Logic Gate Chain

TRUE or FALSE — that's all a computer thinks about

Year 8 Computer Science — Boolean Logic

Stand up if...

Stand up if you are a student AND you have a bag with you.

Now stand up if you are taller than 160cm OR you are wearing something blue.

Why did more people stand for the OR rule?

AND, OR, NOT

AND

T+T→T

T+F→F

F+T→F

F+F→F

OR

T+T→T

T+F→T

F+T→T

F+F→F

NOT

T→F

F→T

Flips the value

Build the Chain!

5 volunteers. Each person has a role:

[Input A] → [NOT] → [AND with Input B] → [Output]

Inputs: hold up TRUE or FALSE card
NOT gate: flip the value, pass it on
AND gate: take two values, apply AND rule, pass result
Output: show the final result

Run 4 different input combinations!

At Your Desks

Draw this gate chain and complete the full truth table:

A → [NOT] → [AND with B] → [OR with C] → Output

Complete all 8 rows (A,B,C can each be T or F).

Challenge: which input combinations give Output = TRUE?

Reverse Challenge

Given: the output must be TRUE.

Work BACKWARDS through the chain.

Find ALL valid input combinations that produce TRUE.

How many are there? Check your truth table.

Real World

Logic gates are used in:

CPUs — arithmetic and comparisons
Memory chips — storing and reading bits
Security systems — access control logic
Your code — every IF statement compiles to gates

A modern CPU has billions of gates, switching billions of times per second.

Key Takeaway

Every decision a computer makes is built from AND, OR, and NOT gates.

Billions of them. Nanosecond speed.

Every IF statement you write compiles down to gate logic.

Discussion

Can you design a gate chain that outputs TRUE only when exactly one input is TRUE?
What is a NAND gate? (NOT + AND combined)
Write IF (age >= 18 AND has_ID = True) — which gates does this use?