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David Vassallo's Blog

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Android Linux / Raspberry Pi Bluetooth communication


This post explores my attempt to get bidirectional bluetooth communication going between an android device and a linux computer (in this case a Raspberry Pi running Raspbian). This post is part of the documentation of knowledge acquired while building an “AquaPi” – a raspberry pi that will act as a control center for an aquarium. Here is a video showing a working example:


https://www.youtube.com/watch?v=hvykOO73LbM


Hardware:

Software:

  • On the raspberry pi: setting up and using pybluez

Note: these instructions have also been confirmed to work on an ubuntu workstation with in-built bluetooth support.

First, plugin the bluetooth dongle and fire up the raspberry pi. Next, install the pre-requisites to get bluetooth support on the raspberry pi by running:

sudo apt-get install bluez python-bluez

There are plenty of guides in the internet on how to get bluetooth working, but the only method that worked consistently for me is the following:

1. Disable bluetooth pnat support as there seems to be a bug which stops proper operation with pnat enabled. Full details can be found here: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=690749

A workaround is to add the following to /etc/bluetooth/main.conf:
   DisablePlugins = pnat

Not following the above will give errors similar to:

 (111, 'Connection refused')

Incidentally, the “Steps to Reproduce” section of the bug link are a very good way of testing your bluetooth stack and making sure you have the proper groundwork in place

2. Make note of the following two commands which will be useful when pairing your raspberry pi to the android phone:

sudo hciconfig hci0 piscan [make your device discover-able]
sudo hciconfig hci0 name 'Device Name' [change your device name to something else you fancy]

3. Download the following python script someplace on your raspberry pi. The script is a modified version of the “simple rfcomm server” script found under /usr/share/doc/python-bluez/examples/simple/rfcomm-server.py

Note that as previously stated, this script stems from a build to control an aquarium. So you may need to modify the if/elif/else statements beginning at line 60 to do something more meaningful in your particular case. Simply run the server via:

sudo python /path/to/script.py
  • On android: testing bluetooth and writing an app

Before proceeding, you must make sure to pair your android device to your raspberry pi. To do this, ensure that your raspberry pi is discover able (see step 2 above), and search for the device in the bluetooth settings in your android. After successful pairing, you should be able to continue.

1. Install “BlueTerm” from the playstore (https://play.google.com/store/apps/details?id=es.pymasde.blueterm). This will allow you to test your bluetooth setup up this stage. Your raspberry pi should accept connections and even respond to typed commands

2. Write an android app to establish a bluetooth rfcomm channel and send commands to the raspberry pi as well as read information. Following is a sample main activity from the mobile app written for the video demo:

Notes on the android script:

  • Line 33: note the UUID. This must match the UUID being advertised by the rfcomm server – which is the python script (line 46)
  • Lines 31-51: this is a function which will send information to the raspberry pi (hence the android here is acting as a “client”) . If not already connected, the function creates an RFCOMM socket, gets a writeable stream, and pushes the message through the socket. The message to be sent is passed as an argument to the function
  • Lines 67-141: this is the runnable (thread) which is used to send and receive information over the bluetooth RFCOMM channel. We need to run this in a seperate thread so as to avoid ANR (application not responding) notifications, and hence leaving the GUI responsive. The thread first sends a message using the sendBtMsg function (line 77), and then opens an input stream and waits for response data. The app waits for response data by creating a buffer (of 1024 bytes in this case – line 95). When data is received, the app checks if the delimiter character is present (in this case the delimiter character is the exclamation mark [!] – ascii character 33 – line 27). We implement this pattern so as to be sure that we received the entire intended data stream before displaying the information to the end user. Obviously, you may want to change my choice of using ! as a delimiter to something more suited to your needs.
  • Line 113: here we set a text view to the received data from the raspberry Pi
  • Line 129: if the data is received and displayed to the user, close the bluetooth connection and break out of the listener loop to finish the thread
  • Lines 144-179: here we are simply attaching onClickListeners to buttons on the GUI to send commands to the raspberry pi. These differ only in the string which is sent over the bluetooth RFCOMM connection to the raspberry pi
  • Lines 184-185: here we check if bluetooth is enabled. If not, we fire off an intent to ask the user to allow bluetooth use
  • Lines 188-199: go through the list of paired devices, and find the raspberry pi. Note, you may need to change the name in the code if you didnt use the standard (line 193). Once the raspberry pi is found, assign it as the device to be used (line 196)
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