Yet Another Update to the AcuRite 5N1 Weather Sensor

The previous post on this project is here <link>

Just when you thought I'd finally shut up about the AcuRite weather sensor I decide to post again. I promise, at some point I'll actually be satisfied with this device, but not quite yet. Over the months I've been working with this I've received comments and emails from folk that tried it and discovered things. One reader had an especially interesting situation; he had a neighbor that had a sensor within range. This meant that the code would pick them both up and present data from both sensors. To add insult to injury, when he tried changing his station to channel C, he found out that rtl_433 had a bug that prevented his receiving data on that channel. He must have been really annoyed at that point.

Since wind and temperature matter to placement, this was causing him grief and he set off to fix it. He isolated the sensor ID, and while he was at it, decoded the battery level, channel and found a better way to read the RF that allowed channel C to work.

During his trials he also found that the checksum was allowing bad data into the decoding and would present unusual readings. I've seen this over time as well. Things like a wind gust of 147 mph, a temperature spike of 30 or so degrees for no reason, the usual outlier readings, but with weather, it's hard to tell what an outlier really is.

I totally stole his ideas and some of his code and incorporated into my weather station,; I haven't been running it too long, but it looks like the vast majority of the outlier readings are gone. There may be some turn up, but so far it's been good.

First, inspection of the data showed that the bytes holding the various readings were all even parity. I added code to check the parity and discard the message if it didn't pass. Note that I only check the readings bytes because the first, second and last bytes are sensor, status, and checksum; these don't incorporate parity. In the c code I add the decoded sensor id, battery level, and channel to the JSON string I output, then check for the proper values in the python code that receives it. I did it this way because some folk may want to catch multiple sensors and average or compare them; especially if they can see a neighbors device for free.

I also noticed at my house that I receive a couple of devices from other folk that are not AcuRite devices. These things can cause problems as well, especially since they could easily be using the same checksum algorithm.

All the changes meant that I had to get the very latest version of rtl_433 from the original github repository. This was required because the rtl_433 folk made a new decoding method available that works better with the AcuRite data stream. That decoder wasn't there in January, but it is now. So, I tried again to fork it and failed miserably, and wound up just updating my copy of rtl_433. But, another good thing happened as well. The folk that wrote rtl_sdr, the actual radio software have improved it a lot and it installed first time, and I got to remove that from my copy.

Now, you get rtl_sdr, install it, build it, and then get my rtl_433, install it and build it. That will give you the version I'm using with all the changes to support the newest findings. My python code for the weather station I run is in GitHub as well. The specific stuff I use is:

rtl_sdr <link>
rtl_433 <link>
weather station <link>

The reason I linked to a particular commit in GitHub is so you can have exactly the same code to work with. The two rtl projects are in flux and could change dramatically at any time. You can always get the latest version if you think it will serve you better.

The latest version of rtl_433 not only has the new decoder software, it also allows you to specify the device you expect to receive. That makes it nice for me since I only have the AcuRite device I'm interested in. I changed my start up command to support this feature:

rtl_433 -f 433.915e6 -R9 2>> saveweather.log | saveweather.py  >> saveweather.log 2>&1

The '-R9' specifies the 5N1 device. Notice how I save the stderr output to a file? That helps me understand how many errors I'm getting and could help me fix a problem. Right now I'm also using the -D for debug option as well to get more information. Try '--help' on rtl_433 to see the other options it has.

I owe this latest version to Pcjunky (yes, I know his name, but I ain't gonna post it without his permission). He took my code and beat on it until it worked to his satisfaction. Thank's a lot.

Now, I want to show you the parity routine I ran across. Here's the code:

static int acuriteParity(uint8_t v){
    // returns 1 if parity odd, 0 if parity even
    v ^= v >> 4;
    v &= 0xf;
    return (0x6996 >> v) & 1;
}

This calculates the parity for a byte in around five cpu operations. It's an extremely clever implementation that I leave the explanation as an exercise for the student.

Have fun.