Electronics · Semester 1 — Analog · Lesson 2 · 55 min · build 2026.09.23-1644
Big idea
The board makes nodes permanent, the meter makes them provable — and the switch gives you the loop's first control: a break you command.
Safety — same two rules
No shorts (red to black with nothing in between = a finger-burning wire and a knocked-out supply) and no LED without its resistor — a naked LED can pop like a firecracker. You'll be rearranging circuits all class today, so after every change, trace each path from the + rail to the − rail and check it still goes through the resistor before you reconnect power.
Last lesson your fingers held every node together — and fingers cramp. In your kit today is the cure: the breadboard. Every row of five holes is joined by a hidden spring-metal strip: push a lead into any hole and the strip grips it, and everything in the same row is electrically connected. One row, one node. The whole board is just rows — plus a trench down the middle that rows never cross, and two striped side columns — the rails, full-length nodes that carry power the whole length of the board. Your supply plugs onto the board's end and feeds them: red stripe = +5 V, blue stripe = −, also called ground.
The #1 mistake
A component's two legs must land in different rows. Both legs in one row = both ends on the same node — the strip short-circuits the component, as if it weren't there. A bypassed resistor is how LEDs pop (safety rule 2).
Retrieval first — 90 seconds, no peeking: sketch last lesson's schematic from memory on scrap paper. Three parts, three lettered nodes, the LED's arrow pointing the right way. Then check it against the drawing below.
First, retire lesson 1's finger leads: pull them off the supply's pins — their job is done. The supply itself now rides the end of the board, feeding the rails: + to the red stripe, − to the blue. Build last lesson's circuit — same schematic, same three nodes, one row per node — with jumpers carrying power from the rails. Say each node's letter out loud as you plug it in:
Dead circuit? You know the drill
Debugging Loop card, top to bottom → check your build against the schematic → ask a neighbor → then a teacher, findings first. This applies to every build today.
You've taken our word about the rows and the rails — but in this room, nobody takes anyone's word about wiring. The interrogation tool is a multimeter: it measures the invisible. Today you'll use its simplest mode: continuity — the beep. On our meters it shares a dial position with Ω: turn the dial to the position with the Ω and sound symbols, then press Select once — you're there when the sound symbol shows on the display. Touch the two probe tips to two points, and:
Meter rule #1
Continuity mode is for unpowered circuits only. Switch the supply off — or slide it off the board — before beep-testing; a powered circuit confuses the meter and can damage it.
Supply off. For each pair of points below, predict first, then touch the probes and listen:
The probes won't fit the holes
Meter tips are fatter than breadboard holes — don't force one in, or the spring clip underneath never grips right again. Plug a short wire (or a spare component leg) into the row and touch the probe tip to that instead: everything in a row is one node, so probing the wire is probing the row. The Making Wires card at the wire station shows how to cut and strip your own.
| Probe these two points | Predict: beep? | Result |
|---|---|---|
| Two holes in the same 5-hole row | ||
| Two holes in the same column, different rows | ||
| Same row number, across the trench in the middle | ||
| Top and bottom hole of one striped edge column | ||
| The red-striped column and the blue-striped column | ||
| The two ends of one jumper wire from your new bundle |
The wire hospital
That last row matters: a wire can be broken inside its insulation — looks fine, conducts nothing. Beep-test every jumper in your new bundle end to end. Somewhere at your table there's a dead one. Find it, and hand it in for a healthy replacement. From now on, when a circuit fails, your wires are innocent — you tested them. The copper is proven; only a loose seating can still betray it, and that's Loop step 4, not a bad wire.
If a rail goes quiet halfway
Some breadboards split their rails in the middle — top half and bottom half are two separate nodes. (It's a feature, for circuits with two supplies; today it's a trap.) If your top-to-bottom rail test stays silent: bridge the middle of the rail with a jumper, beep again, and you've rejoined it into one node. Leave the bridge in — future-you will forget.
The math, for the curious — always optional
The beep is secretly a resistance measurement. The meter pushes a tiny test current through the probes and beeps if it flows almost freely — resistance near 0 Ω. A wire is ≈ 0 Ω, so it beeps. Your body, hand-to-hand, is roughly 100,000 Ω — no beep, and also why your finger couldn't light the LED in lesson 1. Air is billions of ohms: the ultimate insulator.
Compare your expedition table to the diagram in section 3 — every claim checked out. (And if your board hides a split rail, you may have just caught it.) Rows are little nodes, rails are giant ones, the trench keeps the banks apart. That's the entire board: trust, replaced by proof.
The convention
From today on: the supply rides the end of the board, + feeding the red-striped rail, − the blue. Every circuit for the rest of the course starts this way — and anyone can look at anyone's board and instantly know where power lives.
The payoff
Plug a jumper into the + rail at the far end of the board, as far from the supply as you can get. Supply off, beep-test its free end against node A's row. Same node. The rail delivers power anywhere on the board, no matter how big a circuit grows.
Predict
If a board's rails are split in the middle, what happens to a circuit powered from the top half but built in the bottom half?
Start from your warm-up circuit. Seat the switch across three fresh rows (say 24-25-26 — its legs are close together, one row each). Then rewire the loop's tail end:
Predict
The switch is "after" the LED in the loop. Would it still work between the power supply and the resistor instead? At the top of the loop?
Stretch
1. Move the switch somewhere else in the loop — before the resistor, at the very top. Last lesson you proved a break anywhere stops everything; a switch is just a break you control. 2. Map the switch like you mapped the board: supply off, beep across each pair of its three legs, flip it, beep again. Which legs connect in which position? You just read the switch's secret wiring.
On scrap paper: (1) sketch one 5-hole row and its hidden metal strip; (2) from memory, draw Build A — the full switched loop, nodes lettered — and mark the two nodes — C and D — that were one node before the switch moved in. Hand it in at the door.
Cleanup: if the switch experiments left Build A rearranged, restore it exactly as the schematic shows. Then power off and close the box with the circuit still standing — lesson 3 starts by checking it, then grows it.