How to Read Circuit Boards and Identify Components [Solve]

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Please help identify a component on my NodeMCU. I don't know how to read a schematic yet, help appreciated.

The question " how to read circuit boards " or " how to read schematics" often arises for beginners. Sometimes " what is this component" may also arise. These questions can be solved altogether in this article guiding how to read a circuit board and identify components.

circuit board

Part 1: How to Read Circuit Boards - Standards for Electronic Symbols

A printed circuit board is a bunch of electronic components interconnected via conductive paths printed on a baseboard. The electronic components and conductive paths are based on a map, the schematic diagram. This diagram is drawn based on widely-accepted rules and symbols. The symbols used in schematic diagrams are according to standards which are defined at the National and International level by professional organizations such as, Institute of Electrical and Electronics Engineers (IEEE), the International Electrotechnical Commission (IEC), and the American National Standards Institute (ANSI).

Following are some of the common standards for electronic symbols.

  • IEC 60617
  • ANSI Y32.2-1975
  • IEEE Std 91/91a


Part 2: Where to Start Reading Circuit Boards - Power Source

If you got a schematic diagram or a circuit board, the best and easiest way to start analyzing it is from the power source. Every  electronic component depends on some sort of power supply. Commonly the designing process of a circuit also begins from there. The most common type of failure in electronic devices is also the power supply failure. Following are the common symbols associates with the power source.

Symbols related to the power source

Direct Current (DC) Supply Symbols
Alternative Current (AC) Supply Symbols

dc supply  dc-supply-2.jpg

ac supply  ac supply

Current Source Symbol
Battery Symbols

current source

battery source  battery source

Controlled Voltage Source
Controlled Current Source


controlled current source

Solar Cell Symbol

solar cell

Ground (Earth) Symbols

ground earth

solar cell symbol

ground  ground  ground





fuse symbol

transformer symbol

The connections within those symbols are shown with lines. And these lines (conductive paths) have joints and crossings. These are represented by the below symbols.

Trace Junction
Trace Crossing

trace junction

trace crossing

Part 3: Reading Schematics - Passive Components

After identifying the power supply, the next most common electronic components are the passive components. The name "passive component" is used for electronic components that  cannot inject power or amplify the power in a circuit. These can only absorb, dissipate, or store energy. These components do not need a set level of voltage (energy) to perform a task. Common component types that fall into this category are resistors, capacitors, inductors, and transformers.

  • Resistors

Resister is a component that acts as a barrier to the current flow. It dissipates energy as heat and creates a voltage drop in the circuit. The resistance value (R) is given in terms of "Ohms", and the voltage drop can be calculated from Ohm's Law Equation.

V = IR

(V = voltage, I = current, and R = resistance)



Variable Resistor (Rheostat)

variable resistor

resistor symbol 1  resistor symbol 2

rheostat  rheostat



Termister or Varistor


potentiometer symbol 1  potentiometer symbol 2


  • Capacitors

Capacitors are components that store energy in an electric field. The capacitance value (C) is given in terms of "Farads". The governing primary equation of capacitors is,


(C =capacitance, Q = charge in qoulomb, V = Voltage)

Unpolarized Capacitor

unpolarized capacitor

Polarized Capacitor

polarized capaciotr

unpolarized capacitor symbol

polarized capaciotr symbol

Variable Capacitor

variable capacitor

Trimmer Capacitor

trimmer capacitor

variable capacitor symbol

trimmer capacitor symbol

  • Inductors

Inductors are components that store energy in a magnetic field. Inductance (L) is measured in "Henry". The governing primary equation of Inductors is,

inductor format

( Φ = magnetic flux linkage, I = Current, L = Inductance)

Air Cored Inductor

air cored inductor

Magnetic Core Inductor

magnetic core inductor

air cored inductor symbol

magnetic core inductor symbol

Tapped Inductor

tapped inductor

Ferrite Bead

ferrite bead

tapped inductor symbol

ferrite bead symbol

  • Transformers

Transformers are used to step up or step down voltages and currents. Energy is transferred in a transformer via a varying magnetic flux. Primary winding induces this magnetic flux, and there are one or many secondary windings which get induced currents from this core magnetic flux. Voltage and current-induced here are proportional to the number of turns in the windings.

The governing primary equations of transformers are,

format 1   format 2

(Vp = Primary Winding Voltage, Vs = Secondary Winding Voltage, Np = Number of coils turns in the primary winding, Ns = Number of coils turns in the secondary winding, Ip = Primary Winding Current, Is = Secondary Winding Current.)



Transformer with tapping

transformer with tapping

transformer symbol

transformer with tapping symbol

Current Transformer

currrent transformer

Voltage Transformer

voltage transformer

currrent transformer-symbol

voltage transformer symbol

  • RLC circuits

These are circuits only composed of passive components. These are common in power transmission systems, audio noise filtering systems, EMI filters, and passive frequency filters.

EMI Filters
Audio Filters

emi filter 1   emi filter 2

audio filter 1   audio filter 2

Part 4: Reading Schematics - Common Active Components

Active components are the heart of modern electronics. They are usually made of semiconductors. To perform tasks these components need a set level of voltage or they supply energy to the circuit. Voltage sources, current sources, generators, all components made of transistors, and all types of diodes are examples of active components.

Let's see some common symbols of active components.

  • Transistors

There are tons of transistor types, and each has a unique symbol. Transistor type cannot be determined from its appearance because different types of transistors have the same package type. Transistor type can only be accurately recognized from its model number and symbol. Here we mention some of their most common types.


N-channel junction gate field-effect transistor (JFET)
P-channel junction gate field-effect transistor (JFET)
Metal-oxide-semiconductor field-effect transistor (MOSFET)

n jfet transistor

p jfet transistor

mosfet transistor

Enhancement mode, N-channel MOSFET
Enhancement mode, P-channel MOSFET
NPN bipolar junction transistor (BJT)

n mosfet transistor

p mosfet transistor


PNP bipolar junction transistor (BJT)
NPN Darlington transistor
PNP Darlington transistor

pnp bjt



  • Diodes

Diodes are gates that allow current to flow in one direction only. These are commonly used for rectifying AC currents into DC currents in the power supply. There are other types of diodes that emit light which is called light-emitting diode (LED) and there are diodes that capture light and convert it into a current which is called photodiode. All diodes are made of semiconductors and have a P-N junction.

Rectifier Diode

rectifier diode

Schottky diode

schottky diode

Zener diode

zener diode

rectifier diode symbol

schottky diode symbol

zener diode symbol

Light Emitting Diode (LED)




Tunnel diode

tunnel diode

led symbol

photodiode symbol

tunnel diode symbol

Varicap diode

varicap diode

Shockley diode

shockley diode

Silicon-controlled rectifier (SCR)


varicap diode symbol

shockley diode symbol

scr symbol

Constant-current diode

constant current diode



Bridge rectifier

bridge rectifier

constant current diode symbol

diac symbol

bridge rectifier symbol

  • Integrated Circuits (ICs)

Integrated Circuits (IC) consist of few transistors to a few billion transistors. In the modern-day, they perform all the main tasks of any circuit. They provide logical computational power, storage, switching tasks, and many more. Integrated circuits are easily identifiable by their package and number of pins. These usually have a higher number of pins than any of the above components. The building block of most ICs is the operational amplifier (opamp). Since there are too many variants of ICs, here we look into only the basic IC symbol which is the OP-AMP.


Operational Amplifier (Op-Amp) or Comparator

op amp   op amp-symbol 1  op amp symbol 2

Part 5: Reading Schematics

Other Components

Apart from all the above primary components, there are many other key components in circuits that play vital roles. These can be electro-mechanical components such as relays, speakers, solenoids, connectors, and switches. Or they can be other types of the electronic component which do not fall into above main categories like, crystal oscillators, hall-effect sensors, displays and etc.

Crystal Oscillator
Hall-Effect Sensor

crystal oscillator

half effect sensor



microphone  microphone




switch   switch



Part 6: From Reading Circuit Boards to PCB Manufacturing

Now that you may know how to read a PCB and identify components. It's time to have the PCB manufactured by a reliable PCB company - PCBONLINE. This is a custom PCB manufacturer, its services include advanced PCB manufacturing, assembly, layout, SMT stencil, and circuit board component sourcing. If you need more technical assistance, you can also ask PCBONLINE free of charge, whose engineers have been dealt with PCB projects for decades.

Why choose PCBONLINE for PCB fabrication:

  • We offer free tech support, including reading PCB, checking Gerber and BOM, providing advice, and more.
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  • Free PCB sample, free PCBA first piece test, free functional test, fast delivery.

PCBONLINE's PCB manufacturing capability:

  • Layer: 1 ~ 42
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  • Surface finishing: OSP/HASL/LF HASL/immersion ENIG/immersion tin/immersion silver
  • Board thickness: 0.15 ~ 3.2 mm
  • Max board size: 500 × 580 mm
  • Copper thickness: (inner finish copper) 1-4oz, (outer finish copper) 1-7oz
  • Min. line width/spacing: 0.0635 mm/0.0635 mm
  • Min. CNC drilling size: 0.15 mm
  • Min. laser drilling size: 0.075 mm
  • HDI stack up: 1+N+1, 2+N+2, 3+N+3

Last thing

Now that you can understand how to read schematic diagrams and identify components. PCBONLINE not only provides high-quality PCB manufacturing but also sources all kinds of electronic components according to your BOM. Get a free quotation now!

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