How are the AND, OR and NOT operators drawn as logic gates and combined into circuits?
Logic gates: the AND, OR and NOT gates and their symbols, reading and drawing simple logic circuit diagrams, and producing the truth table for a combination of gates.
An OCR J277 2.4.1 answer on logic gates: the AND, OR and NOT gates and their symbols, reading and drawing simple logic circuit diagrams, and working out the truth table for a combination of gates.
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What this dot point is asking
OCR wants you to know the three logic gates, AND, OR and NOT, and their symbols, to read and draw simple logic circuit diagrams, and to work out the truth table for a combination of gates. Logic gates are the physical building blocks that carry out Boolean logic inside a processor. This is examined in Paper 2, usually as naming gates, completing a circuit's truth table, or writing the Boolean expression for a circuit.
The three logic gates
Reading and drawing logic circuits
A simple circuit drawn as a Mermaid diagram, where A and B feed an AND gate whose output is inverted by a NOT gate:
This circuit gives .
Truth table for a combination of gates
Try this
Q1. State which logic gate outputs 1 only when both inputs are 1. [1 mark]
- Cue. The AND gate.
Q2. State how many inputs a NOT gate has and what it does. [1 mark]
- Cue. One input; it outputs the opposite (inverts it).
Q3. Write the Boolean expression for a circuit where A and B go into an OR gate, and the output goes into a NOT gate to give Q. [1 mark]
- Cue. .
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR 20213 marksName the logic gate described by each statement: (a) the output is 1 only when both inputs are 1, (b) the output is the opposite of the input, (c) the output is 1 when at least one input is 1.Show worked answer →
(a) AND gate: the output is 1 only when both inputs are 1.
(b) NOT gate (also called an inverter): it has a single input and outputs the opposite value.
(c) OR gate: the output is 1 when one or both inputs are 1.
Markers reward the three correct gate names matched to the behaviour. The NOT gate is the only single-input gate of the three, which is a useful check.
OCR 20224 marksA logic circuit takes inputs A and B. A and B go into an AND gate. The output of the AND gate goes into a NOT gate, giving output Q. Write the Boolean expression for Q and complete its truth table.Show worked answer →
Expression (1): the AND gate gives , then the NOT gate inverts it, so .
Truth table (up to 3): work out first, then NOT it.
: AND = 0, Q = 1. : AND = 0, Q = 1. : AND = 0, Q = 1. : AND = 1, Q = 0.
So Q is 1 except when both inputs are 1. Markers reward the correct expression with the NOT applied to the whole AND, an intermediate AND column, and the correct Q column.
Related dot points
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Sources & how we know this
- OCR GCSE (9-1) Computer Science (J277) specification — OCR (2020)