Electronics, from the ground up.
A free, plain-English course on electronics, circuits, and PCB design. Nine units, starting with what voltage actually is and ending at high-speed layout.
Electricity, in plain terms.
Everything else builds on these ideas. Get comfortable with voltage, current, and resistance and the rest of electronics gets much easier.
Voltage, current & resistance
Voltage is electrical pressure. More precisely, it’s the difference in electrical potential between two points, measured in volts (V). The “between two points” part matters. A voltage never exists at a single point, the same way water pressure only means something as a difference between two tanks. A 9 V battery holds its + terminal nine volts above its − terminal, and if you connect a path between them, that pressure pushes charge around it.
Current is the flow itself: the amount of charge passing a point each second, measured in amperes (A). Resistance, in ohms (Ω), is how hard a material fights that flow. Copper wire has almost none. A resistor adds a deliberate, known amount. Push the same voltage through more resistance and less current flows, which is the relationship the next lesson pins down.
This shapes how you use a multimeter on a real board. You read voltage across a component, with a probe on each side while the circuit runs, but you read current through it, with the meter inserted into the path. Probing “the voltage at a point” only works because the black probe supplies the second point, usually ground.
- Voltage (V) is a difference in potential between two points, never a property of one point alone.
- Current (A) is the rate charge flows past a point, and it’s the same everywhere along one unbroken path.
- Resistance (Ω) opposes current. Wires have almost none; resistors add a controlled amount.
- Measure voltage across a part and current through it. Wiring an ammeter in parallel is the classic way to blow its fuse.
Ohm’s law & power
Ohm’s law ties the three together: V = I × R. Know any two and you can find the third. Put 5 V across 1 kΩ and exactly 5 mA flows. Need 10 mA through an LED from a 5 V supply? The same equation tells you which resistor to use. Most everyday circuit math is some rearrangement of this one formula.
Power is the rate energy gets converted, in watts: P = V × I. Combined with Ohm’s law you also get P = I² × R and P = V² / R. Power becomes heat in most components, and heat is usually what you’re worried about. A resistor dropping 5 V at 100 mA dissipates half a watt, enough to cook a small part rated for a quarter watt.
A good habit: whenever current flows through a resistance, ask where the power goes. Run P = V × I on anything that drops real voltage at real current (regulators, power resistors, transistors) and you’ll catch parts that would run hot before the board is even built.
- V = I × R. Any two values give you the third.
- P = V × I gives power in watts; P = I² × R and P = V² / R are the same law rearranged.
- Every component has a power rating. Exceed it and the part overheats and fails.
- Doubling the current through a fixed resistance quadruples the heat, since power scales with I².
Ground & references
Because voltage is always a difference, every circuit needs an agreed zero point to measure from. That point is called ground: the common 0 V reference that every other voltage in the circuit is compared to. There’s nothing physically special about it. No exotic charge, and no actual connection to soil required. It’s a convention, like using sea level for altitude.
The word covers a few related ideas. Signal ground is the reference node on a board. Earth ground is a literal rod in the dirt, there for mains safety. Chassis ground ties a metal enclosure in. They’re often connected to each other, but not always, and mixing them up causes real trouble in mains-powered equipment.
The classic beginner mistake goes like this: power two boards from separate supplies, run a signal wire between them, and forget the ground wire. Each board measures its “3.3 V” against its own reference, so without a shared ground the signal means nothing and the receiver sees noise. When two circuits talk, their grounds have to be connected.
- Ground is the agreed 0 V reference point, not a special kind of charge.
- Node voltages on a schematic are all implicitly measured against ground.
- Signal ground, earth ground, and chassis ground are different things that share a name.
- Two circuits exchanging signals need a shared ground, or the signal has no meaning.
AC vs DC
DC, direct current, flows steadily in one direction, like the output of a battery or a USB port. AC, alternating current, reverses direction periodically; wall power swings back and forth 50 or 60 times a second. Nearly everything on a circuit board runs on DC internally, even when the device plugs into an AC outlet.
AC won the power grid for one reason: transformers. A transformer can step AC voltage up or down with very little loss, but it only works with changing current. Utilities transmit at hundreds of thousands of volts because high voltage means low current for the same power, so thin cables waste little energy as heat. The voltage gets stepped down near your house. When the grid was built, there was no efficient way to do any of that with DC.
One subtlety worth knowing: “120 V AC” is an RMS figure, an effective average, while the waveform actually peaks near 170 V. That matters when you pick parts that touch mains. For board-level work the takeaway is simpler. A power supply’s first job is turning wall AC into the clean low-voltage DC your circuit actually runs on.
- DC flows one way at a steady level. AC reverses direction periodically (50/60 Hz for mains).
- AC dominates power distribution because transformers can change its voltage efficiently.
- Boards run on DC internally; converting AC to DC is the power supply’s job.
- AC voltages are usually quoted as RMS. The instantaneous peak is about 1.4× higher.
Shorts, opens, conductors & insulators
Materials sit on a spectrum. Conductors like copper, gold, and aluminum let charge move freely. Insulators like plastic, glass, and the FR-4 fiberglass in a PCB barely let it move at all. A circuit is really just conductors routed through insulators so current goes exactly where you want and nowhere else.
A short circuit is an unintended low-resistance path between two points that were never meant to connect, like a solder blob bridging power to ground. Ohm’s law explains the damage: with resistance near zero, the current tries to become enormous, and something overheats, whether that’s a trace, a regulator, or a battery. An open circuit is the opposite failure, a break in the intended path (a cracked joint, a cut trace) so no current flows at all.
Between them, these two faults cover most board debugging. A multimeter’s continuity mode (the beep) finds both. A beep where there shouldn’t be a connection means a short. Silence where there should be one means an open. Checking the resistance from power to ground before first power-up is a habit that saves boards.
- Conductors let charge flow; insulators block it. A PCB is copper conductors on an FR-4 insulator.
- A short is an unintended low-resistance path. Near-zero resistance means huge current and heat.
- An open is a break in the path, so no current flows and the circuit simply does nothing.
- Continuity mode on a multimeter is the fastest way to hunt down both faults.
