Digital electronics and logic gates Unit 10.4

L.O: To be able to differentiate between analogue and digital. Describe the action of NOT, AND, OR, NAND and NOR gates Recall and use the symbols for logic gates Design and understand simple digital circuits combining several logic gates Use truth tables to describe the action of individual gates and simple combinations of gates

Solve the following problem.

What do we mean by digital and analogue ?

The difference… Analog refers to circuits in which quantities such as voltage or current vary at a continuous rate. When you turn the dial of a potentiometer, for example, you change the resistance by a continuously varying rate. In digital electronics, quantities are counted rather than measured. There’s an important distinction between counting and measuring. When you count something, you get an exact result. When you measure something, you get an approximate result.

Think about the following examples, are they digital or analogue? Clocks Baking Age Temperature Weight Test results

So, An analogue output continually changes and can have any value in a given range. Digital signals are a series of pulses consisting of just two states, ON (1) or OFF (0). There are no values in between But…Why is it useful? Reducing errors! analog communication copies are quality wise not as good as their originals while due to error free digital communication, copies can be made indefinitely. Simply put, digital is more accurate!

How do we convert from analogue to digital?

What do we do with the digital signal now?

What are logic gates? Many electronic circuits have to make decisions. They look at two or more inputs and use these to determine the outputs from the circuit. The process of doing this uses electronic logic, which is based on digital switches called gates. Logic gates allow an electronic system to make a decision based on a number on its inputs. They are digital electronic devices. Digital means that each input and output of the gates must be one of two states: high, also known as 1 or 'on' low, also known as 2 or 'off'

The OR gate The ‘truth table’ for an OR gate looks like this

The AND gate The truth table for an and gate.

The NOT gate A NOT gate is slightly different because it has just one input. It will give a high output if the input is low. This could be represented by a simple lighting circuit with a push-to-break switch: if the switch is pressed then the lamp will turn off. NOT gates are often used in emergency-stop buttons on machine tools.

Draw the truth tables for NAND and NOR

Logic gates are used to build complex logic systems, such as burglar alarms and combination locks. These may be built up of many different primitive logic gates. Lets try and design a few circuits together, within your groups 

Q1. Which logic gate would you use to design a door bell system for your house, assuming you have a front door and a back door?

Q2. You want to design a fire alarm system Q2. You want to design a fire alarm system. The conditions are: The alarm only goes off 1- if the temperature is HIGH (ON) 2- The smoke detector is ON

Q3. You are going on holiday, and you want a design an alarm security system. The conditions are: The alarm only goes off 1- if the front or the back door is opened (or both!) and… 2- if motion is detected in the house (motion sensor).

**HOMEWORK worksheet + questions pg 235-237

Summary: What is the difference between analogue and digital ? What are logic gates? Describe the action of NOT, AND, OR, NAND and NOR gates Recall and use the symbols for logic gates Use truth tables to describe the action of individual gates and simple combinations of gates