does kind of work

door-thing-app/src/App.tsx

@@ -1,8 +1,16 @@
 import { useState } from "preact/hooks";
 
+const doorServiceUUID: BluetoothServiceUUID = 0x1811;
+const doorSwitchUUID: BluetoothCharacteristicUUID = 0x2ae2;
+
 export function App() {
   const [connecting, setConnecting] = useState(false);
-  const [device, setDevice] = useState<BluetoothDevice>(null);
+  const [device, setDevice] = useState<{
+    device: BluetoothDevice;
+    service: BluetoothRemoteGATTService;
+    characteristic: BluetoothRemoteGATTCharacteristic;
+  } | null>(null);
+  const [doorOpen, setDoorOpen] = useState<boolean | null>(null);
 
   const onConnectClick = async () => {
     try {
@@ -13,12 +21,30 @@ export function App() {
             namePrefix: "Nano",
           },
         ],
+        optionalServices: [doorServiceUUID],
       });
       console.log("Found device:");
       console.log(foundDevice);
-      setDevice(foundDevice);
       await foundDevice.gatt.connect();
       foundDevice.ongattserverdisconnected = onDisconnect;
+
+      const doorService = await foundDevice.gatt.getPrimaryService(
+        doorServiceUUID,
+      );
+
+      const doorSwitch = await doorService.getCharacteristic(doorSwitchUUID);
+      setDevice({
+        device: foundDevice,
+        service: doorService,
+        characteristic: doorSwitch,
+      });
+
+      const data = await (await doorSwitch.readValue()).getUint8(0);
+
+      doorSwitch.oncharacteristicvaluechanged = onDoorUpdate;
+      doorSwitch.startNotifications();
+
+      setDoorOpen(Boolean(data));
     } catch (error) {
       console.log("Error connecting:");
       console.log(error);
@@ -27,13 +53,21 @@ export function App() {
     }
   };
 
+  const onDoorUpdate = async (ev: Event) => {
+    const characteristic = ev.target as BluetoothRemoteGATTCharacteristic;
+    const parsed = Boolean(await characteristic.value.getUint8(0));
+    setDoorOpen(parsed);
+    new Notification("Door update", { body: parsed ? "Open" : "Closed" });
+  };
+
   const onDisconnect = async () => {
     setDevice(null);
   };
 
   const onDisconnectClick = async () => {
     try {
-      device.gatt.disconnect();
+      await device.characteristic.stopNotifications();
+      device.device.gatt.disconnect();
     } catch (error) {
       console.log("Error disconnecting:");
       console.log(error);
@@ -50,6 +84,7 @@ export function App() {
         <>
           <span>connected</span>
           <button onClick={onDisconnectClick}>Disconnect</button>
+          <span>{doorOpen ? "open" : "closed"}</span>
         </>
       )}
     </div>

door-thing-firmware/src/main.cpp

@@ -4,10 +4,10 @@
 // Door switch pin
 #define SWITCH_PIN 2
 
-BLEService switchService("180A"); // BLE Switch Service
+BLEService switchService("1811"); // BLE Switch Service
 
 // BLE Switch Characteristic - custom 128-bit UUID, read by central
-BLEByteCharacteristic switchCharacteristic("1234", BLERead | BLEIndicate);
+BLEByteCharacteristic switchCharacteristic("2AE2", BLERead | BLEIndicate);
 
 void setup()
 {