NOT
One input. The output is its opposite: 0 becomes 1 and 1 becomes 0.
OCR J277 · AQA 8525 · Cambridge 0478
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Free GCSE Computer Science guide
Recognise the gate symbols your board requires, draw a circuit from a Boolean expression and check every output with a truth table. Try original GCSE questions with indicative answers.
Content owner: Michael Print · Written for GCSE learners · Checked against official specifications
A logic gate receives one or more binary inputs, each 0 or 1, and produces one binary output. A truth table shows the output for every input combination.
A two-input gate needs four truth-table rows: 00, 01, 10 and 11. A three-input circuit needs eight rows.
Follow the syllabus for the year of your exam, especially when revising from older papers.
Draw inputs entering from the left and a single output leaving on the right. The small circle on NOT means “reverse the signal”. XOR looks like OR with a second curve behind the input side.
One input. The output is its opposite: 0 becomes 1 and 1 becomes 0.
OCR J277 · AQA 8525 · Cambridge 0478
Two inputs. Output 1 only when both are 1.
OCR J277 · AQA 8525 · Cambridge 0478
Two inputs. Output 1 when either or both inputs are 1.
OCR J277 · AQA 8525 · Cambridge 0478
Two inputs. Output 1 when exactly one input is 1.
AQA 8525 · Cambridge 0478
Each is the corresponding AND or OR shape with an inversion circle at its output. Leave out that circle and you have drawn a different gate.
An AND gate with an inversion circle. Output 0 only when both inputs are 1.
An OR gate with an inversion circle. Output 1 only when both inputs are 0.
For X = (A AND NOT B) OR C, work from the brackets and the individual inversion towards the final output.
Worked example
For A = 1, B = 0 and C = 0: NOT B = 1; A AND NOT B = 1; 1 OR C = 1. Therefore X = 1.
Use the standard symbol outlines. A box with a gate name may not earn a drawing mark; make every wire meet the correct gate and label the inputs and output.
List every input combination first, then calculate each gate in circuit order. Intermediate columns help you find a mistake and show how you reached the output.
| A | NOT A |
|---|---|
| 0 | 1 |
| 1 | 0 |
| A | B | AND | OR | XOR |
|---|---|---|---|---|
| 0 | 0 | 0 | 0 | 0 |
| 0 | 1 | 0 | 1 | 1 |
| 1 | 0 | 0 | 1 | 1 |
| 1 | 1 | 1 | 1 | 0 |
XOR is included for AQA 8525 and Cambridge IGCSE 0478. OCR J277 learners can use the AND and OR columns only.
These questions are independently written and aligned to the named specifications; their marking guidance is indicative, not an official mark scheme.
OCR J277, AQA 8525 and Cambridge 0478 · identify gates. An alarm turns on when sensor A or sensor B is active. A second alarm turns on only when both are active. Name the gate for each alarm and explain the difference in their outputs.
Answer: OR (1), because either input can make the output 1 (1); AND (1), because both inputs must be 1 (1).
OCR J277, AQA 8525 and Cambridge 0478 · circuit drawing. Draw a circuit using standard gate symbols for Y = (P OR Q) AND NOT R. Label all inputs and the output.
One valid circuit: connect P and Q to an OR gate; send R through a NOT gate; connect both intermediate outputs to an AND gate; label its output Y.
Indicative marks: correctly drawn OR symbol taking P and Q (1); correctly drawn NOT symbol taking R, including its inversion circle (1); both outputs connected to a correctly drawn AND symbol (1); final output labelled Y (1).
AQA 8525 and Cambridge 0478 · truth tables. Complete a four-row truth table for A XOR B and A OR B. State the input combination for which their outputs differ.
| A | B | A XOR B | A OR B |
|---|---|---|---|
| 0 | 0 | 0 | 0 |
| 0 | 1 | 1 | 1 |
| 1 | 0 | 1 | 1 |
| 1 | 1 | 0 | 1 |
Indicative marks: correct four XOR outputs (2); correct four OR outputs (2); explains that the gates differ when both inputs are 1 (1).
Cambridge IGCSE 0478 only · inverted gates. Complete a four-row truth table for A NAND B and A NOR B. Explain how each output relates to AND or OR.
| A | B | A NAND B | A NOR B |
|---|---|---|---|
| 0 | 0 | 1 | 1 |
| 0 | 1 | 1 | 0 |
| 1 | 0 | 1 | 0 |
| 1 | 1 | 0 | 0 |
NAND reverses AND; NOR reverses OR. Indicative marks: two marks for each correct output column and one for the explanation.
The named boards share a core idea but require different sets of gates. For Cambridge IGCSE 0478, circuits can be drawn from a statement, expression or truth table and are limited to three inputs and one output. Check the syllabus for your exam year.
These are independently written explanations and practice questions. CompSciTutoring.co.uk is not affiliated with or endorsed by an examination board. The marking guidance is indicative; always check the syllabus for your examination year.
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Use the free examples first, then explore one-to-one tuition if guided practice would help.