Little Relay

An Android application that acts as a bidirectional bridge between BLE devices and an MQTT broker, supporting both Central and Peripheral roles.

BLE ↔ MQTT Bridge for Android

An Android application that acts as a bidirectional data bridge between Bluetooth Low Energy (BLE) devices and an MQTT broker.

The app supports both BLE roles:

• Peripheral (GATT Server)
• Central (GATT Client)

This allows BLE devices to publish data to MQTT topics and receive MQTT data back over BLE, with the Android device acting as the intermediary.

Architecture Overview

+-------------+       BLE        +----------------+       MQTT       +--------------+
| BLE Device  | <--------------> | Android Bridge | <--------------> | MQTT Broker |
+-------------+                  +----------------+                  +--------------+

The Android app:

• Translates BLE writes & notifications into MQTT messages
• Translates MQTT messages into BLE notifications or writes

BLE Roles

The app can operate in one BLE role at a time.

Peripheral Role (GATT Server)

In Peripheral Mode, the Android app advertises itself and allows other BLE devices to connect to it.

graph LR
    BLE[BLE Device]
    APP[Android App]
    MQTT[MQTT Broker]

    BLE -->|WRITE or WRITE_NO_RESPONSE| APP
    APP -->|Publish| MQTT

    MQTT -->|Subscribed Data| APP
    APP -->|NOTIFY| BLE

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Advertised BLE Service

Service UUID

d2dfc1a2-9f53-494b-a2a1-1efd6148aa81

Characteristics

Purpose	UUID	Properties
MQTT → BLE	007a7899-dc76-4778-b33b-3460bf250eed	NOTIFY
BLE → MQTT	eeb6d18b-863b-4fb5-81b6-a38902b985df	WRITE, WRITE_NO_RESPONSE

Data Flow (Peripheral Role)

BLE Device → MQTT Broker

1. Remote BLE device connects to the app
2. Remote BLE device writes data to following characteristic exposed by the app

eeb6d18b-863b-4fb5-81b6-a38902b985df

3. App publishes the received payload to the configured MQTT topic(s)

✔ Supported write types:

• WRITE
• WRITE_NO_RESPONSE

MQTT Broker → BLE Device

1. Remote BLE device subscribes to notifications on:

007a7899-dc76-4778-b33b-3460bf250eed

2. App subscribes to MQTT topic(s)

3. When MQTT data arrives:

   • App sends it via BLE Notification
   • Device receives data instantly

ℹ️ Indications are not supported (for now) , only Notifications.

Peripheral Role Summary

Direction	Mechanism
BLE → MQTT	Write to writable characteristic
MQTT → BLE	Notification on notifiable characteristic

Central Role (GATT Client)

In Central Mode, the Android app scans for nearby BLE devices and connects to them.

This mode is ideal when:

• BLE devices already advertise services
• Android initiates and manages connections

graph LR
    BLE[BLE Device]
    APP[Android App]
    MQTT[MQTT Broker]

    BLE -->|NOTIFY or INDICATE| APP
    APP -->|Publish| MQTT

    MQTT -->|Subscribed Data| APP
    APP -->|WRITE_NO_RESPONSE| BLE

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Device Requirements (Central Role)

BLE Device → MQTT Broker

The BLE device must expose at least one characteristic with:

• NOTIFY or
• INDICATE

Once found, the app UI allows enabling notifications/indications.

When data arrives:

• App publishes it to the configured MQTT topic(s)

MQTT Broker → BLE Device

The BLE device must expose a characteristic supporting:

• WRITE_NO_RESPONSE

When MQTT data arrives:

• App writes the payload to the selected characteristic

Central Role Summary

Direction	Requirement
BLE → MQTT	Characteristic with NOTIFY or INDICATE
MQTT → BLE	Characteristic with WRITE_NO_RESPONSE

MQTT Configuration

The app allows configuring:

• Broker URL
• Port
• Credentials (if required)
• Publish topics
• Subscribe topics

Topic Mapping

Source	Destination
BLE Write / Notification	MQTT Publish
MQTT Subscribe	BLE Notify / Write

Payload Handling

• Payloads are forwarded as bytes
• No transformation is applied

Android 11 and Lower – Location Permission Requirement

On devices running Android 11 (API level 30) or lower, the system requires location permission to perform Bluetooth Low Energy (BLE) scans. This is an Android platform restriction, as BLE scanning can potentially be used to infer a user’s location. To use the app in Central role and scan for nearby BLE devices, you must grant the appropriate location permission (typically ACCESS_FINE_LOCATION) and ensure that location services are turned ON on the device. If location permission is denied or location services are disabled, BLE scanning will not return any results, even if Bluetooth is enabled.