GCSE · Computer Science · AQA · Spec 8525
Logic circuit diagrams
“If I’m not tired and I’ve finished my homework, I’ll watch a film.” Secretly, that’s a circuit. Turn its words into gates and find out when the film happens.
Computer Science · Logic circuits
The film decision, gate by gate
The word ‘not’ becomes a NOT gate and the word ‘and’ becomes an AND gate. Tired goes into the NOT gate, and its result meets ‘homework finished’ at the AND gate. Every wire carries a 1 (true) or a 0 (false).
Before you press Next, predict it: in each row, what will the NOT gate hand to the AND gate?
| tired | homework finished | after NOT gate | watch a film |
|---|---|---|---|
| 0 | 0 | 1 | 0 |
| 0 | 1 | 1 | 1 |
| 1 | 0 | 0 | 0 |
| 1 | 1 | 0 | 0 |
Output
Step 1: Not tired (0) and homework not finished (0). The NOT gate flips its input, so the 0 for tired comes out as 1. That 1 and the 0 for homework reach the AND gate, which only outputs 1 when both inputs are 1. They are not both 1, so no film.
Step through the four rows. In each one, find the ‘after NOT gate’ value first, then the output.
Computer Science · Logic circuits
Which gate is it?
Sort each description into the gate it belongs to.
Still to sort
AND (0)
Two inputs, one output
OR (0)
Two inputs, one output
NOT (0)
One input, one output
XOR (0)
Two inputs, one output
You have already used NOT and AND. Two more gates join them: OR and XOR. Commit to each card first, then read why.
Predict, then check
A chain of only AND gates can be rearranged freely, because every input has to be 1 whichever way you wire it. What about a mixture?
Circuit 1: A and B go into an OR gate, and its output goes into an AND gate together with C. Circuit 2: A and B go into an AND gate, and its output goes into an OR gate together with C. Both circuits get A = 0, B = 0, C = 1. Do they give the same output?
WHAT YOU'VE LEARNED
A quick recap of today's lesson.
Join gates so each answer feeds the next one, then work out exactly when the output is true.
What you need to know
- In a logic circuit, one gate’s output becomes the next gate’s input, so you work in stages.
- Label the value between the gates, fill that column first, then find the final output.
- A truth table lists every input combination and the output for each row.
- OR includes the case where both inputs are 1. XOR is the gate that leaves that case out.
- Rearranging gates can change the output once different gates are mixed.
The big picture
A logic circuit joins gates so that one gate’s output becomes the next gate’s input. You build it from a sentence by spotting the gate words, naming the inputs and the output, then connecting the gates. You test it with a truth table, working through the in-between values stage by stage until you reach the final output and can say when it is true.
Key points
Worked example
Problem
A fan controller has two sensors, S1 and S2. Both feed an AND gate, and the AND gate’s output feeds a NOT gate. The NOT gate’s output is F. Work out F for every sensor combination and say when F is 0.
⚠ Watch out
Treating OR as ‘one or the other but not both’. That is XOR. An OR gate also outputs 1 when both inputs are 1. A second easy slip is assuming you can swap gates around freely: once different gates are mixed, the order changes the result.
Memory hook
Stage by stage: label the wire between the gates, fill that column, then the output. And remember: OR says ‘and both is fine’, XOR says ‘exclusive: one or the other, never both’.
Check yourself
Cover the page and say from memory: (1) what an OR gate outputs when both inputs are 1, and (2) the one thing XOR does differently. Then check against the OR and XOR sort.
Flashcards
(12)What does a NOT gate do?
When does an AND gate output 1?
When does an OR gate output 1?
When does an XOR gate output 1?
How can you spot an AND gate in a diagram?
How can you spot a NOT gate in a diagram?
How does the XOR symbol differ from the OR symbol?
True or false: OR means ‘one or the other but not both’.
In a two-gate circuit, why label the wire between the gates?
What do you do first when drawing a circuit from a statement?
Does rearranging the gates in a circuit change its output?
How do you say when a circuit’s output is true?
Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.
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