Hey there! Sounds like you're already halfway there with connecting to your BLE weight scale—great job on getting the service and characteristic sorted out. Let's walk through exactly how to send that command and pull the weight reading.

Step 1: Confirm the writable characteristic #

First up, you need to make sure the characteristic you're targeting supports write operations. BLE characteristics have properties that define what you can do with them, so check if yours has either PROPERTY_WRITE or PROPERTY_WRITE_NO_RESPONSE set. You can do this with a quick check:

BluetoothGattCharacteristic targetCharac = ...; // Your pre-found characteristic
int characProperties = targetCharac.getProperties();
if ((characProperties & BluetoothGattCharacteristic.PROPERTY_WRITE) != 0 || 
    (characProperties & BluetoothGattCharacteristic.PROPERTY_WRITE_NO_RESPONSE) != 0) {
    // This characteristic accepts writes—perfect!
} else {
    // You might have picked the wrong characteristic. Double-check the official docs for the correct write command UUID.
}

Step 2: Build your command byte array #

The official docs mention a specific command to trigger the weight reading—you'll need to convert that command into a byte[] that Android's BLE API can send. For example, if the docs say to send the hex command 0x01 0x03 0x00, you'd create it like this:

byte[] weightRequestCommand = new byte[]{0x01, 0x03, 0x00};

Stick strictly to the format in the docs—BLE devices are super picky about missing bytes or wrong order.

Step 3: Send the command to the scale #

BLE writes are asynchronous, so you'll need to handle the result in your BluetoothGattCallback. Here's how to trigger the write:

// Set the characteristic's value to your command
targetCharac.setValue(weightRequestCommand);
// Send the write—use WRITE_TYPE_DEFAULT if you need a confirmation response, WRITE_TYPE_NO_RESPONSE if not
boolean writeSuccess = gatt.writeCharacteristic(targetCharac);

Then, override onCharacteristicWrite in your callback to confirm the command went through:

@Override
public void onCharacteristicWrite(BluetoothGatt gatt, BluetoothGattCharacteristic characteristic, int status) {
    super.onCharacteristicWrite(gatt, characteristic, status);
    if (status == BluetoothGatt.GATT_SUCCESS) {
        // Command sent successfully! Now wait for the weight data to come in.
    } else {
        // Something went wrong—check if the device is still connected, or try re-sending the command.
    }
}

Step 4: Listen for the weight response #

Most BLE weight scales send weight data as a notification after receiving your command (some might require an explicit read, but notifications are more common). To set this up:

1. Find the characteristic meant for weight notifications (check the docs for its UUID).
2. Enable notifications for that characteristic:

BluetoothGattCharacteristic weightNotifyCharac = ...; // The notification characteristic from docs
gatt.setCharacteristicNotification(weightNotifyCharac, true);
// Set up the Client Characteristic Configuration descriptor to enable notifications
BluetoothGattDescriptor descriptor = weightNotifyCharac.getDescriptor(UUID.fromString("00002902-0000-1000-8000-00805f9b34fb"));
descriptor.setValue(BluetoothGattDescriptor.ENABLE_NOTIFICATION_VALUE);
gatt.writeDescriptor(descriptor);

3. Override onCharacteristicChanged in your callback to catch the incoming data:

@Override
public void onCharacteristicChanged(BluetoothGatt gatt, BluetoothGattCharacteristic characteristic) {
    super.onCharacteristicChanged(gatt, characteristic);
    byte[] rawWeightData = characteristic.getValue();
    // Parse this data using the official docs! Example:
    // If docs say weight is in bytes 1-2, big-endian, multiplied by 0.1kg
    int rawValue = ((rawWeightData[1] & 0xFF) << 8) | (rawWeightData[2] & 0xFF);
    float weightInKg = rawValue * 0.1f;
    // Update your UI or process the weight value here
}

Pro Tips to Avoid Headaches #

• Stay off the UI thread: All BLE operations (connecting, writing, reading) should run on a background thread (like a HandlerThread)—Android will throw errors if you do this work on the main thread.
• Verify connection state: Always check that the device is in BluetoothGatt.STATE_CONNECTED before sending commands.
• Follow the protocol exactly: Every scale has its own data format. The docs should tell you:
  • Which bytes represent the weight
  • Byte order (big-endian vs little-endian)
  • Multipliers (e.g., 0.1kg per unit)
  • Any offset values (e.g., subtract 1000 to get the actual weight)

If you hit snags (like failed writes or no notifications), double-check your UUIDs and command format against the docs—those are the most common culprits.

内容的提问来源于stack exchange，提问作者Mallik J