Dial-a-Brightness

If it doesn't work

The boring ways this circuit actually breaks — check these first before you unwire everything.

Knob does nothing — LED stays at full brightness. You wired the pot's two OUTER legs (fixed 10 kΩ). Use the middle (wiper) + ONE outer leg — that's what varies as you turn the knob.

LED dark at every knob position. Flip the LED (long leg toward +), then check the 220 Ω is in the lane, then that the wiper leg (middle) actually feeds the LED.

One side of the knob does nothing. That end is the 0 Ω corner — the pot is basically a wire. The 220 Ω is still in the lane, so the LED still gets ~11 mA and doesn't pop.

LED at 0 Ω end burns way too bright. That means the 220 Ω safety resistor is missing. Put it back — it's what stops full-battery current.

Knob jumps or crackles. Dusty pot. Rotate it end-to-end a few times to clean the wiper — cheap trimmers do this.

What you're building

A dimmer switch for your LED. Turn the knob one way and it glows bright; turn the other way and it fades to almost dark. Somewhere in the middle it's breathing. No chips, no code — just a resistor you can change by hand.

The big idea: the knob IS a resistor

The potentiometer (say "puh-TEN-shee-OH-meh-ter" — most engineers just say pot) is a resistor with a knob on it. Turn the knob and its resistance slides smoothly — from almost zero all the way up to its full size, which for yours is 10 kΩ.

A pot is a resistor you can change.

Now think back to the Brightness Lab: bigger resistor in the loop → less current → dimmer LED. So if you put the pot in series with your LED, twisting the knob twists the brightness.

Do the math

Same battery, same LED, same Ohm's law from First Light — only the resistor changes. The battery gives 4.5 V and the LED "eats" 2.0 V, so the resistor half of the loop always has 2.5 V to spend.

Knob at the low end (pot ≈ 0 Ω): only the 220 Ω safety resistor is left. Current = 2.5 V ÷ 220 Ω ≈ 11 mA. Full-brightness First-Light glow.
Knob in the middle (pot ≈ 5 kΩ): total = 5220 Ω. Current ≈ 0.48 mA. Barely a glow — but you can still see it in a dim room.
Knob at the high end (pot ≈ 10 kΩ): total ≈ 10220 Ω. Current ≈ 0.24 mA. Essentially off.

Because the pot changes smoothly, the brightness changes smoothly too. Lamp dimmer switches use the same basic idea.

Why the 220 Ω is still there

Fair question — the pot already has resistance, right? Yes, but only most of the time. At one extreme of the knob's travel the pot is basically 0 Ω, and without the 220 Ω safety resistor your LED would be across the battery with nothing to hold back the current. LEDs pop in that situation (you've seen this on the Workbench). The 220 Ω stays in the lane as a guaranteed minimum speed bump. Careful engineers call this a defensive part — it costs almost nothing and saves the expensive part next to it.

Meet the three legs

Look at the pot: three legs sticking out, and we only need a two-legged variable resistor. Here's what the third leg does.

Inside, the pot is a strip of resistance material with the two outer legs on the ends and a middle leg that's a slider (called the wiper) touching the strip somewhere between them. Turn the knob and the wiper slides.

Outer leg ↔ outer leg = always the full 10 kΩ. Boring.
Outer leg ↔ middle leg = "how much strip between them right now" — changes as you twist. This is the variable resistor you want.

The other outer leg just dangles. (Level 3 you'll do the divider trick where all three legs finally get a job.)

Build it

Pot pushed into the breadboard, three legs in three different rows.
Battery red (+) → one outer leg of the pot.
Pot's middle leg → 220 Ω → LED long leg.
LED short leg → battery black (−).
Twist the knob and watch the LED breathe.

Tiny reliability tip: short the middle leg to the other outer leg with a small jumper. If the wiper ever loses contact for an instant (dust, an old pot), the LED won't blink off — it'll just hit maximum resistance instead. Real electronics ships with this trick.

Where else you'll see it

Volume knob on any radio or speaker — same part, just tapped for tiny audio signals instead of LED current.
Wall dimmer on a lamp — bigger, and rated for mains power, but the idea is identical.
Game controller stick — an X-pot and a Y-pot, one for each direction the stick moves; the game reads their wiper voltages.

Once you've wired a pot, a whole family of "how much" input devices makes sense: sticks, sliders, and dials.

🧠 Your challenge

No single right answer. That's the point.

Wire a second LED (with its own 220 Ω!) also fed from the pot's middle leg. Do both LEDs dim equally as you turn the knob, or does one fade faster? Why? (Hint: think about the two 220 Ω lanes as sharing the pot's current — same idea as the two lanes in the Tabletop Traffic Light.)
Swap the LED lane for the buzzer you already own. Does the pot change the loudness the way it changed the brightness? (Active buzzers are surprising — try to find the knob position where it goes silent all at once. Report what you hear.)
Wire all three legs: one outer to battery +, the other outer to battery −, middle leg to the LED lane. Predict what the LED does as you turn before you power up. (You just invented the voltage divider, which is Level 3's superpower.)

For grown-ups: safety notes

Battery-safe throughout. Even at the "brightest" end of the knob the LED gets its normal 11 mA — the 220 Ω safety resistor guarantees it.
Pot bodies are metal on many types. If you're building on a metal surface, put a bit of paper under the breadboard so nothing shorts.
If the LED stays fully dark no matter how you turn the knob, the middle leg is probably wired to a row that isn't connected to the wiper — pull that jumper and try the other two legs one at a time until turning the knob changes the brightness.
If the LED stays fully bright no matter how you turn, you probably bridged the two outer legs by mistake — the pot is being bypassed. Trace the loop with your finger.

Dial-a-Brightness

What you need

Circuit map