Cambridge O Level Computer Science · Syllabus 2210 · Boolean Logic
NOT Gate
What is NOT Gate?
A single-input logic gate, also called an inverter, whose output is the inverse of its input: an input of 0 gives an output of 1 and an input of 1 gives an output of 0; its symbol is a right-pointing triangle with a small inversion circle at the tip, and it is written X = NOT A.
This definition is part of the Boolean Logic chapter in Cambridge O Level Computer Science.
NOT Gate in context
The NOT gate, also called an inverter, has one input only. Its output is the inverse of its input: an input of 0 gives an output of 1, and an input of 1 gives an output of 0. It is the only single-input gate in this syllabus.
Common mistakes with NOT Gate
- “A NOT gate can take two inputs.”ActuallyNOT is a single-input gate. The syllabus states it explicitly, and there is no two-input NOT symbol.What you have really gotTwo wires arriving at a NOT means an AND or an OR has been left out. Draw the missing gate, then feed its single output into the NOT.
- “A NAND needs a NOT gate drawn after it to do the inverting.”ActuallyThe inversion is already inside the NAND symbol — that is what the circle is. Adding a NOT gate afterwards inverts a second time and turns the pair back into an AND.Same for NORA NOR followed by a NOT is an OR.
Examiner tips on NOT Gate
- Read the limits as instructions, not as background. “Circuits must be drawn for the statement given, without simplification” is not a hint about how examiners mark — it is the mark scheme. If the expression says NOT (A AND B), the circuit has an AND gate followed by a NOT gate. Replacing the pair with a single NAND gate changes the structure, and the structure is what is being assessed.
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