Circuit board components explained: A beginner's guide to PCB electronic components and their functions
Pull the lid off almost any gadget and you will find a green or black board covered in little parts. Some are the size of a grain of rice. It looks like something you would need a degree to understand.
You do not. There are about nine kinds of part on that board, they each do one job, and once you can name them you can look at almost any circuit board and get the gist of it.
This guide goes through them one at a time. For each one: what it does, what it looks like so you can find it on a real board, and a simple example of it doing its job. No maths until you need it.
What are circuit board components?
Two different things are going on, and it helps to separate them.
The board is the flat green thing. It is fiberglass with thin copper lines printed on it, and those lines are just wires: they carry electricity from one place to another and do nothing else. That board on its own is completely useless. It cannot do anything.
The components are the parts stuck on top of it. They are what actually does the work, and the copper lines connect them to each other. So when someone says "PCB components", they mean the parts, not the board.
Sticking them on is called assembly. Small flat parts get glued down with solder paste and baked in an oven. Parts with legs have the legs poked through holes in the board and soldered on the back. Most boards have both, which is why you will see parts too small to pick up sitting next to a chunky USB socket.
One more thing worth knowing early, because it explains why boards look so crowded: no part can do more than its one job. A microcontroller runs your code, but it cannot give itself the right voltage, or clean up a noisy power supply, or protect itself if you plug the battery in backwards. It needs other parts around it to do all that. Most of what you see on a board is this support crew.
The most common circuit board components
Nine types of electronic circuit board components cover almost everything you will meet. Here they are, roughly in the order a beginner runs into them.
Resistors
What it does: a resistor holds electricity back. Think of a narrow section in a hosepipe. Less gets through.
How to spot it: if it has legs, it is a small beige or blue cylinder with coloured stripes around it. If it is stuck flat to the board, it is a tiny black rectangle with a three-digit number on it. Either way the label next to it on the board starts with R.
Why you need one: connect an LED straight to a battery and it burns out in a second, because nothing is limiting how much electricity flows through it. Put a resistor in the way first and the LED runs happily for years. This is the single most common job a resistor does, and it is why every beginner kit comes with a bag of them.
Resistors do two other jobs you will meet quickly. Two of them in a row can turn a light sensor into a number your board can read, because a board can only measure voltage and two resistors together turn "how much light" into "how much voltage". And a resistor connected to a button stops the button from giving nonsense readings when nobody is pressing it, which we come back to in the mistakes section.
The numbers, when you want them: resistance is measured in ohms (Ω). For an LED on a 3.3 V board, 330 Ω is the usual choice and gives you about 4 mA of current, which is bright enough to see and gentle enough to be safe. For buttons, 10 kΩ.
Try it yourself: 1.2 LED Blink uses one to protect an LED, 2.3 Photoresistor Analog Read uses two to read light, and 2.1 Button Counter uses one on a button.
Capacitors
What it does: a capacitor is a tiny rechargeable battery that fills and empties very fast. It holds a little electricity and gives it back the instant something needs it.
How to spot it: the small ones are flat brown or tan rectangles, easy to mistake for resistors, labelled C. The big ones are unmistakable: little aluminium cans standing up off the board, with a pale stripe down one side.
Why you need one: when a Wi-Fi board sends something, it suddenly needs a big gulp of power for a fraction of a second. The power supply cannot react that fast, so the voltage dips and the board resets itself. A capacitor sitting right next to the chip covers that gulp from its own little store, and nothing dips. That is why boards have capacitors sprinkled all over them rather than in one tidy corner: each one has to sit next to the chip it is protecting.
They also tidy up messy power. Electricity coming from a cheap USB charger is full of fast little wobbles, and a capacitor drains those away to ground before they reach anything that would be upset by them.
The one thing to be careful about: the big can-shaped ones only go in one way round. The stripe marks the negative side. Put one in backwards and it can pop, loudly, and it will smell terrible.
You will not be fitting these yourself for a while, and that is fine. Your development board came with all of them already on it. That is exactly why it works reliably when a bare chip on a breadboard would not.
Diodes
What it does: a diode is a one-way street for electricity. Forwards, it lets current through. Backwards, it blocks it.
How to spot it: a small glass or black cylinder with a band painted near one end. That band marks the end electricity comes out of. Labelled D.
Why you need one: mostly, to save you from yourself. Put a diode in the power input and connecting the battery backwards simply does nothing, instead of destroying everything on the board. A lot of the diodes you see are there purely for that.
The other common job is soaking up the kick from a motor. When you cut power to a motor or a relay, the coil inside fires a brief voltage spike backwards down the wire, and that spike will eventually kill whatever was switching it. A diode fitted across the motor gives that spike somewhere harmless to go.
Diodes also turn wall power into the kind of power electronics can use, four of them working together inside every phone charger you own.
LEDs
What it does: an LED is a diode that glows. Same one-way behaviour, plus light.
How to spot it: on a finished product it is the little light that tells you the thing is on or charging. On a board, look for a clear or coloured dome, or a tiny rectangle that lights up. Also labelled D, because it is a kind of diode.
Why it matters to you: an LED is the easiest way to find out whether your code is working. Make it blink, and if it blinks you know your program is running and you are controlling the pin you think you are. Half of learning electronics is finding out which part is broken, and a blinking LED answers that faster than anything else.
Since an LED is a diode, it only works one way round, and the legs are different lengths so you can tell which is which. The long leg is the positive one. And like any LED it needs a resistor in front of it or it will not last.
Something that catches everyone out: different colours need different amounts of voltage to light up at all. Red is easy and glows off almost anything. Blue and modern green need noticeably more. So a circuit that happily lights a red LED can leave a blue one completely dark, and it looks for all the world like you wired it wrong. Try a red one before you start pulling the circuit apart.
Stick a lot of LEDs together and you get scoreboards, digital clock displays, and the addressable strips people put behind their TVs, which our WS2812 LED strip guide covers if you want to go there next.
Try it yourself: 1.2 LED Blink is the classic first project, and 7.4 RGB LED Controller mixes colours with an LED that has three colours inside one dome.
Transistors
What it does: a transistor is a switch with no moving parts. A small electrical signal flips it, and it switches something much bigger.
How to spot it: with legs, it is a small black half-cylinder or a black rectangle with three legs. On a board it is often a tiny three-legged speck. Labelled Q.
Why you need one: a pin on your microcontroller can only supply a trickle of power. Enough for an LED, nowhere near enough for a motor, a relay, or a metre of LED strip. So the pin does not drive the motor. The pin flips a transistor, and the transistor drives the motor. Every project that moves something works this way.
You will also hear that transistors amplify, which is true and is how they got famous, but for the projects you are likely to build first they are simply switches.
The type you will see most on modern boards is called a MOSFET, because it wastes almost no power as heat. Four of them arranged cleverly can run a motor forwards and backwards, and that arrangement is exactly what is inside a motor driver module. If you want to try one on a breadboard, our BC547 transistor guide walks through which leg is which and a couple of simple circuits.
Integrated circuits (ICs)
What it does: an IC is millions of transistors shrunk down and sealed into one black rectangle. It is a whole circuit sold as a single part.
How to spot it: the black rectangles. They are the biggest things on the board, they have legs down the sides or pads underneath, and they have text printed on top. Labelled U.
How to tell what one does: read the text on top and search that number followed by the word "datasheet". That is not a shortcut, it is genuinely how engineers do it. Nothing about the outside of a chip tells you what is inside.
You will meet four kinds again and again:
- Microcontrollers run your code. This is the main chip, the brain. An ESP32-C6 is one, with Wi-Fi and Bluetooth built in.
- Regulators turn one voltage into another, usually the 5 V from a USB cable down into the 3.3 V the chips actually want.
- Drivers sit between a delicate chip and something demanding, like a motor or a screen.
- Sensor chips measure something and hand back a tidy number.
The first IC most people wire up by hand is a shift register, because it fixes a problem you will actually hit: running out of pins. You want to light eight LEDs and you have three pins left. A shift register takes those three pins and gives you eight outputs.
A development board like an ESP32 or a Raspberry Pi is really just a board that already has the right ICs on it, wired up correctly and tested, so you can get on with writing code instead of designing a power supply.
Try it yourself: 7.5 Shift Register Binary Counter turns three pins into eight.
Connectors
What it does: a connector is how the board talks to the outside world. Nothing clever, but getting the right one saves a lot of grief.
How to spot it: the sockets and the rows of pins. Labelled J or P.
Four you will meet:
- USB-C brings in power and carries your code onto the board.
- Pin headers are the two rows of pins along the edges. They are what lets a board push into a breadboard.
- JST is the small white plug for a rechargeable battery.
- Qwiic is a little four-pin socket that carries power and data together.
Why Qwiic is worth caring about as a beginner: connecting a sensor by hand means four separate wires, and two of them look identical. Swap those two and nothing happens at all, with no error message to tell you why. A Qwiic cable only fits one way, so you plug it in and it is right. When the sensor then does not respond, at least you know the problem is in your code and not in the wiring.
Crystal oscillators
What it does: a crystal is a sliver of quartz that vibrates at an extremely steady rate when you put electricity through it. The board counts those vibrations, and that is how it keeps time.
How to spot it: this is the easiest part on the whole board. It is a small silver metal capsule. On the bigger ones the number printed on top is its speed, something like 16.000; the tiny surface-mount ones are too small to print on. Labelled Y or X.
Why you need one: a chip has a rough sense of timing built in, which is fine for "wait half a second". It is useless for a clock, because it drifts as the room warms up and cools down. Anything that needs to know the actual time uses a crystal.
There is a shortcut, though, and it is worth knowing because it explains a design choice you will see a lot. A board with Wi-Fi does not need to keep time itself. It can just ask the internet. That is how the alarm clock project in our kit works: no timekeeping crystal, no backup battery, it simply asks a time server when it switches on. The trade-off is obvious the moment your router goes down.
Sensors
What it does: a sensor turns something in the real world into something electrical. Temperature, light, distance, movement.
How to spot it: for hobby projects a sensor usually arrives as its own little board, called a breakout, with the sensor chip in the middle and mounting holes at the corners. Somebody has already added all the supporting parts that chip needs, so you can just plug it in.
The one distinction worth learning: some sensors hand you a raw voltage and leave you to work out what it means. A light sensor does this: you get a number that is higher in bright light, and what counts as "bright" is up to you to figure out. Others do the work themselves and hand you a finished answer, like a temperature sensor that simply reports 21.4 degrees. The second kind is far easier to start with, and most modern sensors are that kind.
Distance sensors are worth a special mention because there are several sorts and they fail in different ways. The cheap ultrasonic ones bounce a sound off things, which works in total darkness but gets confused by anything soft, like a jumper. Our guide to distance sensor types compares them if you need to pick one.
Try it yourself: 3.1 Measuring Distance and 5.1 Reading Temperature and Humidity.
Mistakes beginners make with these components
Almost every "my board is broken" turns out to be one of these. None of them are your fault the first time, and all of them look identical to a dead board.
Putting something in backwards
Plenty of components only work one way round, and they do not warn you.
- An LED in backwards simply does not light. It is not damaged, it is just facing the wrong way. The long leg is positive. Turn it round.
- A capacitor in backwards is the serious one. The big can-shaped ones have a stripe marking the negative side, and fitting one the wrong way round can make it pop.
- A diode in backwards blocks the current it was supposed to pass, so the circuit is simply dead. The painted band shows which way it goes.
- A battery in backwards is what those protection diodes are for. On a board that does not have one, this is how boards die.
Fitting the wrong component
Parts of very different value look identical, and this trips up everyone.
- The wrong resistor. A 330 Ω and a 33 kΩ resistor are the same size and shape and only the stripes differ. Fit the big one where the small one belongs and your LED will be so faint you will assume it is broken. The 330 Ω reads orange-orange-brown.
- The wrong kind of buzzer. There are two, and they look the same. One plays whatever note you send it. The other has a fixed note built in and ignores your code completely. If your musical project plays one flat beep, you have the second kind.
- A chip in the wrong way round. Black rectangles have a notch or a dot at one end marking pin 1. Get it backwards and nothing works, and the chip may get hot. Check the notch against the outline printed on the board.
Forgetting the part that is not obviously needed
- No resistor with an LED. It will light. Briefly.
- No resistor on a button. A pin with nothing but a button attached is connected to nothing at all while the button is up, so it drifts and reports random presses. A resistor holds it steady. Most boards have suitable resistors built in that you can switch on in code, which is why some button projects need no resistors while others need two. Follow whichever guide you are using rather than mixing them.
Trusting a reading that cannot be right
Sensors do not say "I do not know". An ultrasonic distance sensor pointed at an empty room will report something in the region of ten metres, wandering about by a few centimetres, because that is what it does when no echo comes back. Treat an impossible number as no reading rather than as a real measurement.
How circuit board components work together
None of these parts is interesting on its own. Here is a small real project and what each part contributes.
Say you want a weather station: something that measures the temperature and puts it on the internet.
- The microcontroller runs the program and handles the Wi-Fi.
- The sensor measures the temperature.
- The regulator turns the 5 V from the USB cable into the 3.3 V the chips need.
- The capacitors keep the power steady every time the Wi-Fi transmits.
- The resistors keep the sensor's data lines behaving and protect the status LED.
- The connector brings the power in.
Take any single one away and it breaks differently. Skip the regulator and 5 V destroys the chip. Skip the capacitors and it restarts every time it sends anything. Skip the resistors on the sensor lines and the sensor never answers at all. The microcontroller gets all the credit, but it only works because of the plain little parts around it.
This is the argument for a development board when you are starting out. All of that is already done and tested, so the weather station project in our kit is one cable and about twenty lines of code, instead of a circuit you have to design first.
How to identify components on a circuit board
Pick up any board and you can work out most of it in a couple of minutes. There are three steps.
1. Read the white writing. Next to each part there is a letter and a number printed on the board. The letter tells you what kind of part it is. This is the single most useful thing to know, and it is the same on boards from every manufacturer in the world.
| Marking | Component |
|---|---|
| R | Resistor |
| C | Capacitor |
| D | Diode or LED |
| U | Integrated circuit |
| Q | Transistor |
| L | Inductor |
| Y or X | Crystal |
| J or P | Connector |
| SW | Switch or button |
| F | Fuse |
2. Read the part itself. Chips have a part number on top, and searching it with the word "datasheet" tells you everything. Small resistors have a three-digit code where the last digit is how many zeros to add, so 103 means 10,000 Ω. Resistors with legs use coloured stripes instead. Capacitors use the same three-digit trick, counted in different units.
3. If it has no markings, measure it. A multimeter set to continuity beeps when two points are connected, which is how you check whether a wire is actually doing anything before you start blaming your code. It will also read a resistor's value and tell you which way round an LED goes.
Circuit board components used in maker projects
The same handful of PCB parts keeps reappearing. What changes is the mix.
Smart home
A Wi-Fi board, a temperature and humidity sensor, and something to switch a light or a heater on and off. The switching is the part to be careful with: a microcontroller pin cannot drive a lamp directly, so it drives a relay or a transistor which drives the lamp.
Robotics
A microcontroller for the thinking, a motor driver so the small signals can command big motors, a regulator to keep the logic supply steady while the motors draw a lot of current, and diodes across the motors to absorb the spikes. A robot that resets itself every time it moves is almost always missing that regulator.
Environmental monitoring
A temperature sensor, a current sensor to see how much power the thing is using itself, and a clock chip so readings are still correctly timestamped after the board has been asleep for hours. Battery life is usually the hard part, which is why measuring your own power draw matters as much as measuring the weather.
E-paper display
A Wi-Fi board to fetch the data, a driver chip to run the display, and power management so it can sleep for days between updates. E-paper keeps showing its picture with the power completely off, so the only electricity you spend is on changing it.
Which circuit board components should beginners learn first?
In an order where each step gives you something to look at:
- LEDs, because you can see straight away whether it worked.
- Resistors, because you needed one the moment you used an LED.
- Buttons, which is how your circuit starts listening to you.
- Capacitors, which explain why fast circuits misbehave.
- Diodes, for one-way current and protection.
- Transistors, where something small controls something big.
- Sensors, where the project starts noticing the real world.
- Microcontrollers, where all of it comes together in code.
Every step here answers a problem the previous step just handed you, and that is why it sticks better than reading definitions.
Where you practice: the breadboard
You do not need to solder anything to start. A breadboard is a plastic block full of holes with metal clips inside, and you just push parts and wires into it. Nothing is permanent, so a wrong circuit costs you thirty seconds.
The one thing to learn about it is which holes are joined to which. The long strips down the sides are joined all the way along and are for power. The short rows in the middle are joined in groups of five. Get that wrong and your circuit either does nothing or shorts out, and it looks the same either way. Our guide to what a breadboard is and how to use it goes through the layout.
Learn each component by building with it
Reading about a resistor is nothing like watching an LED stay stubbornly dark because you forgot one. Every component in this article turns up in the Zero to Hero kit documentation in a project you can finish in an evening, in roughly the order above:
| Component | What you learn | Project |
|---|---|---|
| Breadboard | Which holes are joined to which | 0.1 Breadboard Fundamentals |
| LED and resistor | Limiting current, polarity, switching a pin on | 1.2 LED Blink |
| Push button | Reading an input, and why pins drift | 2.1 Button Counter |
| Push button, again | Why one press counts as five, and the fix | 2.2 Button Debounce |
| Photoresistor | Turning a resistance into a measurement | 2.3 Photoresistor Analog Read |
| Buzzer | Driving something that makes a noise | 2.4 Buzzer Beep |
| Ultrasonic sensor | Reading a sensor over a data connection | 3.1 Measuring Distance |
| LED, dimmed | Getting in-between brightness from an on/off pin | 3.2 Distance Fade LED |
| LCD display | Talking to a display chip | 4.1 Print Message |
| Temperature sensor | A sensor that hands you a finished number | 5.1 Reading Temperature and Humidity |
| RGB LED | Three LEDs in one dome, mixing colours | 7.4 RGB LED Controller |
| Shift register | Turning three pins into eight | 7.5 Shift Register Binary Counter |
| All of the above | Running four things at once, and getting the time | 7.7 Alarm Clock |
The kit those pages are written for contains a NULA Mini ESP32-C6, 13 LEDs including a colour-mixing one, 40 resistors, 3 light sensors, 5 buttons, a buzzer, a shift register, a mini breadboard, jumper wires, and three plug-in modules: a small screen, a temperature and humidity sensor, and a distance sensor. All 20 projects are written up for both Arduino and MicroPython. If you would rather start from a finished project and work backwards, our 15 ESP32 projects for beginners is the other way in.
Why Soldered is a great place to learn electronics
Buying parts one at a time is a slow way to learn, mostly because you lose your evenings hunting a missing resistor instead of building something. A kit and a development board get you past that. The Embedded Journey Kit, a NULA Mini and a breadboard cover the whole progression above, and because the modules use Qwiic, adding a sensor is one cable rather than four wires and a diagram.
Everything we make also comes with its circuit diagrams published openly, libraries for both Arduino and MicroPython, and full documentation, so when something does not work you can go and look at why rather than guessing. If you are not sure which language to start with, our comparison of MicroPython and Arduino lays out the trade-offs.
FAQ
What are the main components of a circuit board?
Resistors, capacitors, diodes, LEDs, transistors, integrated circuits, connectors, crystal oscillators, and sensors. Most boards also have inductors, switches, and fuses.
What is the difference between a PCB and its components?
The PCB is the flat board with copper lines printed on it. Those lines are just wiring. The components are the parts soldered on top, and they do the actual work. A bare PCB does nothing at all.
How do you identify circuit board components?
Read the letter printed next to the part: R is a resistor, C a capacitor, D a diode or LED, U a chip, Q a transistor. Then read the part itself and search any part number along with the word "datasheet". A multimeter sorts out anything with no markings left.
What are the most common electronic components?
Resistors and capacitors, by a wide margin. Most boards carry more of those two than everything else put together, because nearly every other component needs some around it to work properly.
Which PCB components should beginners learn first?
LEDs and resistors, then buttons and capacitors. They are cheap, hard to break, and you can see them working, which is what makes the ideas stick.
What components do I need to start learning electronics?
A development board, a breadboard, jumper wires, some LEDs, a set of 330 Ω and 10 kΩ resistors, and a few buttons. That covers the first several projects anyone builds. A multimeter is the next thing worth adding, and a beginner kit works out cheaper than buying the list separately.
Do I need to solder to learn about circuit board components?
No. Everything in this article can be built on a breadboard, which holds parts by friction and pulls apart again. Soldering matters when you want a circuit to survive being carried around, not when you are learning what the parts do.
What does an integrated circuit do?
It packs a whole job into one chip: running code, storing data, driving a motor or a screen, or converting one voltage into another. Building the same thing out of individual transistors would take hundreds of parts.
What is the purpose of a resistor on a PCB?
To hold back current. Resistors stop LEDs burning out, bring voltages down to a level a chip can measure, and hold unused input pins steady so they do not drift and report nonsense.
Are sensors considered PCB components?
Yes. A sensor is a component like any other, and for hobby projects it usually arrives as a breakout: the sensor chip plus all the supporting parts it needs, already assembled onto a small board of its own.
