diff --git a/transmitter/transmitter.ino b/transmitter/transmitter.ino
new file mode 100644
index 0000000..4d14a07
--- /dev/null
+++ b/transmitter/transmitter.ino
@@ -0,0 +1,152 @@
+/*
+ * transmitter.ino  --  PS4 gamepad -> UART bridge (ESP32 "A")
+ *
+ * Board package: esp32_bluepad32  (NOT the plain esp32 package)
+ * Board:         "ESP32 Dev Module" listed under esp32_bluepad32
+ * Libraries:     none (Bluepad32 ships inside the board package)
+ *
+ * Reads a Bluetooth Classic gamepad via Bluepad32 and streams the stick /
+ * button state out of Serial2 as plain ASCII lines. The second ESP32 (see
+ * ../receiver) picks those up and drives the LEGO hubs over BLE.
+ *
+ * Wiring to the receiver board:
+ *   TX GPIO17  ->  RX GPIO16 on receiver
+ *   RX GPIO16  <-  TX GPIO17 on receiver   (unused today, reserved for acks)
+ *   GND        ->  GND                      (mandatory - common ground)
+ *
+ * Line format:  G,<lx>,<ly>,<rx>,<ry>,<buttons>*<XX>\n
+ *   lx/ly/rx/ry : -512..511   (Bluepad32 raw axis range)
+ *   buttons     : 16-bit mask, decimal
+ *   XX          : XOR checksum of everything before the '*', two hex digits
+ *
+ * Created by: Jess Rogerson (yelling commands at Claude.AI)
+ */
+
+#include <Bluepad32.h>
+
+// ---------------------------------------------------------------- settings
+static const int  LINK_TX_PIN      = 17;
+static const int  LINK_RX_PIN      = 16;
+static const long LINK_BAUD        = 115200;
+static const int  STATUS_LED_PIN   = 2;
+static const unsigned long SEND_INTERVAL_MS = 20;   // 50 Hz
+static const unsigned long HEARTBEAT_MS     = 200;  // resend even if idle
+
+// ------------------------------------------------------------------ state
+ControllerPtr myControllers[BP32_MAX_CONTROLLERS];
+
+static unsigned long lastSend      = 0;
+static unsigned long lastHeartbeat = 0;
+static int  lastLx = 0, lastLy = 0, lastRx = 0, lastRy = 0;
+static uint16_t lastButtons = 0;
+
+// -------------------------------------------------------------- callbacks
+void onConnectedController(ControllerPtr ctl) {
+    for (int i = 0; i < BP32_MAX_CONTROLLERS; i++) {
+        if (myControllers[i] == nullptr) {
+            myControllers[i] = ctl;
+            Serial.printf("Controller connected in slot %d\n", i);
+            digitalWrite(STATUS_LED_PIN, HIGH);
+            return;
+        }
+    }
+    Serial.println("Controller connected but no free slot");
+}
+
+void onDisconnectedController(ControllerPtr ctl) {
+    for (int i = 0; i < BP32_MAX_CONTROLLERS; i++) {
+        if (myControllers[i] == ctl) {
+            myControllers[i] = nullptr;
+            Serial.printf("Controller disconnected from slot %d\n", i);
+            digitalWrite(STATUS_LED_PIN, LOW);
+            return;
+        }
+    }
+}
+
+// ----------------------------------------------------------------- helpers
+static uint8_t xorChecksum(const char *s) {
+    uint8_t c = 0;
+    while (*s) c ^= (uint8_t)*s++;
+    return c;
+}
+
+static void sendFrame(int lx, int ly, int rx, int ry, uint16_t buttons) {
+    char payload[64];
+    snprintf(payload, sizeof(payload), "G,%d,%d,%d,%d,%u", lx, ly, rx, ry, buttons);
+    Serial2.printf("%s*%02X\n", payload, xorChecksum(payload));
+}
+
+// Only bother transmitting when something actually moved, or on heartbeat.
+static bool worthSending(int lx, int ly, int rx, int ry, uint16_t buttons) {
+    const int DELTA = 4;   // ignore stick jitter of a few counts
+    if (buttons != lastButtons) return true;
+    if (abs(lx - lastLx) >= DELTA) return true;
+    if (abs(ly - lastLy) >= DELTA) return true;
+    if (abs(rx - lastRx) >= DELTA) return true;
+    if (abs(ry - lastRy) >= DELTA) return true;
+    return (millis() - lastHeartbeat) >= HEARTBEAT_MS;
+}
+
+// -------------------------------------------------------------------- main
+void setup() {
+    Serial.begin(115200);
+    Serial2.begin(LINK_BAUD, SERIAL_8N1, LINK_RX_PIN, LINK_TX_PIN);
+
+    pinMode(STATUS_LED_PIN, OUTPUT);
+    digitalWrite(STATUS_LED_PIN, LOW);
+
+    Serial.printf("Bluepad32 firmware: %s\n", BP32.firmwareVersion());
+
+    BP32.setup(&onConnectedController, &onDisconnectedController);
+
+    // Clear stored pairings so a controller that was last paired to a phone
+    // will bind here. Comment this out once you are happy with the pairing,
+    // otherwise you re-pair on every boot.
+    BP32.forgetBluetoothKeys();
+
+    // We only care about real gamepads, not the mouse/keyboard virtual device.
+    BP32.enableVirtualDevice(false);
+
+    Serial.println("Ready - put the PS4 pad in pairing mode (SHARE + PS)");
+}
+
+void loop() {
+    bool dataUpdated = BP32.update();
+
+    ControllerPtr gp = nullptr;
+    for (int i = 0; i < BP32_MAX_CONTROLLERS; i++) {
+        if (myControllers[i] && myControllers[i]->isConnected() &&
+            myControllers[i]->isGamepad()) {
+            gp = myControllers[i];
+            break;
+        }
+    }
+
+    unsigned long now = millis();
+    if (now - lastSend < SEND_INTERVAL_MS) return;
+    lastSend = now;
+
+    if (gp == nullptr) {
+        // Nothing connected: keep sending neutral so the receiver's failsafe
+        // never has to guess.
+        sendFrame(0, 0, 0, 0, 0);
+        lastHeartbeat = now;
+        lastLx = lastLy = lastRx = lastRy = 0;
+        lastButtons = 0;
+        return;
+    }
+
+    int lx = gp->axisX();
+    int ly = gp->axisY();
+    int rx = gp->axisRX();
+    int ry = gp->axisRY();
+    uint16_t buttons = gp->buttons();
+
+    if (dataUpdated || worthSending(lx, ly, rx, ry, buttons)) {
+        sendFrame(lx, ly, rx, ry, buttons);
+        lastLx = lx; lastLy = ly; lastRx = rx; lastRy = ry;
+        lastButtons = buttons;
+        lastHeartbeat = now;
+    }
+}