How do logic gates make decisions, and how are flowcharts used to control a sequence?
Logic gates and digital control: the AND, OR, NOT, NAND and NOR gates with truth tables, combining gates, and flowcharts for program control.
A CCEA GCSE Technology and Design answer on digital control: the AND, OR, NOT, NAND and NOR logic gates with their truth tables, combining gates to make a decision, and using flowcharts (sequence and decision) for program control.
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What this dot point is asking
CCEA wants you to know the logic gates - AND, OR, NOT, NAND and NOR - and their truth tables, how to combine gates to make a decision, and how flowcharts (sequence and decision) describe program control. Digital control works with two states, 1 (high/on) and 0 (low/off).
The answer
Digital signals and logic gates
The gates and their truth tables
The 2-input truth tables (inputs A, B to output):
- AND: 0,0 to 0; 0,1 to 0; 1,0 to 0; 1,1 to 1.
- OR: 0,0 to 0; 0,1 to 1; 1,0 to 1; 1,1 to 1.
- NAND: 0,0 to 1; 0,1 to 1; 1,0 to 1; 1,1 to 0.
- NOR: 0,0 to 1; 0,1 to 0; 1,0 to 0; 1,1 to 0.
- NOT (1 input): 0 to 1; 1 to 0.
Choosing a gate for a condition
A safety interlock that starts only when the guard is closed AND the button is pressed uses an AND gate; an alarm that sounds if the door OR the window opens uses an OR gate.
Flowcharts for program control
Worked example: designing a logic decision
Examples in context
- Example 1. A two-hand press
- An AND gate makes the machine operate only when both buttons are pressed, keeping the operator's hands clear - a safety use of logic.
- Example 2. A burglar alarm
- An OR gate sounds the siren if the door OR the window sensor is triggered, because either should raise the alarm.
- Example 3. A microcontroller program
- A flowchart with a decision diamond ("is it dark?") and a loop describes how the controller checks the sensor and switches a light, the bridge to programmed control.
Being able to give truth tables and match a gate to a condition lets you answer both the "draw the truth table" and "which gate is needed" questions.
Try this
Q1. For a 2-input AND gate, what inputs give an output of 1? [1 mark]
- Cue. Only when both inputs are 1 (inputs 1 and 1).
Q2. What does a NOT gate do? [1 mark]
- Cue. It inverts the input (0 becomes 1, 1 becomes 0).
Q3. A circuit must give an output if either of two sensors is triggered. Which gate is needed? [1 mark]
- Cue. An OR gate.
Q4. What is the output of a NOR gate when both inputs are 0? [1 mark]
- Cue. 1 (NOR gives 1 only when all inputs are 0).
Q5. What flowchart symbol represents a yes/no decision? [1 mark]
- Cue. A diamond.
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA style4 marksDraw the truth table for a 2-input AND gate and a 2-input OR gate.Show worked answer →
AND gate (output 1 only when both inputs are 1):
Inputs 0,0 give 0; 0,1 give 0; 1,0 give 0; 1,1 give 1 (2 marks for a correct table).
OR gate (output 1 when either or both inputs are 1):
Inputs 0,0 give 0; 0,1 give 1; 1,0 give 1; 1,1 give 1 (2 marks for a correct table).
CCEA style3 marksA machine should only start when a guard is closed AND a start button is pressed. State the logic gate needed and explain why.Show worked answer →
An AND gate is needed (1). Its output is 1 (start) only when both inputs are 1 (1).
So the machine starts only when the guard-closed input AND the start-button input are both true, which gives the required safe behaviour (1).
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Sources & how we know this
- CCEA GCSE Technology and Design specification — CCEA (2017)