Systems: input, process, output
Design & Technology · GCSE · The School
Three blocks, in order
INPUT senses: switch, LDR, thermistor, moisture probe, microphone. PROCESS decides: a microcontroller, or a fixed circuit. OUTPUT acts: LED, buzzer, motor, solenoid, heater, display. Draw any product as those three boxes with arrows and most of the design questions answer themselves, because each box has a short list of candidates.
The same hardware, different behaviour
A microcontroller with a thermistor and a relay is a THERMOSTAT if the process says "below 18, switch on", and a FIRE ALARM if it says "above 40, sound the buzzer". Nothing physical changed. That is why programmable control dominates modern products: the behaviour is the cheapest part to change, and the last part that has to be decided.
Feedback closes the loop
An OPEN-LOOP system acts without checking: a toaster runs for the time you set, whatever the bread is doing. A CLOSED-LOOP system measures its own output and corrects: an oven measures its temperature and cuts the element. Open loop is cheaper and drifts; closed loop costs a sensor and holds. Say which one you have designed and why.
INPUT: a thermistor reports 31 °C, and a light sensor reports that it is daytime. PROCESS: the program compares 31 with the 25 °C target and finds it too warm, then checks the light sensor — because opening the vent at night lets in cold and pests, so the rule is "too warm AND daylight". OUTPUT: a signal to the motor that opens the vent. FEEDBACK: the thermistor keeps reading, so when the temperature falls back through 24 the vent closes — and note the CLOSING threshold is lower than the opening one on purpose. Without that gap the vent would chatter open and shut every few seconds around exactly 25, wearing out the motor. That gap has a name, hysteresis, and it is the difference between a system that works and one that is technically correct and useless.