Understanding symbols, nets, and connections before you start designing

Introduction

So you want to design a PCB. Or maybe you just need to understand an existing circuit.

Either way, you need to read a schematic.

A schematic is the blueprint of your circuit. It shows every component, every connection, and how everything fits together — without worrying about physical placement or board shape.

If you’ve never seen one before, it can look like a mess of random symbols and lines.

This guide will teach you how to read one.

The Basics: What Every Schematic Shows
Every schematic has three fundamental things:

1. - Components(symbols representing resistors, chips, connectors, etc.)
2. - Nets(lines representing electrical connections)
3. - Reference designators (labels like R1, C2, U3)

Once you can identify these three things, you can understand any schematic.

Reading Component Symbols

Different components have different symbols. Here are the most common ones you‘ll encounter:
Resistors

Reference designator: R (R1, R2, R3...)

Capacitors

Reference designator: C (C1, C2, C3...)

Diodes
Symbol: —▶|— (triangle pointing to a line)

Note: The line (cathode) is the negative side. Current flows toward the line.

Reference designator: D (D1, D2, D3...)

LEDs (Light Emitting Diodes)
Symbol: Same as diode with two arrows pointing away (indicating light)

Reference designator: LED (LED1, LED2...) or D

Integrated Circuits (ICs / Chips)
Symbol: A rectangle or box with pins extending from all sides. Pins are numbered.

Reference designator: U (U1, U2, U3...)

Pro tip: The schematic won‘t show what’s inside the IC — only what connects to each pin. You need the IC‘s datasheet to understand what each pin does.

Connectors
Symbol: Usually a rectangle or a series of small circles/numbers.

Reference designator: J (J1, J2...), CON (CON1...), or P (P1...)

Power and Ground
These are so common they have special symbols:

Pro tip: Instead of drawing power and ground lines across the whole schematic, designers use these symbols. Any component connected to the same power symbol is connected together.

Understanding Reference Designators
Every component has a unique label. This label is how you cross-reference the schematic to the physical board and the BOM.

Common prefixes:

Example: R1 is the first resistor. C4 is the fourth capacitor. U3 is the third IC.

Understanding Nets
Nets are the lines connecting components. They represent copper traces on the physical board.

What nets tell you:

• Straight lines = direct connection
• Junction dots = lines are connected (without a dot, crossing lines are not connected)
• Net labels = text names that connect points without drawing lines

Net labels (very important):

When a net label (like “SCL” or “MISO”) appears in multiple places, those points are all connected — even if no line is drawn between them.

This keeps schematics clean and readable.

Common Schematic Features
Test Points
Symbol: A small circle or pad labeled “TP1”, “TP2”, etc.

They are exposed copper pads for probing during debugging.

Jumpers and Configuration
Symbol: Three pads with a box outlining two of them. Used to set board options by selecting which pads to bridge with solder.

Reading a Schematic: Step by Step
Let‘s walk through a simple example — an LED blinker circuit.

What you see:

• A battery symbol (power)
• A resistor (R1)
• An LED (LED1)
• A ground symbol

How to read it:

1. Identify the power source. The battery provides voltage.
2. Follow the path. Positive battery goes to one end of R1.
3. Flow through. The other end of R1 connects to the LED’s positive (anode).
4. Finish the loop. The LED‘s negative (cathode) connects to ground.
5. Return. Ground connects back to the negative side of the battery.

Circuit behavior: Current flows from battery → resistor → LED → ground → back to battery. The LED lights.

Common Beginner Mistakes

Mistake 1: Ignoring net names
All same-name nets connect. GND is GND everywhere.

Mistake 2: Missing connection dots
Four lines meeting in a plus shape. Are they all connected? Only if there’s a dot at the intersection.

Mistake 3: Forgetting pull-up or pull-down resistors
Many IC pins require a resistor (typically 4.7kΩ or 10kΩ) to VCC or GND. Check the datasheet.

Mistake 4: Misreading pin numbers
Always verify the datasheet for pin numbering — chips have notches, dots, or chamfers to indicate pin 1.

From Schematic to PCB Layout

The schematic is your map. The PCB layout is the actual board.

How to move from one to the other:

1. Capture the schematic in your EDA software (KiCad, Eagle, Altium, EasyEDA, etc.)
2. Run Electrical Rules Check (ERC) — catches floating pins, missing connections, duplicate designators
3. Generate a netlist — tells the PCB layout tool how to connect everything
4. Place components on the board (physical arrangement)
5. Route traces following the net connections

The golden rule: Fix all errors in the schematic before starting layout. It‘s much harder to fix later.

Quick Reference Card

Final Thoughts
Reading a schematic is like learning a new language. At first, it looks foreign and confusing. But once you learn the symbols and patterns, you can understand any circuit.

Start with simple schematics (LED blinker, power supply, Arduino shield). Practice identifying components, following nets, and understanding how signals flow.

Within a few hours of practice, you‘ll be able to read most beginner-to-intermediate schematics confidently.

About the Author
This article was written by AnyPCBA, a China-based PCB manufacturer specializing in small-to-medium volume production. We help hardware startups and engineering teams bring their designs to life.