GCSE-ready- Home
- Lessons
- KS3 Computing
- Logic gates and circuits
Logic gates and circuits
🎬 The doodle video for this lesson is coming soon. Subscribe on YouTube to see it first.
Inside every processor are billions of tiny logic gates. Each gate takes 1s and 0s in and gives a 1 or 0 out.
Three gates
Combining gates
Gates can be joined so the output of one is the input of another.
We can write a circuit as an expression, such as Q = (A AND B) OR C.
Work out the brackets first, just like in maths.
If A = 1, B = 0, C = 1: A AND B = 0, then 0 OR 1 = 1.
We can write a circuit as an expression, such as Q = (A AND B) OR C.
Work out the brackets first, just like in maths.
If A = 1, B = 0, C = 1: A AND B = 0, then 0 OR 1 = 1.
A real use: seatbelt warning
A car shows a warning light when the engine is on AND the seatbelt is NOT fastened.
E = engine on (1 = on), S = seatbelt fastened (1 = fastened).
Light = E AND (NOT S)
E = engine on (1 = on), S = seatbelt fastened (1 = fastened).
Light = E AND (NOT S)
Q = NOT (A OR B). Find Q when A = 0 and B = 1.
- A OR B = 0 OR 1 = 1.
- NOT 1 = 0.
Answer: Q = 0
AND gates output 1 only if both inputs are 1; OR gates output 1 if either input is 1; NOT gates invert. Work out combined circuits from the inside out.
The interactive lesson includes the diagrams for this topic.
Check you have got it
Answer 6 quick questions with instant marking. If you get one wrong, GCSE-ready shows you why and gives you another go. It is free, and you do not need an account.