We drew what mattered and left out the rest. Not the bench, not the wire’s wander: only what joins to what. Any hand that can read the map can build the machine. Here is the map.
⟨ transmission ends ⟩By the end of this page you will read a schematic and build it on a board, and look at a board and draw its schematic. A schematic is a map of one thing only: what connects to what. Where the parts sit on the page means nothing; the lines between them mean everything.
Below is First Light as a schematic: the battery, the 470 Ω resistor and the LED, each drawn as its symbol, joined by 3 lines. Because only the connections matter, you can drag any symbol anywhere and the circuit cannot change: the wires follow and never let go.
Battery 9 V, resistor 470 Ω, red LED: the loop from First Light, about 15 mA, so the LED symbol is drawn lit. The battery symbol’s long thin line is the + end. Positions and wire routes are yours to mangle; the 3 connections are the circuit.
Messy, and still the same circuit: 3 connections before, 3 after. On a page with more parts, wires must cross each other, and one rule keeps every crossing honest. A filled dot where lines meet means joined; a bare crossing means strangers passing. Below, a second branch with its own resistor reaches the supply wire and stops right on it, dark, because touching is not joining. An ammeter sits in the supply wire, counting everything the battery sends. Put the dot where the wires meet and watch its number.
The circle marked A is an ammeter, the meter from The Meter drawn into the loop; a meter in a schematic is a real part like any other. Branch 1 is 470 Ω and bright, about 15 mA. Branch 2 is 2.2 kΩ, so when the dot joins it, its LED glows dim on about 3.2 mA and the ammeter climbs to 18.2 mA: the dot is 3.2 mA of real current. The lower crossing never gets a dot: branch 2’s return passes over branch 1’s wire without touching, which is exactly what a bare crossing says. Old drawings marked it with a little hop; the hop went out decades ago, the bare crossing took its place.
Dot: joined, and the branch lights. No dot: strangers, however long the wires lie across each other. One more piece of shorthand and you can read most pages. Big schematics do not drag every return wire across the page to the battery. They end each return in a ground symbol, and the rule is: every ground symbol on the page is the same point, the shared − everything is measured against. The board below took its schematic too literally: both branch returns dangle in the air. Wire each one to the − column.
Only the board’s used top half is in frame; the bottom half sits below the crop, unused. Battery + feeds column 1, its − lead waits in column 17, so the − column is 17a to 17e. Branch A is 470 Ω and a red LED, about 15 mA; branch B is 1.0 kΩ and a red LED, about 7.1 mA, so it lands visibly dimmer. The 3 ground symbols in the drawing are one electrical point; wiring them all to the − column is what the symbol asks for.
Now the whole skill, one direction at a time. Schematics read like a page: + at the top, ground at the bottom, so power falls top to bottom; signals enter at the left and leave at the right. Below is a schematic drawn in that order, its 3 nets tinted so you can see them, and an empty board. The drawing also decides the parts: it says 1.0 kΩ, and the bench holds 2 resistors that only their color bands tell apart. Build it.
Battery 9 V with 2 Ω inside, red LED 1.9 V when lit. The drawing asks for 1.0 kΩ: brown black red, 7.1 mA, a gentler glow than First Light’s 15. The other resistor on the bench is 470 Ω, yellow violet brown; seat it and the LED lights brighter, but that is a different circuit than the drawing. Each tint in the drawing is one net: one clip group on the board. The LED’s + leg is on the left; if it ever sits backwards, click it in its seat to turn it round. An LED wired straight across + and − with no resistor takes about 3.5 A and dies at once, loudly.
And back again. Below is a build you have never made, pictured as it sits on the bench: a resistor, an LED, and a slide switch that turns the loop on and off. Under it, a schematic pad: the 4 symbols already placed, unwired, each lead ending in a small pad. The switch is drawn as 2 contacts with a blade lying across them, closed, because the switch on the bench is on. Tap 2 pads to run a wire between them; tap a wire to take it back. Read the board and draw it.
The board carries a 470 Ω resistor, a red LED and an SPST slide switch in one loop: about 15 mA while the switch is on. A closed switch is just a piece of wire, which is why the loop runs; drawn open, the blade lifts off its contact and the same drawing means a dead loop. The pad accepts any true drawing of this board: the switch before the LED or after it lights either way, because one loop has one current everywhere. The LED symbol wired backwards stays dark, because the triangle points + to −. A wire that reaches the LED with no resistor in its loop is marked hot: a real build of it takes about 3.5 A and kills the LED. A wire from + straight to − is a short, and a real build of it would cook the battery.
That is the whole skill: a circuit is its connections, and the schematic is the honest list of them wearing symbols. You now own it in both directions.
When you are ready, test yourself:
Two schematics show the same 3 parts in different corners of the page, wires routed differently, connections identical. They describe…
Two wires cross with no dot. How much current passes from one to the other?
A schematic shows 4 ground symbols. On the board that means…
Your build of a schematic is dark. The drawing shows the LED triangle pointing toward ground; your LED sits with its long leg in the net nearest ground. What is wrong?
Every circuit from here on will be handed to you as a schematic first, and you will build it without a second thought. What the drawings will not tell you is where the power comes from. Batteries, USB, wall bricks, bench supplies, and the safety rules that go with them: next, The Power.
Lesson 6 done.
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