# How A Computer Works

## 1 Learning Objectives

By the end of the lesson …

• All Should Understand the role of a switch as a human interface with an electrical circuit.
• All Should Understand that the behaviour of elecrical circuits is reliable, predictable and logical
• All Should Be able to define how a transistor can act as a switch.
• Most Should Be able to complete a truth table for an NAND gate and describe it's operation.
• Most Should Be able to complete the truth tables for AND, OR and NOT.

## 2 Transistors

### Learn It

The Big Question

• What is man's greatest invention… Fire? The wheel? The internet? Sliced bread?
• Transistors can come in many shapes and sizes.
• They all have something in common - three pins.
• The transistors we find in computer chips and memory are much smaller - 22nm to be exact which is 0.000000022m
• The circuit diagram for a transistor is shown below, with the three pins labeled B for Base, C for Collector and E for Emitter.
• As mentioned in the previous lesson, a transistor can act like a switch. When there is current entering the Base(B) of the transistor, current is allowed to flow through the Collector(C) to the Emitter(E).

### Try It

• Have a play with the transistor below so you understand how applying a voltage to the base of a transistor is equivalent to turning on a switch.

## 3 Assessment

• In your own words write up a description of how a transistor can act as a switch. Make sure you are using the words Collector, Base, and Emitter.

## 4 NAND Gates

### Learn It

• We can combine transistors to make some pretty useful things. One combination of transistors is known as a NAND gate.
• A NAND gate is made from two transistors combined as shown below.
• The basic concept of this arrangement, is that if current can't flow through the collector and base of both resistors to Earth, then instead it will flow through the OUT
• If both transistors allow current through their Collector and Emitter, then no current will flow through OUT
• Have a play with the arrangement below and make sure you understand how the NAND gate works.

## 5 Assessment

• Below is a table showing the states that A and B can have, with a column left blank for the OUT (which has been labeled Q). This is called a Truth Table.
A B Q
0 0
1 0
0 1
1 1
• Copy the table and complete the column labeled Q. (An input or output is ON (1), if it current is flowing through it (red)
• Now copy and fill in the missing spaces in the description below.

NAND GATES

NAND gates have two inputs (A and B) and an output (_). When A and B are both __, then Q is __. If either A or B are __ then Q is __. If A and B are both __ then Q is __.