Modelling electronic devices with the input, process and output system model, and distinguishing analogue from digital signals.

What this key area is asking

The SQA wants you to analyse any electronic device using the input, process, output system model, and to tell apart analogue signals (continuously varying) from digital signals (discrete levels). This is the framework for the whole electronics area: every circuit you meet is an input, process or output sub-system, and knowing whether a signal is analogue or digital tells you what kind of circuit must handle it.

Modelling an electronic system

The model lets you break an unfamiliar circuit into manageable pieces and reason about each in turn. The rest of this area then fills in the standard sub-systems: input sub-systems built around the potential divider, processing built from amplifiers, comparators and logic gates, and output sub-systems driven through transistor switches.

Analogue signals

An analogue signal varies continuously and can take any value within its range. Most natural quantities are analogue: temperature, light level, sound pressure, position. Sensors that respond to these produce analogue voltages, for example the smoothly changing voltage across a thermistor as a room warms, or the waveform from a microphone. Analogue signals are processed by analogue circuits such as potential dividers and operational amplifiers, which work with the whole continuous range of values.

Digital signals

A digital signal has only discrete levels. In the digital electronics of this course there are just two levels: a high voltage (logic 1) and a low voltage (logic 0). A switch is the simplest digital input (pressed or not), and the output of a logic gate is digital (high or low). Digital signals are processed by logic circuits, which combine two-state inputs to produce two-state outputs.

Bridging analogue and digital

Many real systems contain both kinds of signal, so the boundary matters. A comparator (later in this area) takes an analogue input voltage and produces a digital output: high when the input is above a threshold, low when below. This is exactly how an analogue sensor signal gets turned into a clean two-state signal that logic can use, which is why the comparator sits at the meeting point of the analogue and digital parts of a control system.

Examples in context

A digital thermometer shows the journey across the boundary. The temperature sensor produces a continuously varying analogue voltage; this is converted to a digital value so it can be processed and shown on a numeric display. A dimmer switch stays analogue: it varies the voltage to a lamp continuously across its whole range. A burglar alarm is largely digital: door and window switches give two-state inputs that logic gates combine to decide whether to sound the alarm. Recognising which parts of a product are analogue and which are digital tells you which circuits the rest of this area applies to each part.

Try this

Q1. Name the three sub-systems of the electronic system model in order. [1 mark]

• Cue. Input, process, output.

Q2. State whether the output of a logic gate is analogue or digital. [1 mark]

• Cue. Digital: it is only ever high (logic 1) or low (logic 0).

Q3. Give one example of an analogue input signal in an electronic system. [1 mark]

• Cue. For example the continuously varying voltage from a thermistor, a light-dependent resistor or a microphone.