Introduction

System diagrams show how the different parts of a complex electronic system connect and interact. System diagrams are sometimes called block diagrams.

The boxes represent electronic sub-systems but do not give details of how the sub-system works. The arrows between the boxes represent the flow of information, not physical wires or other types of connection.

Basic System Diagram

The most basic system diagram has an INPUT, a PROCESS and an OUTPUT ... and that's it. All systems can be expressed in terms of inputs, processes and outputs at the most basic level.

Minimal System Diagram

A more detailed system diagram includes the names and functions of each sub-system and shows how they are connected together. However, details of the circuits used, component values and other technical data is not included in the system diagram.

The system diagram shown is for an outside light that comes on when movement is detected at night. The system diagram shows what sub-systems are required but not how they are implemented electronically.

The advantage of using a systems based approach to electronics is that each sub-system can be designed and tested separately which makes finding problems and mistakes much easier. In a large system different sub-systems can be designed and tested by different engineers as it is clear what each sub-system needs to do and how it relates to all the other sub-systems. Another advantage of a systems based approach is that one sub-system can be upgraded or modified as necessary without having to change the whole circuit.

Input Transducers

Input Transducers convert information in the real physical world into electrical signals.

Input Transducers can produce either a digital or an analogue signal.

Many Input Transducers (LDRs, Thermistors, variable resistors) change their resistance as the environmental conditions change - these input transducers need to be used as part of a potential divider to convert the changing resistance into a changing voltage or current that can be processed.

Examples of Input Transducers include:

• Used as part of a potential divider, LDRs convert light level into an analogue voltage.
• Used as part of a potential divider, thermistors convert temperature into an analogue voltage.
• Variable resistors or potentiometers - produce an analogue voltage that depends on position or angle.
• Push buttons and switches are used with pull-up or pull-down resistors to produce a digital signal that depends on the user input e.g. Logic 1 when the button is pressed.
• Microphones produce an analogue signal that relates to sound level.
• Magnetic field detectors produce an analogue signal that depends on the strength of the magnetic field. Magnetic field detectors can give information about direction or orientation when used to detect the Earth's magnetic field.
• Motion sensors such as PIR sensors give a digital output when movement is detected.
• A pressure pad gives a digital signal when pressure is applied e.g. a Logic 1 when someone stands on the pad.
• A moisture detector, as part of a potential divider, gives an analogue signal that depends on the moisture (of the soil for example).

Processes

Processes respond to and process electrical signals from the input transducers. The processed electrical signals are then interpreted by the output transducers.

Process sub-systems can be analogue (amplifiers etc) or digital (logic circuits, for example). The process sub-systems are constructed from active components meaning they need a power supply to function. The power supply is an essential part of an electronic system but is not shown on system diagrams, it is simply assumed to be part of the system.

Examples of Processes include:

• Combinational Logic operations (AND, OR, NOT etc)
• Timers (monostables)
• Oscillators producing pulses (astables)
• Memory and Latches (bistables, flip flops)
• Comparators (op-amp circuits)
• Amplifiers
• Counters (binary and decade)
• Analogue to Digital Convertor (ADC) - an Op-Amp circuit
• Digital to Analogue Convertor (DAC)
• Micro-controllers (PICs etc)

Transducer Drivers

Transducer Drivers are necessary to increase the voltage, current (and consequently) power of the process sub-systems so that the output transducers can function correctly. For example, a logic gate can drive an LED sufficiently well but cannot drive a 24 W bulb. In this case a transducer driver is required to increase the current available for the bulb.

Examples of Transducer Drivers include:

• MOSFETs
• Bipolar Transistors
• Electromagnetic relays
• Power Amplifiers

Output Transducers

Output Transducers convert electrical signals into actions in the real physical world. Output transducers can require high voltages or a large current to operate and so a transducer driver may be required to connect the process sub-system to the output transducer.

Output Transducers can be either digital or analogue.

Examples of Output Transducers include:

• Bulbs and LEDs produce light and can be either analogue or digital.
• Motors produce motion and movement, as the speed can vary these are analogue devices.
• Solenoids produce movement but are digital devices, the solenoid is either in one position or the other e.g. a door lock.
• Actuators or Servos produce motion related to a position and are analogue.
• Loudspeakers and Buzzers produce sound. Loudspeakers are usually connected to an amplifier of some sort and are therefore analogue whereas a buzzer is either on or off and is therefore digital.
• Displays such as LED bar graphs, 7 segment displays or LCD and OLED displays can produce letters and numbers.
• Heaters increase the temperature and can be either analogue or digital.