How A Computer Works

1 Learning Objectives

By the end of the lesson…

  • All Should Be able to complete the truth table for a NOT gate.
  • All Should Be able to describe the function of a NOT gate as an inverter.
  • Most Should Know that computer circuits are built from Transistors.
  • Most Should Understand the term Boolean Logic (such as AND, OR and NOT).
  • Most Should Be able to complete a truth table for a NOR,OR and XOR gate.
  • Some Could Evaluate that the output from a gate depends on the pattern of transistors in its circuitry.
  • Some Could Explain how a D Latch works in a circuit with two possible stable states.

2 Simplifying NAND gates

Learn It

Let's look at other gates

  • NAND gates are really useful, but it would be annoying to have to draw the transistors every time.
  • Instead we use a symbol like the one shown below. You can click the buttons and compare it to the transistor version below it, to see that they are identical in function.

3 Combining NAND gates

  • NAND gates are said to be universal logical gates because you can make all other types of logic gate out of them.
  • We can take a single NAND gate like this one.
  • And use it to make this
  • This is called an AND gate.
  • Here is an online gate builder which is a nice way to visualise what is happening.

4 Assessment

Badge It - Silver

  • Have a look at the below. This is the arrangement for a NOT gate, sometimes called an inverter.
  • Construct a Truth table for the arrangement, showing input A output Q.
  • Describe why the NOT gate is often called an Inverter?



5 Making other gates from NAND gates

  • We've already seen one other gate that can be made using NAND gates - it was called an AND gate.
  • Below is the diagram for the NAND gate arrangements for an AND gate again, and a simplified symbol for an AND gate that is commonly used.
  • Hopefully you can see that the two are identical. When inputs A and B are on, output Q is on. When either or both A and B are off, Q is off.
  • The AND gate has a very similar symbol to the NAND gate (it's just missing the little circle near the output) so make sure you don't get the two confused.
  • The truth table for an AND gate looks like this
0 0 0
1 0 0
0 1 0
1 1 1

6 Assessment

Badge It - Gold

  • OR gate


  • NOR gate


  • XOR gate


7 Assessment

Badge It - Platinum

  • There is a special arrangement of NAND gates called a D-Latch.
  • Use the Web to find out what the arrangement of NAND gates in a D-Latch is.
  • Write a description of a D-Latch and explain why it is useful in computing.