Feedback and closed-loop control systems
Design & Technology · Key Stage 3 · The School
Open-loop: no checking
An OPEN-LOOP system carries out an instruction without checking the result. A simple toaster on a timer runs for a fixed time regardless of how the toast actually looks partway through — it has no way to know, and no way to adjust.
Closed-loop: checking and adjusting
A CLOSED-LOOP system includes a SENSOR that measures its own output, compares that measurement to a target, and adjusts accordingly. A thermostat-controlled heater measures the room's actual temperature, compares it to the target set by the user, and switches the heater on when the room is too cold and off when it reaches the target.
What feedback means here
FEEDBACK is the information sent back from the output to the controller, used to adjust future behaviour. In a heating system, the measured room temperature IS the feedback — it feeds back into the decision about whether to keep heating. Without that feedback loop, the system could only guess, exactly as the open-loop toaster does.
A room thermostat set to 20°C works through a repeating loop. STEP 1: a temperature sensor measures the room, reading 17°C. STEP 2: the controller compares 17°C to the 20°C target, and finds the room is too cold. STEP 3: the controller switches the heater on. STEP 4: the room slowly warms as the heater runs. STEP 5: the sensor measures again, now reading 20°C. STEP 6: the controller compares again, finds the target is met, and switches the heater off. The loop then repeats indefinitely, checking and adjusting continuously — which is exactly why a thermostatically controlled room settles near its target temperature rather than overheating or staying cold, the way a simple, uncontrolled heater switched on once by hand would do.