1. Resistors
What it does: restricts how much current flows through a circuit.
The analogy: a narrowing in a pipe. Same pressure behind it, less flow through it.
These are the small cylinders with coloured bands. The bands encode the value in ohms (Ω) — and reading them is a skill you'll never fully trust, which is why every technician I know just measures them with a multimeter instead.
You'll use them constantly: protecting LEDs, setting the brightness or speed of things, and as "pull-up" or "pull-down" resistors that hold an input at a known voltage instead of letting it float randomly.
2. Capacitors
What it does: stores a small amount of charge and releases it.
The analogy: a tiny water tank in the line. It smooths out sudden surges and dips rather than letting them reach the rest of the circuit.
Two types turn up in kits. Ceramic capacitors are the small disc-shaped ones, unpolarised — they go in either way round. Electrolytic capacitors are the little cylinders that look like miniature cans, and these are polarised: they have a positive and negative leg, marked with a stripe down the negative side.
The everyday use is smoothing power supplies — a capacitor across the supply rails absorbs the current spikes a motor or servo makes, so they don't drag the whole circuit's voltage down with them.
3. LEDs
What it does: emits light when current flows through it the right way.
The workhorse of every first project. Two things matter: they're directional (long leg positive, and there's a flat spot on the rim marking the negative side), and they always need a resistor in series to limit current.
There's a full walkthrough in the first Arduino project guide, including how to work out the resistor value.
4. Diodes
What it does: lets current pass one way and blocks it the other.
The analogy: a one-way valve.
An LED is a diode that happens to glow; a plain diode is the same idea without the light. The band printed at one end marks the cathode — the side current flows out of.
The classic use is protection. Put one across a relay coil or a motor and it safely absorbs the voltage spike those generate when they switch off — a spike which is otherwise quite capable of damaging whatever is driving them. In that role it's called a flyback or freewheeling diode, and it's cheap insurance.
5. Transistors
What it does: lets a small current control a much larger one.
The analogy: a tap. A small movement of your hand controls a much bigger flow of water.
This is what you reach for the moment something needs more current than a microcontroller pin can supply — a motor, a bright lamp, a long strip of LEDs. The pin controls the transistor, and the transistor does the actual heavy lifting.
Kits usually include small signal transistors like the 2N2222 or BC547. They have three legs and the order matters, so check the specific part's datasheet rather than assuming — the pin order genuinely differs between types that look identical.
6. Potentiometers
What it does: a resistor you can adjust by turning a knob.
Three legs: the outer two connect across the full resistance, and the middle one — the wiper — gives you a voltage that varies as you turn it. Feed that middle leg into an analog input and your microcontroller can read the knob position.
Useful for volume controls, brightness, calibration, or just having something physical to fiddle with while testing rather than re-uploading code with different values.
7. Push buttons (tactile switches)
What it does: completes a circuit while you're pressing it.
The small square four-legged clickers. The four legs catch people out: they're really only two connections, with each pair internally joined. If your button seems permanently pressed, you've probably wired across a pair that's already connected — rotate it 90° on the breadboard.
Two things worth knowing. First, an input pin with a button attached needs a pull-up or pull-down resistor so it reads a definite high or low when the button isn't pressed — both Arduino and Raspberry Pi can do this in software, so you rarely need a physical resistor. Second, mechanical buttons "bounce": one press can register as several. The fix is debouncing, either a short delay in code or a small capacitor across the switch.
8. Integrated circuits (ICs)
What it does: a whole circuit, packaged into one chip.
The black rectangles with legs down both sides. Most kits include a 555 timer (generates pulses and delays) and often an op-amp or a shift register.
There's a dot or notch at one end marking pin 1; from there pin numbering runs anticlockwise. Getting an IC in backwards is a very common way to destroy one, and there's usually no visible sign it happened.
And the things that aren't components
Your kit also contains a breadboard — the white block full of holes that lets you build circuits without soldering — and a bundle of jumper wires. The breadboard's internal connections aren't obvious from looking at it: the long rails down the sides are connected lengthways for power and ground, while the main area is connected in short rows across the central channel. Wiring as though the whole board is connected is a rite of passage.
Working out what a part needs
Every one of these components has a datasheet telling you its limits. Knowing which five numbers to look up is the skill that ties all of this together.
What to buy
If you don't have a kit yet, or you're wondering which tools are worth adding, the home bench guide covers what's genuinely worth buying and what to skip.
Gear that helps
A component assortment kit with a decent spread of resistors, capacitors and semiconductors covers everything above. A basic multimeter is what turns the colour bands into a number you can actually trust.