I have a task where I need to read 2 parameters from a BLE Beacon. The documentation was seriously lacking and after a fair amount of effort, I managed to get some basic information about reading the data from the BLE Beacon.
The parameters to read are
1) Battery Voltage of the sensor
2) Temperature the beacon has a built in temperature sensor.
I think I have tried almost every popular Python BLE library out there but I just can't seem to get the temperature reading out of the beacon. "I think" I am able to read the voltage. The reason why I said "I think" is because the value seems to match what was provided in the minimal document. And also when I put the beacon into the charger, I can see the value go up - an indication that it is the voltage reading. As I could not read the temperature ( because the UUIDs that are mentioned in the document, the value doesn't seem to change ). I have tried enabling the sensor in every possible way and method described - by writing 01:00 etc. I spent a fair amount of time to reverse engineer the thing. I ran a packet sniffer and managed to capture the data that was being transferred between the beacon and the mobile app ( They have a mobile app ). But then again I am not able to figure out how the temperature readings are being communicated between the beacon and the app. Let me break the whole stuff in smaller blocks.
Hardware: BLE beacon from which voltage and temperature can be read. The temperature sensor is built into the beacon. And the beacon itself is from Texas Instruments but the temperature, voltage sensing part is done by a third party. They provided us with some minimal information and it was difficult to make sense of some of the sentences as they have trouble communicating in English.
The sequence to get the data goes like this
I have been able to do the first 4 real fast, and "half" of No. 5, i.e getting the voltage part. When I say real fast I mean I got that stuff with nearly no documentation available at that time.
As per the info that I have the data resides in these characteristics/UUIDs. Also please note that the UUID are not standard 128 bit and this caused me issues when using certain libraries. But after some tries I got to read/write to them using handles etc. The handles and other stuff I printed are ones that I read using PYGATT (A Python wrapper for gatttool).
The UUIDs are marked as 1st, 2nd, 3rd and 4th parameters and it has the following to say about the parameters
- A: 1 byte (2nd Param)
- B: Maj + Min values, 4 bytes (4th Param)
- C: 4 bytes (3rd Param)
- D: Enable/disable notification ( I have been able to turn this on )
- E: Set notification interval ( I have been able to set this and can notice the change in notification interval )
This is minimal so as to not have a large file. All it does is this - the mobile app connects to the beacon, then the notifications start and the temperate readings are retrieved by the mobile app. Like I had mentioned, I don't seem to have problem reading the voltage, it's only the temperature that I am getting stuck at. I have been at it for a week now. I think I have tried nearly everything that I could think of. I even enumerated all the writable characteristics and tried writing numbers like 1 ( enables the sensor? ). I could have offered a bounty for this straight away if it were possible. I rarely get stuck for so long with a problem. This is driving me a little crazy. I am getting close to my wits end - I guess it's time for a super hero - anyone out there? :) I can provide for every bit of information needed if someone could indicate what is wrong. I even wrote a cordova app ... and tried a bunch of stuff from my Android phone. I can connect ... write to characteristics, read stuff etc but temperature ready, nah!!! It just won't budge. All I get is the same set of values ( I used a JSON.stringify to display A, B and C). I can bother about the byte order later. I guess that is a smaller problem.
The communication between the beacon and a third party mobile app is fine, it is able to read the temperature info just fine.
I have been looking at wireshark data and I am fairly sure that the temperature data is being communicated at this stage. But then when I decode the "value", it looks like it's the voltage. It mentions l2cap but I am not sure how that is being used here to send the temperature readings ( if it is using that in the first place ).
Update: Wrote to every writable characteristics. Wrote values like 1, 0100, 2, 7 on every writable characteristics. At the same time I was reading every readable characteristic ( in a loop ) and doing a comparison (just true/false) with the previous set of values. This seemed like a quick and easier way to know if something changed. Didn't want to take chances with converting the hex to a float. I can figure out the byte order later.
From the sniffed data (wireshark) I can only see 3 writes happening on the beacon.
I am not fully sure, even after a long discussion, but it seems that the four bytes of the notification are used for the voltage as well as the temperature, since the temperature can most probably be derived from the voltage.
From the values it seems that those four bytes represent the voltage in float (if you ignore the absurd factor of 10^-38 that comes in because only 4 bytes instead of 8 bytes are used).
Since typically the temperature T is derived from a resistivity measurement, where the resistivity R is proportional to the voltage U (if the current is constant), you can in principle calculate the temperature T from the voltage U.
The problem is that T(R) is relatively linear, but not perfectly (in contrast to U(R) which is assumed to be U=RI). So you may need to plot the values for T(U) to find out the curve that they are using.
To add to the confusion, I got the best results when only using the first five bits of the third byte and the eight bits of the fourth byte. I am not aware why this is the case, and it might point to some trouble still.
The best option is to ask for their function T(U) that they are using. If they can and will provide it for you...