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Control Systems

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A Generic Diagram

Here is a diagram representing a generalised control system.

Open loop control systems do not have any feedback or an error detector so the diagram can be simplified.



Input Instructions

Control systems have input sensors which generate signals that instruct the system to act. The instructions specify what the control system should do. It could be something simple like a voltage used to specify how bright a lamp should be. It could be more complex like a binary code used to change a TV channel. Here are some examples ...

  • Resistive Transducers:
    • Switches:
      • Start, Stop, Reset, Sense machinery position.
      • Combination Lock.
      • Pressure mat.
    • Resistors
      • LDR for light level measurement.
      • Thermistor for temperature measurement.
      • Potentiometer for angle measurement.
      • Potentiometer for setting a reference voltage.
      • Strain gauge.
  • Sound sensors:
    • Microphone, Ultrasound sensor.
  • Time information:
    • Astable produces timed pulses which trigger the control system to act.
    • A microcontroller that has computer code to instruct the control system to act.


  • Digital logic circuit
    • Logic gates, Counters, Shift registers, Latches.
  • Microcontroller
    • Analogue input.
    • Digital inputs and outputs.
  • Analogue circuits
    • Comparator.
    • Schmitt trigger.
    • Summing Amplifier.
    • Difference Amplifier.


These are needed to drive the output transducers. Mostly the processor has insufficient power to drive these output devices. If higher power circuits like railway locomotives need to be controlled, complex high power driver circuits are needed. Most output devices need some sort of driver.

  • MOSFET Switch
  • MOSFET Source Follower
  • BJT Switch
  • BJT Emitter Follower
  • Voltage Follower
  • Relay
  • H Bridge Driver


It could be anything from a single light bulb to the avionic controls for an entire aircraft.

  • Motors
    • Conventional
    • Stepper
    • Servo
  • Light
    • Bulbs
    • LEDs
    • LASERs
    • LED Displays
      • Seven Segment
      • LED matrix
    • LCD Displays
  • Sound
    • Piezoelectric Sounder
    • Loud Speaker
    • Buzzer
    • Siren



Closed Loop Control System


Negative Feedback

Closed loop control systems always include negative feedback. This is where the output is measured in order to control the output.

Beware of trick feedback scenarios where the thing being measured is not the output.


Vehicle Cruise Control Example


Open Loop Control Systems



Sewage Pumping Remote Control Vertically Polarised Yagi Antenna

Here is a pumping station, linked by radio to a central control station. If the pumps failed, sewage could flow in the streets so keeping track of the state of the pumps is important for public health, never mind the complaints if it all went wrong.

Broadcast TV is mostly horizontally polarised so this antenna is mounted vertically to minimise the chances of interference between the TV and Water Company services.

Sewage Pumping Remote Control Vertically Polarised Dipole Antenna




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