SAASTE Technology/Activities/AND and OR Gates

Electrical System

Grade 8

AND & OR Logic Gates
There are two types of electrical systems, viz. Digital and Analogue. Digital systems operate on logic circuits. The logic state or logic level of a system can either be HIGH or LOW.

This high and low state is referred to as ON or 1 to represent High and OFF or 0 to represent Low. We use logic gates in electrical systems to decide in a logical way our desired output.

Understanding a logic level

The table is called a truth table. It shows the possible logic levels of the circuit by using a 1 to indicate ON and a 0 to indicate OFF.

This circuit can also be represented as a Systems Diagram as follows:

Activity 1:
Practical, truth table and systems diagram

Practical

Gather the necessary components and connect up the circuit shown below:

Truth Table

Complete the truth table alongside the circuit

This is an AND circuit &shy the lamp will only light up if S1 AND S2 are closed.

Systems Diagram

Draw a systems diagram to represent the AND circuit.

Activity 2:
Practical and truth table

Practical

Gather the necessary components and connect up the circuit shown below:

Truth Table

Complete the truth table below:

This is known as an OR circuit &shy for the lamp to light up either S1 OR S2 need be closed

Glossary:
Switch: a device which allows or prohibits electricity to flow.

Conductor: a material which allows electricity to flow through it. All metals are conductors.

Circuit diagram: a drawing which uses circuit symbols to represent components.

Truth Table: represents the functioning of a digital circuit. This table uses a `1' to indicate a closed switch (or an `on' lamp, motor etc.), and `0' indicates an open switch (or an `off' lamp, motor etc.).

Logic state or logic level: tell us if the system is high or low or on or off.

Analogue system: has a range of values within its limits

Digital system: have discreet values within its limits

`AND' gate: produces 1 or high when all of its inputs are 1 or high

`OR' gate: produces 1 or high when any of its inputs is 1 or high

Author: Ricardo Miller Contributions by Osman Sadeck Editor: Osman Sadeck