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Add 3-board relay wiring diagram

blue — ESP32/PlatformIO firmware 2026-06-10 12:10:04 -04:00
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3-Board Relay Wiring.-.md Normal file

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# 3-Board Bluetooth Relay — Wiring
Relays iPhone audio to **two** Bluetooth speakers (JBL Charge 5 + Cardo "Tangerine EDGE") at once.
**Board A** (the A2DP sink — the board the iPhone connects to) is the I2S **master** and drives three signals. **Boards B and C** (the A2DP sources) both **listen** to those same three signals in parallel. Plus one shared ground.
## Pin-to-pin
| Signal | Board A (SINK) out | Board B (JBL) in | Board C (Cardo) in |
|---|---|---|---|
| Bit clock (BCK) | **GPIO5** | GPIO19 | GPIO19 |
| Word select (WS/LRCK)| **GPIO25** | GPIO18 | GPIO18 |
| Data | **GPIO23** | GPIO22 | GPIO22 |
| Ground | **GND** | GND | GND |
Each of A's three pins splits to **both** B and C:
- A·**GPIO5** -> B·GPIO19 **and** C·GPIO19
- A·**GPIO25** -> B·GPIO18 **and** C·GPIO18
- A·**GPIO23** -> B·GPIO22 **and** C·GPIO22
- A·**GND** -> B·GND **and** C·GND
## Diagram
```
BOARD A (SINK / I2S MASTER) iPhone connects here
+---------------------------+
| GPIO5 (BCK) o------------+----------+-------------+
| GPIO25 (WS) o----------+ | | |
| GPIO23 (DAT) o--------+ | | | |
| GND o------+ | | | | |
+---------------------|-|-|-|----------+ |
| | | | |
+---------------+ | | +------+ +-------------+ (BCK to C)
| +-------------+ +-----+ | | +--------- (WS to C)
| | +---------------+--+-+---+---+--------- (DAT to C)
v v v v v v v
BOARD B (JBL) BOARD C (Cardo)
+-----------------+ +-----------------+
| GPIO19 (BCK in) | | GPIO19 (BCK in) |
| GPIO18 (WS in) | | GPIO18 (WS in) |
| GPIO22 (DAT in) | | GPIO22 (DAT in) |
| GND | | GND |
+-----------------+ +-----------------+
sends to JBL sends to Cardo
```
## Important
- **Only share GND** between boards — do **not** tie their 3.3V/5V together. Each board runs off its **own USB**.
- The GND link is **mandatory** (common clock reference).
- Keep wires short (< ~20 cm); it's a ~1.4 MHz clock.
- One-way: A **outputs**, B and C **only listen**.
## Power-on order
Bring up **Board A first** so the I2S bus is clocking before B and C read it.
## Limitations
- The two speakers are **not** sample-synchronized (independent BT buffering -> tens of ms skew). Fine for music/intercom; not for tight stereo L/R.
- Audio is fixed at SBC **44.1 kHz / 16-bit / stereo**.
- This relay carries **music (A2DP) only** — no microphone, no phone-call (HFP) audio.