Electronics · Semester 1 — Analog · Lesson 4 · 55 min · build 2026.09.23-1644
Big idea
Every component in a loop takes a share of the supply's voltage — its voltage drop — and the drops always add up to the supply's voltage. The meter turns that invisible bookkeeping into numbers you can check.
Kits closed. Draw the plain single-LED loop from memory — supply on the rails, 220 Ω, LED, three lettered nodes. Then open the box: lesson 3's circuit is standing there, and your job is surgery, not construction. Reduce it to exactly your drawing — pull every part the drawing doesn't show (the whole second branch, any switch, the blue budget-breaker if it stayed in), closing each gap you create — and say the node letters as you confirm what remains. Five minutes. Leave it standing; it's today's laboratory.
New dial position: DC volts — the V with straight lines, not the wavy ~. Volts get measured on a live, powered circuit: two probes, two points, like this:
One habit from the very first reading: red probe on the side nearer the supply's +, black on the side nearer −. Backwards hurts nothing — the reading just comes out negative; swap the probes and carry on. (Today's tables want the plain, no-minus numbers.)
Meter rules
Volts: circuit powered ON, probes touch two nodes — measuring across is safe and normal. Ohms & continuity: power OFF — and for ohms, the part out of the circuit. The current jacks (mA and 10 A): off-limits — leave the red probe in the V/Ω jack all year. (Teachers will demo current measurement once; ask us why it kills meters — or read the Multimeter card in the extras.)
Read the unit, not just the number
The meter picks its own scale — and announces which one, in small letters at the edge of the display: V or mV (thousandths of a volt) for voltage; Ω, kΩ (thousands), or MΩ (millions) for resistance. A display saying 3.2 means nothing by itself: 3.2 V is an LED drop, 3.2 mV is a thousand times smaller — basically zero. Glance at the unit before you write any number down.
Power your warm-up loop and measure — predict before each reading, and swap who holds the probes each time so both of you get the practice:
| Measurement (probes across…) | Predict | Reading |
|---|---|---|
| The supply — + rail to − rail | ||
| The resistor (node A to node B) | ||
| The LED (node B to node C) | ||
| Resistor drop + LED drop = |
Try to break the rule
Power off, swap in a different LED color, and remeasure both drops. Both numbers move — the new LED claims a different share, and the resistor's share shifts to match — but the sum refuses to budge from the supply's own reading. Nothing you can build on this board will break this rule; engineers bet their designs on it every single day, and its fancy name is in the vocab list. Done? Power off and put the red LED back — the rest of today counts on it.
Because voltage isn't at a place — it's a difference between two places. Asking "what's the voltage of node B?" is like asking "what's the distance of Chicago?" — distance to where? Every voltage is a between. That's why the measurement is called across, and why the meter can't do it with one probe.
Back in lesson 3, red + blue in series went dim, or dark, and we said their two drops broke the supply's 5-volt budget. Today you can audit the books. Power off, make sure the red LED is the one in your loop, and grow it — blue LED in after the red (four rows: power → resistor → red LED → blue LED → back).
Predict
The pair is dim — maybe dark — but the supply still pushes 5 V. Where is that voltage? What will the meter read across the resistor? Across each LED?
Now measure all three drops. The headline is the resistor: its drop collapsed — compare the ~3 V it claimed in your working loop. A resistor only develops a drop when current flows through it, so for a given resistor its reading is a current report: smaller drop across your 220 Ω, less current through it. How far it fell depends on your pair. A dim glow: a few tenths of a volt — a small fraction of the current you had. Dark: a few mV — millivolts, thousandths of a volt — small, but measurable, and the meter can tell small from nothing. Either way the two LEDs claim almost the entire 5 V between them and leave the resistor next to nothing to push with. Nothing is broken; much less is flowing. The meter can also tell a trickle from a break: a resistor reading a flat 0 V, with the whole 5 V showing up across one spot, is the #1 clue that the loop is open there. (That's the "drop check" on your Debugging Loop card.)
Back to the beep's dial position from lesson 2 — it wakes up in plain Ω, resistance, so this time there is no Select press at all. Rules: power off, part out of the circuit, probes on its two legs (fingers off the metal — you're a resistor too, and you'll pollute the reading). And watch the display's unit: the same digits with Ω, kΩ, or MΩ after them are three wildly different resistors.
| Color | Digit | Color | Digit |
|---|---|---|---|
| black | 0 | green | 5 |
| brown | 1 | blue | 6 |
| red | 2 | violet | 7 |
| orange | 3 | gray | 8 |
| yellow | 4 | white | 9 |
New this lesson: no steps. Just a schematic with a hole in it and a mission. You've got this.
First, power off and put the loop back to a single red LED — the blue one comes out and the gap closes — and turn the dial back to DC volts (§7 left it on Ω). The schematic is that same loop, but now it says R = ▢. Mission: pick a resistor from your assortment so the LED is clearly dimmer than with 220 Ω, but still definitely lit. Predict which one will do it, build it, then measure both drops and write them on the schematic's dotted lines. What did the resistor's drop do as its ohms went up?
After you've measured: the resistor's drop barely moved — if anything it crept up a hair. It still reads about 3 V whichever resistor you chose, because the LED still takes its ~2 V and the resistor gets whatever is left. Yet the LED is plainly dimmer, so less current is flowing. Both are true: the same 3 V pushing through more ohms moves less current, and the dimmer LED gives up a little of its share. So a resistor's drop reports current only when you also know its ohms — change the resistor and the report changes scale. (The exact exchange rate is Ohm's law, in the math box below.)
First in line is a stretch build: the split. Two identical resistors in series with the LED — predict how the twins will share before you measure.
| Drop | Predict | Measured |
|---|---|---|
| First 220 Ω (A→B) | ||
| Second 220 Ω (B→C) | ||
| LED (C→D) | ||
| Sum |
Twin resistors split their share in half — as close to it as real parts allow, anyway: ±5% "twins" aren't perfectly identical, so your two readings may sit a few hundredths of a volt apart. File the even split away carefully — next lesson we make it adjustable, and it turns out half of electronics is built on this one trick.
Stretch
1. In the split, replace the second 220 Ω with a 1 kΩ. Predict: which resistor takes the bigger share now? Measure. 2. Rebuild the single-LED loop with the LED before the resistor. Does the resistor have to come first to do its job? Predict, then let the drops settle it. 3. Ohms mode, a probe tip squeezed in each fist: measure yourself. The number is huge — hundreds of thousands of ohms, or millions (watch the display flip to MΩ) — and it's the real answer to lesson 1's "why didn't my finger light the LED": you conduct, but you squeeze the flow down to almost nothing. 4. Draw a fat, dark pencil stripe on paper and put your probes on it. Slide them closer together — what happens to the reading, and why? You just made a resistor. 5. Ohm the mystery bin: measure everything from lesson 1's conductor hunt and rank them by ohms — the beep only told you yes/no; now you get the whole spectrum.
The math, for the curious — always optional
Ohm's law ties today together:
One LED, one 220 Ω — the leftover is 5 − 2 = 3 V:
The split: the same 3 V now pushes through 440 Ω of resistor, so the current halves to ≈ 7 mA — and each resistor's drop is
— exactly what your meter said. Deeper (needs a little algebra): in a series pair, each resistor takes this fraction of the available voltage:
— the voltage divider formula, the star of the next lesson.
Draw today's single-LED loop from memory and label it with your three measured numbers: the supply's voltage, the resistor's drop, the LED's drop. Show they add up.