Here is a diagram representing a "generalised control system". All real-life control systems are similar to this generic one. Open loop control systems do not have any feedback so the diagram can be simplified.

Generalised Control System

Open Loop A - instructions 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.   B - processor this might be based on a PIC chip or hard wired   C - driver most control systems produce a small output power. 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.   D - output device this is the device being controlled. It could be anything from a single light bulb to an entire aircraft.   Examples : Light dimmer, Vacuum cleaner bag full alarm, Chemical reactor too hot warning alarm.

Closed Loop   An example of an input instruction could be a 6 Volt signal used to represent 60 miles per hour.   The output sensor measures the actual output (eg actual vehicle speed - 5 volts might represent 50 miles/hour).   The error detector measures the difference between the required and actual output (-1 volts represents 10 miles/hour too slow)   The processor interprets the signal from the error detector and generates an output signal.     This could be a very crude process giving either maximum throttle or none - uncomfortable.     The processing could be much smoother, increasing the throttle until the vehicle is accelerating at a comfortable rate.   The output transducer driver converts the processor output to the correct format to control the output device.     In this case the driver uses the small signal from the processor to control the throttle setting.   The feedback loop is the path from the output back to the input. In virtually all closed loop systems, this is negative feedback. It is used to correct errors in the output.   Examples : Vehicle cruise/speed control, Too cold so turn the heating on, Water level too low so turn on the inlet valve.

Beware

Some control systems appear, at first glance, to be closed loop but are actually open loop. One example is the computer monitor that adjusts its brightness depending on the light level in the room. Here the room light level is measured and used to control the monitor brightness. A closed loop control system would have to measure the brightness of the monitor (not the room).