Duty Cycle and Presence

I can think of 2 cases where I want to record binary data:

• Duty-Cycle of the boiler
• Presence of the members of the familly at home

Use Cases

Boiler

As said in a previous post about Z-Wave, the status of the switch on the actuator is available and will be dumped every 5 minutes in a JSON file.

Presence

This is a little bit more tricky. One of the best way to know if someone (and who) is home might be to check if the mobile phones are around. I looked into possible solutions but every phone has its peculiarities: ping doesn't respond on iOS but does on Android. iOS has Bonjour/mDNS activated by default but Android hasn't, etc...

Then, I discovered that my ISP's Box has an JSON API listing all the deviced connected with a flag called "active" bearing the values true or false. Unfortunately, this flag is not very reliable but combined with another one called "lastConnection" (returning a date), it seems possible to check if a given mobile was around in the last 10 minutes. The value 10 minutes comes from iOS which can be silent for up to 10 minutes. By running a check every 5 minutes, I can check if a phone is back home with a 5-minute resolution or has left home with a 10-minute resolution.

One day, I might investigate the bluetooth tag/tiles on keyfobs but in the meantime, mobiles detection rules!

Filtering is done by the MAC address so an associative array MAC address => owner is all what is needed.

Storage

In this case, we are not talking about a rate (something per second) nor a gauge (temperature, humidity, ...) but I still believe that RRD can be used here:

At the end of the 5-minute period, we have a binary value (on/off, in/out, ...). Once consolidated (30 min ou 1 day), the only relevant information is either the total time for each state or simply the percentage.

So if "always on for a given period" is represented 100%, then the easiest way to store the information is to use the value 30000 as base.

rrdtool update presence1.rrd 1407153600:30000 # if on/present
rrdtool update presence1.rrd 1407153600:0     # if off/away

Why 30000?

Remember that RRD will (for non gauge) always store a rate, hence will divide that value by the base period (called step).

Here 30000 / 300s = 100.

When consolidating to 30-minute periods, 6 values will be taken for the average calculation. If the heating is on for half an hour, then average will be 100. If it is on for only 15 minutes, the average will be 50 and so on. These values look like nice percentages :-)

The same apply to the daily value. We end-up with the duty cycle of the heating or the percentage of presence at home day by day. Useful? No idea but that's not the point here!

RRD creation parameters

rrdtool create presenceX.rrd --step 300 \
DS:personX:ABSOLUTE:600:0:30000 \
RRA:AVERAGE:0.9:1:24 \
RRA:AVERAGE:0.9:6:35064 \
RRA:AVERAGE:0.9:288:1826

rrdtool create heating.rrd --step 300 \
DS:heating:ABSOLUTE:600:0:30000 \
RRA:AVERAGE:0.9:1:24 \
RRA:AVERAGE:0.9:6:35064 \
RRA:AVERAGE:0.9:288:1826

New RF 433Mhz receiver

Following my post about the receiver for La Crosse sensors, I started to doubt about the quality of the reception.

On my left: XY-MK-5V

As I said before, I found this one on ebay for 1 € (inc. postage)!

But it is not specially noted for its quality of reception and output signal is very noisy.

On my right: RXB6

I decided to break the piggy (hum!) and ordered a RXB6 module which has a far better reputation. Cost from China is $5 (inc. pp). Obviously it took 3 weeks to arrive but finally, I was able to try it on the Arduino.

Surprise! It didn't work at all. Nothing, no signal on the PIN 13.

After a while, and really thinking I got a duff one, I hooked it on the Digispark and... miracle!

Performances

But if I had some readings, it occured to me that something must be wrong as TX sensors are quite chatty (a repeted signal every minute or so) and I had only 20 values for a whole evening.

Something was definitely amiss. OK pulseIn is not necessarily the best method ever but since the microcontroller is not doing anything else, this should not be a problem.

While decoding, I ignore the LOW part of the signal (the 1000μS gap) but still...

After a lot of trial and error, I finally understood where the lock was: A tiny little bug!

The Bug

The first loop validate the preamble by checking the value is 10d (00001010b) but the variable was defined by int so depending on the preceding bits it could have held a different (higher) value despite a correct preamble.

To correct this behaviour I could either do a AND 255 with the value but I simply defined by variable by 8-bit byte type.

I also changed the boundary timings to be less restrictive.

Pin 13

Reading about different receivers, it seems that the RXB familly (at least) doesn't cope well with pull-ups. Since the pin 13 of the Arduino has a LED, this might be the reason why it didn't work when I first tried it. It works fine on any other pin...

Additional items

Adding an external antenna to the RXB6 doesn't seem to make much difference but when hooked on the digispark, a decoupling capacitor (100nF) seems mandatory for the RF receiver!

And the winner is...

RXB6 module is definitely better, especially with the outdoor sensor, but it also uses twice as much power (6.7mA vs 3.8mA). And the XY-MK-5V has an unbeatable value for money.

Lacrosse TX2/TX3 Sensors and Digispark

Digispark

The Digispark is a Atmel Attiny85 based microcontroller development board similar to the Arduino line but cheaper and smaller. They had an extremely successful Kickstarter campaign 2 years ago and clones are available on ebay a few euros.

They seem to now have their own ecosystem and can emulate a USB device. The USB is also used to programme the microcontroller, the same way it is done on Arduino.

Wiki pages quickref and basics have some information about pins, differences from the Arduino and limitations.

For reference, here is the schematic of digispark.

Pins

Here I need only two pins for the project : 1 for the RF receiver and the other one for the serial connection to to Raspberry Pi.

I decided to go serial instead of I²C mainly because of the asynchronous nature of the signal and to avoid polling from the I²C master (Raspberry Pi in my case). Here the digispark will simply send its data as soon as it decodes something from the RF receiver.

So PIN 0 will be the RF receiver IN while PIN 2 (default for Serial Debug) will be used for the Serial Communication. 38400 bit/s seems to be the recommended value. Fair enough.

Code is available on github :https://github.com/guillier/la_crosse_sensors

Raspberry Pi

Serial communication

In order to allow serial communication, the serial port has to be freed from any usage as a console. Once it is done (and the Raspberry rebooted), minicom can be used to check if everything is alright.

minicom -b 38400 -o -D /dev/ttyAMA0

Note that the port is /dev/ttyAMA0 while the one for the Z-Wave static controller is /dev/ttyACM0.

Storing values

Same as the values from the airpi sensors, these ones can be stored in RRD databases:

rrdtool create temperature_device33.rrd --step 300 \
DS:temperature:GAUGE:600:0:U \
RRA:AVERAGE:0.9:1:24 \
RRA:AVERAGE:0.9:6:35064 \
RRA:MIN:0.9:288:1826 \
RRA:AVERAGE:0.9:288:1826 \
RRA:MAX:0.9:288:1826