# 4.4.1 Digital Electronics

Extended Only

### Analogue Signals vs Digital Signals

• There are two types of signals:
• Analogue
• Digital
• An analogue signal is continuously varying and can take any value

An analogue signal is continuously varying, taking any value

• A digital signal can only take one of two (discrete) states – commonly referred to as 1s and 0s (or highs and lows, or ons and offs)

A digital signal can only take one of two values – 0 or 1

Extended Only

### Logic Gates

• A logic gate is a digital electronic component that gives a particular output, depending on the input(s) to it
• A NOT gate:
• Has a single input and one output.
• Will output a 1 if the input is NOT 1

Outputs a 1 if the Input is NOT 1

• An AND gate:
• Has two inputs and one output
• Will output a 1 if both the first input AND the second input are 1

Outputs a 1 if both A AND B are 1

• An OR gate:
• Has two inputs and one output.
• Will output a 1 if either the first input OR the second input is 1

Outputs a 1 if either A OR B are 1

• A NAND gate:
• Has two inputs and one output
• Will output a 1 so long as the first input and the second input are not both 1 (NOT AND)

NAND is short for NOT AND – the opposite of an AND gate

• A NOR gate:
• Has two inputs and one output
• Will output a 1 if neither the first input NOR the second input is 1

NOR is short for NOT OR – the opposite of an OR gate

Extended Only

### Truth Tables

• Given a simple circuit involving several logic gates, you should be able to construct a truth table showing how the outputs depend upon the inputs to the circuit
• To solve problems like this, tackle them one step at a time, figuring out how the state of the wires in the middle depends upon the input to the circuit

Example:

• Consider the following circuit, consisting of NOT, AND and OR gates:

A digital circuit, consisting of three different logic gates

• There are two inputs to the circuit, and so there are four different possible inputs:

(Note how we are also including the states of wires X and Y in our truth table)

• Start by considering how the state of wire X depends on the inputs to the AND gate:

• Next let’s consider how the state of wire Y depends upon the input to the NOT gate (input B):

• Finally, we can complete the truth table by considering the inputs to the OR gate (X and Y):

### Author: Jenna

Jenna studied at Cardiff University before training to become a science teacher at the University of Bath specialising in Biology (although she loves teaching all three sciences at GCSE level!). Teaching is her passion, and with 10 years experience teaching across a wide range of specifications – from GCSE and A Level Biology in the UK to IGCSE and IB Biology internationally – she knows what is required to pass those Biology exams.
Close