Question 5
A logic gate circuit is shown.
Which truth table shows the correct output column for the logic gate circuit?
That is it. Step through the hints below to see why. In 2024, 82.7% of Queensland students chose (A).
Not quite. 0, 1, 1, 1 is a plain OR of A and B, as if the NOT gate were not there. The NOT gate flips A before it reaches the OR, so every row with A = 0 sends a 1 into the OR. In 2024, 8.9% of Queensland students chose (B).
Not quite. 0, 1, 0, 0 is what you get with an AND as the second gate, because NOT A and B are both 1 only at A = 0, B = 1. The second gate has the curved back and pointed front of an OR. In 2024, 5.4% of Queensland students chose (C).
Not quite. 0, 0, 0, 1 is a plain AND of A and B. It ignores the NOT gate and reads the OR as an AND, two errors at once. In 2024, 2.7% of Queensland students chose (D).
Fill the NOT A column first, then X. Click a cell to cycle it through ?, 0 and 1. Click the A and B cells of a row to send that row to the circuit.
Read the circuit left to right. The first gate is the NOT gate on A, so give its output a column of its own before you touch the OR.
$$\text{NOT }A=1,\ 1,\ 0,\ 0$$
The rows are in binary order, so A is 0 in the top two rows and 1 in the bottom two. The NOT gate flips that.
The second gate has the curved back and pointed front of an OR, with no bubble. Its inputs are NOT A and B, not A and B.
An OR gate gives 0 only when both of its inputs are 0. NOT A = 0 and B = 0 together happens in one row, A = 1 and B = 0.
$$X=0\text{ only for }AB=10$$
So the X column reads 1, 1, 0, 1 from the top. Only (A) has it.
Each wrong option is the same circuit with a gate changed. (B) leaves out the NOT gate, (C) swaps the OR for an AND, and (D) does both. Use the toggles on the left to see each one.
Give the NOT gate its own column, then apply the OR rule to NOT A and B.
The most common wrong answer was (B), the plain OR table, from reading straight past the NOT gate. A column for every gate, even a one-input NOT gate, stops that happening.
Only mark this done when you could do it without help. Reading the solution does not count.
Question wording and any figures are from the 2024 QCAA Engineering external assessment, © State of Queensland (QCAA) 2024, licensed under CC BY 4.0, and have been adapted, except where a third-party source is named. Tangent Tuition is not affiliated with the QCAA.
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