A-level Computing/AQA/Paper 2/Fundamentals of computer systems/Building circuits

Template:CPTAmbox A common question in the exam is to be given some boolean algebra and be asked to express it as logic gates. Let's take a look at an addition and subtraction example that you should be familiar with:

$9-(7+1)$

First we are going to deal with the inner-most brackets

$(7+1)=8$

Finally we combine this answer with the $9-$

$=9-(8)=1$

It will work exactly in the same way for boolean algebra, but instead of using numbers to store our results, we'll use logic gates: Template:CPTExample

$C.(A+B)$

As with any equation, we are going to deal with the inner-most brackets first$(A+B)$, then combine this answer with the $C.$
Template:Robox/Close

A common question in the exam is to give you a description of a system. You'll then be asked to create a boolean statement from this description, and finally build a logic gate circuit to show this system: Template:CPTExample Using boolean algebra describe the following scenario: Template:Cquote Where:

• A = being towed,
• B = window broken,
• C = engine on,
• D = senses movement

Before you rush into answering a question like this, let's try and break it down into its components. The questioner will often be trying to trick you. The two occasions that the alarm will sound are: $B+D$ but there is a caveat, the alarm will sound if either of these are true AND two things are also true, namely the engine is NOT on, and the car is NOT being towed: $\bar{A}+\bar{C}$

Combining both we get (remember the brackets!): $(B+D).(\bar{A}+\bar{C})$

The next step is to create a diagram out of this: