Question 15
An automated machine fabricates a metal component using a punching tool. The machine includes sensors that allow it to operate only when the punching tool is detected as correctly inserted with the safety guard in the punching position, and when either a component is detected as correctly positioned or with the manual shut-down switch not activated. All sensors send a 1 input for a yes/activated condition and a 0 input for a no/not activated condition.
Develop a logic gate circuit to control the given function of the automated punching machine to produce a 1 output from the circuit.
Clearly annotate all logic gate inputs and outputs, and label all sensors.
The punch fires only when both conditions hold, and the shut-down switch always stops it
The machine is on the left and QCAA’s circuit is on the right. Watch which sensor lamps are lit each time the ram comes down, and what the shut-down switch does to the lower AND gate.
Each condition is its own small AND, and a final AND joins them. The NOT gate is what lets an activated switch, a 1, stop the machine.
Build the circuit gate by gate and see which marks it earns
It starts as a literal reading of the stem, with an OR for “either … or”. Change the gates, add the labels and annotations, then run the safety tests. The marks panel uses the six dot points of the 2021 marking guide.
Orange wires carry 1 for the inputs you have set. Blue numbers are the annotations a marker looks for: the values that give a 1 output. On a phone, scroll the circuit sideways.
Work through the solution one mark at a time
Try each step yourself before you reveal it. Four marks are for gates and two are for what you write on the diagram.
Before you draw a gate, write each sensor on its own input line with its letter and its 1 condition.
- P: punching tool detection (correctly inserted = 1)
- S: safety guard (in punching position = 1)
- C: component detection (correctly positioned = 1)
- M: manual shut-down switch (activated = 1)
The guide’s notes list exactly these four labels. Adding the 1 condition costs nothing and satisfies the stricter label rule in the 2022 marking guide, which wants the yes and no conditions shown.
Marker: correct sensor labels for logic circuit inputs
Condition 1 is “the punching tool is detected as correctly inserted with the safety guard in the punching position”. Both must be true, so P and S go into an AND gate.
$$\text{Condition 1}=P\text{ AND }S$$
Marker: includes use of the AND gate for P and S sensors
The machine needs the shut-down switch “not activated”. The switch sends 1 when it is activated, so put a NOT gate on M. Its output is 1 exactly when the switch is off.
$$M=0\ \Rightarrow\ \text{NOT }M=1$$
Marker: NOT gate for M sensor prior to AND gate
Condition 2 joins C and NOT M. QCAA’s key uses an AND gate here.
$$\text{Condition 2}=C\text{ AND NOT }M$$
Read the stem closely and you will see a mismatch. It says “either a component is detected as correctly positioned or with the manual shut-down switch not activated”, and “either … or” reads as an OR. The marking guide credits an AND. The AND is the safe machine: with an OR, a detected component would let the press fire while the shut-down switch is activated, and an empty die would be punched whenever the switch is off. Test both in the interactive above. The published guide lists only the AND for this mark and says nothing about an OR answer.
Marker: AND gate for C and M sensors
The stem says the machine operates “only when” condition 1 holds “and when” condition 2 holds. Both outputs feed one final AND gate.
$$\begin{aligned}X&=(P\text{ AND }S)\text{ AND }\\&(C\text{ AND NOT }M)\end{aligned}$$
Marker: AND gate linked to an AND gate and an AND gate to acknowledge the two conditions of machine operation
Annotate every line with the value that gives a 1 output, the way QCAA’s sample response does: P = 1, S = 1, C = 1 and M = 0, then 1 after the NOT gate, 1 out of each AND and 1 at the output.
Only one of the sixteen input rows gives a 1, which is what a safety interlock should look like.
Marker: provides an appropriately annotated logic gate showing inputs and outputs to produce a logic circuit 1 output
Each phrase of the question gave you something
Four labelled sensors, two conditions and three AND gates: the relationships are spelled out and every label is supplied in the table. QCAA did not publish a band for this question. It reads as simple familiar, and the 2021 subject report says students did well on simple familiar logic control problems “where relationships and interactions were obvious and had few elements”.
If I had to guess where you might lose marks, it would be the two marks for writing on the diagram. A correct circuit with bare lines earns four of six. The other trap is the “either … or” wording, which pulls you towards an OR the marking guide does not list. Ask what happens when the shut-down switch is activated: if your machine can still punch, the circuit is wrong.
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 2021 QCAA Engineering external assessment, © State of Queensland (QCAA) 2021, 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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