Key Stage 3 design and technology · Technical knowledge

KS3 design and technology: sensors, actuators and microcontrollers

In Years 7 to 9, pupils make products that respond to the world around them. A sensor collects information, a small programmable computer called a microcontroller decides what to do, and an actuator, such as a motor or buzzer, carries out the decision. The curriculum calls this embedding intelligence, and it brings computing and electronics into design and technology.

Sensors, actuators and microcontrollers, explained

Many everyday products work this way: a car that beeps when you reverse towards a wall, a hand dryer that starts when it detects hands, or a central heating thermostat. The curriculum asks pupils to apply computing and use electronics to embed intelligence in products that respond to inputs, for example sensors, and control outputs, for example actuators, using programmable components, for example microcontrollers. Those examples are only examples, and each school picks its own boards, sensors and software. Some use block-based programming and others use a text language.

A microcontroller is a tiny computer on a single chip, usually fitted to a small circuit board with pins that connect to other parts. It has no screen or keyboard of its own. A program is written on a computer and sent to the board, which then runs it by itself. Inputs arrive through its pins from sensors such as light, temperature, sound, distance, tilt, moisture and buttons. Outputs go to actuators, the parts that make something happen: LEDs, buzzers, motors, servos (motors that turn to a set angle) and heating elements.

Programs follow a pattern. The board reads an input, usually as a number, compares it with a threshold using a selection statement (if this, then that, otherwise something else), and sets an output. This all sits inside a loop that repeats many times a second. Variables store values such as a threshold or a counter. Some sensors give a digital signal, either on or off, like a button, while others give an analogue value that varies smoothly, like a light sensor, which the board turns into a number. Pupils learn to test their code with the board connected, change values and debug when it does not behave as planned.

The microcontroller connects to the wider electronics pupils learn about in the same key stage. Its pins can only supply a small current, so a motor or other power-hungry output is usually driven through a transistor or a motor driver board, with its own power supply. Pupils also look at how a programmed product is made, tested and improved: a design brief, a flowchart or pseudocode, a prototype on a breadboard, and then a more permanent circuit. They consider the user too, asking whether the product is easy to use, safe, and worth the energy it uses.

Words your child will hear

Microcontroller: A small programmable computer on one chip, built into products to control them.
Sensor: An input component that measures something, such as light, heat, sound or distance.
Actuator: An output component that causes movement or action, such as a motor, servo or buzzer.
Threshold: A set value at which a program changes what it does, for example “below 300 means it is dark”.
Selection: An “if … then … else” statement that makes a program choose between actions.
Loop: A set of instructions that repeats over and over.
Debugging: Finding and fixing mistakes in a program or circuit.

What the National Curriculum says

The programme of study for Key Stage 3 says:

When designing and making, pupils should be taught to apply computing and use electronics to embed intelligence in products that respond to inputs [for example, sensors] and control outputs [for example, actuators] using programmable components [for example, microcontrollers]
When designing and making, pupils should be taught to understand how more advanced electrical and electronic systems can be powered and used in their products [for example, circuits with heat, light, sound and movement as inputs and outputs]

Objectives from the National Curriculum in England, © Crown copyright, used under the Open Government Licence v3.0.

Try it: programming an automatic night light

A pupil wants an LED to switch on automatically when a room gets dark. A light sensor is connected to a microcontroller. On this board the reading is a number from 0 (darkest) to 1023 (brightest); other boards use different ranges. How should the program work?

State the system in words. Input: light sensor. Process: compare the reading with a threshold. Output: LED on or off. Write a plan before touching the keyboard.
Find the threshold by experiment. Display the sensor reading on screen in a bright room, a dim room and with a hand over the sensor. Suppose the readings are about 800, 450 and 100. A threshold of 300 would switch the light on only when it is properly dark.

Write the first version of the program as pseudocode.

forever:
  light = read sensor
  if light < 300:
    LED on
  else:
    LED off

Test it in different conditions. The pupil notices that in dusk light, with readings around 300, the LED flickers on and off, because tiny changes in the reading keep crossing the threshold.
Debug it. Use two thresholds: switch on below 250, but only switch off again above 350. In between, the LED stays as it was. This gap is a simple form of hysteresis, and it is what stops the LED flickering.
```
if light < 250: LED on
if light > 350: LED off
```

The finished program turns the LED on below 250, off above 350 and leaves it alone in between. A strong answer describes the testing, names the fault (flickering near a single threshold) and explains how the change fixes it.

Common mistakes and how to help

The sensor decides what to do

A product seems to react by itself, so children credit the sensor with the decision.

What helps: The sensor only reports a number. The program compares it with a threshold and decides. Changing the program changes the behaviour without touching the circuit. Ask your child to point to where the decision is made.

A motor can be connected straight to a board pin

An LED can be joined directly (with a resistor), so a motor seems the same.

What helps: A pin supplies only a small current, and a motor can draw far more and send voltage spikes back. A transistor or motor driver and a suitable power supply protect the board. Check the board’s documentation.

If the program works once, it is finished

A successful first test feels like proof, especially under classroom lights.

What helps: Sensors behave differently in different conditions. Test in bright, dim and changing light, with a low battery, and with a different person using it. Record what happens each time.

Digital and analogue inputs are read in the same way

Both come in through pins, so children assume the program treats them alike.

What helps: A button is digital: pressed or not. A light sensor is analogue: a range of values that must be compared with a threshold. Look at which kind of pin the code reads from and what kind of value it returns.

How to help at home

Sensor-spotting around the house. Ask your child to list every device that senses something and reacts: automatic lights, smoke alarms, thermostats, doorbells with cameras. For each, name the sensor, what decision is made and the actuator.
Write the logic in plain English. Pick a product, such as an automatic plant-watering sensor or a bedside clock that dims with the room light, and ask your child to write the rules in “if … then … else” form. Then ask what could go wrong, such as flickering or not reacting.
Support a school project. If a board comes home, let your child explain what each part does and demonstrate it to you. Talking it through is excellent revision, and you don’t need to understand the code to ask “what is the input, and what happens next?”
Keep the electronics low-voltage. Home experiments should use batteries or a computer’s USB connection only, never mains electricity. Use a light, a buzzer or a small motor from a hobby kit, and keep wires tidy so nothing short-circuits.

Builds on

Key Stage 2: Strengthening and reinforcing structures
Key Stage 2: Gears and pulleys
Key Stage 2: Cams, levers and linkages
Key Stage 2: Series circuits in products
Key Stage 2: Programming and controlling products