Logic Gates
What are Logic Gates?
Logic gates are electronic components that perform logical operations on binary inputs (0 and 1).
Each gate produces a single binary output based on its input values.
They form the foundation of digital systems such as CPUs, memory, and storage devices.
The main logic gates are:
AND gate
OR gate
NOT gate
XOR gate
What is an AND Gate?
An AND gate outputs 1 only when all inputs are 1.
If any input is 0, the output is 0.
Input A | Input B | Output |
0 | 0 | 0 |
0 | 1 | 0 |
1 | 0 | 0 |
1 | 1 | 1 |
Example: Used in a security system that only unlocks when both the PIN and fingerprint are correct.
AQA Boolean Expression: A • B (read as A AND B)
OCR Boolean Expression: A ∧ B (read as A AND B)
Image to include:
Symbol with two inputs labelled A and B, curved right output labelled Q.
Title: AND gate symbol.
What is an OR Gate?
An OR gate outputs 1 when at least one input is 1.
It only outputs 0 when all inputs are 0.
Input A | Input B | Output |
0 | 0 | 0 |
0 | 1 | 1 |
1 | 0 | 1 |
1 | 1 | 1 |
Example: A light controlled by two switches in parallel turns on if either switch is on.
AQA Boolean Expression: A + B (read as A OR B)
OCR Boolean Expression: A ∨ B (read as A OR B)
Image to include:
Symbol with curved left, pointed right, two inputs A and B, output Q.
Title: OR gate symbol.
What is a NOT Gate?
A NOT gate (or inverter) has one input and one output.
It outputs the opposite of the input.
Input 0 → Output 1
Input 1 → Output 0
Input | Output |
0 | 1 |
1 | 0 |
Example: An alarm that activates when a sensor does not detect motion.
AQA Boolean Expression: Ā or NOT A
OCR Boolean Expression: ¬A
Image to include:
Symbol with triangle and small circle at the tip, input A, output Q.
Title: NOT gate symbol.
What is an XOR Gate?
An XOR (exclusive OR) gate outputs 1 only when the inputs are different.
It outputs 0 when both inputs are the same.
Input A | Input B | Output |
0 | 0 | 0 |
0 | 1 | 1 |
1 | 0 | 1 |
1 | 1 | 0 |
Example: A light controlled by two switches turns on only when one switch is on, not both.
AQA Boolean Expression: A ⊕ B (read as A XOR B)
OCR Boolean Expression: A ∨ B (read as A XOR B)
Image to include:
Symbol similar to OR gate with an extra curved input line, inputs A and B, output Q.
Title: XOR gate symbol.
Summary of Logic Gate Functions
Gate | Symbol | AQA Expression | OCR Expression | Description |
AND | • | A • B | A ∧ B | Outputs 1 only if both inputs are 1 |
OR | + | A + B | A ∨ B | Outputs 1 if at least one input is 1 |
NOT | – | Ā / NOT A | ¬A | Outputs the opposite of the input |
XOR | ⊕ | A ⊕ B | A ∨ B | Outputs 1 only if inputs are different |