View Index by Level





Systems and Subsystems

This is the AQA version closing after June 2019. Visit the the version for Eduqas instead.

To gain access to revision questions, please sign up and log in.



Simplifying Complex Systems

Complex electronic systems can be difficult to design, understand and test.

It is a good idea to modularise systems so each module is relatively simple.

This makes it easier for designers and fabricators to work in teams. Each team member can work on their own module. Many very useful modules are available pre-designed and built.


Working from the top down you have a 555 timer, an LM380 Audio Amplifier, an eight bit Darlington Driver and two Programmable Integrated Circuits (PICs).



One of these modules might contain several subsystems. For example, a 555 timer contains six subsystems, all of which are on the AQA specification.


555 Timer Module - Subsystems


More Subsystems and Modules

As well as single chips, it is possible to get complete modules, sometimes with input and output devices. Examples include ...


Subsystems and System Diagrams

To describe the subsystems or modules in a complex system, block diagrams are used. Most systems consist of the following elements.







Input Sensors or Transducers

The input typically comes from a measurement sensor or transducer. Examples include ...



The input signal is processed in some way. A decision might be made. The input signal might be amplified. The input signal could be converted into a new form ready for transmission to another system elsewhere.


Output Transducers

Output transducers are used to produce a visible or audible signal. Alternatively actuators convert the electrical output into movement used to control machinery.


Information Flows

The flow of information through a system is shown with labelled arrows. Don't forget the labels. Don't forget the arrow heads to show the direction. The information is carried by currents, voltages, sound waves, ultra-sound, radio waves, pulses of light or by some other method. There are lots of ways to get information from one place to another.


Feedback - A2 Topic

Open Loop control systems do not use feedback and the output is not measured.

Closed Loop control systems measure the output. This measurement is fed back to the input where it is used to correct errors in the output.

Negative Feedback - Usually a good thing. The feedback is used to correct errors in the output.

Positive Feedback - Useful in the Schmitt trigger, bistable, monostable, astable and other oscillator circuits. Otherwise it can be a bad thing. The feedback makes the error in the output worse. If a microphone picks up the signal from loudspeakers there is often a loud howling noise. This is an example of unwanted positive feedback. Carefully controlled positive feedback in radio frequency amplifiers (regeneration) increases the gain and selectivity of the circuit without unwanted oscillations.

Closed Loop Example - Expensive cars have cruise control.


Analysing a System

To Analyse a system, break it down into smaller parts (sub-systems), each of which is simpler and easier to understand than the whole system.

For example, you might have a complete system such as a radio. First locate the input, the process and the output.

Here is the simplified block diagram of a radio showing sub-systems without too much detail.


This simple diagram can be expanded to show extra detail. Often a module contains several subsystems. These can be shown on the more detailed diagram.


Each subsystem can be analysed in further detail. The highest level of detail consists of Layout and Circuit Diagrams - These show all the details with every component shown.



Design is not easy for beginners. For starters, complete beginners will not know what is possible and will not be familiar with the basic building blocks used in electronics.

Some serious learning of the basics helps a lot.

To design a system ...


Example Specification Including Testing

  1. If it's too hot, sound the alarm.
  2. Work on 12 to 15 Volts. (Test with a volt meter)
  3. If the temperature is above 35 C, sound the alarm (Test with a thermometer)
  4. The alarm will be a loud tone on a frequency between 3500 and 4000 Hz (Test with an oscilloscope)
  5. The alarm will be louder than 80 dB. (Test with a sound level meter)
  6. The 35 C temperature will still be correct at any power supply voltage between 12 and 15 Volts.

Real Life: You get paid for your work if your circuit meets the specification and passes all the tests.

A Level Work: AQA gives you a good project mark if your circuit meets the specification and passes all the tests.

Important: The specification contains actual electronics measurements and values that are possible to test. Without these, you have not got a specification.


A Temperature and Light Sensor

An electronic system sounds a warning signal when the temperature and the light intensity both fall below set levels.

For example this could warn a horticulturist about night-time frosts.

The system contains ...

The task is to label the diagram.





Reference Levels

These are set so the system responds to the correct light level and temperature.


Process 1

This control system uses two comparators for the processing. Comparators are based on Operational Amplifiers.

  1. Temperature comparator - box b
  2. Light level comparator - box d

Process 2

If both comparators are active (too cold) and (night time) these signals are processed with an AND Logic Gate - box e
Depending on the sensor outputs and driver input, different gates might be needed to get the right behaviour.

More about Logic Gates


Transducer Driver

The AND Gate output is not sufficient to drive the alarm. A driver is needed - box f



The audible warning device - box g.


Another Example

In this circuit ...

Simple System Diagram


A More Complex Buoy Flasher Example

Here is a fairly complex system. It breaks down into much simpler subsystems.

System Diagam

  1. This is a decoupling capacitor.
  2. This is a Voltage Divider providing a fixed or set level.
  3. This is a second Voltage Divider providing a voltage that increases if the light level increases.
  4. This is a comparator subsystem. If the ANALOGUE input light level goes below the set level, the DIGITAL output goes high.
  5. This is an Astable designed to flash the lamp at regular intervals but only if it's dark. If the output from "O" is high, the astable oscillates.
  6. Q this is a transistor switch used as a transducer driver. The output transducer is the lamp. It converts the electrical signal into light.

Buoy Flasher Subsystems


Voltage Dividers

The Comparator

The Astable

The Driver Transistor

Complete Circuit Behaviour




reviseOmatic V3     Contacts, ©, Cookies, Data Protection and Disclaimers Hosted at, London