Schematic Reading II — Arduino L2

Learning Goals 5 min

Re-meet the schematic symbols from L01-13 (LED, resistor, ground, power) plus the L2 additions: IC chip, button, servo, transistor, LCD, SPI bus.
Read a multi-IC schematic — Arduino + SD card module + LCD + sensor on one diagram — and identify the shared rails, the bus signals, and each device's connection points.
Sketch your own schematic for one of your L2 projects, using standard symbols, in 20 minutes.

Warm-Up 10 min

L01-13 introduced the LED + resistor + GND symbols. That covered single-component circuits. Today the schematics get bigger: multiple chips connected by buses (SPI, I²C), shared power rails, named connection points instead of literal wires. The engineer's view of the projects you've already built.

Why schematics, not wiring photos

A photo of a wired-up breadboard shows the physical layout but hides the function. A schematic ignores the physical layout but shows the function clearly. Both are useful — photos for the bench, schematics for the design.

New Concept · The L2 symbol set 25 min

The basics, recapped from L01-13

You met these five in Level 1. They have not changed, and they are drawn here from the same catalogue every diagram in this course uses — so the zigzag below is the identical zigzag you will meet in the worked example.

Symbol	Name	What it means
	Resistor	Limits current. Its value is written beside it. A slash through it, or an arrow, means the value varies.
	LED	Triangle and bar — a one-way valve — with two arrows leaving it. Current flows the way the triangle points.
	+5V	Where power comes in. Always at the top.
	GND	Where current returns. Always at the bottom.
	Junction dot	These wires join. No dot means they merely cross.

The Level 2 additions

Three of the new ones are components you have already wired this level:

Symbol	Name	What it means
	Potentiometer	A resistor with a third leg — the wiper, drawn as an arrow — that slides along the track. The arrow is what says "you can move this".
	LDR	A resistor in a circle with arrows pointing in. Inwards is the point: light arriving is what changes it. The LED's arrows point out for the opposite reason.
	Thermistor	A resistor with a slash and a `t°`. Same idea, varied by heat instead of light.
	Capacitor	Two plates with a gap. Stores charge; blocks steady current and passes changes.

The big one: a module as a box

Everything else you met this level — the HC-SR04, the LCD, the SD card, the DHT11 — has no symbol of its own, because it is not one component. It is a whole board, and a schematic draws it the way a datasheet does: a rectangle with its pin names on it, and a wire off each one.

The box is deliberately empty. That is not laziness — it is what a module is. Somebody else has guaranteed the insides work, and your job is the four wires. Power on the left, signals on the right, every time: once you have read three of these you can read any of them.

Connection point labels

Instead of drawing a wire from one corner of the page to the other, schematics use named connection points. Two boxes labelled +5V are connected even though no wire is drawn between them. Same for GND, SDA, SCL, MOSI — buses with consistent names. Keeps diagrams readable.

The Arduino UNO as a black box

In a diagram with several modules, the UNO itself becomes a box too — there is no point drawing a microcontroller's 28 pins when you are using nine of them. In this course we go one step further and do not draw the UNO at all: every wire that would reach it simply ends in its pin name, in amber. D9 on one diagram and D9 on another are the same pin, whether or not a line is drawn between them.

Connection point labels

That is the same trick, generalised. Rather than run a wire from one corner of a page to the other, a schematic gives the wire a name and writes that name at both ends. Two points labelled +5V are connected even with no line between them. Same for GND, SDA, SCL, MOSI.

This is why the diagrams in this course have a supply rail across the top and a ground bar along the bottom: those two are so common that they get a symbol instead of a name.

Worked Example · Read the Weather Station v2 schematic 25 min

The Weather Station v2, drawn properly

Here is the L02-43 build as a schematic. Four modules and one divider, and the whole thing fits in a glance — which is the entire argument for schematics.

Every module hangs off the same two rails, and every signal wire ends in a pin name. Follow a current dot from the supply, through a module, and back to the ground bar: that loop is the same for all four, which is exactly the thing a photograph of this board would hide behind a tangle of jumper wires.

What this tells you that a wiring photo doesn't

Power topology. All five things on the board — three modules, the divider and the button — hang off the same +5 V and the same GND, which is what makes the current budget a single sum.
The LDR is in a divider with a 10 kΩ resistor. Photos hide that detail; schematics make it the centrepiece.
SD and LCD use different buses (SPI vs I²C). Both fit because they don't share pins.
The DHT11 needs no partner resistor — its 3-pin module has the pull-up built in, which is why its box has three wires and not four. On the bare 4-pin sensor the diagram would show a 10 kΩ to +5 V.
The button uses the internal pull-up — the resistor above it sits inside the chip, which is why it is drawn but not on your breadboard.

Spot the bugs

Now the same board wired by somebody in a hurry. Three things are wrong, and every one of them is invisible in a photograph — the wires would look identical. Find them before you open the answer.

Look at what is missing, not at what is drawn. A schematic makes an absent wire into an absent line; a photograph makes it into nothing at all.

Reveal the 3 bugs

The SD module has no VCC and no GND. Not one wire reaches its power pins, so nothing about it can work. On the drawing its box simply has fewer lines than the others — count them.
Nothing reaches ground. There is no ground bar at the bottom at all, so no module can complete its circuit. Current has no way home.
The LDR has no partner resistor. It runs from +5 V straight into A0 and stops. With nothing below it there is no divider and no split to measure, so A0 sits near 5 V whatever the light does — the flat reading from L02-15's warm-up.

Each bug is invisible in a photo (the wires would look the same!), but blindingly obvious in a schematic. That's the schematic's value.

Trace a Smart Bin Lid schematic

For practice, try the same exercise for the L02-26 Smart Bin Lid in your head:

HC-SR04 → 4 wires (VCC, TRIG, ECHO, GND).
SG90 servo → 3 wires (red to +5V, brown to GND, orange to D11).
That's the whole circuit.

Now sketch that on paper using proper symbols — the schematic should fit on a quarter-page.

Basic 7 min

🟢 Easy

Goal: On paper, draw the schematic for the L01-43 Auto Night Light (LDR divider + LED + transistor or just LED on a PWM pin).

Challenge 1 7 min

🟡 Medium

Goal: Draw the schematic for the L02-44 Digital Combination Lock. Include all 4 buttons, both LEDs, the buzzer, the servo, and the Arduino as a labelled box.

Challenge 2 6 min

🔴 Stretch

Goal: A classmate planned the pins for a Smart Bin Lid that also logs to an SD card and shows a count on an I²C LCD. Their plan has four mistakes. Find them, then write a corrected pin table.

Part	Part pin	UNO pin
HC-SR04	TRIG	D9
HC-SR04	ECHO	D10
Servo	signal	D11
microSD	MOSI / MISO / SCK	D11 / D12 / D13
microSD	CS	D10
LCD	SDA / SCL	A5 / A4
Lid button	—	D13

Use the Weather Station v2 schematic in the worked example to check the bus pins.

It works if your corrected table uses each UNO pin only once, and the SPI and I²C pins match the worked example.

Challenge 3 · Schematic your favourite L2 project 15 min

Pick the L2 project you're proudest of and draw its complete schematic on paper. Requirements:

Use standard symbols (LED triangle, resistor zigzag, ground three-bar, IC rectangle).
Use named connection points (+5V, GND, SDA, SCL etc.) rather than drawing every wire.
Label every pin connection on the Arduino-as-box (e.g. "D2 → Button", "A4 → LCD SDA").
Add a small title block: project name, your name, date, sketch version.
The whole diagram should fit on one A4 sheet, readable from arm's length.

It's done when:

Another student (who didn't build the project) could in principle wire the breadboard from your schematic alone.
Every pin used has its purpose obvious from the diagram.
No wires cross without either a junction dot or a hop indicator.
The title block makes it clear which project + version this is.

Recap 5 min

Schematics are the engineer's view of a circuit — logical, not physical. The L2 symbol set adds ICs, buses, and named-point shorthand to the L1 LED-and-resistor basics. Multi-IC schematics make pin sharing (SPI bus), power topology (rails), and missing-component bugs visible at a glance — things a wiring photo hides. Drawing your own schematic for a project you built is one of the fastest ways to deepen your understanding of how the project works. We'll see Level 3 schematics with motor drivers, transistors as switches, and dedicated power rails — but the symbol set and conventions don't change.

Schematic diagram: The logical drawing of a circuit using standardised symbols. Shows function and connectivity; doesn't reflect physical placement.
IC (integrated circuit): A chip containing many transistors. In schematics, drawn as a rectangle with pin numbers/names on the sides.
Bus: A set of related wires drawn together (often labelled). SPI = MOSI + MISO + SCK + CS; I²C = SDA + SCL. Multiple devices share the bus.
Power rail: A wire (or label) connecting everything that shares a voltage. +5V and GND are the most common.
Named connection point: A label like +5V or SDA implying every place with that name is electrically connected, even without a drawn wire. Keeps complex diagrams readable.
Junction dot: A small filled circle at a wire crossing to indicate the wires connect. No dot = they cross without touching (one hops over).
Title block: The labelled box (usually bottom-right of a sheet) with project name, author, date, version. Standard engineering practice.
Hop / bridge symbol: A small semicircle where one wire crosses another without connecting. Clarifies non-connection. Useful when a junction dot is ambiguous.
Decoupling capacitor: A small (0.1 µF) capacitor placed close to an IC's power pins to filter noise. Schematics always show them; breadboard projects often skip them and get away with it.

Extra Mission 5 min

Part 1 — Design a gadget from modules

Pick an everyday problem that two or three Level 2 modules could solve together. Ideas: a door counter with an LCD, a lamp that logs how bright the room was, a plant pot that shows its soil reading. On paper, design it as a pin plan.

Your design must include:

A one-sentence job for the gadget.
Each module or part, and why it is needed.
A pin table that uses every UNO pin once at most, with SPI and I²C on their fixed pins.
A block drawing: the modules as boxes, with named +5V and GND points.

Part 2 — Make it

Wire the gadget from your pin table and test each part on its own first. Then combine them in one sketch. Finally, draw its full schematic with standard symbols and modules as labelled boxes.

Bring back next class: your pin table, your schematic, your hw-l02-46.ino sketch, and a photo of the wired gadget.

Learning Goals 5 min

Re-meet the schematic symbols from L01-13 (LED, resistor, ground, power) plus the L2 additions: IC chip, button, servo, transistor, LCD, SPI bus.
Read a multi-IC schematic — Arduino + SD card module + LCD + sensor on one diagram — and identify the shared rails, the bus signals, and each device's connection points.
Sketch your own schematic for one of your L2 projects, using standard symbols, in 20 minutes.

Warm-Up 10 min

Why schematics, not wiring photos

New Concept · The L2 symbol set 25 min

The basics, recapped from L01-13

Symbol	Name	What it means
	Resistor	Limits current. Its value is written beside it. A slash through it, or an arrow, means the value varies.
	LED	Triangle and bar — a one-way valve — with two arrows leaving it. Current flows the way the triangle points.
	+5V	Where power comes in. Always at the top.
	GND	Where current returns. Always at the bottom.
	Junction dot	These wires join. No dot means they merely cross.

The Level 2 additions

Three of the new ones are components you have already wired this level:

Symbol	Name	What it means
	Potentiometer	A resistor with a third leg — the wiper, drawn as an arrow — that slides along the track. The arrow is what says "you can move this".
	LDR	A resistor in a circle with arrows pointing in. Inwards is the point: light arriving is what changes it. The LED's arrows point out for the opposite reason.
	Thermistor	A resistor with a slash and a `t°`. Same idea, varied by heat instead of light.
	Capacitor	Two plates with a gap. Stores charge; blocks steady current and passes changes.

The big one: a module as a box

Connection point labels

The Arduino UNO as a black box

Connection point labels

This is why the diagrams in this course have a supply rail across the top and a ground bar along the bottom: those two are so common that they get a symbol instead of a name.

Worked Example · Read the Weather Station v2 schematic 25 min

The Weather Station v2, drawn properly

Here is the L02-43 build as a schematic. Four modules and one divider, and the whole thing fits in a glance — which is the entire argument for schematics.

What this tells you that a wiring photo doesn't

Power topology. All five things on the board — three modules, the divider and the button — hang off the same +5 V and the same GND, which is what makes the current budget a single sum.
The LDR is in a divider with a 10 kΩ resistor. Photos hide that detail; schematics make it the centrepiece.
SD and LCD use different buses (SPI vs I²C). Both fit because they don't share pins.
The DHT11 needs no partner resistor — its 3-pin module has the pull-up built in, which is why its box has three wires and not four. On the bare 4-pin sensor the diagram would show a 10 kΩ to +5 V.
The button uses the internal pull-up — the resistor above it sits inside the chip, which is why it is drawn but not on your breadboard.

Spot the bugs

Look at what is missing, not at what is drawn. A schematic makes an absent wire into an absent line; a photograph makes it into nothing at all.

Reveal the 3 bugs

The SD module has no VCC and no GND. Not one wire reaches its power pins, so nothing about it can work. On the drawing its box simply has fewer lines than the others — count them.
Nothing reaches ground. There is no ground bar at the bottom at all, so no module can complete its circuit. Current has no way home.
The LDR has no partner resistor. It runs from +5 V straight into A0 and stops. With nothing below it there is no divider and no split to measure, so A0 sits near 5 V whatever the light does — the flat reading from L02-15's warm-up.

Each bug is invisible in a photo (the wires would look the same!), but blindingly obvious in a schematic. That's the schematic's value.

Trace a Smart Bin Lid schematic

For practice, try the same exercise for the L02-26 Smart Bin Lid in your head:

HC-SR04 → 4 wires (VCC, TRIG, ECHO, GND).
SG90 servo → 3 wires (red to +5V, brown to GND, orange to D11).
That's the whole circuit.

Now sketch that on paper using proper symbols — the schematic should fit on a quarter-page.

Basic 7 min

🟢 Easy

Goal: On paper, draw the schematic for the L01-43 Auto Night Light (LDR divider + LED + transistor or just LED on a PWM pin).

Challenge 1 7 min

🟡 Medium

Goal: Draw the schematic for the L02-44 Digital Combination Lock. Include all 4 buttons, both LEDs, the buzzer, the servo, and the Arduino as a labelled box.

Challenge 2 6 min

🔴 Stretch

Part	Part pin	UNO pin
HC-SR04	TRIG	D9
HC-SR04	ECHO	D10
Servo	signal	D11
microSD	MOSI / MISO / SCK	D11 / D12 / D13
microSD	CS	D10
LCD	SDA / SCL	A5 / A4
Lid button	—	D13

Use the Weather Station v2 schematic in the worked example to check the bus pins.

It works if your corrected table uses each UNO pin only once, and the SPI and I²C pins match the worked example.

Challenge 3 · Schematic your favourite L2 project 15 min

Pick the L2 project you're proudest of and draw its complete schematic on paper. Requirements:

Use standard symbols (LED triangle, resistor zigzag, ground three-bar, IC rectangle).
Use named connection points (+5V, GND, SDA, SCL etc.) rather than drawing every wire.
Label every pin connection on the Arduino-as-box (e.g. "D2 → Button", "A4 → LCD SDA").
Add a small title block: project name, your name, date, sketch version.
The whole diagram should fit on one A4 sheet, readable from arm's length.

It's done when:

Another student (who didn't build the project) could in principle wire the breadboard from your schematic alone.
Every pin used has its purpose obvious from the diagram.
No wires cross without either a junction dot or a hop indicator.
The title block makes it clear which project + version this is.

Recap 5 min

Schematic diagram: The logical drawing of a circuit using standardised symbols. Shows function and connectivity; doesn't reflect physical placement.
IC (integrated circuit): A chip containing many transistors. In schematics, drawn as a rectangle with pin numbers/names on the sides.
Bus: A set of related wires drawn together (often labelled). SPI = MOSI + MISO + SCK + CS; I²C = SDA + SCL. Multiple devices share the bus.
Power rail: A wire (or label) connecting everything that shares a voltage. +5V and GND are the most common.
Named connection point: A label like +5V or SDA implying every place with that name is electrically connected, even without a drawn wire. Keeps complex diagrams readable.
Junction dot: A small filled circle at a wire crossing to indicate the wires connect. No dot = they cross without touching (one hops over).
Title block: The labelled box (usually bottom-right of a sheet) with project name, author, date, version. Standard engineering practice.
Hop / bridge symbol: A small semicircle where one wire crosses another without connecting. Clarifies non-connection. Useful when a junction dot is ambiguous.
Decoupling capacitor: A small (0.1 µF) capacitor placed close to an IC's power pins to filter noise. Schematics always show them; breadboard projects often skip them and get away with it.

Extra Mission 5 min

Part 1 — Design a gadget from modules

Your design must include:

A one-sentence job for the gadget.
Each module or part, and why it is needed.
A pin table that uses every UNO pin once at most, with SPI and I²C on their fixed pins.
A block drawing: the modules as boxes, with named +5V and GND points.

Part 2 — Make it

Wire the gadget from your pin table and test each part on its own first. Then combine them in one sketch. Finally, draw its full schematic with standard symbols and modules as labelled boxes.

Bring back next class: your pin table, your schematic, your hw-l02-46.ino sketch, and a photo of the wired gadget.