Key Stage 3 design and technology · Technical knowledge

KS3 design and technology: electronic circuits and systems

In Years 7 to 9, pupils move beyond a battery, a bulb and a switch to electronic systems that sense something, process it and respond. They learn how circuits are powered, how inputs such as light or temperature sensors connect to outputs such as lights, buzzers and motors, and how to choose components with simple calculations so that the circuit works and nothing burns out.

Electronic circuits and systems, explained

In Key Stage 2 children build series circuits with switches, bulbs, buzzers and motors. Key Stage 3 builds on this by looking at a circuit as a system with three parts. An input detects or receives something, such as a press, light, heat or sound. A process decides what to do with it. An output then does something useful. The curriculum gives circuits with heat, light, sound and movement as inputs and outputs as an example, so a school might make an automatic night light, a temperature alarm or a buggy that stops when it senses a wall. Schools choose their own projects.

Power comes first. Pupils learn that cells connected in series add their voltages (four 1.5 volt cells make 6 volts), while cells connected in parallel keep the same voltage but last longer. They compare power sources such as batteries, rechargeable packs, solar cells and mains adaptors, and think about cost, size, lifetime and impact on the environment. In school, pupils work only with low-voltage supplies. Mains electricity is dangerous and is never part of their experiments, and the same rule applies at home.

Pupils meet a wider set of components. Common inputs include push switches, reed switches, tilt switches, light-dependent resistors (LDRs), whose resistance falls as light increases, and thermistors, whose resistance usually falls as temperature rises. Common outputs include light-emitting diodes (LEDs), buzzers, speakers, motors and heating elements. Resistors limit current, capacitors store a little charge, diodes allow current in one direction only, and a transistor acts as an electronic switch, letting a small current from a sensor control a larger current through a motor or lamp. Circuit diagrams use standard symbols, and pupils learn to read and draw them.

Simple calculations help pupils choose the right parts. The most common is Ohm’s law: voltage equals current multiplied by resistance (V = I × R). Pupils use it to work out the resistor to put in series with an LED, because an LED left on its own will pass too much current and burn out. They also build circuits on a breadboard or simulator first, test them with a multimeter and fix faults. Many projects add a programmable component to the process stage, which is the focus of the topic on microcontrollers.

Words your child will hear

Input: A component that detects something and sends a signal into a circuit, such as a switch or light sensor.
Output: A component that does something when the circuit tells it to, such as an LED, buzzer or motor.
Resistance: How much a component opposes the flow of current. It is measured in ohms (Ω).
Series and parallel: In series, components are in one loop. In parallel, the current has more than one path.
LDR: A light-dependent resistor, whose resistance goes down as it gets brighter.
Thermistor: A resistor whose resistance changes with temperature.
Transistor: A tiny electronic switch that lets a small signal control a larger current.

What the National Curriculum says

The programme of study for Key Stage 3 says:

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]
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]

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

A worked example

A pupil wants to light a red LED from a 9 volt battery. The LED needs about 2 volts across it and should carry no more than 20 mA (0.02 amps). What resistor should go in series with it?

Find the voltage the resistor must take up. The battery supplies 9 V and the LED will use about 2 V, so the resistor has to take the other 7 V.
```
9 V − 2 V = 7 V
```
Use Ohm’s law, R = V ÷ I, with the current converted to amps (20 mA = 0.02 A).
```
R = 7 ÷ 0.02
  = 350 Ω
```
Resistors only come in standard values, so 350 Ω cannot be bought. The nearest values are 330 Ω and 390 Ω. Check 330 Ω first: 7 ÷ 330 is about 0.021 A, or 21 mA, which is just over the limit.
```
7 ÷ 330 ≈ 0.021 A (21 mA)
```
Check 390 Ω. This keeps the current under 20 mA, so it is the safe choice, and the LED will be only slightly dimmer.
```
7 ÷ 390 ≈ 0.018 A (18 mA)
```
Draw it. Battery, resistor and LED in a single series loop, with the LED the correct way round: its longer leg goes towards the positive side.

Choose a 390 Ω resistor, which gives about 18 mA. A calculated 350 Ω is rounded up to the next standard value, never down, so the LED stays within its safe current.

Common mistakes and how to help

Current gets used up as it passes through each component

A battery does run down, so children imagine the current shrinking as it travels round.

What helps: In a series circuit the same current flows everywhere. What is used up is energy, which the battery supplies and the components transfer as light, heat or movement. An ammeter placed before and after a bulb would give the same reading.

An LED doesn’t need a resistor if the battery is small

LEDs look like tiny bulbs, and a bulb works fine without anything extra.

What helps: Once an LED lights, a tiny rise in voltage makes the current shoot up, so without a resistor too much current flows and it can burn out in a flash. The resistor sets the safe current. Always calculate or look it up.

Joining batteries in parallel makes a stronger supply

More batteries sounds like more power, so children join them side by side expecting a higher voltage.

What helps: Parallel keeps the voltage the same and makes the supply last longer. For a higher voltage, join cells in series, positive to negative.

It doesn’t matter which way round a component goes

Bulbs, switches and resistors work either way, so children assume every component does.

What helps: LEDs, diodes, electrolytic capacitors and transistors are polarised or have set pins. Look for the longer leg, a flat edge or a stripe, and check the datasheet or diagram before connecting.

How to help at home

Spot inputs and outputs. Around the house, ask what each device senses and what it does in response: a lamp that comes on at dusk, a toaster that pops, a fridge light and a doorbell. Have your child sketch each as input, process, output.
Safe circuits with a battery. A kit of cells in a holder, wires, an LED, a resistor and a switch is cheap and low-risk because it uses only a few volts. Never join the battery terminals directly with a wire, as this short circuit makes the wire and battery hot. Let your child build it, then swap in a larger resistor and describe what happens to the brightness, and why.
Read the numbers. Battery and charger markings give volts, amps and sometimes milliamp-hours. Ask your child what the volts and amps on a charger mean, or why a toy that needs 6 volts takes four 1.5 volt cells.
Talk about mains safety. Make the rule clear: experiments only use batteries. Never take the cover off, open or poke anything plugged into a wall socket, and never use damaged leads. This is a good point to teach your teenager what the symbols and warnings on appliances mean.

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