Build a circuit from a problem statement
Design a one-output gate circuit that follows a short condition exactly.
Exam scope and assessment notes
This original learning sequence follows Topic 10 of Cambridge IGCSE Computer Science 0984, 2026–2028 syllabus Version 4. It covers standard symbols and functions for NOT, AND, OR, NAND, NOR and XOR/EOR, and translation among statements, expressions, circuits and truth tables. NOT has one input; other gates have at most two. Circuit practice is limited to three inputs and one output and keeps the requested design without simplification. Lessons and checks support learning but do not award an official Cambridge grade. Full timed-paper practice, practical programming in an executable workspace and human subject-teacher review remain outstanding; the separate 2029–2031 syllabus is not mapped here. Cambridge states that 0984 has the same 2026–2028 subject content as 0478, with 9–1 grading. In June and November, Cambridge lists 0984 in Administrative zone 3 and for centres in Oman and the UAE. Confirm availability for your centre and exam series with Cambridge International. The separately published 2029–2031 syllabus changes to Python 3 only and removes pseudocode; this 2026–2028 route does not cover those changes.
Before you begin
- Be able to read binary 0 and 1 as the two possible logic values. Each lesson introduces the gate or translation skill it uses.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What should you do before drawing a circuit from prose?
What you will learn
- Translate a stated rule into an expression
- Draw gates with correctly connected inputs
- Check the circuit against the scenario
Core idea
A problem statement describes the circumstances that make an output 1. Translate that rule into an expression first; the expression makes the gate choices and order explicit before you draw wires.
In a logic circuit, 1 represents TRUE and 0 represents FALSE. A gate applies a defined rule to its input value or values and produces one binary output.
A reliable method
Define each input, identify words such as both, at least one and exactly one, write an expression, then draw the gates in its order. Finally test one allowed case and one disallowed case against the story.
Keep the given statement, expression, diagram or table in view while working. Check each row or connection against the rule, and preserve the stated circuit structure; the syllabus does not ask you to simplify it.
Pause and explain
A motor runs if either control A or control B is pressed. Which gate combines them?
Worked example
A greenhouse vent opens only when temperature T is high and rain R is absent. Describe its circuit.
Show the worked solution
- Rain absent is NOT R.
- Both high temperature and no rain are required.
- Use V=T AND NOT R.
Answer R enters NOT; T and NOT R enter AND; the output is V.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
A lamp turns on only if switch A and switch B are both on. Give the expression and circuit description.
Give me a hint
The two conditions must both be true.
Compare my reasoning
- Write L=A AND B.
- Connect both switches to an AND gate.
- Use the gate output as L.
L=A AND B; A and B feed AND, whose output is L.
Look for these in your work
- Expression
- Gate
- Output
A buzzer sounds if window W is open or door D is open. Write and describe the circuit.
Give me a hint
Either condition is enough, including both.
Compare my reasoning
- Use OR to combine W and D.
- Connect the two inputs to OR.
- Name the output Buzzer.
Buzzer=W OR D; W and D feed OR.
Look for these in your work
- Inclusive condition
- Circuit described
A warning light comes on when exactly one of two independent locks L1 and L2 is unlocked. State input meanings, expression and gate.
Give me a hint
Exactly one means the inputs must differ.
Compare my reasoning
- Define 1 as unlocked for each input.
- Exactly one true input is XOR.
- Connect both inputs to XOR and label the output.
Let L1 and L2 equal 1 when each lock is unlocked. Warning=L1 XOR L2; connect both to XOR.
Look for these in your work
- Input meanings
- XOR chosen
- Output stated
Common mistakes
- Forgetting to invert an absent condition
- Drawing a circuit before clarifying input meanings
- Adding conditions not stated in the problem
What can you explain now?
If a statement says an output requires A and not B, which input is inverted?
Compare with the explanation
B
The word NOT applies to B.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Why translate a problem statement into an expression before sketching gates?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.