A feedback system uses a sensor to check the effect of its own output, then adjusts. A fixed sequence runs the same steps in the same order whatever happens. The test is simple: does the next decision depend on a new reading?
Extension: feedback loops go slightly beyond the Cambridge syllabus list. They are included to help you understand how automated systems correct themselves.
This lesson builds on tracing the stages and sits inside automation and emerging systems. Exam scenarios often describe one system and ask whether it uses feedback, so the habit of looking for the reading is worth building.
How do you tell them apart?
Ask three questions about the scenario:
- Is there a sensor that measures the thing being controlled?
- Does the processor use that reading to choose the next action?
- Does the action change what the sensor reads next?
If all three answers are yes, it is a feedback system. If the actions follow a timetable or a counter and ignore what the sensor reads, it is a fixed sequence.
Worked example
A room starts at 18 °C. The target is 22 °C. Each step, a heater that is ON raises the temperature by 2 °C, and a heater that is OFF lets it fall by 1 °C.
Feedback version. The processor reads the temperature and applies IF temp < 22 THEN heater ← "ON" ELSE heater ← "OFF".
temp ← 18
FOR step ← 1 TO 6
IF temp < 22
THEN
heater ← "ON"
newTemp ← temp + 2
ELSE
heater ← "OFF"
newTemp ← temp - 1
ENDIF
OUTPUT step, temp, heater, newTemp
temp ← newTemp
NEXT step| step | temp read | heater | temp after |
|---|---|---|---|
| 1 | 18 | ON | 20 |
| 2 | 20 | ON | 22 |
| 3 | 22 | OFF | 21 |
| 4 | 21 | ON | 23 |
| 5 | 23 | OFF | 22 |
| 6 | 22 | OFF | 21 |
Fixed version. The heater is ON for steps 1 to 3 and OFF for steps 4 to 6, whatever the temperature is.
| step | temp read | heater | temp after |
|---|---|---|---|
| 1 | 18 | ON | 20 |
| 2 | 20 | ON | 22 |
| 3 | 22 | ON | 24 |
| 4 | 24 | OFF | 23 |
| 5 | 23 | OFF | 22 |
| 6 | 22 | OFF | 21 |
Both runs end at 21 °C, but the fixed version pushed the room to 24 °C, two degrees over target, because it never looked at the reading. The feedback version stayed within 1 °C of the target after step 2.
The mistake to watch for
A common slip is to call a system “feedback” because it has a sensor.
Mistaken answer: “A sprinkler has a moisture sensor, so it is always feedback.”
The student stopped at the sensor and did not check what the processor does with the reading.
If the sprinkler logs the moisture but still waters for 20 minutes at 06:00 every day, the reading never changes the action, so it is a fixed sequence. If it stops watering when the moisture reaches a set level, the output changes the next reading and it is feedback.
Check yourself
1. A school bell rings at set times each day. Feedback or fixed sequence?
Show answer
Fixed sequence. The bell follows a timetable and does not read the effect of its own output.
2. A water tank pump switches off when a level sensor reads full. Feedback or fixed sequence? Give a reason.
Show answer
Feedback. The pump raises the level, the sensor reads the new level, and the processor uses it to stop the pump.
3. In the feedback heater trace above, why is the heater OFF at step 3 although the room is still below 24 °C?
Show answer
The rule compares with the target 22, not 24. At step 3 the reading is 22, and 22 < 22 is false, so the heater is OFF.
Where this leads next
Next, move from one rule to several inputs in explaining a simple decision model. To step through loops like the one above, use the pseudocode trace trainer.
A teacher in Computer Science tuition can build you a fresh scenario each time, so you practise the three-question test on systems you have not seen.