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
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.
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 | |