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+# BikeAudio — Small Build (the buildable v1 target)
+
+The focused, buildable unit. Beast (6/12 broadcasters) is deferred — this drops
+all the hard scaling parts. See [Hardware Design v0.1] for the full picture and
+the detailed [Power and Battery] / [Carrier PCB and BOM] / [Stack Connector and
+Addressing] / [LED and Music Sync] pages.
+
+## Config
+**5 boards:** 1 HUD + 1 SINK + **3 BROADCASTERS** (channels: Headset, Speaker 1, Guest).
+- SINK: iPhone A2DP in -> I2S master.
+- 3 BROADCASTERS: I2S -> A2DP to each speaker (I2C 0x11 / 0x10 / 0x12).
+- HUD: ESP32-S3 round LCD, control over I2C.
+All audio boards = ESP32-WROOM-32E (one carrier PCB, SINK vs BROADCASTER assembly).
+
+## Runtime target: ~8 h (start)
+Load ~2.9 W @ 5 V (5 boards). At ~61% usable derate, 8 h needs ~38 Wh nameplate.
+
+**Battery: 2S2P 18650 (4 cells, ~50.4 Wh)** -> ~10 h with margin (comfortably clears 8 h).
+- ~225 g of cells; compact. (Upgrade path to 2S3P/6 cells later for >12 h with no design change.)
+- Charger: TI **BQ25887** (2S, I2C, balancing, ~2 A) over USB-C 5 V — fine at this pack size (no USB-PD needed for the small build).
+- Reg: single **TPS563201 (3 A)** buck to the 5 V bus. NTC on pack -> charger (JEITA). Reverse-polarity P-FET + TVS + fuse; soft-latch power switch.
+
+## What the small build SIMPLIFIES (vs beast)
+- **No backplane, no I2C mux** — a single vertical 5-board stack on the 26-pin 2.54 mm pass-through connector.
+- **I2S:** one buffer at the SINK (e.g. 74LVC125) + 22-33 R series term, fanning to just **3** broadcaster inputs — short, easy.
+- **I2C:** 100 kHz, a single **2.2-4.7 kOhm** pull-up pair on the SINK. ~5 nodes -> well under the cap budget.
+- **Addressing:** **4-bit solder straps** (GPIO13/14/27/26) — only 3 broadcasters, so no auto-enumeration needed. Headset=0x11, Speaker1=0x10, Guest=0x12.
+- **Power:** single 2S buck + a basic 2 A USB-C charger; no PD, no high-current multi-phase.
+
+## Small-build BOM (rough)
+| Block | Parts | ~$ |
+|---|---|---|
+| 4x carrier PCB (1 sink + 3 bcast) | WROOM-32E + connectors + passives, ~$7 ea | ~28 |
+| HUD | ESP32-S3-Touch-LCD-1.28 | ~15 |
+| Battery | 4x INR18650-35E + 2S2P holder/weld | ~35 |
+| Power electronics | BQ25887 + TPS563201 + USB-C + BMS + fuse/TVS/switch | ~20 |
+| Misc | I2S buffer (74LVC125), stacking connectors, standoffs, enclosure | ~15 |
+| **Total (excl. speakers)** | | **~$113** |
+
+## Wiring (single stack)
+- **I2S:** SINK GPIO5/25/23 (BCK/WS/DATA) -> buffer -> bus -> each broadcaster GPIO19/18/22. Common GND.
+- **I2C:** HUD GPIO15/16 + each board GPIO32/33 on the shared bus; one pull-up pair at the SINK.
+- **Power:** 2S pack -> buck -> 5 V bus up the stack; each board's onboard LDO makes 3V3.
+- **Antenna:** still mind WROOM antenna keep-out / orientation even at 4 boards (overhang + rotate). Lower risk than beast but real.
+
+## Next engineering step
+Hand this + the detailed pages to a **KiCad layout** of the single carrier PCB (45x45 mm, 4-layer, antenna keep-out), then fab 4 + assemble. LEDs (APA102) are an optional later add per [LED and Music Sync].