1 and E=1, so P=1. Naming D and E reduces working-memory load and gives you columns for a truth table.

Fill in the blanks3 marks
For A = 1, B = 0 and C = 1: D = entry 1, E = entry 2 and P = entry 3.
Multiple choice1 mark

For the same circuit, what is P when A=0, B=0 and C=0?

  • A0, because D=0 even though E=1
  • B1, because C=0
  • C1, because D and E are equal
  • D0, because NOT C is 0
Written answer5 marks

On paper, redraw the model circuit but replace its first OR gate with XOR. For A=1, B=1 and C=0, state D, E and P and explain the changed outcome.

Use the XOR rule before the NOT and final AND stages.

Students type their answer here.

Multiple choice1 mark

A small circle appears on the output of a gate. What does it mean?

  • AThe signal is stored
  • BThe signal is inverted
  • CThe inputs are swapped
  • DThe gate has no output

Closed-book checkpoint

Use the shape and signal language from this worksheet. No answer bank is provided.

Fill in the blanks4 marks
XOR is distinguished from OR by an extra entry 1 line at its inputs. A NOT gate normally has entry 2 input. A named value between gates is an entry 3 signal. Wires and gates form a logic entry 4, not a flowchart.

Review your responses

Check every response against its command word and the supplied constraints. Strengthen unsupported answers with accurate method, mechanism, state or contextual consequence before submitting.