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Complex electronic systems can be broken down into simpler sub-systems. This makes it easier to understand, design and test the circuits.

Systems typically have ...

1. INPUT/S - Sensing
   1. Light,
   2. Temperature
   3. Magnetic field
   4. Pressure
   5. Moisture
   6. Sound
   7. Movement or Rotation
2. PROCESS/ES
   1. Comparator
   2. Logic gates
   3. Latch
   4. Time delay
   5. A to D Converter
   6. D to A Converter
   7. Microcontroller
3. OUTPUT/S
   1. Lamp or LED
   2. Warning buzzer
   3. Solenoid
   4. Actuator, Motor or Servo
   5. Loudspeaker
   6. Most output transducers need a DRIVER.

Sub-systems and System Diagrams (Labelled boxes joined with arrows)

System diagrams are used to describe systems and sub-systems.

Dividing a system into sub-systems is a good idea because a whole system can be hard to understand.
Each sub-system can be quite simple.
Complex systems are only possible because different teams of designers and testers are responsible for each sub-system.
Many sub-systems can be purchased pre-built and tested.

Example: Turn on a lamp when it gets dark.

To achieve this you'd need ...

• A light sensor in a voltage divider circuit.
• A comparator to sense when the light sensor detects it's too dark
• A driver because the comparator is not able to drive the relay without some help.
• An electromagnetic relay because this can switch high voltage AC mains.
• A lamp

Circuit Diagram (Don't confuse circuit and system diagrams)

• It's possible to design and test each sub-system on its own. 
• Frequently needed sub-systems like the relay are mass produced at low cost.
• The same sub-systems can often be used to solve many different problems.