Electronics · Semester 1 — Analog · Lesson 5 · 55 min · build 2026.09.23-1644
Big idea
Put two resistors in series across the supply and the voltage at the point between them is a predictable fraction of the whole — a voltage divider. Half of electronics is exactly this: making the right voltage appear at a node.
Kits closed. Draw the plain single-LED loop from memory — supply, 220 Ω, LED, three lettered nodes. Then open the box and pare the standing circuit down to your drawing: Challenge A left your chosen resistor in the loop, so the 220 Ω goes back in, and any split leftovers go home to the box. Then, without instructions, measure both voltage drops and show they sum to the supply's voltage. Five minutes: draw, pare, measure, done. That skill is today's foundation.
Take the LED out of the picture entirely. Two resistors in series, straight across the supply:
New convention — voltage at a node
Until now you always measured across a component, two probes on its two sides. From today there's a shortcut: park the black probe in the ground rail and leave it there. Then "the voltage at a node" means what the red probe reads there — every "at" is secretly an "across, measured to ground." Same physics, one wandering probe. And "ground" itself is just the negative rail: the node we choose to call 0 V and measure from. (The name is a leftover from circuits that literally connect to the earth; ours only borrows the word.)
Build it: R1's top leg in a row jumpered from the + rail, R1's bottom leg and R2's top leg sharing the tap row, R2's bottom leg jumpered to ground. Then predict before each measurement:
| R1 (top) | R2 (bottom) | Predict tap voltage | Measured |
|---|---|---|---|
| 1 kΩ | 1 kΩ | ||
| 220 Ω | 1 kΩ | ||
| 1 kΩ | 220 Ω | ||
| 4.7 kΩ | 4.7 kΩ |
The pattern: the bigger resistor takes the bigger share, and the tap reads R2's share — within the gold stripe's few percent. Equal resistors split the supply in half — your own first table row just proved it; now it's a tool. And compare rows one and four: 1 k over 1 k and 4.7 k over 4.7 k read the same 2.5 V. Only the ratio sets the tap voltage — remember that when a 10 kΩ knob shows up in a minute.
Predict
Swap R1 and R2 without changing anything else. What happens to the tap voltage, and why?
It's your divider from section 4, made continuous: the wiper splits the pot's 10 kΩ into a top part and a bottom part, and the knob trades resistance between them. Build the proof:
Predict
Before you turn: what will the meter read at one end? The other end? Exactly half-way?
Now make the knob do something. Here the pot works as a plain adjustable resistor — wiper plus one outer leg — in series with the LED and its 220 Ω bodyguard:
No steps — you've built series loops before. Wire it, then turn the knob: fewer ohms in the loop means more current means brighter (lesson 4's rule, now with a handle on it). Which direction brightens? Predict, then turn.
Two legs vs. three
Use all three legs and the pot is a divider (splits voltage at the wiper). Use the wiper plus one end and it's an adjustable resistor (varies current in a loop). Same part, two jobs — and the schematic tells you which by where the wires go: three connected legs = divider; wire through the wiper and one end = adjustable resistor.
Stretch
1. Build the 3.3: your supply's other switch position outputs 3.3 V — a voltage half the world's chips run on. Choose R1 = ▢ and R2 = ▢ from your assortment so a tap of your own reads as close to 3.3 V as you can get. House rule: before every attempt, write the direction first — "too low → grow R2 (or shrink R1)" — then swap, then measure. Guess-and-shuffle doesn't count. (And a warning: your 3.3 V node is a reading, not a power supply — stretch #3 shows why.) 2. Get the tap to 1.0 V — harder than 3.3. 3. Try to power the LED (with its 220 Ω) from the wiper of the pot-as-divider. The dimming is strange and weak — measure the wiper while it happens and compare against the unloaded sweep. Something about connecting the LED changes the divider itself; the purple box has the word for it. 4. Measure the wiper voltage at five knob positions (off, quarter, half, three-quarter, full) and sketch the curve.
The math, for the curious — always optional
The divider has a formula:
Check it against your table — 220 on top, 1 k on the bottom:
Swap them and the bottom's share shrinks:
The 3.3 stretch inverts it: you want the fraction to come out 0.66, so R2 ≈ 2 × R1 — try 4.7 kΩ over 10 kΩ. Deeper: stretch #3 fails because the LED draws current from the tap, changing the effective R2 — the datasheet word is loading, and it's why real circuits buffer their dividers.
From memory: draw the divider schematic, label the tap and ground — and write down which resistor you'd make bigger to raise the tap voltage.
Cleanup: power off and take the dimmer apart — pot and resistors home to the box, board down to the supply and its bare rails. Lesson 6 opens with a fresh build, and its new part arrives with rules of its own.