Monday, September 29, 2014

Gas sensor prototype explained

The "We know RFDuino" contest has not ended yet but its end is sufficiently close so that I can explain our prototype application. Our entry is a Bluetooth Low Energy-connected gas sensor and it is presented in the video below. Make sure that you watch it, you help us win the competition.

The prototype demonstrates a unique capability of Bluetooth Low Energy device advertisement messages: you can embed user data into these broadcasts. These come handy if you just want to send out some measurement data to whoever cares to listen without creating a session between the BLE client and server. This broadcast-type data transfer may support unlimited number of clients with very low energy consumption on the sensor side.

Click here to download the Android client application project.

Click here to download the RFDuino source code.

The prototype works like the following. The microcontroller presented in the video measures the Lower Explosion Limit and sends this value to the RFDuino microcontroller over a super-simpe serial protocol. A message of this protocol looks like this:

0xA5 <seq_no> <LEL%>

where seq_no is an increasing value and LEL% is the measured Lower Explosion Limit value. The microcontroller code is not shared here but you can get the idea. The RFDuino code receives the LEL% value over the serial port it creates on GPIO pins 3 and 4, creates a custom data structure for BLE advertisements consisting of the site ID and the LEL% value then starts advertising. This is performed cyclically so the LEL% value is updated in the sensor's BLE advertisement every second.

Now let's see what happens on the Android side. This is a non-trivial application with multiple activities but the Real Thing (TM) happens in the MapScreenActivity, in the onLeScan method. This method is called every time the Android device's BLE stack discovers a device. In this case we check whether the device's name is "g" (this is how we identify our sensor) and we retrieve the LEL% data from the advertisement packet.  We also handle the Received Signal Strenght Indicator (rssi) value for proximity indication. Bluetooth device discovery is restarted in every 2 seconds so that we can retrieve the latest LEL% value. The rest is just Plain Old Android Programming.

The identification of the sensor and the encoding of the sensor data is obviously very naive but this is not really the point. You can make it as complex as you like, e.g. you can protect the sensor data with a hash and place that hash also into the advertisement so that the receiver can make sure that it gets data from an authorized sensor and not a fake one. The important thing is that the entire framework is sufficiently flexible so that relatively complex functionality can be implemented and RFDuino really simplifies sensor programming a lot.

If you enjoyed the example application, make sure you watch the video (many times if possible :-)) and if you happen to be in London on 2014 November 19, you might as well come to the Londroid meetup where I present this and another BLE project (a connection-oriented one, called MotorBoat).

Saturday, September 6, 2014

Camera shot on charger connection

Somebody came to me with an idea whether a cheap Android phone can be turned into an automatic camera. Some external sensor would send a signal to the phone and the phone would take a picture automatically. We started to discuss the possible connection of the external sensor and an interesting idea came up: the charger connection.

Android delivers an event whenever the charging power is connected or disconnected: can it be used to send a binary signal to an application in a very simple way, without fiddling with Bluetooth or USB?

Click here to download the example application.

You have to start the application once. Then whenever you connect the charger, it takes a picture. When the application is in the foreground, a preview is shown but as long as the application is active (not destroyed) it works from the background too.

Here are the experiences:

  • On my high-end device the application reacted quickly to charger connection, the reaction time from connecting the charger to the camera shot was less than a second. But when the application was tested on the very low-end Android target device, the picture was much less rosy: the delay increased to 3-4 seconds, effectively making the solution unusable.
  • In order for this application to work, it has to be started at least once manually. This pretty much kills all unattended use cases.
  • The shutter sound is almost impossible to remove. Update: on certain devices (Nexus 4 and Nexus 7 confirmed) there is no shutter sound in silent mode.
The takeaway for us was to reject the idea. But I share the example program anyway, maybe it can be useful for somebody.

Tuesday, September 2, 2014

Android gas sensor application with Bluetooth Low Energy/RFDuino

I have always had a fascination with sensors linked up with mobile devices so it seemed just a good opportunity to try out the latest fashionable technology in the area, Bluetooth Low Energy in the context of a competition. announced the "We know RFDuino" competition for applications of the RFDuino module. RFDuino is an Arduino module with Bluetooth Low Energy (BLE) support. It is ideal to act as an interface between a sensor and a BLE-enabled mobile device like the Nexus 7.

Eventually I will publish the entire source code of this prototype application on this blog. But as this is a contest, I will wait until the contest ends (Sept. 30). Till then, watch the (very amateurish) video we have prepared about our sensor and the Android application. The entry with the most views wins the contest so if you like the concept, share the video with others! Thanks in advance. :-)