Spectacles BLE — Reference Guide

Spectacles can act as a BLE Central (scanner + connector) to communicate bidirectionally with any BLE Peripheral — an Arduino, a game controller, a phone app, or any custom BLE device.

Official docs: Spectacles Home · Experimental APIs (BLE enablement)

Enable BLE: Project Settings → Spectacles → Experimental APIs → ✅ Bluetooth Low Energy

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Core Concepts

Term	Meaning
Central	The device that initiates connections (Spectacles)
Peripheral	The device that advertises and accepts connections (Arduino, phone, etc.)
Service	A logical grouping of related functionality, identified by a UUID
Characteristic	A specific data value within a service; readable, writable, and/or notifiable

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BLE Module API

const bleModule = require('LensStudio:BleModule')

Scanning for peripherals

const TARGET_SERVICE_UUID = '12345678-1234-1234-1234-1234567890ab'

bleModule.startScan([TARGET_SERVICE_UUID], (peripheral: BlePeripheral) => {
  print('Found device: ' + peripheral.name + ' (' + peripheral.id + ')')
  bleModule.stopScan()  // stop immediately — scanning drains battery
  connectToPeripheral(peripheral)
})

Connecting

function connectToPeripheral(peripheral: BlePeripheral): void {
  peripheral.connect((success: boolean, error: string) => {
    if (success) {
      print('Connected to ' + peripheral.name)
      discoverServices(peripheral)
    } else {
      print('Connection failed: ' + error)
    }
  })
}

Discovering services and characteristics

const SERVICE_UUID = '12345678-1234-1234-1234-1234567890ab'
const CHAR_UUID    = 'abcdef01-1234-1234-1234-1234567890ab'

function discoverServices(peripheral: BlePeripheral): void {
  peripheral.discoverServices([SERVICE_UUID], (services: BleService[]) => {
    const service = services.find(s => s.uuid === SERVICE_UUID)
    if (!service) return

    service.discoverCharacteristics([CHAR_UUID], (characteristics: BleCharacteristic[]) => {
      const char = characteristics.find(c => c.uuid === CHAR_UUID)
      if (char) {
        activeCharacteristic = char
        subscribeToNotifications(char)
      }
    })
  })
}

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Reading & Writing Characteristics

Read once

activeCharacteristic.read((data: Uint8Array, error: string) => {
  if (error) { print('Read error: ' + error); return }
  const value = new DataView(data.buffer).getFloat32(0, true) // little-endian float
  print('Sensor value: ' + value)
})

Subscribe to notifications (streaming data)

function subscribeToNotifications(char: BleCharacteristic): void {
  char.setNotifyValue(true, (data: Uint8Array, error: string) => {
    if (error) { print('Notify error: ' + error); return }
    parseIncomingData(data)
  })
}

function parseIncomingData(data: Uint8Array): void {
  if (data.byteLength < 12) {
    print('[BLE] Unexpected packet length: ' + data.byteLength)
    return
  }
  const view = new DataView(data.buffer)
  const roll  = view.getFloat32(0, true)  // bytes 0–3, little-endian
  const pitch = view.getFloat32(4, true)  // bytes 4–7
  const yaw   = view.getFloat32(8, true)  // bytes 8–11
  print(`Roll: ${roll}, Pitch: ${pitch}, Yaw: ${yaw}`)
}

Write to a characteristic

// Send a single byte command
function sendCommand(command: number): void {
  const buffer = new Uint8Array([command])
  activeCharacteristic.write(buffer, true, (success: boolean, error: string) => {
    if (!success) print('Write error: ' + error)
  })
}

// Send a float value
function sendFloat(value: number): void {
  const buffer = new ArrayBuffer(4)
  new DataView(buffer).setFloat32(0, value, true)  // little-endian
  activeCharacteristic.write(new Uint8Array(buffer), false, () => {})
}

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MTU Negotiation

The default BLE MTU is 20 bytes per write. For larger payloads, negotiate a higher MTU first:

peripheral.requestMtu(512, (negotiatedMtu: number, error: string) => {
  if (error) {
    print('MTU negotiation failed: ' + error)
    return
  }
  print('Negotiated MTU: ' + negotiatedMtu + ' bytes')
  // Now you can write payloads up to (negotiatedMtu - 3) bytes at once
  // (3 bytes reserved for ATT overhead)
})

If MTU negotiation isn't possible, split large payloads into 20-byte chunks and send sequentially.

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Arduino Pattern

Arduino side (C++):

#include <ArduinoBLE.h>
#include <Arduino_LSM9DS1.h>  // IMU

BLEService imuService("12345678-...");
BLECharacteristic imuChar("abcdef01-...", BLERead | BLENotify, 12); // 3 floats = 12 bytes

void loop() {
  float roll, pitch, yaw;
  IMU.readGyroscope(roll, pitch, yaw);
  byte data[12];
  memcpy(data,     &roll,  4);
  memcpy(data + 4, &pitch, 4);
  memcpy(data + 8, &yaw,   4);
  imuChar.writeValue(data, 12);
  delay(20); // 50 Hz
}

Lens Studio TypeScript reads the three Euler angles and applies them to a scene object's rotation to mirror the physical board's orientation.

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Game Controller Pattern

function parseControllerInput(data: Uint8Array): void {
  const buttons = data[0]                       // bitmask: bit 0 = A, bit 1 = B, etc.
  const joyX = (data[1] - 128) / 128           // signed normalised [-1, 1]
  const joyY = (data[2] - 128) / 128

  const buttonA = (buttons & 0x01) !== 0
  if (buttonA) onJump()
  moveCharacter(joyX, joyY)
}

Haptic feedback:

function rumble(durationMs: number): void {
  sendCommand(0x01)  // start rumble
  const timeout = this.createEvent('DelayedCallbackEvent')
  timeout.bind(() => sendCommand(0x00))  // stop
  timeout.reset(durationMs / 1000)
}

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Spectacles Mobile Kit

const mobileKit = require('LensStudio:SpectaclesMobileKit')

mobileKit.onMessage.add((message: string) => {
  const data = JSON.parse(message)
  print('From phone: ' + JSON.stringify(data))
})

mobileKit.sendMessage(JSON.stringify({ event: 'scoreUpdate', score: 42 }))

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Connection Lifecycle

let retryCount = 0
const MAX_RETRIES = 5

peripheral.onDisconnected.add(() => {
  print('Device disconnected — retrying in 3s')
  hideConnectedUI()
  if (retryCount >= MAX_RETRIES) {
    print('[BLE] Max retries reached. Stopping reconnect.')
    return
  }
  retryCount++
  const retry = this.createEvent('DelayedCallbackEvent')
  retry.bind(() => connectToPeripheral(peripheral))
  retry.reset(3)
})

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Common Gotchas

• Enable via: Project Settings → Spectacles → Experimental APIs → Bluetooth Low Energy. Lenses using Experimental APIs cannot be published to a wider audience — BLE is development and sideloading only.
• Scan drains battery — always call stopScan() once you find your target device.
• Service UUID must match exactly between lens and peripheral (case-insensitive, dashes required).
• Data endianness: Arduino's memcpy and DataView.setFloat32 must agree on byte order (true = little-endian on most MCUs).
• Always check packet length before reading with DataView — a peripheral sending fewer bytes than expected causes silent out-of-bounds reads.
• Default MTU is 20 bytes — negotiate with requestMtu() for larger payloads; expect up to 3 bytes of ATT overhead.
• Cap the reconnect loop to avoid infinite retries against a device that keeps dropping (e.g., a spoofed peripheral).
• iOS background mode: if building a companion mobile app, enable CoreBluetooth background mode in the iOS app's entitlements.

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Reference Examples

• BLEArduino.ts - Shows scanning, connecting, and reading/writing GATT UUIDs from a generic embedded device.