Systems thinking using input, process and output, including sensors and other input devices, the processing of signals and control, and output devices such as motors, lamps and buzzers.

What this dot point is asking

This is AQA section 3.1.4. AQA wants you to think about products as systems made of three blocks: input, process and output. You need to name common input devices (sensors), describe how a process stage handles the signal and makes a decision, and name output devices that produce a result. You should be able to read and draw a simple system block diagram. In Paper 1 this is examined by asking you to label or complete a block diagram and to describe the role of each stage for a given product.

The three blocks of a system

The input detects a change, the process decides what to do, and the output makes something happen. The great value of the systems approach is that you can design and understand a product without drawing the full electronic circuit: you treat each block as something with a known input and output, then design the parts separately. Some systems also use feedback, where a sensor monitors the output and feeds the result back to the process so it can correct itself, as in a thermostat that switches a heater off once the target temperature is reached.

Input devices (sensors)

Common inputs include:

• Switch: detects a press or contact; types include push-to-make, push-to-break, toggle, tilt and reed (magnetic).
• Light-dependent resistor (LDR): its resistance falls as light increases, so it senses brightness and is used in dusk-to-dawn lighting.
• Thermistor: its resistance changes with temperature, so it senses heat or cold and is used in thermostats.
• Moisture or humidity sensor: detects water or damp, used in plant waterers and rain sensors.
• Pressure, infra-red and ultrasonic sensors: detect force, body heat (PIR for movement) and distance, used in alarms, automatic doors and parking aids.

Processing and control

The process block, often a microcontroller (a small programmable chip), compares the input signal against a set condition and decides what the output should do. Many systems use simple logic: an AND condition (act only if two inputs are both true, such as dark AND movement), an OR condition (act if either is true), or a NOT condition (act if an input is not present). Using a microcontroller means one chip can replace many separate components, reducing size and cost, and the behaviour can be changed by reprogramming rather than rewiring, which makes products quicker to develop and update.

Output devices

• Motor: produces rotary movement, as in a fan or a pump; a solenoid or actuator gives linear movement.
• Lamp or LED: produces light; an LED is efficient and long-lived and signals status.
• Buzzer or speaker: produces sound for an alarm or alert.
• Heater or relay: a heater produces warmth; a relay lets a small signal switch a much larger current, such as mains power.

A block diagram is the standard way to communicate a system. Each stage is a labelled box and arrows show the signal flow from input through process to output. Drawing it makes the function clear and helps spot which sensor, decision and output device each stage needs.