Logic gates, circuits and truth tables — GCSE Computer Science practice questions

Question 1

Lena is reading the circuit diagram of a model railway signal. One of its symbols is shown, with inputs A and B and output X.

What is the name of this logic gate?

Accepted Answer

B: NOR. The body has a curved back and a pointed front, so the gate is in the OR family, and there is no extra curved line at the inputs, so it is not XOR. The small circle on the output inverts the result, which makes it a NOR gate: X is 1 only when both A and B are 0.

Question 2

A fridge has an alarm A with three inputs:

| Input | Binary value | Meaning |
|---|---|---|
| D | 0 | Door closed or open for less than 2 minutes |
| D | 1 | Door open for more than 2 minutes |
| T | 0 | Temperature normal |
| T | 1 | Temperature too high |
| F | 0 | Defrost cycle not running |
| F | 1 | Defrost cycle running |

The truth table shows when the alarm sounds (A = 1).

| D | T | F | A |
|---|---|---|---|
| 0 | 0 | 0 | 0 |
| 0 | 0 | 1 | 0 |
| 0 | 1 | 0 | 1 |
| 0 | 1 | 1 | 0 |
| 1 | 0 | 0 | 1 |
| 1 | 0 | 1 | 1 |
| 1 | 1 | 0 | 1 |
| 1 | 1 | 1 | 1 |

Which statement describes when the alarm sounds?

Accepted Answer

C: Whenever the door has been open too long, and also when the temperature is too high while the defrost cycle is not running. A = 1 in all four rows where D = 1, so a door left open always sounds the alarm. The only other row with A = 1 is D = 0, T = 1, F = 0, so a high temperature sounds the alarm only when the defrost cycle is not running.

Question 3

A delivery drone checks three inputs before take-off:

| Input | Binary value | Meaning |
|---|---|---|
| G | 0 | No GPS signal |
| G | 1 | GPS locked |
| B | 0 | Battery above the minimum level |
| B | 1 | Battery below the minimum level |
| W | 0 | Wind speed safe |
| W | 1 | Too windy |

The drone may take off (T = 1) only if GPS is locked, the battery is not below the minimum level and it is not too windy.

Which logic expression gives T?

Accepted Answer

C: T = G AND (B NOR W). Take-off needs G = 1, B = 0 and W = 0. 'B = 0 and W = 0' is NOT B AND NOT W, which is 1 only when both inputs are 0: that is B NOR W. So T = G AND (B NOR W), and the drone takes off only for G = 1, B = 0, W = 0.

Input	Binary value	Meaning
D	0	Door closed or open for less than 2 minutes
D	1	Door open for more than 2 minutes
T	0	Temperature normal
T	1	Temperature too high
F	0	Defrost cycle not running
F	1	Defrost cycle running

Input	Binary value	Meaning
G	0	No GPS signal
G	1	GPS locked
B	0	Battery above the minimum level
B	1	Battery below the minimum level
W	0	Wind speed safe
W	1	Too windy