IGCSE · Practice questions · Drawing logic circuits

Wire it up.

Six original Cambridge-style questions on drawing logic circuits, with the gates and connections given in each solution.

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

Left to right.

Each bracket is a gate; the operator outside the brackets is the final gate.
Statements: and = AND, or = OR, not or = 0 means NOT. Draw what is written; do not simplify.
Truth tables: one AND term per row where X = 1, joined with OR; or spot a single gate.
Two inputs per gate (one for NOT), at most three inputs and one output; label everything.

Write

[2 marks]

Draw the logic circuit for X = (A NOR B) AND C. Describe the gates and how they are connected.

A NOR gate with inputs A and B. ✓
An AND gate with inputs from the NOR gate and C; its output is X. ✓

Write

[4 marks]

Draw the logic circuit for X = NOT (A AND B) OR (B XOR C). Describe the gates and how they are connected.

An AND gate with inputs A and B. ✓
A NOT gate on the output of the AND gate. ✓
An XOR gate with inputs B and C. ✓
An OR gate with inputs from the NOT gate and the XOR gate; its output is X. ✓

Apply

[4 marks]

A water pump P switches on (P = 1) if the tank is low (L = 1) and the mains supply is on (S = 1), or if the manual switch is on (M = 1) and the mains supply is on (S = 1). Write the logic expression and describe the circuit. Do not simplify it.

P = (L AND S) OR (M AND S). ✓
An AND gate with inputs L and S. ✓
A second AND gate with inputs M and S. ✓
An OR gate with inputs from the two AND gates; its output is P. ✓

Apply

[2 marks]

A truth table has inputs A and B. The output X is 1, 0, 1, 1 for the rows A B = 00, 01, 10, 11. Draw a logic circuit with two gates that produces this truth table.

X is 0 only for A = 0, B = 1, so X = A OR NOT B. ✓
A NOT gate on B, and an OR gate with inputs A and NOT B; its output is X. ✓

Analysis

[2 marks]

A student draws X = (A AND B) AND C as a single AND gate with three inputs. Explain why this is not acceptable at IGCSE and describe a correct circuit.

Logic gates other than NOT are limited to two inputs. ✓
Use one AND gate for A and B, then a second AND gate with its output and C. ✓

Apply

[3 marks]

A truth table has inputs A, B and C. The output X is 1 only in the row A = 0, B = 1, C = 1. Write a logic expression for X and describe a circuit for it.

X = (NOT A AND B) AND C. ✓
A NOT gate on A, and an AND gate with inputs NOT A and B. ✓
A second AND gate with inputs from the first AND gate and C; its output is X. ✓

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.