Electronics · Under the hood · Optional — the deep story · build 2026.09.23-1644
The idea
DC flows in one constant direction — every circuit in this course. AC reverses direction on a strict rhythm: 60 complete cycles each second at a household outlet. Same electrons, run two different ways.
You can make AC yourself, right now: swap a supply's red and black leads back and forth by hand, and that is alternating current — at about one reversal per second. Wall power is the same idea performed 60 times a second, by generators the size of school buses, forever. The unit for "how often" is the hertz; the grid's heartbeat is 60 Hz. It's a pitch, not just a rate: the famous low "mains hum" that sneaks into audio gear is exactly this 60 Hz, made audible.
Because of one device: the transformer — two coils of wire sharing an iron core, no moving parts, no connection between them. A changing current in one coil creates a changing magnetic field, and the changing field pushes current in the other coil. (Sound familiar? It's the capacitor's law wearing magnetic clothes: only change gets across.) Wind the second coil with more turns and the voltage steps up; fewer turns, down — almost for free, but only for AC. Steady DC makes no changing field, and the transformer goes silent.
That's the whole strategy of the grid: step voltage way up (hundreds of thousands of volts) to cross the countryside — high voltage means low current for the same power, and low current wastes far less in the wires (the P = IV sheet's I²R story) — then step down, and down again, to the 120 V at your outlet.
The war of the currents
In the 1880s–90s this was a genuine commercial war: Edison sold DC systems; Westinghouse, armed with Tesla's AC designs, sold transformers and distance. AC lit the 1893 Chicago World's Fair, then harnessed Niagara Falls — and the grid has been AC ever since. (DC is quietly returning for some very-long-distance lines — engineering arguments are rarely over forever.)
Your kit runs on DC, so somewhere between the outlet and the BreadVolt's battery, AC must be tamed. Every charger and wall-wart on Earth does it with the same three moves — all parts you know:
Your body, hand to hand, is very roughly 100 kΩ dry. Ohm's law, both ways: at 5 V that's ≈ 0.05 mA — far below anything you can feel. At 120 V it's ≈ 1–2 mA — already at the threshold of sensation, and wet skin can drop your resistance ten-fold or more, pushing current into genuinely dangerous territory. Worse, 60 Hz AC is unluckily well-suited to disrupting the heart's own electrical rhythm. That's the whole safety stance of this course in one line: 5 V DC is a safe teacher; 120 V AC can be lethal and is never touched here.
Concepts, not procedures
Everything on this page is physics anyone may learn. Actually working on mains wiring — outlets, panels, fixtures — is a licensed trade, learned through real apprenticeship under people who do it daily. If that trade calls to you: electricians run on exactly what this course teaches — meters, Ohm's law, Kirchhoff's laws — and a student who arrives at an apprenticeship already fluent in them is years ahead.