Three blocks — input, process, output
A control system has three blocks in a fixed order: INPUT → PROCESS → OUTPUT. Input: a sensor or switch that DETECTS a change and makes an electrical signal (push switch, LDR, thermistor, PIR). Process: RECEIVES the signal and DECIDES the output — a transistor, IC or PIC. Output: PRODUCES the effect — a lamp/LED, buzzer, motor. Name a real device for each block, in order.
Feedback — open-loop vs closed-loop
Feedback is a fourth feature: information about the OUTPUT is sent BACK to the input so it compares the result with the target and adjusts itself. A system WITH feedback is CLOSED-LOOP — self-correcting; e.g. a thermostat heater that senses the temperature and switches off at the target, on again as it cools. A system with NO feedback is OPEN-LOOP — a fixed action, never checking the result (a timer traffic light, a toaster).
Transducers and the block diagram
Input and output devices are TRANSDUCERS — they convert one form of signal into another: an input transducer turns a physical quantity into an electrical signal (LDR: light; thermistor: temperature), an output transducer turns it back into an effect (lamp: light; motor: movement). The process between is not a transducer — it decides. Drawn as a block diagram: input → process → output boxes and arrows, plus a feedback arrow from output back to input if closed-loop.
Drawn from real examiner reports.
Feedback described too vaguely
Asked what feedback is, weaker answers just say "it is the output". Feedback is information about the OUTPUT sent BACK to the input, so the system can COMPARE the result with the target and ADJUST the output. Examiners want back, compare/adjust, automatically — without a signal RETURNING and self-correction it is one-way, not feedback.
Generic device or wrong order
Two linked faults. Generic naming: "a sensor" for the input or "electricity" for the output scores little — name a SPECIFIC device (LDR, thermistor or switch for input; lamp, buzzer or motor for output). Wrong order or a missing block: the path is fixed as input → process → output, and a diagram must show the arrows that way. Give a specific device for each block, in order.
Switch names must be exact
Where a question asks for a specific input switch, the exact words carry the mark. A toggle switch "locks in position"; a press-to-make (PTM) switch suits a doorbell (on only while pressed); an SPDT switch changes a circuit over. Vague names — "a button", "a switch" — or the wrong type lose the mark. Match the switch to the job described.
s23 P42 Q8
Feedback arrow added or reversed
In a block diagram, candidates add a feedback arrow to an open-loop system that has none, draw it the wrong way, or forget the process block. The feedback path runs from the OUTPUT back to the INPUT, and only a closed-loop, self-correcting system has one. A fixed-timer system has no feedback arrow — do not add one to look complete.
Not every system has feedback
Believing all control systems have feedback. Feedback distinguishes a CLOSED-loop system from an OPEN-loop one. A closed-loop system (a thermostat heater) senses its result and corrects itself; an open-loop one does not — traffic lights on a fixed timer run a set action and never check the output. The self-correcting system is the one with feedback.
Device in the wrong block
The commonest slip is putting a device in the wrong block. An LDR, thermistor or switch is always an INPUT (it detects); a lamp, buzzer or motor is always an OUTPUT (it produces the effect). The process is the decider in the middle — a transistor, IC or PIC. Test each: does it DETECT, DECIDE, or DO something you see or hear?
Sort each device by its job
Sort each device by its job: does it DETECT (input), DECIDE (process), or DO something visible/audible (output)? Then name a specific device per block — LDR/switch (input), transistor/IC/PIC (process), lamp/buzzer/motor (output) — not "a sensor".
Diagram arrows and the feedback path
Draw the arrows along the signal: input → process → output. Add a feedback arrow (output back to input) ONLY for a closed-loop system that checks its result (e.g. thermostat); an open-loop fixed-timer has none. The feedback path wins marks.
Explain feedback with the right words
For an "explain feedback" part, use the words examiners reward: information about the OUTPUT is sent BACK, COMPARED with the target, the output ADJUSTED AUTOMATICALLY. Name a closed-loop example (a thermostat holding a steady temperature) and point to the RETURN path.
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