Question 15
A truth table for the reversing safety system of a forklift is shown.
| Inputs | Intermediate signals | Output | |||||
|---|---|---|---|---|---|---|---|
| A | B | C | D | E | F | G | X |
| 0 | 0 | 0 | 0 | 0 | 1 | 0 | 0 |
| 0 | 0 | 0 | 1 | 0 | 1 | 0 | 0 |
| 0 | 0 | 1 | 0 | 0 | 0 | 0 | 0 |
| 0 | 0 | 1 | 1 | 0 | 0 | 0 | 0 |
| 0 | 1 | 0 | 0 | 1 | 1 | 1 | 0 |
| 0 | 1 | 0 | 1 | 1 | 1 | 1 | 1 |
| 0 | 1 | 1 | 0 | 1 | 0 | 0 | 0 |
| 0 | 1 | 1 | 1 | 1 | 0 | 0 | 0 |
| 1 | 0 | 0 | 0 | 1 | 1 | 1 | 0 |
| 1 | 0 | 0 | 1 | 1 | 1 | 1 | 1 |
| 1 | 0 | 1 | 0 | 1 | 0 | 0 | 0 |
| 1 | 0 | 1 | 1 | 1 | 0 | 0 | 0 |
| 1 | 1 | 0 | 0 | 1 | 1 | 1 | 0 |
| 1 | 1 | 0 | 1 | 1 | 1 | 1 | 1 |
| 1 | 1 | 1 | 0 | 1 | 0 | 0 | 0 |
| 1 | 1 | 1 | 1 | 1 | 0 | 0 | 0 |
The forklift has a motion sensor on each side at the back. If either sensor detects an obstacle while the forklift is reversing, a safety brake is activated.
Key
Input A = Left sensor (motion detected = 1)
Input B = Right sensor (motion detected = 1)
Input C = Gear (forward selected = 1, reverse selected = 0)
Input D = Engine (engine on = 1)
E, F and G are intermediate input/output signals to the logic gates.
Output X = Safety brake (brake on = 1)
Construct a logic gate circuit, based on the truth table, that meets the requirements for the safety brake to activate. Clearly label all inputs and outputs.
The brake needs a sensor, reverse gear and a running engine all at once
A forklift seen from above backs towards a crate. Watch the eight signals underneath as the gear shifts into reverse and the left sensor picks up the crate.
A sensor, reverse gear and the engine all have to agree before X = 1. The paragraph never mentions the engine, so only the table tells you that D joins the circuit.
Build the circuit and test it against the printed table
Pick a gate for each slot and add the labels. Then press the inputs, or run all sixteen rows. The table fills in as you test, and the marking guide ticks off each criterion you earn.
Press a row to set its inputs. An orange cell disagrees with the printed table.
Work through the solution one mark at a time
Try each step yourself before you reveal it.
Start with column E. It is 0 in the first four rows, where A = 0 and B = 0, and 1 in every other row. A gate that gives 0 only when both inputs are 0 is an OR. That is the “either sensor” in the paragraph.
The mark is for the whole path, from both sensors through the OR and the two ANDs to the brake, so it only lands once Step 3 is in place.
Marker: provides a correctly applied logic gate circuit including left motion sensor and right motion sensor => OR => AND1 => AND2 => safety brake operation
Column F is the opposite of column C in every row. That is a NOT gate on the gear input. The brake cares about reverse, and reverse is C = 0, so the gear signal has to be inverted before it can switch an AND on.
Marker: gear => NOT => AND1 => AND2 => safety brake operation
Column G is 1 in exactly the six rows where E = 1 and F = 1, so G comes from an AND gate (AND1). Column X is 1 in exactly the three rows where G = 1 and D = 1, so the engine joins at a second AND gate (AND2), and AND2 drives the brake.
The guide’s Method 2 wires it the other way round. The engine and F go into AND1, then E and that output go into AND2. It gives the same X on all sixteen rows and earns the same marks.
Look at what Method 2 calls G. Its AND1 output is F AND D, which is not the printed G column. The two disagree in four rows (0001, 0100, 1000 and 1100), yet the guide still labels it G. Build Method 1 and every column you label matches the table.
Marker: engine => AND2 => safety brake operation. Method 2: engine => AND1 => AND2 => safety brake operation
Label each input line with what it is and what 1 means for it. Copy the conditions from the key.
The labels are marked apart from the gates. The guide says “Allow FT marks for an incorrect logic gate circuit”, so label every line even if you are unsure of a gate.
Marker: correctly labels left and right motion sensor inputs · correctly labels gear input · correctly labels engine input
Letter the three wires between the gates: E out of the OR, F out of the NOT and G out of AND1. The table names these columns, so the marker expects to find them on your circuit.
Marker: correctly labels intermediary inputs/outputs E, F and G
Label the output: X, safety brake (brake on = 1). That completes the circuit.
Marker: correctly labels safety brake output
Each phrase of the question gave you something
QCAA did not publish a band for this question. It reads as complex familiar because the four gates are the usual safety interlock, an OR, a NOT and two ANDs, but you have to read them out of a sixteen-row table with three intermediate columns rather than a short paragraph.
If I had to guess where you might lose marks, it would be the engine. The paragraph only talks about the sensors and reversing, so a circuit built from the words alone leaves out D and drops the engine path and the engine label. Build from the table, then use the words to name the inputs. The other trap is labelling with letters only, when five of the eight marks are labels.
Only mark this done when you could do it without help. Reading the solution does not count.
Question wording, any figures and marking-guide steps 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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