Years 7–10 · Week 5 of 12
Bits, Logic and Classical Circuits
Learning goals
By the end, you can…
- Represent small whole numbers and messages using binary bits.
- Construct truth tables for NOT, AND and XOR.
- Trace a bit string through a simple classical circuit.
- Explain what deterministic computation means.
What you already know
Connect to a familiar idea
You use devices that store text, images and numbers. At their lowest logical level, classical digital systems encode and transform patterns of two symbols, usually labelled 0 and 1.
- A switch can be in one of two labelled states.
- Place value gives symbols different numerical weights.
- A table can list inputs and outputs.
Opening story
Start with something familiar
A stage manager needs to signal four lighting instructions through a cable that carries only off or on. Two bits provide four patterns—00, 01, 10 and 11—enough to encode all four instructions.
Plain-English explanation
Build the idea carefully
Bits and binary representation
A bit is a classical two-state information unit labelled 0 or 1; sequences of bits can encode numbers, characters, colours and instructions. Binary place values are powers of two, so the bit string 101 represents four plus zero plus one, which is five in decimal notation.
Rules built from logic gates
NOT flips one bit, AND outputs 1 only when both inputs are 1, and XOR outputs 1 when its two inputs differ. A truth table lists the output for every possible input combination and lets us check a circuit without guessing. An ideal classical logic circuit is deterministic: a fixed valid input and gate sequence produce a fixed output, even though real hardware can suffer faults and noise.
Try the model
Classical logic playground
Toggle input bits, connect NOT, AND and XOR gates, then decode the binary output to complete the message challenge.
Ready. Adjust a control, then run the model.
What this model shows: The playground uses ideal classical logic; output bits update deterministically for each input.
Text alternative for this interactive
Keyboard truth-table challenge Choose each input pair from a labelled table and read the resulting NOT, AND and XOR outputs without dragging circuit parts.
Expected observation: The same input bits and gate arrangement always produce the same output, and a truth table predicts every case.
Guided activity
Design a one-bit difference alarm
- List all four input pairs for bits A and B.
- Choose a gate that outputs 1 exactly when A and B differ.
- Test every row rather than testing only one example.
- Explain how the output could act as an alarm signal.
Evidence to collect: A complete four-row truth table and a tested XOR circuit whose output is 1 for 01 and 10 only.
Glossary
Words to know
- bit
- A classical information unit with two labelled states, 0 and 1.
- binary
- A base-two representation using the symbols 0 and 1.
- truth table
- A table listing outputs for all possible inputs.
- NOT
- A one-input gate that swaps 0 and 1.
- AND
- A gate that outputs 1 only when both inputs are 1.
- XOR
- A gate that outputs 1 when its two inputs differ.
- deterministic
- Producing the same output for the same specified input and process.
Short recap
Keep these ideas
- Bit strings can encode many kinds of classical information.
- Truth tables define and test the rules applied by logic gates.
- Ideal classical circuits transform fixed inputs into predictable fixed outputs.
Knowledge check
6 clear questions
Choose an answer for immediate feedback. You may retry, and your best submitted score is kept.
Go Further Optional extension for Years 9–10
For Years 9–10, combine XOR and AND as the two outputs of a half-adder. XOR gives the sum bit and AND gives the carry bit when adding two one-bit numbers.
Try this
Complete the half-adder truth table, then explain why adding longer binary numbers requires carrying information between positions.
Adult support Teacher and parent notes
Discuss
- Separate a logical state from its physical implementation.
- Ask students to justify circuits with exhaustive truth tables for two inputs.
Answer guidance
Students should state that XOR detects unequal bits and should verify 00, 01, 10 and 11.
Offline activity
Students act as gates holding 0/1 cards while classmates pass inputs through a human-sized circuit.
Safety
Use cards or low-voltage school-approved equipment only; do not connect activities to mains electricity.
Sources and further reading
Checked references for this lesson
These sources support the lesson’s main scientific claims. Links open on the source organisation’s site.
- Classical information IBM Quantum Learning · official learning module · checked 2026-08-02
- Circuits IBM Quantum Learning · official learning module · checked 2026-08-02
Content review: Reviewed on 2026-08-02.