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Foundation3 past questions

Logic gates and Boolean algebra

Master the basic logic gates, their truth tables, Boolean expressions, universal gates, and how logic circuits are implemented.

Why this shows up in the exam

You must be able to analyze, design, and interpret digital logic circuits for NEET.

How NEET tests this

Application · 6 QsDirect recall · 3 Qs

Learn the idea

Logic gates are physical realizations of Boolean operations; the key insight is that a single universal gate (NAND or NOR) can be combined to reproduce any other gate, so mastering its truth table unlocks all digital logic.

🧠 Memory hook: NAND = Not‑AND, the Swiss‑army knife of logic – one tool does it all

Get this one clearly and it pays off every single time it shows up in the paper. 🎯

Formulas & facts to keep ready

  • OR gate: output 1 if any input is 1
  • AND gate: output 1 only when all inputs are 1
  • NAND gate = NOT (AND); its truth table is opposite of AND
  • Universal gate: a gate that alone can implement any Boolean function
  • De Morgan’s theorem: (AB)¯ = A¯ + B¯ and (A+B)¯ = A¯ B¯
  • NOT using NAND: A NAND A = A¯

How to approach it

  1. 1Read the question and note what is asked – truth table, expression or gate identification
  2. 2Write or recall the relevant truth table from memory
  3. 3Apply Boolean identities (e.g., De Morgan’s theorem) to simplify or convert the expression
  4. 4If the question is about universality, check whether the gate can produce NOT, AND and OR using only that gate

Worked example — watch it click

Which of the following gate is called universal gate?

  • A)OR gate
  • B)AND gate
  • ✅NAND gate
  • D)NOT gate

The concept behind this problem

The example asks which gate is universal; it tests the student’s knowledge that NAND alone can realize NOT, AND and OR, the defining property of a universal gate.

Step by step

  1. 1A universal gate is one that can be used to implement any Boolean function.
  2. 2Both NAND and NOR gates are universal gates.
  3. 3Using only NAND gates, we can construct NOT (A NAND A = Ā), AND (A NAND B, then NAND with itself), and OR gates (NOT A NAND NOT B).
  4. 4Since all logic functions can be built from these basic gates, NAND is universal.
  5. 5NOR is also universal, but among the given options, NAND gate is the correct answer.

Watch out

Choosing OR gate because it is familiar, overlooking the universal‑gate definition.

Common slip-ups that cost marks

  • •Confusing NAND with NOR – both are universal but only one is listed in the options
  • •Assuming OR or AND is universal because they are common
  • •Forgetting that a NOT can be obtained by tying the two inputs of a NAND together

🌟 That's the whole idea — you've got this. Try the practice set below; every question you attempt makes it stick a little harder.

Practise it

These are real questions from past NEET papers that test this exact idea.

Question 1 of 3NEET 2004

The output of OR gate is 1:

Push further

More challenging

18 harder questions built from the past papers above — a step up in difficulty, with distractors designed so you can't get there by elimination. Written and checked by our reviewers, not from a real paper.

Question 1 of 18

Consider a logic circuit with inputs P and Q, and output R. The truth table for this circuit is given below: P | Q | R --|---|--- 0 | 0 | 0 0 | 1 | 1 1 | 0 | 0 1 | 1 | 1 Which of the following Boolean expressions correctly represents the output R?

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