A night-light that comes on when the room goes dark.
A doorbell that buzzes when you press it. Every gadget is the same
shape underneath: it takes something in, thinks about it, and does
something out.
For Florence, circuit in mind.
Florence's Design Technology · Electronics
The big idea
Input, process, output.
You do not need to understand every wire to understand an electronic
gadget. There is a shape that almost all of them share, and once you
can see it, the whole subject gets simpler. The shape is just three
boxes in a row: something comes in, something happens in the
middle, and something useful comes out.
Designers call this the
systems model,
and they draw it as a block diagram — three labelled boxes joined
by arrows. The arrows show the signal flowing from left to right. The
beauty of it is that you can describe a gadget without opening it: name
what goes in, what decides, and what comes out.
The systems model. Read it left to right: a signal comes in, the
circuit decides what to do, and something useful comes out.Original schematic
Stage
What it does
Examples
Input
Takes a signal in from the world or the user.
A switch, a light sensor, a temperature sensor.
Process
Makes a decision about that signal.
The circuit, often a tiny chip or microcontroller.
Output
Does something you can see, hear or feel.
A light, a buzzer, a motor turning.
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A detail worth knowing
30–45 seconds · MF 1
Cool fact
A modern car runs on more than a hundred tiny
input-process-output systems at once — one watching the wheels for
a skid, one reading the engine's heat, one deciding when an airbag
should fire. Each is the same three boxes: a sensor in, a decision,
an action out. A car is really a swarm of small systems agreeing
with each other.
Florence's Design Technology · Electronics
The pieces
Components, and the symbols for them.
A circuit is built from
components.
Rather than draw each one as a little picture, engineers use an agreed
set of symbols — a shared shorthand so that a circuit drawn in
one country reads the same in any other. Learn these eight and you can
read most beginner circuits. Notice which are inputs (they sense
or switch), which are outputs (they light, sound or move), and
which simply supply or control the flow.
Cell / battery
supplies the power
Switch
input — opens & closes the circuit
Resistor
limits the flow of current
LDR
input — light-dependent resistor
Thermistor
input — senses temperature
LED
output — a light that shines
Buzzer
output — makes a sound
Lamp
output — a filament bulb
Wire
carries the current
Two sensors worth knowing by name
An LDR (light-dependent resistor) lets more current through
when it is bright and less when it is dark — so a circuit
can tell night from day. A thermistor does the same trick for
heat — its resistance changes as it warms or cools, so a circuit
can tell hot from cold. Both are inputs: they turn a change in the
world into a change the circuit can read.
Cool fact
The first LED that glowed visible light was made
in 1962 and was a dim, sulky red. For decades nobody could make a
blue one — and without blue you cannot mix white light. Three
scientists finally cracked it in the 1990s and won a Nobel Prize for
it in 2014. The bright white LED bulb in your lamp exists because of
that blue.
Florence's Design Technology · Electronics
Watch
A circuit, built and lit.
A short film of a simple circuit being put together — a cell, a switch,
a bulb. Watch the moment the switch closes and the current can finally
complete its loop, and the bulb lights.
Video to be placed here: a short institutional clip on building a simple circuit.
Florence's Design Technology · Electronics
A worked example
A night-light that switches itself on.
Let us build a real design in our heads — a night-light that
comes on by itself when the room goes dark. It is the perfect example
because it fits the systems model exactly. Ask the three questions:
what is the input? what is the process? what is the
output?
The three stages
Input — an LDR watching the light in the room. As
the room darkens, its resistance climbs.
Process — the circuit notices the LDR's change and decides:
"it is dark now — switch on."
Output — an LED lights up, giving a soft glow.
And in reverse
Morning comes; the room brightens.
The LDR lets more current through again.
The process decides "it is light now" and the LED switches
off. The night-light minds itself.
The night-light as a block diagram. The LDR is the input, the
circuit is the process, the LED is the output — the systems model,
made real.Original schematic
▶
A detail worth knowing
30–45 seconds · MF 1
Cool fact
The street lamps along most British roads work in
exactly this way. Each one has its own tiny LDR pointing at the sky.
Nobody flicks a switch at dusk — every lamp watches the daylight fade
and turns itself on, then watches the dawn and turns itself off. A
whole city lighting up, one independent night-light at a time.
Florence's Design Technology · Electronics
Two ways to wire it
Series, or parallel?
When a circuit has more than one component, there are two ways to
join them up, and they behave very differently. In
series, the components sit in a single loop, one after another —
the current has only one path. In parallel, each component sits
on its own branch — the current splits and flows through several paths
at once.
Left, a series circuit: one loop, so if one bulb fails the whole
loop breaks. Right, a parallel circuit: each bulb has its own branch,
so one can fail and the rest stay lit.Original schematic
Why it matters in a real design
The lights in your house are wired in parallel — that is why
one blown bulb does not plunge the whole room into darkness. Old
Christmas-tree lights were wired in series, in one long loop,
which is why a single dead bulb used to switch off the entire string
and send everyone hunting for the culprit.
▶
A detail worth knowing
30–45 seconds · MF 1
Cool fact
In a series circuit, adding more bulbs makes each one
dimmer — they share the same push from the battery. In a
parallel circuit, each bulb gets the full push, so they all stay
bright; but the battery drains faster because it is feeding several
full paths at once. Every wiring choice is a trade.
Florence's Design Technology · Electronics
Watch
Systems in the real world.
A short film showing the input-process-output idea inside everyday
gadgets. Watch for the three boxes hiding in each example — sensor in,
decision, action out.
Video to be placed here: a short institutional clip on electronic systems and the input-process-output model.
Florence's Design Technology · Electronics
Question · match the label
Name the circuit symbols.
Four circuit symbols are numbered below. Pick a label, then drop it on
the matching pin. Two of the labels are not symbols on this diagram at
all — read carefully before you place them.
Four standard circuit symbols. 1 — a cell. 2 — a switch. 3 — a
resistor. 4 — an LED.Original schematic
Match each label to its pin
Pin 1—
Pin 2—
Pin 3—
Pin 4—
Two chips name components that are not drawn here — read first, then place.
Florence's Design Technology · Electronics
Question 1 · circle the answer
The three stages.
The systems model breaks a gadget into three stages,
in order. What are they?
Question 2 · circle the answer
Input, process or output?
A buzzer sits at the end of a circuit and makes
a sound when the circuit decides it should. Which stage of the systems
model is it?
Question 3 · type your answer
The dark-sensing part.
In the night-light, one component senses how light or
dark the room is — its resistance changes with the light falling on it.
What is this input component called? (Three letters.)
input =
Fresh one. A different input component changes its resistance with heat — a circuit uses it to sense temperature. What is it called? (One word.)
input =
Florence's Design Technology · Electronics
Question 4 · circle the answer
What a resistor is for.
A circuit includes a resistor. What is its job?
Question 5 · circle the answer
The night-light's input.
In the night-light design, which part is the
input?
Question 6 · circle the answer
One bulb fails.
Two bulbs are wired in series — one single loop.
One bulb blows. What happens to the other?
Question 7 · circle the answer
Why parallel for the house.
The lights in a house are wired in parallel. Why
is that the sensible choice?
Florence's Design Technology · Electronics
Question 8 · extended response
Design a fridge-door alarm — as a system.
A fridge left open wastes power and spoils food. Design a gadget that
beeps if the fridge door is left open. Describe it as a
system: name the input, the process, and the
output, and say what each one is. The move that lifts an answer
is using the right words — input, process, output — and tying each to a
real component you have met today.
Lay it out as three stages — input, process, output — and name a real
component for each. 120–180 words is plenty, Florence. No rush.
0 words
reading what you wrote…
On your system, Florence
strong
You laid it out as three clear stages and named a real component for
each — a switch on the door as the input, the circuit deciding as the
process, a buzzer as the output. Using the systems words and tying
each to a real part is exactly how this kind of answer is read.
try this
One stage carries two ideas at once, so the structure blurs a
little. When the worry creeps in that you have not said enough, the
three boxes can run together. Give input, process and output each its
own short line and the system stands out clearly.
to add
You have the three stages; now add the thoughtful touch. A real
designer would add a short delay so a quick open-and-close
does not beep. Naming one improvement like that turns a working
circuit into a designed one. That is the part worth reaching for
next time.
Florence's Design Technology · Electronics
Glossary
The words from today.
Systems model
A way of describing any gadget in three stages: input, process and output.
Input
The part that senses or switches — a switch, an LDR, a thermistor. The signal comes in here.
Output
The part that acts — a light (LED), a buzzer, a motor. Something you can see, hear or feel.
LDR
A light-dependent resistor — an input that changes its resistance with the light around it.
Resistor
A component that limits how much current flows in a circuit.
Series & parallel
Series — one single loop. Parallel — each component on its own branch, current splitting between them.
Watch
Worth watching.
A short film to watch alongside today's lesson.
See how circuit symbols build a working system, input to output.BBC Bitesize · YouTube
End of lesson
You can see the system now.
You learned the three boxes — input, process, output — and the
symbols for the parts that fill them. You followed a night-light from
a dark room to a soft glow, and you know why your house lights are
wired in parallel and not in series. Next time a gadget does something
clever, you will ask: what is the input, what decided, what came out?
Florence, this is electronics.
F.M. · Design Technology · Electronics
Cool fact
The first pocket calculator of 1967 needed a chip
the size of a stamp to do sums you now carry a thousand of in a
phone. Every one of those chips is still just input, process, output —
keys in, a decision, a number out. The idea has not changed in sixty
years; only the size of the boxes has.
Images
· All diagrams on this page — the systems model, the circuit-symbol grid,
the night-light block diagram, the series-and-parallel comparison, and the
symbol-labelling schematic — are original SVG line-art, drawn for this
lesson. The circuit symbols follow the standard BS / IEC conventions.
Feel free to use them freely.
Videos are marked as slots for short institutional clips, added
when the lesson is voiced and finished.