Learning Goals 5 min
By the end of this lesson you will be able to:
- Recognise the five core schematic symbols — resistor, LED, +5V, ground and junction dot — and name what each one stands for.
- Trace the path of current through a vertical schematic, starting at the top supply and ending at the ground symbol at the bottom.
- Take any schematic of a circuit you've already built (L01-07 single LED, L01-10 three-LED chase) and identify which breadboard layout it represents.
Warm-Up 10 min
You've built six working circuits over the last six lessons. When a real engineer in Penang wants to send a circuit to a friend in KL, she doesn't post a photo of the breadboard — she sends a schematic. Today we learn to read those.
Quick-fire puzzle
Look at this small drawing. You've never been taught the symbols, but you've built the circuit it shows.
- Which of the four symbols is the LED — and how did you know?
- Which is the 220 Ω resistor?
- The whole picture stands for a circuit you wired in which previous lesson?
Reveal the answer
- The second from the bottom, with the triangle and the bar. The triangle is the diode shape (current only flows in the direction of the point) and the two little arrows beside it stand for the light it emits. That's the LED.
- The zigzag just below the +5V label. Engineers all over the world draw resistors as zigzags. That's the 220 Ω.
- This is exactly the always-on LED circuit you built in ARD-L01-07's Mini-Challenge: +5V → 220 Ω resistor → red LED → GND. Same circuit, different drawing style.
New Concept 20 min
The big idea — two languages for the same circuit
From the very first wire of L01-04 we have been drawing circuits as pictorial diagrams: pictures of the actual breadboard, with rails, holes, jumpers and component bodies. Those drawings tell you exactly where to build.
Engineers use a second drawing called a schematic. A schematic doesn't show the breadboard at all — only the logic of the circuit: which components are in series, which are in parallel, where the power comes in and where the current leaves. It is an abstract drawing for thinking, not for building. Once you can read a schematic, you can build it on any breadboard.
The five core symbols
For everything in Level 1, you only need these five symbols. Memorise them.
| Symbol | Name | What it stands for |
|---|---|---|
| Resistor | A component that limits current. The zigzag stands for "current has to wiggle through". The value (e.g. 220 Ω) is written next to it. | |
| LED | Triangle + bar = diode (one-way valve). Two little arrows = it emits light. Current only flows in the direction the triangle points — top (anode) to bottom (cathode). | |
| +5V (supply) | "Power comes in here at 5 V". This is the top of the schematic, where current enters. The terminal sits at the bottom of the symbol; the label sits above. | |
| GND (ground) | "Current goes out here". Always at the bottom of the schematic. The three lines, shrinking as they go down, are the universal symbol for ground. | |
| Junction dot | "These wires join here". Without a dot, wires that cross are not connected. The dot is the difference between a busy crossroad and a flyover. |
Layout rules every schematic follows
- Power at the top, ground at the bottom. Current flows top to bottom on the page. This is so consistent across the world that flipping it would confuse any electrical engineer who has ever lived.
- Wires are straight lines with right-angle bends. Curves are for the pictorial breadboard view; schematics are abstract, so they use only horizontal and vertical lines.
- Crossings vs joins. Two wires that cross with no dot are not connected — imagine one passes over the other like a flyover. Two wires meeting with a dot are connected.
- No breadboard. The schematic doesn't care which hole you used. It only cares which components are wired to which.
- Pin labels at the top mark wires that leave the schematic to reach an Arduino pin:
D9,5V,A0,GND. The label is the "name of the cable that goes off-page".
How to read a schematic in three steps
- Find +5V (or a pin label) at the top. That's where the current starts.
- Follow the wire downward. Whenever you hit a component, the current goes through it (and "uses up" some voltage). Whenever you hit a junction dot, the current can split or merge.
- End at the ground symbol. The current always returns to ground. If you can trace a path from top to bottom, the circuit is complete.
Worked Example 20 min
Goal: read the schematic for the single-LED circuit you built in L01-07 and use it to describe the build in your own words.
The schematic
Step 1 — find the start
The +5V symbol at the top tells us: this is where 5 V of electrical pressure enters the circuit. On the real Arduino, that wire would plug into the 5V pin.
Step 2 — follow the wire down
From +5V, the wire travels downward into the resistor. The "R = 220 Ω" label tells us its value. The resistor "uses up" some of the 5 V — specifically, the part the LED doesn't use. By Ohm's Law from L01-05: V_R = 5 − 2 = 3 V across the resistor, I = 3 ÷ 220 ≈ 13.6 mA.
Step 3 — the current arrives at the LED
The triangle points downward, so current enters the LED at the anode (top) and exits at the cathode (bottom). The two little arrows pointing away from the bulb tell us light comes out.
Step 4 — back to ground
From the LED's cathode, the wire continues down to the GND symbol. The current has returned. The circuit is complete and the LED glows.
Step 5 — translate to a breadboard
The schematic doesn't tell you which holes to use, only what to put in series. Three valid breadboard builds for this exact schematic:
- L01-07 Mini-Challenge:
5V→ + rail → resistor → LED → − rail →GND. - Resistor in row A or row D — both are inside the same column tie strip, so electrically identical.
- Use cols 1–6 (where we put it in L01-07) or cols 20–25 (anywhere else on the breadboard). The schematic is silent on this.
That silence is the schematic's power. The same drawing covers every valid breadboard layout you'll ever make for this circuit.
Compare: schematic vs breadboard view
| Aspect | Breadboard view (L01-07) | Schematic view (today) |
|---|---|---|
| Shows the breadboard? | Yes — every rail, hole and column | No — there is no breadboard in a schematic |
| Component bodies look like… | The real component | An abstract symbol |
| Wires drawn as… | Curves, suggesting flexible jumpers | Straight lines with right-angle bends |
| Best for… | Building the circuit | Understanding the circuit and sharing it |
| Used by… | Beginners, tutorials | Engineers, datasheets, professional manuals |
Try It Yourself 20 min
Goal: Name the symbol. Without looking back at the table, fill in the name of each:
Question: Two of these symbols always sit at a specific position on a schematic — never in the middle. Which two, and where do they go? ____
Goal: Trace the current. The schematic below has a pin label instead of +5V at the top — this is what L01-07's main build (not the Mini-Challenge) looks like as a schematic. The LED only glows when pin D13 is HIGH.
D13 replaces +5V at the top.Questions:
- When pin
D13isHIGH, how much current flows through the LED? (Reuse the maths from the Worked Example.) ____ mA - When pin
D13isLOW, what happens to the LED? Why? ____ - Why is the symbol at the top a label (
D13) and not a+5Vbar? ____ (Hint: is D13 always at 5 V?)
Goal: Read a parallel circuit. Two LEDs, each with its own resistor, both wired between +5V and GND. Notice the junction dots that show where the wires fork and where they re-join.
Questions:
- How many junction dots are on the schematic, and what does each one mean? ____
- If LED 1 burns out (open circuit), does LED 2 keep glowing? Why? ____ (Hint: is there still a complete path from +5V to GND through LED 2?)
- Each branch draws ≈ 13.6 mA. What is the total current the +5V source has to supply? ____ mA
- On the breadboard, where would you plug the bottoms of both LEDs to make this parallel circuit work? ____ (Hint: think of the GND bus from L01-10.)
Mini-Challenge 15 min
Identify the L01-10 chase circuit
Below is the schematic for the three-LED chase circuit you built in L01-10. Each LED is driven from its own digital pin instead of from a common +5V — that's why the schematic has three pin labels at the top instead of one.
Your task — answer these by reading only the schematic above, without looking back at L01-10's breadboard view:
- How many junction dots are on the schematic?
- Which pin drives the green LED?
- If you cut the wire between the merge bar and the GND symbol — at the central GND drop — what happens to all three LEDs? Why?
- If you cut the wire between the merge bar and LED 1 only — the bottom of the red LED's branch — what happens to red, yellow and green? Why is your answer different from question 3?
- On the breadboard, the schematic's "merge bar" is realised by which physical part of the board?
Reveal the answers
- Three junction dots, one at each branch's landing on the merge bar.
- D11 — the rightmost label.
- All three go dark. The single GND drop is the only path back from any branch. Cutting it leaves every LED with nowhere for its current to return.
- Only red goes dark. Cutting the red branch's wire to the merge bar isolates the red LED from GND. The yellow and green branches still have their own complete paths from D10/D11 through the merge bar to the central GND drop.
- The breadboard's − rail. The merge bar in the schematic is exactly what the − rail does in a real build: gather the bottoms of all LED branches into one common GND node.
Recap 5 min
A schematic is the engineer's abstract drawing of a circuit. Five symbols carry you through every circuit in Level 1: resistor (zigzag), LED (triangle + arrows), +5V (bar at top), GND (shrinking lines at bottom), junction dot (small filled circle). Read from top to bottom: power comes in at the top, components in series along the way, ground at the bottom. The schematic is silent about which breadboard hole you use — that's your call.
- Schematic
- An abstract drawing of a circuit using standard symbols. Shows what is connected to what, but not where on a breadboard. Used by engineers, datasheets and professional manuals.
- Pictorial diagram
- The breadboard-view drawings used in this course since L01-04. Shows the physical layout — holes, rails, jumpers. Used for building.
- Junction dot
- A small filled circle on a schematic showing that two or more wires are electrically connected at that point. Crossings without a dot are not connected.
- Pin label
- The name of an Arduino pin (e.g.
D9,5V,A0,GND) written where a wire leaves the schematic. The "off-page cable name". - Merge bar
- The horizontal wire at the bottom of a parallel circuit's schematic that gathers all branch ends into one node before reaching GND. On the breadboard, this is realised by the − rail.
Homework 5 min
Draw your first schematic. Take the three-LED chase circuit from L01-10 (you've already built it; you've just seen its schematic in the Mini-Challenge) — and re-draw it on a clean sheet of paper, by hand, in pencil.
- Use a ruler. Schematic wires are straight, with right-angle bends only.
- Place the pin labels
D9,D10,D11at the top of the page, spread out across the width. - Below each pin, draw a zigzag for the 220 Ω resistor, then the LED triangle-bar-with-arrows.
- Below all three LEDs, draw a horizontal merge bar and connect each branch into it. Put a junction dot at each landing point.
- Drop one wire from the merge bar to a GND symbol at the very bottom centre.
- Write the value
220 Ωnext to each resistor and the LED colour (red, yellow, green) next to each LED.
Also: a short reading exercise. Look at the parallel 2-LED schematic from the 🔴 stretch task and answer on paper:
- If you swap one of the LEDs around (anode and cathode flipped), what happens to that LED? What about the other LED? ____
- If you remove the resistor on the right branch entirely and just join the wires through, what happens? (Hint: remember the warm-up question from L01-07.) ____
Bring back next class:
- Your hand-drawn schematic of the L01-10 chase circuit (on paper or a phone photo of the page).
- Your reading-exercise answers.