IGCSE Physics 4.4 Digital Electronics MCQs – 30 Practice Questions with Answers (Cambridge 0625)
Free practice questions for Cambridge IGCSE Physics (0625), Section 4.4 Digital Electronics, covering the AND, OR, NOT, NAND and NOR logic gates, truth tables, gate combinations and simple control systems such as alarms and heating controls. Tap Show Answer under each question to reveal the correct answer with a step-by-step explanation. Most of this section sits in the Supplement part of the syllabus, and combination questions are tagged [Supplement].
Key facts recap
AND: output 1 only when both inputs are 1. OR: output 1 when at least one input is 1. NOT: output is the opposite of the single input. NAND = AND followed by NOT. NOR = OR followed by NOT.
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Single Logic Gates
Q1. In digital electronics, logic level 1 stands for which condition in a circuit?
- (A) Zero voltage everywhere in the circuit
- (B) A low voltage
- (C) A high voltage
- (D) A broken connection
Show Answer
Answer: C – Logic 1 is the higher of the two voltage levels used in the circuit. Logic 0 is the low level, close to 0 V.
Q2. An AND gate has inputs A = 1 and B = 0. What is its output?
- (A) 1
- (B) 0
- (C) 2
- (D) 10
Show Answer
Answer: B – An AND gate gives output 1 only when BOTH inputs are 1. Here B = 0, so the output is 0.
Q3. An OR gate has inputs A = 0 and B = 1. What is its output?
- (A) 0
- (B) 2
- (C) 1
- (D) 0 and 1 at the same time
Show Answer
Answer: C – An OR gate gives output 1 when at least one input is 1. Since B = 1, the output is 1.
Q4. A NOT gate receives an input of 0. What is its output?
- (A) 0
- (B) 1
- (C) 2
- (D) It depends on the frequency of the input
Show Answer
Answer: B – A NOT gate (inverter) reverses its single input: 0 → 1 and 1 → 0. So an input of 0 gives an output of 1.
Q5. A NAND gate can be built by connecting…
- (A) an OR gate followed by a NOT gate
- (B) a NOT gate followed by an OR gate
- (C) an AND gate followed by a NOT gate
- (D) two OR gates in series
Show Answer
Answer: C – NAND is short for NOT-AND: the inputs pass through an AND gate first, and the AND output is then inverted by a NOT gate.
Q6. A NAND gate has inputs A = 1 and B = 1. Its output is…
- (A) 0
- (B) 1
- (C) 2
- (D) 10
Show Answer
Answer: A – Step 1: the AND part gives 1 (both inputs are 1). Step 2: the NOT part inverts this to 0. A NAND gate gives output 0 only when both inputs are 1.
Q7. A NOR gate produces an output of 1 only when…
- (A) both inputs are 1
- (B) at least one input is 1
- (C) the two inputs are different
- (D) both inputs are 0
Show Answer
Answer: D – NOR is OR followed by NOT. The OR part gives 0 only when both inputs are 0, and the NOT part turns that 0 into 1. For every other input pair the output is 0.
Q8. For a two-input AND gate, which pair of inputs is the ONLY pair that gives an output of 1?
- (A) A = 0, B = 1
- (B) A = 1, B = 0
- (C) A = 0, B = 0
- (D) A = 1, B = 1
Show Answer
Answer: D – The full truth table for AND is: (0,0) → 0, (0,1) → 0, (1,0) → 0, (1,1) → 1. Only the last row gives output 1.
Q9. A two-input logic gate gives output 1 for the input pairs A = 0, B = 1; A = 1, B = 0; and A = 1, B = 1. It gives output 0 only for A = 0, B = 0. Which gate is it?
- (A) AND
- (B) OR
- (C) NAND
- (D) NOR
Show Answer
Answer: B – The output is 1 whenever at least one input is 1, and 0 only when both inputs are 0. That is the truth table of an OR gate.
Q10. A NOR gate has inputs A = 0 and B = 1. Its output is…
- (A) 0
- (B) 1
- (C) 2
- (D) 10
Show Answer
Answer: A – Step 1: the OR part gives 1, because B = 1. Step 2: the NOT part inverts this to 0.
Combining Gates
Q11. Inputs A and B feed an AND gate, and the output of the AND gate feeds a NOT gate. If A = 1 and B = 0, the final output is… [Supplement]
- (A) 0
- (B) 1
- (C) 2
- (D) 10
Show Answer
Answer: B – Step 1: AND gate: AND(1, 0) = 0, because the inputs are not both 1. Step 2: NOT gate inverts this: NOT(0) = 1. Final output = 1.
Q12. Inputs A and B feed an OR gate. The output of the OR gate and a third input C then feed an AND gate. If A = 1, B = 0 and C = 1, the final output is… [Supplement]
- (A) 0
- (B) 2
- (C) 1
- (D) It cannot be worked out
Show Answer
Answer: C – Step 1: OR gate: OR(1, 0) = 1. Step 2: AND gate receives this 1 and C = 1: AND(1, 1) = 1. Final output = 1.
Q13. An AND gate followed by a NOT gate behaves exactly like which single gate? [Supplement]
- (A) OR
- (B) NOR
- (C) NOT
- (D) NAND
Show Answer
Answer: D – NAND means NOT-AND. Building an AND gate and then inverting its output gives the NAND truth table: output 0 only when both inputs are 1.
Q14. Input A feeds a NOT gate. The output of the NOT gate and input B then feed an AND gate. If A = 1 and B = 1, the final output is… [Supplement]
- (A) 1
- (B) 0
- (C) 11
- (D) 3
Show Answer
Answer: B – Step 1: NOT gate: NOT(1) = 0. Step 2: AND gate: AND(0, 1) = 0, because one input is 0. Final output = 0.
Q15. Inputs A and B feed an AND gate to give a signal P. Signal P and input C then feed an OR gate. If A = 0, B = 1 and C = 0, the final output is… [Supplement]
- (A) 1
- (B) 2
- (C) 0
- (D) 10
Show Answer
Answer: C – Step 1: P = AND(0, 1) = 0. Step 2: OR gate: OR(0, 0) = 0, because neither input is 1. Final output = 0.
Q16. An LED must light up (output 1) only when switch X AND switch Y are both closed. Each closed switch provides logic 1. Which gate should connect the switches to the LED? [Supplement]
- (A) OR
- (B) NOT
- (C) NAND
- (D) AND
Show Answer
Answer: D – The LED needs output 1 for one input pair only: X = 1 and Y = 1. Only the AND gate does this; an OR gate would also light the LED when just one switch is closed.
Q17. An OR gate is followed by a NOT gate, making a NOR arrangement. Both inputs of the OR gate are 0. The final output is… [Supplement]
- (A) 1
- (B) 0
- (C) 2
- (D) 10
Show Answer
Answer: A – Step 1: OR gate: OR(0, 0) = 0. Step 2: NOT gate: NOT(0) = 1. Final output = 1, which matches the NOR truth table.
Q18. Inputs A and B feed an OR gate, whose output then feeds a NOT gate. Which input pair gives a final output of 1? [Supplement]
- (A) A = 1, B = 0
- (B) A = 0, B = 1
- (C) A = 1, B = 1
- (D) A = 0, B = 0
Show Answer
Answer: D – The final output is 1 only if the NOT gate receives 0, which means the OR gate must output 0. An OR gate outputs 0 only when both inputs are 0.
Q19. Inputs A = 1 and B = 1 feed an AND gate. Its output feeds one input of an OR gate, whose other input is C = 0. The final output is… [Supplement]
- (A) 1
- (B) 0
- (C) 2
- (D) 3
Show Answer
Answer: A – Step 1: AND gate: AND(1, 1) = 1. Step 2: OR gate: OR(1, 0) = 1, because at least one input is 1. Final output = 1.
Q20. Inputs A and B feed a NAND gate. The output of the NAND gate then feeds a NOT gate. If A = 1 and B = 1, the final output is… [Supplement]
- (A) 0
- (B) 2
- (C) 10
- (D) 1
Show Answer
Answer: D – Step 1: NAND gate: AND(1, 1) = 1, inverted gives 0. Step 2: NOT gate: NOT(0) = 1. Final output = 1. (Inverting a NAND output like this gives back the AND result.)
Applications & Sensing
Q21. A burglar alarm must sound (output 1) when a door sensor gives 1 OR a window sensor gives 1 (or both). Which gate connects the sensors to the alarm?
- (A) AND
- (B) OR
- (C) NOT
- (D) NAND
Show Answer
Answer: B – The alarm must respond when either sensor, or both, gives 1. That is exactly the OR truth table: output 0 only when both inputs are 0.
Q22. A boiler must switch on (output 1) only when a thermostat gives 1 (room too cold) AND a timer gives 1 (heating period). Which gate should be used?
- (A) OR
- (B) NAND
- (C) AND
- (D) NOR
Show Answer
Answer: C – Both conditions must be true at the same time, so the AND gate is used: output 1 only for the input pair (1, 1).
Q23. A light sensor gives logic 1 in darkness and logic 0 in daylight. The sensor is connected directly to a street lamp, which is on when it receives logic 1. The lamp is on…
- (A) only in daylight
- (B) only in darkness
- (C) all the time
- (D) never
Show Answer
Answer: B – The lamp receives 1, and so switches on, only when the sensor outputs 1, which happens in darkness.
Q24. A washing machine motor runs only when the door sensor gives 1 (door closed) AND the start button gives 1. The two signals feed an AND gate. The door is closed (1) but the start button has not been pressed (0). The motor…
- (A) does not run
- (B) runs at full speed
- (C) runs at half speed
- (D) runs in reverse
Show Answer
Answer: A – AND(1, 0) = 0, so the motor receives 0 and does not run. This is a safety feature: the machine cannot start with the door open or before Start is pressed.
Q25. Digital signals are well suited to computers because…
- (A) they can take any value in a range
- (B) they use only two levels, 0 and 1, which simple switches can represent reliably
- (C) they are a type of analogue signal
- (D) they need no electrical power
Show Answer
Answer: B – A switch is either on or off, matching the two logic levels perfectly. Small voltage variations do not change a 0 into a 1, so digital data is stored and processed reliably.
Q26. Which statement correctly compares analogue and digital signals?
- (A) An analogue signal can take any value within a range, while a digital signal takes only discrete values such as 0 and 1
- (B) A digital signal varies smoothly between values
- (C) An analogue signal has only two possible values
- (D) Analogue and digital signals are exactly the same
Show Answer
Answer: A – Analogue quantities (such as the voltage from a microphone) change continuously and can have any value. Digital signals jump between fixed levels, usually just two: 0 and 1.
Q27. An alarm system sounds (output 1) when the output of an OR gate, fed by a door sensor and a window sensor, AND a master switch are both 1. The door sensor gives 1, the window sensor gives 0, and the master switch is off (0). The alarm… [Supplement]
- (A) sounds loudly
- (B) sounds quietly
- (C) does not sound
- (D) sounds after a short delay
Show Answer
Answer: C – Step 1: OR gate: OR(1, 0) = 1 (the door is open). Step 2: AND gate: AND(1, 0) = 0, because the master switch is off. So the alarm stays silent.
Q28. A sensor gives logic 1 in bright light and logic 0 in darkness. A lamp must switch on (receive logic 1) in darkness. Which single gate placed between the sensor and the lamp will do this?
- (A) AND
- (B) OR
- (C) NOT
- (D) NAND
Show Answer
Answer: C – In darkness the sensor gives 0, but the lamp needs 1. A NOT gate inverts the signal: 0 → 1 in darkness (lamp on) and 1 → 0 in bright light (lamp off).
Q29. In a car, a warning buzzer is driven by an AND gate. One input is 1 when the headlights are on; the other input is 1 when the driver door is open. The headlights are on (1) and the door is shut (0). The output to the buzzer is…
- (A) 1
- (B) 2
- (C) 11
- (D) 0
Show Answer
Answer: D – AND(1, 0) = 0. The buzzer sounds only when the lights are on AND the door is opened, warning the driver not to leave the lights on.
Q30. Digital signals are preferred to analogue signals for sending data over long distances because…
- (A) small amounts of noise do not turn a 0 into a 1, so the original signal can be regenerated exactly
- (B) they travel faster than the speed of light
- (C) they never need cables or transmitters
- (D) they always use a higher voltage
Show Answer
Answer: A – Noise added during transmission slightly changes the signal level, but a 0 stays clearly a 0 and a 1 stays clearly a 1, so repeaters can rebuild a perfect copy. Analogue signals keep the noise permanently.
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