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J277 2.4.1 Logic diagrams

Part 3 of 4 · J277 2.4 · Boolean logic

Reading and constructing the logic diagrams OCR uses to show how gates are connected.

Students will:

  • trace a Boolean value from input to output through a diagram
  • write the expression a given diagram represents
  • draw a diagram for an expression that includes NOT
  • treat a bracketed section as an input to the next gate
  • find and correct the fault in an incorrect diagram

Inside: 5 explanation cells, 1 multiple-choice question, 1 fill-in-the-blanks cell and 4 written answers. 16 marks, about 45 minutes.

Series: J277 2.4 · Boolean logic, part 3 of 4.

Shared by Coding PathwayVerified teacher

  • 11 cells
  • About 45 minutes
  • CC BY-SA 4.0
  • Shared 17 Aug 2026
  • Updated 9 Sept 2026

Preview

The whole resource, exactly as a class sees it. Answers and marking are held back.

Reading and constructing logic diagrams

A logic diagram shows how gates are connected. Lines carry Boolean values from inputs on the left, through one or more gates, to an output on the right.

To translate an expression into a diagram:

  1. identify the inputs;
  2. draw any NOT operation or bracketed section first;
  3. treat the output of that smaller section as an input to the next gate;
  4. label the final output clearly.

The order of the gates matters. A AND NOT B is different from NOT (A AND B) because a different value is reversed.

Worked diagram: Q = A AND NOT B

Logic diagram for Q equals A AND NOT B Input A connects directly to the upper input of an AND gate. Input B first passes through a NOT gate, then connects to the lower input of the AND gate. The AND output is Q. ABQ NOTAND

Input B is reversed first. The AND gate then combines A with NOT B.

Multiple choice1 mark

In the worked diagram, which value reaches the lower input of the AND gate?

  • AB
  • BNOT A
  • CA OR B
  • DNOT B
Written answer2 marks

For the worked diagram, state the output Q when A = 1 and B = 0. Show the two logical steps.

Calculate NOT B first, then apply AND.

Students type their answer here.

A circuit with three inputs

Logic diagram for Z equals A OR B, combined using AND with NOT C Inputs A and B enter an OR gate. Its output enters the upper side of a final AND gate. Input C passes through a NOT gate and then enters the lower side of the final AND gate. The final output is Z. ABCZ ORNOTAND

The diagram represents Z = (A OR B) AND NOT C. The OR and NOT gates can be calculated independently before their outputs meet at the final AND gate.

Fill in the blanks3 marks
Complete the path description. Inputs A and B first enter an label 1 gate. Input C enters a label 2 gate. Their two results enter the final label 3 gate.
  • AND
  • OR
  • NOT
Written answer3 marks

Calculate Z for the combined diagram when A = 0, B = 1 and C = 0. Show each intermediate result.

Calculate `A OR B` and `NOT C` before the final AND.

Students type their answer here.

Editing an incorrect diagram

The diagram below was meant to show (A OR B) AND NOT C, but the NOT gate has been placed after the AND gate.

Incorrect logic diagram with the NOT gate after the final AND gate A and B enter an OR gate. The OR output and C enter an AND gate. The AND output then passes through NOT. This represents NOT of the whole AND result, not A OR B combined with NOT C. ABC ORANDNOT
Written answer3 marks

Explain how to edit the faulty diagram so it represents (A OR B) AND NOT C.

State which wire should contain the NOT gate and where its output should connect.

Students type their answer here.

Written answer4 marks

Describe how to construct a logic diagram for Y = NOT A OR (B AND C).

Describe the gates in the order they are built and how their outputs connect. You can also sketch it on paper.

Students type their answer here.

Review

Trace every wire from left to right. A gate output can become another gate input. Check the number of inputs, the order of operations, and the small circle on every NOT gate before deciding which expression a diagram represents.