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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.

A More Complex Example

12. Decoupling Capacitor
13. Voltage Divider - a fixed reference voltage
14. Voltage Divider - a light dependent voltage that decreases as it gets dark
15. Op Amp Comparator. In daylight, the output is LOW. At night, the output is HIGH.
16. 555 Astable. At night, the reset pin is HIGH and the astable oscillates causing the lamp to flash. In daylight, reset is LOW and the lamp is OFF.
17. Transistor Switch Transducer Driver. The transducer is the lamp.