Components and Schematics

The last lesson ended with a working picture: voltage pushes current around a loop, and resistance sets how much current flows. Ohms law tied the three together, V equals I times R. That is the physics. It is not yet a circuit anyone can build.

A real circuit is made of parts, each one doing a specific job to the current passing through it. And it is written down on paper as a drawing that looks nothing like the parts themselves: a page of zigzags, triangles, and parallel lines connected by wires. This lesson covers the small set of parts that show up in almost every circuit, what each one does, and how to read the drawing that ties them into a loop.

The battery and ground

A circuit needs a source. The battery is the part that supplies voltage, the push that moves current around the loop. It has two terminals: the positive terminal, marked plus, sits at the higher voltage, and the negative terminal, marked minus, sits at the lower one. Current leaves the positive terminal, travels the loop, and returns to the negative terminal. Cut the loop anywhere and the current stops, exactly as the last lesson described.

Batteries come in fixed voltages that are worth knowing on sight. A single AA alkaline cell is 1.5 volts. A CR2032 coin cell, the flat silver disc in a car key or a motherboard, is 3.0 volts. A single lithium-polymer cell, the kind inside a phone, is 3.7 volts nominal. A USB port delivers 5 volts. The battery does not decide how much current flows. It sets the voltage and lets the rest of the circuit determine the current.

Ground is the other end of the story. It is the reference point every other voltage is measured against, defined as zero volts. On most small circuits ground is simply the battery's negative terminal, and it is the common return path that current flows back through. Voltage is always a difference between two points, so a single number like "3.3 volts" is shorthand for "3.3 volts above ground." Ground is the agreed-upon floor that makes those single numbers mean something.

The resistor

The resistor is a part built to resist current. It converts a bit of electrical energy into heat, and in doing so it holds the current down to a chosen level. Its value is measured in ohms, written with the symbol Ω, and a real resistor's value is printed on it as colored bands or stamped on the body. Common values run from a few ohms up to millions: 220 Ω, 330 Ω, 1,000 Ω (also written 1 kΩ), 10 kΩ.

Ohms law is what makes a resistor useful, because it says exactly how much current a given resistor will allow. Rearranged to solve for current, V equals I times R becomes I equals V divided by R. Put 5 volts across a 1,000 Ω resistor and the current is 5 divided by 1,000, which is 0.005 amps, or 5 milliamps. Swap in a 330 Ω resistor and the current climbs to 5 divided by 330, about 0.015 amps, or 15 milliamps. Smaller resistance, more current. The resistor is the knob that sets it.

That knob is the whole reason resistors sit next to more delicate parts. A part that would pull too much current if wired straight to the source gets a resistor in series with it, sized so the current lands where it should. The next part is the classic case.

The capacitor

A capacitor stores charge. Inside it are two conductive plates held close together but not touching, separated by a thin insulating layer. Apply a voltage across the plates and charge piles up on them, one side positive, the other negative. Remove the source and the charge stays for a while. A capacitor is a small, fast reservoir for electrical charge.

Its capacity is measured in farads, symbol F, but a full farad is enormous. Real capacitors are labeled in fractions: microfarads (µF, a millionth of a farad), nanofarads (nF, a billionth), and picofarads (pF, a trillionth). A 0.1 µF ceramic capacitor, the tiny part scattered across every circuit board, is a completely ordinary value.

The behavior that matters most: a capacitor resists sudden changes in voltage. When the voltage feeding a chip dips for an instant, a capacitor placed across its power pins dumps its stored charge to fill the gap, holding the voltage steady. That job is called decoupling, and it is why boards are covered in small capacitors near every chip. A capacitor also blocks steady, unchanging current once it is fully charged, while still passing a changing signal through. Storing charge and smoothing voltage are the two things to remember it for.

The diode and the LED

A diode lets current flow one way and blocks it the other. It has two terminals, the anode and the cathode, and current passes only when it enters the anode and exits the cathode. Push current the other direction and the diode stops it cold. It is a one-way valve for electricity. On a real diode a printed band marks the cathode, the blocked-return side, so the part can be placed the right way around.

A diode also costs a little voltage to push current through in its allowed direction. For an ordinary silicon diode that forward drop is about 0.7 volts. The current gives up 0.7 volts crossing the diode, which is a fixed toll rather than something Ohms law sets.

An LED, light-emitting diode, is a diode that glows when current flows through it the right way. It behaves like any diode, one-way with a forward voltage, except its forward drop depends on the color: a red LED drops about 2.0 volts, a blue or white one about 3.0 to 3.4 volts. And here is the trap. Once the voltage across an LED passes its forward drop, the LED barely resists current at all. Nothing inside it limits how much flows.

So an LED wired straight across a source with nothing else in the loop pulls current until it destroys itself. This is where the resistor from earlier earns its place. Put a resistor in series with the LED and it takes up the leftover voltage and sets the current. Work the numbers on a real case: a red LED at 2.0 volts forward, run from a 5 volt supply, at a target of 10 milliamps. The resistor has to drop the difference, 5 minus 2.0, which is 3.0 volts. From Ohms law, R equals 3.0 divided by 0.010, which is 300 Ω. The nearest common resistor is 330 Ω, and with it the current settles at 3.0 divided by 330, about 9 milliamps. Safe, bright, and steady.

The transistor

A transistor is a switch with no moving parts, controlled by electricity instead of a finger. It has three terminals. A small signal on one of them, the control terminal, decides whether a much larger current is allowed to flow between the other two. Weak in, strong out: that is the whole idea. Applied one way it acts as a switch, fully on or fully off. Applied another way it acts as an amplifier, where a small changing signal steers a larger copy of itself.

This is why a transistor is the most important part on the list even though it does the least on its own. A microcontroller pin can supply only a tiny current, nowhere near enough to spin a motor or drive a bright lamp. Wire that pin to a transistor's control terminal and the pin's small current switches the transistor, which in turn switches the heavy current the motor needs. The pin decides, the transistor does the lifting.

Stack up enough transistors switching each other and the result is a logic gate, then a memory cell, then a processor. Every chip in every device is transistors, counted in the billions. The takeaway here is simpler: a transistor lets a small electrical signal control a big one, as either a switch or an amplifier.

The schematic

None of these parts is useful alone. A circuit is parts wired into a loop, and the standard way to write down which part connects to which is the schematic: a diagram where every component is drawn as a simple, agreed-upon symbol and every wire is a line. A schematic says nothing about where the parts physically sit or how big they are. It records one thing only, what connects to what.

Each symbol is a shorthand that hardware people read at a glance:

• A resistor is a zigzag line, or a plain rectangle, labeled with its value like 330 Ω.
• A capacitor is two short parallel lines, the two plates, face to face.
• A diode is a triangle pointing at a bar, and the bar is the cathode, the blocked side. An LED adds two small arrows pointing away, for the light it throws off.
• A battery is a stack of alternating long and short lines, the long line positive.
• Ground is a set of shrinking horizontal lines, or a small down-pointing triangle.

The other half of reading a schematic is the wires, and the key idea there is the node. A node is any set of points joined by wire, all sitting at the same voltage because current moves freely between them with nothing in the way. Where lines meet and are connected, the drawing puts a filled dot at the junction. Where two lines merely cross with no dot, they are not connected, like one road flying over another. That dot, present or absent, is the difference between a joint and a flyover, and misreading it is the most common schematic mistake there is.

Put the parts together and a schematic maps straight onto real hardware. Take the LED example and add an Arduino, a small board with a microcontroller whose pins a program can switch on and off. The schematic reads: pin 13 connects through a 330 Ω resistor to the LED's anode, and the LED's cathode connects to ground. Three symbols, three wires, one loop. When the program sets pin 13 high, the pin goes to 5 volts, current flows through the resistor and the LED down to ground, and the LED lights. Set the pin low and the loop drops to zero, and the light goes out.

Here is the program that drives it, the standard Arduino blink sketch:

const int LED_PIN = 13;

void setup() {
  pinMode(LED_PIN, OUTPUT);
}

void loop() {
  digitalWrite(LED_PIN, HIGH);
  delay(500);
  digitalWrite(LED_PIN, LOW);
  delay(500);
}

The setup function runs once and tells the board that pin 13 is an output, a pin the program will drive. The loop function repeats forever: set pin 13 high to put 5 volts on it, wait 500 milliseconds, set it low again, wait another 500. The LED, sitting on the other end of that resistor in the schematic, blinks once a second. Compiling the sketch checks that it is valid before it ever reaches the board.

$arduino-cli compile --fqbn arduino:avr:uno Blink

The exact byte counts shift a little with the compiler version, but the shape holds: a blink sketch is tiny, using a couple percent of the chip. The point is that the schematic and the code describe the same loop from two sides. The drawing says which parts connect; the program decides when the pin that feeds them turns on.

The parts are named and the drawing makes sense, but reading a schematic is not the same as building the loop it describes. The next lesson, your first circuit, takes these exact parts, a resistor and an LED and a source, and wires them up on a breadboard, a reusable board that holds parts and connects them without any soldering. That is where the symbols turn into something that lights up.