Recognise standard logic gate symbols
Identify the six standard gate symbols and use the output bubble to spot inversion.
Section 10 · Boolean logic
Exam scope and assessment notes
This original learning sequence follows Topic 10 of Cambridge IGCSE Computer Science 0478, 2026–2028 syllabus Version 6. 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.
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
Which value can a Boolean input take?
What you will learn
- Identify NOT, AND, OR, NAND, NOR and XOR symbols
- Distinguish the XOR extra input arc from OR
- Read an output bubble as inversion
Core idea
A gate symbol tells you which Boolean operation a circuit performs. NOT has one input. AND, OR, NAND, NOR and XOR each take two inputs in this syllabus. NAND and NOR use the AND and OR shapes with a small output bubble; the bubble means the result is inverted. XOR has a second curved line at its input side.
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
Check the shape and then the output. A curved OR outline with the extra input-side arc is XOR; an OR outline with an output bubble is NOR. Do not infer the operation only from the number of wires, because several two-input gates share that count.
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 curved OR symbol has a second arc near its inputs and no output bubble. Which gate is it?
Worked example
A two-input symbol has an AND-shaped body and a small circle on its output. Name it and explain its rule.
Show the worked solution
- The body is the AND gate shape.
- The output bubble means NOT the result.
- The combined operation is NOT (A AND B).
Answer It is a NAND gate; it outputs 0 only when both inputs are 1.
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.
Describe how to distinguish NAND from AND in a diagram.
Give me a hint
Compare the output end of the two symbols.
Compare my reasoning
- Both use the AND body.
- Look for the small output bubble.
- If present, it inverts the AND result.
NAND has an output bubble; AND does not.
Look for these in your work
- AND shape identified
- Bubble meaning stated
A symbol has an OR body and an output bubble. Name it and state how to interpret it.
Give me a hint
The bubble changes the output of the base gate.
Compare my reasoning
- Identify the OR body.
- Apply inversion to its output.
- The result is NOT (A OR B).
NOR; it outputs the inverse of A OR B.
Look for these in your work
- NOR named
- Inversion applied
A two-input gate symbol has a distinctive extra curve by its input wires, but no output bubble. Explain which gate it represents and what additional check would rule out NOR.
Give me a hint
Use both the input-side curve and output marker.
Compare my reasoning
- The extra input-side curve identifies XOR.
- There is no output bubble, so the output is not inverted.
- XOR returns 1 when exactly one input is 1.
It is XOR/EOR; unlike NOR, it has no output bubble and tests whether exactly one input is true.
Look for these in your work
- Symbol identified
- Function connected
Common mistakes
- Calling an output bubble an input
- Confusing XOR's extra curved line with an inversion bubble
- Treating NAND as an AND gate without inversion
What can you explain now?
What does the small circle at a gate's output mean?
Compare with the explanation
Invert the gate result
An output bubble indicates logical NOT.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Which visual feature marks XOR, and which marks inversion?
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.