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Digital Electronics Fundamentals: 35 MCQs with answers

All 35 multiple-choice questions from this kit, free and with no sign-up.

Full kit · everything Flashcards · 30 Quick Revision · 15

Warm-up · 24 questions

Q1. Which of the following best describes a digital signal?

Beginner
A It takes only two discrete levels, 0 and 1. B It carries only analog audio information. C It takes any value in a continuous range. D It changes only when the clock stops.
Show answer & explanation

It takes only two discrete levels, 0 and 1.

A digital signal is discrete, using only two levels (0 and 1), unlike an analog signal which is continuous.

Q2. A temperature sensor outputs a smooth voltage from 0 V to 5 V. What type of signal is this?

Beginner
A Binary B Analog C Digital D Quantised
Show answer & explanation

Analog

A signal that can take any value in a continuous range is analog. Digital signals have only two discrete levels.

Q3. What is $(1011)_2$ in decimal?

Beginner
A 9 B 11 C 13 D 15
Show answer & explanation

11

$(1011)_2 = 8 + 0 + 2 + 1 = 11$.

Q4. How many distinct values can 8 bits represent?

Beginner
A 128 B 256 C 512 D 64
Show answer & explanation

256

With $n$ bits you can represent $2^n$ values, so 8 bits give $2^8 = 256$.

Q5. Which gate outputs 1 only when both inputs are 1?

Beginner
A AND B OR C NOT D XOR
Show answer & explanation

AND

An AND gate outputs 1 only when all its inputs are 1.

Q6. Which single gate type is functionally complete, meaning any Boolean function can be built from it alone?

Beginner
A NAND B XOR C OR D AND
Show answer & explanation

NAND

NAND (and NOR) is functionally complete — any Boolean function can be implemented using only NAND gates.

Q7. NAND (and NOR) gates are functionally complete: any Boolean function can be implemented using only NAND gates.

Beginner
A True B False
Show answer & explanation

True

NAND (and NOR) gates are functionally complete: any Boolean function can be implemented using only NAND gates.

Q8. What are the two values that a digital circuit typically uses?

Beginner
A 1 and 2 B 1 and 10 C 0 and 255 D 0 and 1
Show answer & explanation

0 and 1

Digital circuits operate using two discrete values, 0 and 1, which are known as binary.

Q9. What is the primary characteristic that distinguishes digital circuits from analog circuits?

Beginner
A Digital circuits can take any value B Analog circuits can take any value C Digital circuits use continuous signals D Analog circuits are more reliable
Show answer & explanation

Analog circuits can take any value

Digital circuits are characterized by their use of discrete values (0 and 1), while analog circuits can take any value in a continuous range.

Q10. Which of the following is a basic logic gate?

Beginner
A NAND B XOR C AND D NOR
Show answer & explanation

AND

AND is one of the fundamental logic gates used in digital circuits.

Q11. What does a half adder produce as its Sum output?

Beginner
A A + B B A · B C A + B + 1 D A ⊕ B
Show answer & explanation

A ⊕ B

The Sum output of a half adder is calculated using the XOR operation, represented as $A \oplus B$.

Q12. What is the output of an AND gate when both inputs are 0?

Beginner
A 1 B Depends on other inputs C 0 D Undefined
Show answer & explanation

0

An AND gate outputs 1 only when both inputs are 1; otherwise, it outputs 0.

Q13. What is the purpose of a truth table in digital electronics?

Beginner
A To measure current flow B To calculate voltage levels C To display circuit diagrams D To list input-output combinations
Show answer & explanation

To list input-output combinations

A truth table lists all possible input combinations and their corresponding outputs for a digital circuit.

Q14. Which theorem allows you to swap AND for OR and vice versa while flipping the inversion?

Beginner
A Distributive theorem B De Morgan's theorem C Idempotent theorem D Absorption theorem
Show answer & explanation

De Morgan's theorem

De Morgan's theorem provides the rules for swapping AND and OR operations while inverting the inputs.

Q15. What type of circuit's output depends only on its present inputs?

Beginner
A Combinational circuit B Memory circuit C Sequential circuit D Analog circuit
Show answer & explanation

Combinational circuit

A combinational circuit's output is determined solely by its current inputs, without any memory elements.

Q16. What are the two values that a digital circuit typically uses?

Beginner
A 1 and 2 B 0 and 5 C 0 and 1 D 1 and 10
Show answer & explanation

0 and 1

Digital circuits use binary values, which are 0 and 1.

Q17. What does the term 'binary' refer to in digital electronics?

Beginner
A A base-16 system B A base-8 system C A base-10 system D A base-2 system
Show answer & explanation

A base-2 system

Binary is a positional number system that uses base-2, consisting of the digits 0 and 1.

Q18. Which of the following is a basic logic gate?

Beginner
A Subtraction B Addition C Multiplication D AND
Show answer & explanation

AND

AND is one of the fundamental logic gates used in digital circuits.

Q19. What is the output of an AND gate when both inputs are 1?

Beginner
A 0 B Undefined C Both 0 and 1 D 1
Show answer & explanation

1

An AND gate outputs 1 only when both of its inputs are 1.

Q20. What is a truth table used for in digital electronics?

Beginner
A To list input combinations and their corresponding outputs B To measure current flow C To calculate voltage levels D To design physical circuits
Show answer & explanation

To list input combinations and their corresponding outputs

A truth table systematically lists all possible input combinations and their corresponding outputs.

Q21. What does a half adder produce as its Carry output?

Beginner
A A + B + 1 B A + B C A · B + 1 D A · B
Show answer & explanation

A · B

A half adder produces the Carry output as the product of its inputs, represented as $A \cdot B$.

Q22. What is the primary characteristic that distinguishes digital circuits from analog circuits?

Beginner
A Discrete levels B Higher voltage levels C Lower power consumption D Continuous signals
Show answer & explanation

Discrete levels

Digital circuits are characterized by discrete levels (0 and 1), whereas analog circuits deal with continuous signals.

Q23. How many distinct values can 4 bits represent?

Beginner
A 16 B 4 C 32 D 8
Show answer & explanation

16

With 4 bits, you can represent $2^4 = 16$ distinct values.

Q24. What is the output of an OR gate when both inputs are 0?

Beginner
A 0 B Both 0 and 1 C 1 D Undefined
Show answer & explanation

0

An OR gate outputs 0 only when both of its inputs are 0.

Building up · 6 questions

Q25. A circuit's output depends only on its present inputs. What type of circuit is it?

Intermediate
A Analog B Clocked C Sequential D Combinational
Show answer & explanation

Combinational

Combinational circuits depend only on present inputs; sequential circuits also depend on past inputs via flip-flops.

Q26. What does a truth table list?

Intermediate
A The gate count of the circuit B The output for every input combination C The clock frequency of the system D Only the rows where the output is 1
Show answer & explanation

The output for every input combination

A truth table lists the output for every possible input combination.

Q27. Which expression is the correct application of De Morgan's theorem to $\overline{A + B}$?

Intermediate
A $A \cdot B$ B $\overline{A} + \overline{B}$ C $A + B$ D $\overline{A} \cdot \overline{B}$
Show answer & explanation

$\overline{A} \cdot \overline{B}$

De Morgan's theorem states $\overline{A + B} = \overline{A} \cdot \overline{B}$.

Q28. Which Boolean identity represents absorption?

Intermediate
A $A \cdot \overline{A} = 0$ B $A + AB = A$ C $A + A = A$ D $A + \overline{A} = 1$
Show answer & explanation

$A + AB = A$

The absorption identity is $A + AB = A$; the other options are idempotent and complement laws.

Q29. What is the Sum output of a half adder with inputs A and B?

Intermediate
A $A \oplus B$ B $A + B$ C $\overline{A \cdot B}$ D $A \cdot B$
Show answer & explanation

$A \oplus B$

A half adder produces Sum $= A \oplus B$ and Carry $= A \cdot B$.

Q30. Why does a copied MP3 sound identical to the original, while a copied cassette does not?

Intermediate
A Cassettes store binary data that gets corrupted. B Bits are discrete and noise-resistant, so copies do not degrade. C MP3 files are smaller than cassette tapes. D MP3 players use analog filters to restore lost signal.
Show answer & explanation

Bits are discrete and noise-resistant, so copies do not degrade.

Digital information is discrete and noise-resistant, so it can be copied, stored, and transmitted without degradation.

Topper level · 5 questions

Q31. A designer needs to simplify a Boolean expression to reduce gate count. Which method is specifically used for visual simplification?

Advanced
A Truth table B Karnaugh map C Clock synchronisation D Sampling and quantisation
Show answer & explanation

Karnaugh map

Karnaugh maps are a visual method for simplifying Boolean expressions, reducing the number of gates needed.

Q32. A student writes $(102)_2$ as a binary number. What is the error?

Advanced
A Binary allows only the digits 0 and 1. B Binary numbers must have exactly four digits. C Binary numbers cannot contain the digit 0. D The number should be written as $(102)_{10}$.
Show answer & explanation

Binary allows only the digits 0 and 1.

In binary only the digits 0 and 1 are legal, so $(102)_2$ is meaningless.

Q33. A circuit must count and store a sequence of steps. Which design choice is required?

Advanced
A Use sequential logic with flip-flops and a clock. B Replace all gates with analog amplifiers. C Use only combinational gates with no memory. D Remove the clock to allow free-running updates.
Show answer & explanation

Use sequential logic with flip-flops and a clock.

Sequential circuits contain flip-flops (memory) and use a clock to synchronise updates, enabling counting and storage.

Q34. A designer is tasked with creating a digital circuit that outputs 1 when either input A is high and input B is low, or both inputs are high. Which of the following Boolean expressions correctly represents this requirement?

Advanced
A $Y = A + B$ B $Y = \overline{A} + B$ C $Y = A \cdot \overline{B} + A \cdot B$ D $Y = A + \overline{B}$
Show answer & explanation

$Y = A \cdot \overline{B} + A \cdot B$

The correct expression is $Y = A \cdot \overline{B} + A \cdot B$, which captures both scenarios: when A is high and B is low ($A \cdot \overline{B}$) and when both A and B are high ($A \cdot B$).

Q35. Given a circuit that implements a full adder using two half adders and an OR gate, if the inputs A, B, and Carry-in are all set to 1, what will be the output of the Sum and Carry outputs respectively?

Advanced
A Sum = 0, Carry = 0 B Sum = 0, Carry = 1 C Sum = 1, Carry = 0 D Sum = 1, Carry = 1
Show answer & explanation

Sum = 1, Carry = 1

For a full adder, the Sum output is calculated as $A \oplus B \oplus Carry-in$. Here, $1 \oplus 1 \oplus 1 = 1$ (Sum) and the Carry output is $A \cdot B + (A \oplus B) \cdot Carry-in = 1 \cdot 1 + (1 \oplus 1) \cdot 1 = 1 + 0 = 1$ (Carry). Thus, the outputs are Sum = 1 and Carry = 1.

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