Electronic systems: input, process, output

From a torch to a smartphone, every electronic product is a system: something goes in, something is done with it and something comes out.

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Systems thinking

A system has three parts:
Input: senses something, such as a switch being pressed or the light level.
Process: decides what to do, such as a timer or a microcontroller.
Output: does something, such as light an LED, sound a buzzer or turn a motor.
A night light: input = light sensor; process = 'is it dark?'; output = LED on.

Circuits and components

Electricity only flows around a complete circuit. A switch makes or breaks the circuit.
A cell provides the energy. Two or more cells joined together make a battery.
An LED (light-emitting diode) only lets current flow one way. It needs a resistor in series to stop too much current damaging it.
A resistor limits the current. Its value is measured in ohms (Ξ©).

Series and parallel

In a series circuit there is one loop. If one bulb breaks, the whole loop is broken and everything goes off.
In a parallel circuit there are separate branches. If one bulb breaks, the others stay on.
This is why the lights in a house are wired in parallel.
ComponentInput, process or output?
Push switchInput
Light-dependent resistor (LDR)Input
MicrocontrollerProcess
LEDOutput
BuzzerOutput
MotorOutput
Worked example

A burglar alarm sounds a buzzer when a door is opened. Identify the input, process and output.

  1. The input senses the door opening, for example a switch on the door frame.
  2. The process decides whether the alarm is set and the door is open.
  3. The output is the buzzer, which makes a sound.

Answer: Input: door switch. Process: the control circuit or microcontroller. Output: buzzer.

Key idea

Systems have an input, a process and an output. Current only flows in a complete circuit. An LED needs a series resistor. In series, one break stops everything; in parallel, the other branches keep working.

Key words
LED
Light-emitting diode: an output that gives light and only lets current flow one way.
Resistor
A component that limits the current in a circuit. Its value is measured in ohms (Ξ©).
Microcontroller
A small programmable computer on a single chip, used to control products.
Sensor
An input component that responds to a change, such as light, temperature or pressure.

All KS3 Design and Technology key terms and definitions β†’

Common mistakes
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Extra practice

2 more questions

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  1. Which one of these options is an output component?

    1. Push switch
    2. Light-dependent resistor
    3. Microphone
    4. LED
    Show the answer

    Answer: LED

    An LED gives out light.

  2. Which component senses temperature?

    1. LED
    2. Buzzer
    3. Motor
    4. Thermistor
    Show the answer

    Answer: Thermistor

    A thermistor's resistance changes with temperature.

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Sources and references

Last updated 2 October 2026. Spotted a mistake? Tell us and we will fix it.