Reading Circuit Diagrams

Learning Goals 5 min

By the end of this lesson you will be able to:

1. Recognise the five core schematic symbols — resistor, LED, +5V, ground and junction dot — and name what each one stands for.
2. Trace the path of current through a schematic, starting at the supply at the top and ending at the ground symbol at the bottom.
3. Take a circuit you have already built on the breadboard and point at the part of the schematic that means the same thing — resistor to zigzag, LED to triangle, − rail to merge bar.

Warm-Up 10 min

You have 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 does not post a photo of her breadboard — she sends a schematic. Today we learn to read those.

Quick-fire puzzle

Look at this small drawing. Nobody has taught you the symbols yet, but you have built the circuit it shows. The moving dots are the current.

1. Which mark is the LED — and how did you know?
2. Which is the 220 Ω resistor?
3. The whole picture stands for a circuit you wired in which previous lesson?

New Concept 20 min

The big idea — two drawings of one circuit

Since L01-04 we have drawn circuits as breadboard pictures: the real board, the real holes, jumpers bent like real cables. Those drawings tell you exactly where to build.

Engineers use a second drawing called a schematic. A schematic has no breadboard in it at all — only the logic of the circuit: what is in series with what, where the power comes in, where the current leaves. It is a drawing for thinking, not for building. Once you can read one, you can build it on any breadboard, in any holes you like.

They are not two topics. They are two pictures of the same thing, and the fastest way to believe that is to look at both at once. Here is the LED circuit from L01-07, twice. Hover any component in either picture and its twin lights up in the other.

The five core symbols

For everything in Level 1 you only need these five. Memorise them.

The rules every schematic follows

1. Power at the top, ground at the bottom. Current reads down the page. This is so consistent worldwide that flipping it would confuse any electrical engineer who has ever lived.
2. Wires are straight, with right-angle bends. Curves are for the breadboard picture, where they suggest a bendy jumper. A schematic is abstract, so it uses only horizontal and vertical lines.
3. Crossings versus joins. Two wires crossing with no dot are not connected — think of a flyover. Two wires meeting with a dot are one wire.
4. No breadboard. A schematic does not care which hole you used. It only cares what is wired to what.
5. Pin labels mark wires that leave the drawing to reach an Arduino pin: D13, D7, A0, GND. They are the name of the cable that goes off the page.

How to read a schematic, in three steps

1. Find the supply or the pin label at the top. That is where the current starts. Watch the dots leave it.
2. Follow the wire downward. Every component the wire meets, the current goes through — and uses up some of the voltage doing it. Every junction dot, the current can split or merge.
3. 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 always-on version of L01-07 and describe the build in your own words.

The schematic

Step 1 — find the start

The +5V bar at the top says: 5 volts of electrical pressure enters here. On the real Arduino that wire plugs into the 5V pin.

Step 2 — follow the wire down

From +5V the wire runs into the resistor. The label R = 220 Ω gives its value. The resistor uses up the volts the LED does not: by Ohm's Law from L01-05, V_R = 5 − 2 = 3 V, so I = 3 ÷ 220 ≈ 13.6 mA.

Step 3 — the current reaches the LED

The triangle points down, so current enters at the anode (top) and leaves at the cathode (bottom). The two arrows pointing away say the light comes out.

Step 4 — back to ground

From the cathode the wire carries on down to the GND symbol. The current has returned, the circuit is complete, and the LED glows.

Step 5 — translate it to a breadboard

The schematic does not name a single hole, only what must be in series. Every one of these is a correct build of that drawing:

• L01-07's Mini-Challenge: 5V → + rail → resistor → LED → − rail → GND.
• The resistor in row A or row C — same column, same tie strip, electrically identical.
• Columns 1–6 (where L01-07 put it) or columns 20–25. The schematic is silent about it.

That silence is the schematic's power. One drawing covers every valid breadboard layout you will ever make of this circuit.

Which drawing is for what?

Try It Yourself 20 min

Mini-Challenge 15 min

Read the L01-10 chase circuit — from the schematic alone

Below is the three-LED chase circuit you built in L01-10, drawn as a schematic. Each LED runs from its own digital pin instead of from a shared +5V, which is why there are three pin labels at the top instead of one supply bar.

Answer these by reading only the schematic, without looking back at L01-10's breadboard picture:

1. How many junction dots are on the schematic?
2. Which pin drives the green LED?
3. If you cut the wire between the merge bar and the GND symbol, what happens to all three LEDs? Why?
4. If you cut the wire between the merge bar and the red LED only, what happens to red, yellow and green? Why is your answer different from question 3?
5. On the breadboard, which physical part of the board is the merge bar?

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 and bar), +5V (bar at the top), GND (shrinking lines at the bottom) and the junction dot. Read top to bottom: power in at the top, components in series on the way down, ground at the bottom. The schematic says nothing about which hole you use — that is yours to choose.

Schematic

An abstract drawing of a circuit using standard symbols. Says what is connected to what, never where on a breadboard. Used by engineers, datasheets and professional manuals.

Breadboard view

The pictorial drawings this course has used since L01-04. Shows the physical layout — holes, rails, jumpers. Used for building.

Junction dot

A small filled circle showing that two or more wires are electrically joined there. Wires crossing without a dot are not connected.

Pin label

The name of an Arduino pin (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 that gathers every branch into one node before reaching GND. On a breadboard this is the − rail.

Homework 5 min

Draw your first schematic. Take the three-LED chase circuit from L01-10 — you have built it, and you have just read its schematic — and redraw it on a clean sheet of paper, by hand, in pencil.

1. Use a ruler. Schematic wires are straight, with right-angle bends only.
2. Put the pin labels D9, D10 and D11 across the top of the page.
3. Under each, draw a zigzag for the 220 Ω, then the LED triangle-bar-and-arrows.
4. Under all three, draw a horizontal merge bar and bring each branch into it. Put a junction dot at each landing.
5. Drop one wire from the bar to a GND symbol at the very bottom, in the middle.
6. Write 220 Ω beside each resistor and the colour (red, yellow, green) beside each LED.

Also a short reading exercise. Look again at the parallel two-LED schematic from the 🔴 stretch task and answer on paper:

• If you turn one LED around (anode and cathode swapped), what happens to that LED? What about the other one? ____
• If you take the right-hand resistor out entirely and join the wires straight through, what happens? (Hint: remember L01-07's warm-up.) ____

Bring back next class:

• Your hand-drawn schematic of the L01-10 chase circuit (paper or a phone photo).
• Your reading-exercise answers.