This topic summary covers Knowledge Organiser: Logic Gates within Logic Gates for GCSE Computer Science. Revise Logic Gates in 3.4 Computer Systems for GCSE Computer Science with 16 exam-style questions and 20 flashcards. This is a high-frequency topic, so it is worth revising until the explanation feels precise and repeatable. It is section 9 of 9 in this topic. Use this topic summary to connect the idea to the wider topic before moving on to questions and flashcards.
Knowledge Organiser: Logic Gates
Key Terms
- Logic gate: An electronic component that takes binary inputs and produces a binary output based on a rule
- AND gate: Output is 1 only when ALL inputs are 1
- OR gate: Output is 1 when AT LEAST ONE input is 1
- NOT gate: Output is the OPPOSITE of the single input (also called an inverter)
- Truth table: A table showing all possible input combinations and the resulting output
- Inversion bubble: The small circle on a gate symbol indicating a NOT (inversion) operation
Must-Know Facts
- AND gate: output is 1 ONLY when both inputs are 1 (one row of 1s in the truth table)
- OR gate: output is 0 ONLY when both inputs are 0 (one row of 0s in the truth table)
- NOT gate: flips the input — 0 becomes 1, 1 becomes 0
- AND symbol: D-shape (flat back, curved front); OR symbol: pointed shape (curved back)
- NOT symbol: triangle with a small circle (bubble) at the output
- A bubble on any gate symbol always means inversion (NOT)
Key Concepts
- AND truth table: 0,0→0 | 0,1→0 | 1,0→0 | 1,1→1 (strict — all must be 1)
- OR truth table: 0,0→0 | 0,1→1 | 1,0→1 | 1,1→1 (generous — any 1 gives 1)
- NOT truth table: 0→1 | 1→0 (simple flip)
- Real-world AND: car starts only when key inserted AND brake pressed
- Real-world OR: alarm triggers if door opened OR window broken
Common Mistakes
- Confusing AND and OR output rules: AND outputs 1 only when ALL inputs are 1 (strict); OR outputs 1 when ANY input is 1 (generous) — a common error is mixing up which has one row of 1s and which has one row of 0s
- Drawing the wrong gate symbol shape: AND has a flat back with a curved (D-shaped) front; OR has a curved back with a pointed front — swapping the shapes loses marks in diagram questions
- Forgetting the inversion bubble on NOT gates: The NOT gate symbol is a triangle with a small circle (bubble) at the output — omitting the bubble makes it look like a buffer, not an inverter
- Not filling in all rows of a truth table: A 2-input gate has 4 combinations (00, 01, 10, 11) — missing any row is an error; a 3-input gate has 8 rows
- Confusing gate names with their operations in combination circuits: When gates are chained together, trace each gate's output step by step — trying to evaluate the whole circuit at once leads to errors
Practice questions for Logic Gates
An AND gate has inputs A=1 and B=0. What is the output?
Complete the truth table for an OR gate with two inputs A and B. | A | B | Output | |---|---|--------| | 0 | 0 | | | 0 | 1 | | | 1 | 0 | | | 1 | 1 | |