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.
Modules on a Chip
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 five subsystems, all of which are on the AQA specification.
555 Timer Module - Subsystems
- a voltage divider
- two comparators
- a flip flop
- an output driver circuit
- a bipolar transistor used as a switch
More Subsystems and Modules
As well as single chips, it is possible to get complete modules, sometimes with input and output devices. Examples include ...
- Ultrasonic Range Finder
- Digital FM UHF Transmitter
- Digital FM UHF Receiver
- Temperature Sensor (more accurate and easier to use than a thermistor)
- Humidity Sensor
- GPS Satellite Receiver
- Infrared Receiver (as used in remote controls)
- Many many more ...
Subsystems
To describe the subsystems or modules in a complex system, block diagrams are used. Most systems consist of the following elements.
Input
The input typically comes from a measurement sensor or transducer. Examples include ...
- Switches (many types)
- Variable resistors
- Light measurement using a Light Dependent Resistor (LDR)
- Temperature measurement using a Temperature Dependent Resistor (TDR)
- Sound wave measurement using a microphone.
- Pressure, speed, position, acceleration, size, thickness, weight and many more.
- More
Process
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.
- Compare two or more signals
- Amplify the input
- Convert an analogue signal to a digital data stream (A to D converter)
- Convert a digital data stream back to analogue (D to A converter)
- Switch a device on or off based on the input signal levels
- Autopilot, cruise control, TV remote control and many more examples.
Output
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 or light or by some other method. There are lots of ways to get information from one place to another.
Feedback
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 (regenertion) increases the gain and selectivity of the circuit without unwanted oscillations.
Closed Loop Example - Expensive cars have cruise control.
- The input is an electrical signal that represents the speed the driver wants.
- The feedback is the actual speed of the vehicle (speed sensor needed).
- The process calculates the difference between the required and actual speeds and generates a control signal that is used to control the engine throttle actuator.
- The output is managed by an actuator used to increase or decrease the fuel flow to the engine.
Analysing a System.
To Analyse a system, break it down into smaller parts, 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.
- INPUT: Antenna
- PROCESS: Radio
- OUTPUT: Headphones (or loudspeaker)
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
Design is not easy for beginners. For starters, complete beginners will not know what is possible and will not be familier with the basic building blocks used in electronics. Some serious learning of the basics helps a lot.
To design a system, identify the Input, The Process and the Output. Occasionally a system might not have an input. An example is a circuit to produce a 440Hz audible tone used for guitar tuning.