This mini-lesson covers OCR J277 · 2.4 Boolean logic: the three logic gates AND, OR & NOT, their truth tables, combining gates, and writing/reading simple Boolean expressions. Every truth-table value here has been double-checked.
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An AND gate outputs 1 only when BOTH inputs are 1. Written A AND B (or A·B).
Think: "both must be true". 1 AND 1 = 1; every other combination = 0.
An OR gate outputs 1 when AT LEAST ONE input is 1. Written A OR B (or A+B).
Think: "one or the other (or both)". Only 0 OR 0 = 0; everything else = 1.
A NOT gate has one input and inverts it: it outputs the opposite. Written NOT A (or Ā).
Think: NOT reverses the input. NOT 0 = 1; NOT 1 = 0.
For inputs A = 1 and B = 0, tap each expression, then tap the output it gives.
Gates can be combined to make bigger expressions. Work from the inside out, doing NOT first, then AND, then OR (brackets first if shown).
If A = 0, B = 1: A AND B = 0 AND 1 = 0; NOT A = NOT 0 = 1; then 0 OR 1 = 1.
Order: do NOT first, then AND, then OR — unless brackets tell you otherwise.
A Boolean expression describes an output using AND, OR and NOT. You should be able to read one and complete its truth table.
Tip: to fill a truth table, add a column for each operation and evaluate step by step (NOT, then AND, then OR).
Tap a gate on the left, then its output rule on the right.
AND: output 1 only when BOTH inputs are 1
OR: output 1 when AT LEAST ONE input is 1 (only 0 OR 0 = 0)
NOT: one input, inverts it (0→1, 1→0)
Combining: evaluate NOT, then AND, then OR (brackets first)
Boolean expressions: read the expression and fill the truth table step by step
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You've worked through Boolean logic for OCR GCSE Computer Science (J277). 🎉
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Next: test yourself in the Evaluate stage Confidence Quiz, then lock it in with Verify.