Friday, July 3, 2020

1-wire Temperature Sensors and Lightning (and other sources of interference)

I use a lot of 1-wire temp sensors around the house for various purposes. One of which was a sort of hack to know when the hot water heater was in use. I placed one on the inlet side and one on the output side of the tank and when the 2 temperatures diverge by enough I know that the hot water is actually flowing. Within a few minutes of someone turning the tap off the temperature has equalized enough that it can tell the hot water is no longer in use. 

The main reason I set this up initially is that I wanted to have my bathroom lights go off after there was no motion in the bathroom for a while, but I didn’t want them to go off while I was in the shower nor did I want to set the timeout to be so long that they would stay on unnecessarily at other times. So now if the hot water is in use the logic of the master bathroom will not turn off the lights on you, but will keep checking back to see if the water is now off or if there has been more motion.

My day job is home automation software so all the logic for that is running in XTension.

Here is the graph of the output from last night:

The blue and red lines are the inlet and outlet temps of the water heater. Physically they are just a standard 1-wire temp sensor taped to the pipes a few inches up from the water heater. The yellow highlight is when the hot water is considered to be in use. The light green line is another 1-wire sensor just hanging in the attic showing the ambient temp in the attic. You can see 2 spikes to 185 degrees between 5:30 and 6pm last evening and one identical spike on the inlet sensor. 

Those were all caused by very sharp close by lightning hits. These particular sensors are connected to a Barix Barionet device which is a fantastic remote host for 1-wire sensors as well as digital inputs and relay outputs. Very handy for home automation and fully supported by my plugin for it in XTension. I have another device that is just basically an arduino that has a similar sensor that is glued to the side of a power supply so that I can make sure it is not overheating as it is in an electrical box with no ventilation. When those lights are on that sensor reads the same 185f or 85c repeatedly until the load decreases. These 1-wire sensors are just prone to interference and when that happens they return a reading of 85c. 

The reading of 185c is within the range of temperatures the device can actually read so it isn’t good to just filter that out in the driver or plugin or hardware. I am considering for a future firmware build of the 1-wire to wifi kit that if I get an 85c reading I will attempt another reading a few seconds later and only report it if the reading is the same twice. For the moment though the solution to filter them is easy enough inside the XTension ON script for the Unit. In the case of the above graph I am not going to filter it as it is interesting and happens rarely and does not throw off any other calculations or anything. For the sensor on the power supply it happens so frequently that it is frustrating to look at the graph. In that case I use the Change Future Value verb to change the value back to the previous reading if it is exactly equal to 85c. That does potentially ignore some valid measurements, but it does let anything through that is 84.9 or below or 85.1 and above and so I’m not terribly worried about that. The script is very simple and just looks like this:

if (future value) is equal to 185 then

change future value to (value of (thisUnit))

write log "ignored bogus 185 value for porch temp sensors"

end if


futureValue tells you the value that the unit will be set to when the script is finished running. The (value of (thisUnit)) portion returns the current value of the unit. This is sometimes initially confusing to people new to working with XTension but it allows you to do things like this or anything where you need to compare the current value with the new value and potentially override or change it. It also lets you do very specific smoothing of readings or throwing out ones that are out of range such as this. The call to ThisUnit just returns the name of the unit that you would normally have to pass to the value of verb so that if you renamed the unit you wouldn’t have to remember to edit the script and if you had many of the devices you want to do this to you could either cut and paste the code, or create a single script that you could call for all of them.

This 185/85 reading is actually a known issue with some 1-wire temp sensors. I don’t see a difference between powered or parasitically powered networks. The sensors in the above graph are parasitically powered but the one that is on the power supply is powered. The parasitically powered ones are on an absolutely horrible network of wires flying through the attic and to say that it is not a single bus or a standard star configuration would be a dramatic understatement. There is no shielding though they are on a single twisted pair of bell wire. Anyplace I wanted to add more sensors I just spliced into it and extended it. That attic network is my test of the absolute worst case I can imagine for testing 1-wire reading devices. The barix does exceptionally well under almost all circumstances except for these lighting storms.

That network now has 10 sensors on it all over the attic and it does sometimes drop a reading for one of them entirely. It also reads the intake and output temperatures from 3 different AC units so that I can verify the temp differential and know if the units are leaking gas and will need a recharge (or in one of the older ones a replacement is coming up I’m afraid) before they actually stop working and we have to make an emergency call to the HVAC folks. That reading also lets me run the fan after an AC or Heat cycle the best length of time to get all the cold or hot out of the unit. I turn the fan to On when a cycle completes and then start watching the temp sensors. As soon as the 2 temperatures are within a few degrees of each other I turn the fan back to Auto. 

I am also going to update the hardware in the next revision of the wifi temp sensor to use the better interface that is outlined in the 1-wire best practices documentation to see if that will help with bad values or interference.  In any case, overall 1-wire sensors work great, are inexpensive and there are a lot of options for getting them into your home automation system. 

Friday, January 3, 2020

Python Motion Detection on a Raspberry Pi Zero

I have been working non-stop the last few months to make my main project software compatible with Apple’s latest OS version. I finally realized how badly this was depressing me and how much I didn’t want to be doing this and how much it was making me hate Apple. I was spending literally man months of time to rebuild from scratch things that weren’t broken before Catalina and it was making me miserable. My rants about the absolute stupidity of everything that Apple chose to do to this alleged OS update will have to wait as I can’t afford the dental bills to fix the damage that gritting my teeth that hard will cause.

So in the interest of sanity and preserving the small quantity of it with which I have been granted  I took a short vacation from everything MacOS. I have been fighting with the horrible firmware of even the best available IP security cams for some time as I struggle to write software that can connect to them and insert their video streams into the web and other interfaces of XTension. I believed that there should be software for the Raspberry Pi that could leverage that camera for a much more reasonable price that would be terrific. Sadly even the best software out there for it just wasn’t doing the things that I wanted or at least with the quality of image and recording that I wanted.

The idea of controlling everything about a security camera is compelling! Not having to setup a firewall to keep it from contacting it’s own “cloud” and making sure it isn’t setting up a UPnP passthrough even when you told it not to have revealed to me several times that even commercial products don’t really do what you think they are. I wanted one based on software that I have written myself and so knew exactly what it’s doing.

It should also run well on a Pi Zero W so that you can make something small, though if it runs better on the latest multi-core Pi4’s thats OK too. Those little machines are worth every penny and very very nice!

The built in “motion” system is really interesting, but it’s remarkably difficult to get it to run at a high framerate and a larger frame size. Who wants a non-HD security cam now days anyway? The separate OS versions available may be spectacular but I want a software package for the stock Pi. After fighting with trying to get openCV to compile on a Pi Zero for the last week I wanted something that didn’t need that either. (Which I suspect is why they went with a totally separate image for their security cam OS as getting that stuff to install properly is almost hilarious)

The python camera support for the Pi is absolutely awesome. I can stream 1080p video and I can record it to disk and then post process it to standard MP4 files in almost real time. Those were not insignificant accomplishments ;) But the real sticking point was going to be the video motion detection. Using the pycamera code as a starting point and the hardware H264 encoders ability to output it’s changes from one frame to another makes it absolutely possible to do this at least as well as any commercial product. I’ll eventually open source this project but for the moment I’m working on it just myself. Here’s the 720p stream, part of the web interface and also a stream of just the motion sensing. This works as high as 1080p but at lower framerates though I’m sure I can optimize the calculations a bit more for that.

The image on the right is a live stream of the motion content and I’m just really enjoying playing with this at the moment.

I am currently building the web interface to the ability to mask out portions of the video for motion detection. The math for that is working great and adds very little if any overhead. Lots of work yet to do of course before something that is download/install ready but it will come before long. This is making me happy! Apple is making me mad.

Lest anyone think I’ll abandon developing software for Apple products over how much they drive me crazy sometimes really needs to spend some time using any of the other truly commercial desktop OS’s available out there. Don’t worry. XTension is undergoing a lot of changes because of this, but it’s not going away and unless I sell enough of it to hire a support team in another time zone I wont even consider doing a windows version. Sorry but if Apple frustrates me then Microsoft would kill me outright.