Spec & Goals 3 min
AQA Spec 3.4.2 · Boolean logic
By the end of this lesson you can:
- Recognise the AND, OR and NOT logic gate symbols.
- State the output of each gate for any given inputs.
- Complete the truth table for each gate.
Warm-Up 5 min
Last lesson you sorted parts into hardware and software. Inside the hardware, the CPU is built from tiny logic gates.
Quick starter
In binary, a switch is either off (0) or on (1). If both of two switches must be on for a lamp to light, when does the lamp stay off?
Reveal the answer
The lamp lights only when both switches are on (1 and 1). It stays off if either one is off. That is exactly how an AND gate behaves.
Key Concept — gates and truth tables 14 min
A logic gate takes one or more binary inputs (0 or 1) and gives one binary output.
An input is a binary value going into the gate. The output is the single binary value the gate produces.
AQA assesses three gates. Each has its own symbol and behaviour.
The AND gate
An AND gate outputs 1 only when all its inputs are 1.
| A | B | Q (A AND B) |
|---|---|---|
| 0 | 0 | 0 |
| 0 | 1 | 0 |
| 1 | 0 | 0 |
| 1 | 1 | 1 |
The OR gate
An OR gate outputs 1 when any input is 1.
| A | B | Q (A OR B) |
|---|---|---|
| 0 | 0 | 0 |
| 0 | 1 | 1 |
| 1 | 0 | 1 |
| 1 | 1 | 1 |
The NOT gate
A NOT gate has one input. It inverts it: 0 becomes 1, and 1 becomes 0.
| A | Q (NOT A) |
|---|---|
| 0 | 1 |
| 1 | 0 |
Worked Example — completing a truth table 12 min
Task: complete the truth table for an AND gate, row by row.
List the inputs A and B in counting order: 00, 01, 10, 11. The output is 1 only when both are 1.
| A | B | Q (A AND B) |
|---|---|---|
| 0 | 0 | 0 — not both 1 |
| 0 | 1 | 0 — A is 0 |
| 1 | 0 | 0 — B is 0 |
| 1 | 1 | 1 — both are 1 |
Try It Yourself 12 min
Goal: State the output of a NOT gate when the input is 0.
Hint: a NOT gate flips its input.
Goal: Complete the truth table for an OR gate, listing inputs in counting order.
Hint: the output is 1 unless both inputs are 0.
Goal: Draw the symbol for each of the three gates (AND, OR, NOT) and label the inputs and output.
📝 Exam Practice 10 min
Answer the way the examiner expects — the command word and the marks tell you how much to write.
Complete the truth table for an OR gate.
| A | B | Q |
|---|---|---|
| 0 | 0 | ? |
| 0 | 1 | ? |
| 1 | 0 | ? |
| 1 | 1 | ? |
Mark scheme
- Row 00 gives 0 (1).
- Rows 01, 10 and 11 all give 1 (1).
State the output of an AND gate when both inputs are 1.
Mark scheme
- 1 (1).
Name the logic gate that inverts its input.
Mark scheme
- NOT (gate) (1).
Which gate outputs 1 only when all its inputs are 1?
- A. OR
- B. NOT
- C. AND
- D. INPUT
Mark scheme
- C — AND (1).
Recap & Key Terms 3 min
A logic gate takes binary inputs and gives one binary output. AND outputs 1 when all inputs are 1; OR outputs 1 when any input is 1; NOT inverts its single input. A truth table lists every input combination.
- Logic gate
- An electronic component that takes one or more binary inputs and produces one binary output.
- AND
- A gate whose output is 1 only when all of its inputs are 1.
- OR
- A gate whose output is 1 when any of its inputs is 1.
- NOT
- A gate that inverts its single input: 0 becomes 1 and 1 becomes 0.
- Truth table
- A table listing every combination of inputs and the output for each.
- Input
- A binary value (0 or 1) going into a gate.
- Output
- The single binary value (0 or 1) a gate produces.
Homework 1 min
Task (≤ 15 min): Draw the truth table for a NOT gate, then write one sentence describing what the gate does.
Model answer
| A | Q |
|---|---|
| 0 | 1 |
| 1 | 0 |
A NOT gate inverts its input: it outputs the opposite of the value put in.
Award marks for: a correct two-row truth table (1), a correct description of inverting (1).