🔴 Three Sisters

What you're building

A row of three red lamps across your breadboard, all lit at full First-Light brightness the moment you flip one slide switch. Flip it off: all three go dark together. Hold a finger over any one lamp — the other two keep glowing exactly as before. The lanes are side by side, but they don't talk to each other.

This is what a bank of indicator lights looks like inside anything you own: dashboards, charger status rows, equaliser bars on a stereo. The topology is: one master switch, three independent lanes, each lane complete in itself.

The big idea: parallel lanes each do their own math

Twin Reds taught you that LEDs in series share the loop current — one LED's physics changes the other's. This project is the opposite move: three LEDs in parallel, each sitting on its own branch, each wired straight back to the battery through its own resistor.

Because every lane is a complete loop from + to − on its own, every lane runs the First Light calculation independently:

I = (V_battery − V_LED) ÷ R = (4.5 − 2.0) ÷ 220 ≈ 0.011 A = 11 mA

Lane A doesn't know lane B exists. Lane B doesn't know lane C exists. All three carry 11 mA when the master is on, same as a single First Light.

Do the math — three lanes, one battery

Per-lane current (identical for all three):

• Battery push: 4.5 V
• LED eats: ~2.0 V (red forward voltage)
• Leftover for the 220 Ω: 4.5 − 2.0 = 2.5 V
• Lane current: I = V ÷ R = 2.5 ÷ 220 ≈ 11 mA

Battery total (what the pack has to supply):

• Three identical lanes, each pulling 11 mA
• I_total = 3 × 11 = 33 mA

Think of the battery like a water tap and the three lanes like three garden hoses off a splitter. Each hose runs at its own flow rate, set by its own nozzle (the 220 Ω). The tap just has to supply the sum of all three flows. Add a fourth hose — the tap supplies four flows. Add ten — ten.

Your 3×AA pack is rated for roughly 1 500 mAh at low loads. At 33 mA this bank runs for ≈ 45 hours of continuous light. A single AA has that much energy in it; three of them in the pack has three times. Nothing is close to stressed.

Why "independence" matters

Three real-world things only work because of this trick:

1. One lamp failing doesn't take the others down. Pull any one LED out of the breadboard mid-run. The other two keep glowing at 11 mA, unchanged. The failed lane just goes dark; the healthy lanes don't notice. Compare with Twin Reds: pull one LED from that series chain and both go dark, because they share the current.
2. You can mix brightnesses lane-by-lane without affecting the others. Swap one 220 Ω for a 1 kΩ and that lane goes dim (2.5 mA). The other two still carry 11 mA. The resistor on a parallel lane controls only its own lane — nothing bleeds across.
3. Lanes can be different colours or output types. Replace the red at lane C with a yellow — it still runs at 11 mA because yellow's forward voltage is also about 2.0 V. The math is lane-local. (A green LED needs ~2.1 V, so its lane runs at ~11.3 mA — tiny difference, same formula.)

Independent-parallel lanes are the fundamental building block of anything that drives multiple outputs at once. Every project from here that lights more than one thing uses this pattern.

Build it

1. Power column. Battery + at the top of column 1 of your breadboard, − at the bottom. Drop a slide switch into column 1 at the very top — this is your master slide.
2. Top rail. A jumper fans out along the top row from the master switch's output, reaching across six columns. This row carries + to every lane. (On a real breadboard, the top power rail runs the whole length — you just need a wire from the master to that rail.)
3. Lane A (col 2). From the top rail, drop a 220 Ω straight down. Below the resistor, insert a red LED, long leg at the top (toward +, matching the resistor's output). Short leg down to the bottom rail.
4. Lane B (col 4). Repeat exactly: top rail → 220 Ω → red LED (long leg up) → bottom rail.
5. Lane C (col 6). Repeat again: top rail → 220 Ω → red LED (long leg up) → bottom rail.
6. Bottom rail. A jumper from each LED's short-leg hole back to the breadboard's − power rail, and from that rail to the battery − terminal.
7. Flip the master slide ON. All three LEDs light at full brightness, same as First Light. Flip OFF: all three dark.

Troubleshoot by lane: if one LED is dark and the others are fine, the fault is in that lane only — almost always a flipped LED. If all three are dark, the master slide is off, the battery pack switch is off, or the + rail isn't fanned out past the first lane.

Where you'll see this

• LED strings in series/parallel combinations. Decorative string lights are a long parallel chain of short series groups — each 2-or-3-LED group has its own tiny resistor, and the groups hang in parallel across the mains supply. If one group fails open, the others stay on; if your string is "when one goes out they ALL go out", it's a straight series string, not a parallel one.
• Charge-level bar graphs. Battery meters, VU bars on an audio mixer, mode indicators on an amp — a horizontal row of 5–10 LEDs, each driven from its own transistor and its own resistor. Lane independence is why one meter LED can go bright while its neighbour is still dark.
• Pilot lights on power strips with per-outlet switches. Each outlet gets its own switch and its own lamp. Flipping one does nothing to the others. Same topology as this build, scaled to mains voltage with a different lamp.

Learn parallel independence once, see it everywhere.