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 (recap)
- Resistor — zigzag (US) or rectangle (Europe). Value labelled (220 Ω, 10 kΩ).
- LED — triangle pointing at a line with arrows leaving (light emitting). The triangle points toward GND.
- Ground (GND) — three horizontal lines decreasing in width, or just "GND".
- Power (+5V, VCC) — labelled wire, often with an "up arrow" symbol or a circle.
- Wire crossing without joining — one wire hops over the other; no dot.
- Wire junction (joining) — a dot at the crossover.
The L2 additions
- IC chip — a rectangle with pin numbers and pin names along the edges. The HD44780, the PCF8574, the ATmega328P all use this shape.
- Pushbutton (momentary) — two open lines with a connecting bar (closed by pressing) or a small button glyph.
- Capacitor — two parallel plates (unpolarised) or with a + sign on one side (polarised electrolytic).
- Transistor — three pins (base, collector, emitter) with arrows showing the type.
- Servo motor / module — typically a box with the 3 pin names (GND, +5V, SIG).
- Bus — a labelled set of wires (e.g. SPI = MOSI + MISO + SCK + CS) drawn as a thick line or labelled set of parallel lines.
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 multi-IC diagram, the UNO usually appears as a labelled rectangle with its pins listed:
+---------------------+ | Arduino UNO | | | | D2 -- BUTTON | | D7 -- DHT11 | | D10 -- SD CS | | D11 -- SD MOSI -----+----- to SD module | D12 -- SD MISO | (shared SPI bus) | D13 -- SD SCK | | A4 -- LCD SDA | | A5 -- LCD SCL | | GND -- common GND | | 5V -- common 5V | +---------------------+
This is just an example sketch — real schematics use proper symbols and KiCad / Eagle / Fritzing software. The key insight: list the chip's relevant pins on the box, label the other end of each line.
Worked Example · Read the Weather Station v2 schematic 25 min
Plain-text schematic for v2
Here's the Weather Station v2 from L02-43, written in ASCII (replace the boxes with proper symbols if you redraw on paper):
+5V ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
│ │ │ │ │
│ │ │ │ │
[LCD] [SD] [DHT11] [Button] [LDR top]
SDA→A4 MOSI→D11 DATA→D7 →D2 │
SCL→A5 MISO→D12 (pullup │
internal) │
SCK→D13 │
CS→D10 │
A0
│
[10 kΩ]
│
GND ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━┻━━━━━━━━━━━━━
The two horizontal bars represent the +5V and GND rails. Everything that touches them connects to the corresponding power node. Each block in brackets is a module; its pins are listed beneath. Arrows show the Arduino pin each module signal goes to.
What this tells you that a wiring photo doesn't
- Power topology. All five modules share +5V and GND — important for current budgeting (sum the currents).
- 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 it built in). On a 4-pin DHT, the diagram would show a 10 kΩ pull-up to +5V.
- The button uses internal pull-up — there's no external resistor on the diagram because it's software-enabled.
Spot the bugs
Now look at this broken version. What's wrong?
+5V ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
│ │ │
│ │ │
[LCD] [SD] [DHT11]
SDA→A4 MOSI→D11 DATA→D7
SCL→A5 MISO→D12
SCK→D13
CS→D10 NO GND CONNECTION
VCC missing!
[LDR—floating, no divider]
│
A0
Reveal the 3 bugs
- The SD module's VCC isn't shown — it'll have nothing to power it. Add a wire from +5V to SD VCC.
- There's no GND rail. None of the modules can complete their circuit. Add the GND rail at the bottom.
- The LDR is "floating" — no partner resistor, so the divider equation collapses and A0 reads garbage. Add the 10 kΩ resistor between A0 and GND.
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.
Try It Yourself 20 min
Goal: On paper, draw the schematic for the L01-43 Auto Night Light (LDR divider + LED + transistor or just LED on a PWM pin).
Hint
Three blocks: +5V rail, GND rail, the divider (LDR top, 10 kΩ bottom, A0 in middle), and an LED with 220 Ω driven from a PWM pin (say D9). Six labelled connections, two rails, two named pins.
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.
Hint
The Arduino sits in the middle; pins fan out to each module. Use named connection points for +5V and GND to avoid criss-crossing wires. The 4 buttons all use INPUT_PULLUP, so no external resistors on them.
Goal: Find a real schematic online — Adafruit, SparkFun, or the Arduino UNO's own schematic — and identify 5 things on it you now recognise. Sketch a simplified version in your notebook.
Hint
The official Arduino UNO schematic is on the Arduino website (search "Arduino UNO schematic", look for the .pdf or .png). It has the ATmega328P chip, the USB-to-serial chip, the voltage regulator, the crystal, the reset button, the power LED. You'll recognise the resistor, capacitor, LED, IC-rectangle, and ground symbols immediately.
Mini-Challenge · 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
+5VorSDAimplying 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.
Homework 5 min
Schematic, no code. Find a real consumer device you have access to (a coffee maker, a hair dryer, a phone charger — open it carefully if it's already broken; otherwise just find an internal photo online). Sketch a simplified schematic of what you see.
- Identify the main IC(s).
- Identify the power input.
- Identify any LEDs, buttons, switches, motors, displays.
- Draw a one-page schematic showing how they connect.
Don't worry about precision — the goal is to practise "reading" a real device with the L2 symbol vocabulary. You'll get plenty wrong; that's fine.
Bring back next class:
- Your sketched schematic (the device + your interpretation).
- A short note: what did you find easiest to identify? Hardest?
- Tomorrow we cover debugging strategies — the systematic process for finding bugs in your own projects.