House Lighting, full wiring (transistor build)

4 zones · PN2222 low-side switch per zone · Arduino Uno R3 · 5V USB · townhouse_lights.ino

+5V wire / GND signal (PWM → base) LED (warm white) FIRE = red LED · crossing lines with no dot = not connected

Running the wire, shared buses, not home-runs

One +5V wire snakes past every fixture (shared by all zones). Each zone has one return wire back to its transistor. Every LED just bridges the two nearest buses.

+5V bus (shared) GF return EXT return · solid dot = tap/join · line crossing with no dot = not connected

Daisy-chain, don't home-run: tap the passing +5V and zone-return buses at each fixture, don't run separate pairs all the way back to the board.
One +5V for the whole house + one return per zone (4 zones → 4 returns; 6 → 6). That's the total wire count out of the board.
Different zones near each other share the +5V bus but must keep separate return wires, never let two zone returns touch, or they switch together.
Put each 220Ω at the LED (heat-shrink it) so the buses stay clean two-conductor runs. Color-code: red = +5V, a distinct color per return.

Zone	Pin → 1kΩ → base	Transistor	LEDs (each +5V→LED→220Ω→collector)
GF_INT	`D3`	Q1 (PN2222)	2, living + kitchen ceilings
UP_INT	`D5`	Q2	3, 2 bedroom ceilings, attic
EXT	`D6`	Q3	5, porch A/B, entry, deck, balcony
FIRE	`D9`	Q4	1, hearth · RED LED (flickers)
scene button	`D2` → button → GND	,	cycles scenes / wakes
section button	`D4` → button → GND	,	spotlight one zone: GF→UP→EXT→FIRE→off

Build & wiring rules

Every zone's transistor: E→GND rail, C→that zone's LED bundle, base←pin through 1kΩ. Legs (flat face toward you): E · B · C.
Every LED: long leg (+) → +5V, short leg (−) → its own 220Ω → the zone's collector. One resistor per LED.
Add more LEDs to a zone by hanging more of these branches in parallel on the same collector.
FIRE zone uses a RED LED; all other zones warm-white/white. (Color is hardware only, the sketch just sets brightness.)
Two buttons, each to GND (internal pull-ups, no resistor): D2 = scene cycle, D4 = section spotlight (one zone at a time). Either wakes from idle.
Power: USB bank → master switch → diode-OR → +5V rail. Bench-test on computer USB alone. Only ONE 5V source live at a time (the diode-OR makes that safe if both are ever connected).
Common ground everywhere: bank −, Arduino GND, all emitters share the one GND rail.