Logic gates, truth tables and logic circuits
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The teaching part (1 min) is free to watch. A free account opens the whole lecture: 5 practice questions, 2 real past-paper questions with the answers explained, the mistakes that lose marks, the full-marks tip, a one-card summary and a quick quiz.
What this lecture covers
- Decisions made of switches
- The basic gates
- The other gates
Read the transcript of the teaching part
Last time
Before we start: last time, in Logic gates, truth tables and logic circuits, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.
Decisions made of switches
A burglar alarm sounds if a door is open AND the alarm is switched on. That decision is made by a logic gate, a tiny circuit that takes ones and zeros in and gives one output. Combine a few gates and you can build any decision, or even a processor.
The basic gates
AND outputs 1 only when both inputs are 1. OR outputs 1 when at least one input is 1. NOT has one input, and flips it. One becomes zero, and zero becomes one.
The other gates
NAND is AND followed by NOT. It outputs 0 only when both inputs are 1. NOR is OR followed by NOT. It outputs 1 only when both inputs are 0. XOR outputs 1 when the inputs are different.
How often it comes up
How often Cambridge has asked each part, counted by ExamYard from every past paper in its question bank: Logic circuits and expressions.
The whole chapter: Hardware.
Chapter 3: Hardware
- Logic gates, truth tables and logic circuits 5 min
- Buffers, embedded systems, RAM and ROM, monitoring and control 7 min
Also for A Level Computer Science 9618
Which topics come up most in A Level Computer Science 9618 past papers, counted from every paper since 2004: Paper 1 topic frequency, Paper 2 topic frequency.