Cover lesson · Computer Science · Year 10 · Intermediate

Logic gates in real life

DCF: Data and computational thinking → Problem-solving and modelling

Name: Class: Date:

Cover teacher — you need no subject knowledge and no computers

Print: this sheet, one per pupil. Everything needed is on it, including a reference table of what each gate does. Rulers are useful for the circuit diagram; nothing else is.

Run: 5 min read the gate reference table · 25 min scenarios A to D, one Boolean expression and one truth table each · 10 min scenario E, the three-input one, plus its circuit diagram · 10 min the "find the fault" question.

Collect: this sheet. Every truth table has exactly one correct set of output values and they are all printed on the worked example sheet, so marking is a tick-and-compare job.

What you are doing

Logic gates are not only inside processors. They are the rule behind a car light, a microwave door, a fire alarm and a landing light. For each of five real situations you will name the inputs, write the situation as a Boolean expression, and complete the truth table that proves your expression behaves the way the real device does.

Step by step

5 min — Read the gate reference table until you can say what each gate outputs without looking.
25 min — Scenarios A to D. For each one: name the inputs, write the expression, complete the truth table.
10 min — Scenario E has three inputs, so its table has eight rows. Complete it and draw the circuit.
10 min — Find the fault in the alarm that was wired up wrongly, and use a truth table to prove what goes wrong.

Gate reference

1 means true, on, pressed or detected. 0 means false, off, not pressed or not detected.

Gate	Output is 1 when…	Behaviour for inputs 00, 01, 10, 11
AND	both inputs are 1	0, 0, 0, 1
OR	at least one input is 1	0, 1, 1, 1
NOT	the single input is 0	NOT 0 = 1, NOT 1 = 0
XOR	exactly one input is 1	0, 1, 1, 0
NAND	the inputs are not both 1	1, 1, 1, 0

Scenario A — the car interior light

The light inside a car comes on if the driver's door is open, or the passenger door is open, or both.

Inputs: D = _______________________ P = _______________________

Expression: L = _______________________

D	P	L (light on)
0	0
0	1
1	0
1	1

Scenario B — the microwave

The microwave only heats when the door is closed and the timer is running. If either is not true, nothing happens.

Expression: M = _______________________

C (door closed)	T (timer running)	M (heating)
0	0
0	1
1	0
1	1

Why is it important that this is AND and not OR?

Scenario C — the fire alarm with a silence button

The sounder goes off when smoke is detected, unless a member of staff is holding the silence button down during a test.

Expression: A = _______________________

S (smoke)	T (silence held)	NOT T	A (sounder)
0	0
0	1
1	0
1	1

Scenario D — the landing light with two switches

There is a switch at the bottom of the stairs and one at the top. Flicking either switch changes the light: if it was on it goes off, if it was off it comes on. Both switches down means off; both up also means off.

Expression: L = _______________________

A (bottom switch up)	B (top switch up)	L (light on)
0	0
0	1
1	0
1	1

Scenario E — the greenhouse fan (three inputs)

The school's polytunnel has an extractor fan. It runs automatically when it is hotter than 28°C and the window is closed — there is no point extracting air through an open window. The gardener can also force it on with an override button at any time.

Inputs: H = hotter than 28°C W = window closed O = override pressed. Output: F = fan runs.

Expression: F = _______________________

H	W	O
0	0	0
0	0	1
0	1	0
0	1	1
1	0	0
1	0	1
1	1	0
1	1	1

Draw the circuit: two gates, three input lines on the left labelled H, W and O, one output on the right labelled F.

Find the fault

An electrician wires the fire alarm from scenario C as A = S OR (NOT T) instead. Fill in the table, then say in one sentence what the caretaker will experience on a normal quiet afternoon.

S	T	NOT T	A = S OR (NOT T)
0	0
0	1
1	0
1	1

Success criteria

I turned each written description into a Boolean expression with clearly defined inputs.
My truth tables list every possible combination of inputs — four rows for two inputs, eight for three — in a systematic order.
The output column of each table matches what the real device would actually do.
I drew a circuit diagram for the three-input problem that matches my expression gate for gate.
I used a truth table as evidence to show that a wrong design fails, rather than just saying it looks wrong.

If you finish early

Rebuild scenario B using only NAND gates. You will need two. Show with a truth table that your version behaves identically to AND.
A lift door must not close if the door sensor is blocked, the emergency stop is pressed, or the lift is already moving. Write the expression and its eight-row truth table.
Scenario D used XOR. What goes wrong if you build a two-way landing light with an OR gate instead? Give the row of the truth table that proves it.