Electronics · Semester 1 — Analog · Lesson 1 · 55 min · build 2026.09.23-1644
Big idea
Electricity only flows around a complete, unbroken loop — and a circuit is nothing but components joined at connection points called nodes.
Safety — two rules
1. Never make a short circuit. If red touches black with no component in between — through a jumper, a pinch, anything — the supply's full strength dumps through that one path. The wire can get finger-burning hot in seconds and melt its insulation, and the power supply itself can be damaged. Bare metal touches only what you chose.
2. Never power an LED without its resistor. A "naked" LED draws far more current than it can survive. It doesn't just quietly die — it can pop like a tiny firecracker and throw hot plastic bits across the table. This really happens. The resistor is the LED's bodyguard: every LED, every circuit, no exceptions. And don't hover face-down over a circuit the first time you power it.
One comfort: at 5 volts, dry skin lets through only a whisper of current — far too little to feel — which is why you can hold today's live circuit in your bare fingers. The real hazard in this kit is indirect: rule 1's short circuit makes wires hot enough to burn.
And one hard line: house wiring is a different world. The 120 volts in wall outlets, cords, and light sockets can push enough current through a body to be fatal. Nothing in this class ever touches household wiring — and neither do you. Everything we build lives on the low-voltage side, full stop.
Every component has a symbol. Symbols are how circuit builders write things down — like sheet music for electricity.
A schematic is the recipe for a circuit. Three rules:
The key move
A node is anywhere parts are squeezed together so firmly they become one electrical point. Today, a node is a finger pinch. Building from a schematic comes down to one job: give every node in the drawing its own pinch, then put the right legs into it.
Partner up: this build takes four hands. One of you reads the schematic aloud and calls the nodes; both of you pinch. The rule of the pinch: your skin can't do the conducting — the metal parts must press directly against each other. Your fingers just provide the squeeze that holds them together.
Dead circuits are normal — engineers spend most of their time exactly here. Your kit includes the Debugging Loop card: six steps, in order, starting with stop, look, touch nothing. When something's dead: run the loop, check your build against the schematic, ask a neighbor — then wave a teacher over. And when you do, report what you found ("power's good, the loop traces, but I'm not sure about node B"), not just "it doesn't work."
Today's usual suspects:
| Symptom | Likely cause | Fix |
|---|---|---|
| Dark LED | Skin inside a pinch — the metals aren't touching each other | Re-pinch: metal pressed on metal |
| Dark LED | LED is backwards (it's a one-way street) | Flip it: long leg toward the resistor side |
| Dark LED | The power supply is off, out of charge, switched to 3.3 V, or knocked out by a recent short | Wave a teacher over to check the supply |
| Dark LED | A pinch slipped apart while everyone watched the LED | Re-set it — squeeze firmly, metal on metal |
The math, for the curious — always optional
Everything you just built has numbers behind it: the supply provides 5 V, the resistor resists with 220 Ω, and a rule called Ohm's law predicts the flow:
— about 0.014 A (14 milliamps) through your LED. You will never be required to compute anything in this class. But the math is real, it lives in these purple boxes every lesson, and it's how engineers size every part before touching one.
You made light. Now the why — and it fits in one picture. Think of a closed loop of water pipes. A pump pushes the water, a narrow spot limits how fast it can flow, and a waterwheel uses the flow to do something. The water goes around and around — it never gets "used up," and if you cut the pipe anywhere, everything stops at once. Keep that in mind — you'll test the "anywhere" part in Experiment 2.
Three words you'll use all year:
Rule for the whole year: say your prediction out loud before you try it. Being wrong is where the learning is.
Predict
If you turn the LED around (swap which leg connects where), what will happen?
Try it
Re-pinch the LED the other way around — swap which leg is in which pinch. What happened, and what does that tell you about LEDs? This one-way behavior has a name: the LED is a diode.
Predict
If you break the loop at node C — after the LED — does the LED stay lit, since the electricity already "got there"?
Try it
Open the loop at A, then B, then C. Anywhere you break it, everything stops instantly. Current doesn't fill up a circuit like water in a cup — no loop, no flow, anywhere.
Turn your circuit into a tester by splitting node C in two: one partner pinches the LED's short leg against one end of the mystery object, while the other pinches the object's far end against the black wire's tip. The object now sits in a gap in the loop:
Predict each one first, then test:
| Object | Predict: light? | Result |
|---|---|---|
| Coin | ||
| Key | ||
| Paperclip | ||
| Pencil — the graphite (sharpened both ends) | ||
| Pencil — the wooden part | ||
| Eraser | ||
| Aluminum foil | ||
| Plastic spoon | ||
| Your finger |
Surprised by any of them? Your body actually does conduct — just not enough for this LED at 5 V. "How much it resists" matters, and that's exactly where we're headed soon.
Stretch
1. Swap the 220 Ω resistor for a 1 kΩ one (brown-black-red) from the spares bin. Brighter or dimmer? What is the resistor really doing to the loop? 2. Grab your spare LED and add it to the loop so both light. Anything you notice about the brightness? (You've just wandered into the next lesson's territory.)
On a scrap of paper, draw today's schematic from memory — power source, resistor, LED, in a loop, symbols only. Label + and −. Hand it in on your way out. (Wobbly lines are fine; correct connections are the point.)