IGCSE · Practice questions · The six logic gates

Know your gates.

Six original Cambridge-style questions on identifying logic gates, describing what they do and completing their truth tables.

Original questions All questions on this page are original work, written in the Cambridge IGCSE style. They are not from past papers. They test the same concepts and skills the syllabus rewards.

Keep these straight

One rule per gate.

AND needs both inputs to be 1; OR needs at least one; XOR needs exactly one.
NAND and NOR are AND and OR with every output inverted.
Symbols: flat back = AND family; curved back = OR family; extra curve = XOR; circle = inverted output.
Rows: list inputs in binary counting order 00, 01, 10, 11 so none is missed.

Recall

[2 marks]

State the output of each gate when both of its inputs are 1: (a) AND, (b) NAND, (c) XOR, (d) NOR.

(a) 1 and (b) 0 (one mark for both). ✓
(c) 0 and (d) 0 (one mark for both). ✓

Explain

[2 marks]

Describe the difference between the functions of an OR gate and an XOR gate.

Both give 1 when exactly one input is 1, and both give 0 when both inputs are 0. ✓
When both inputs are 1, OR gives 1 but XOR gives 0. ✓

Apply

[4 marks]

Complete the truth table for a NOR gate.

A	B	X
0	0
0	1
1	0
1	1

0, 0 gives 1. ✓
0, 1 gives 0. ✓
1, 0 gives 0. ✓
1, 1 gives 0. ✓

Recall

[3 marks]

Describe the standard symbol for each of these gates: (a) NOT, (b) NAND, (c) XOR.

(a) NOT: a triangle with a small circle at the output. ✓
(b) NAND: the AND D shape (flat back, rounded front) with a small circle at the output. ✓
(c) XOR: the OR shape (curved back, pointed front) with an extra curved line behind the back. ✓

Analysis

[2 marks]

A gate has two inputs. Its output is 0 for exactly one combination of inputs: A = 1, B = 1. Identify the gate and give a reason.

NAND. ✓
It is AND inverted: AND is 1 only for 1, 1, so NAND is 0 only for 1, 1. ✓

Apply

[2 marks]

A NOT gate's output is connected to one input of an AND gate. The NOT gate's input is 0 and the AND gate's other input is 1. State the output of the NOT gate and of the AND gate.

NOT gate output: NOT 0 = 1. ✓
AND gate output: 1 AND 1 = 1. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.