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 andE
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
Badge It - Silver
- 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
Badge It - Gold
- 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 __.
Badge It - Platinum
- There is another arrangement of transistors that can produce a table as shown below.
A | B | Q |
---|---|---|
0 | 0 | 0 |
1 | 0 | 1 |
0 | 1 | 1 |
1 | 1 | 1 |
- Using Circuit Wizard (using transistors, power, a LED and switches), or any graphics package on your computer, try to draw the arrangement of transistors that would produce a table like this.