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.