Logic circuits
Logic circuit = inputs (0 or 1) + a logical operation = output. We write those two states as 0 and 1, and logic circuits deal with them. A logic circuit takes inputs, applies a logical operation and gives an output.
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Unit 7 · Lesson 3
Logic circuits
5 steps. Each one is a short story and a question to answer.
A computer does not understand words or pictures. It understands one thing: on or off.
We write those two states as 0 and 1, and logic circuits deal with them.
A logic circuit takes inputs, applies a logical operation and gives an output.
Everything a computer does, from adding to games, is built from circuits like these.
Inputs0 or 1
Logical operation
Output0 or 1
Logic circuit = inputs (0 or 1) + a logical operation = output
Logic circuits work with which values?
Pick an answer
Logic circuits
Logic circuit = inputs (0 or 1) + a logical operation = output. We write those two states as 0 and 1, and logic circuits deal with them. A logic circuit takes inputs, applies a logical operation and gives an output.
The AND gate
AND: output is 1 only if both are 1. The output is 1 only if both inputs are 1 together. If any input is zero, the output is zero.
The OR and NOT gates
OR: 1 if any input is 1. NOT: flips the value. It is zero only when both are zero. NOT takes one input and flips it: 0 becomes 1 and 1 becomes 0.
The half adder
Half adder: S = A XOR B and C = A AND B. It takes two binary digits A and B and gives two things: S, the sum, and C, the carry. When A=1 and B=1: the sum S=0 and the carry C=1, because 1+1=10.
The full adder
Full adder = A + B + Carry In, giving S and Carry Out. The full adder takes three inputs: A, B and Carry In. It gives S and Carry Out, so we can chain many adders and add long numbers.
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Logic circuits
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The lesson on one page
Every idea in the lesson with its rule, ready to screenshot and revise from.
Logic circuits
Logic circuit = inputs (0 or 1) + a logical operation = output
- 2
The AND gate
AND: output is 1 only if both are 1
- 3
The OR and NOT gates
OR: 1 if any input is 1. NOT: flips the value
- 4
The half adder
Half adder: S = A XOR B and C = A AND B
- 5
The full adder
Full adder = A + B + Carry In, giving S and Carry Out
Test yourself
One question from every step. Answer and see your score.
Question 1 of 5
Logic circuits work with which values?
Watch and listen
Unit 7
Learn how a computer thinks with 0s and 1s: AND, OR and NOT gates, truth tables, and the half adder and full adder, in an animated explainer.
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The lesson in writing
The same explanation as the 3D scene, written out for quick review.
The AND gate
AND: output is 1 only if both are 1. The output is 1 only if both inputs are 1 together. If any input is zero, the output is zero.
Page 166
The OR and NOT gates
OR: 1 if any input is 1. NOT: flips the value. It is zero only when both are zero. NOT takes one input and flips it: 0 becomes 1 and 1 becomes 0.
Page 168
The half adder
Half adder: S = A XOR B and C = A AND B. It takes two binary digits A and B and gives two things: S, the sum, and C, the carry. When A=1 and B=1: the sum S=0 and the carry C=1, because 1+1=10.
Page 170
The full adder
Full adder = A + B + Carry In, giving S and Carry Out. The full adder takes three inputs: A, B and Carry In. It gives S and Carry Out, so we can chain many adders and add long numbers.
Page 172
Questions about this lesson
- What are logic circuits?
- Electronic circuits that work with only two values, 0 and 1, and apply logical operations on inputs to produce an output.
- What are AND, OR and NOT gates?
- AND outputs 1 only if both inputs are 1. OR outputs 1 if any input is 1. NOT flips the value.
- What is a truth table?
- A table showing a gate's output for every possible input.
- What does a half adder do?
- It adds two binary digits A and B and gives the sum S and the carry C.