===========================
Building a smart thermostat
===========================
:CreationDate: 2018-03-25 12:47:54
:Id: HW/thermostat
:tags: - hardware
- software
I'd like to have a computer-controller thermostat, that behaves the
way I want to, doesn't need to talk to external servers, can measure
temperature in multiple rooms.
I'll need:
* multiple cheap temperature sensors that can report their readings
wirelessly
* a way to start / stop the boiler
* some clever software to decide when to turn the boiler on/off based
on the temperature readings
* a computer to run that software
The sensors
===========
The current candidate is a ESP32_ running off (probably alkaline)
batteries, using a DHT11_ sensor.
.. figure:: thermometer-diagram.png
:alt: diagram showing a ESP32 microcontroller wired up to a DHT11
sensor
:align: center
Connections:
=============== ======
microcontroller sensor
=============== ======
+5V VCC
GND GND
pin 13 DATA
=============== ======
`The code for the thermometer`_ uses the DHTesp_ library to read the
sensor, and the `esp8266-oled-ssd1306`_ library to drive the
display. The final sensor will most probably not have a display.
.. _ESP32: https://en.wikipedia.org/wiki/ESP32
.. _DHT11: https://www.adafruit.com/product/386
.. _DHTesp: https://github.com/beegee-tokyo/DHTesp
.. _`esp8266-oled-ssd1306`:
https://github.com/ThingPulse/esp8266-oled-ssd1306/
Talking to the boiler
=====================
Using the process described by `Steven Hale`_, `Dave Hope`_, `Ross
Harper`_ and probably others, I connected a 433MHz receiver to my
soundcard:
.. figure:: rf-receiver-diagram.png
:alt: diagram showing a 433MHz receiver wired up to a stereo audio
cable via resistors
:align: center
Connections:
============= =======
From To
============= =======
Power + receiver VCC
Power - receiver GND
Power - microphone cable shielding via 22kΩ resistor
receiver data microphone cable channel via 47kΩ resistor
============= =======
By turning the knob the thermostat that was installed with the boiler,
I generated the on and off signals. I recorded them with Audacity_,
and spent a few hours puzzling them out. This is what the recorded
waveforms look like:
.. figure:: waveforms-small.png
:alt: two similar but not identical saw-toothed waveforms, one
above the other
:target: waveforms-large.png
:align: center
The two signal trains (one for "on" and one for "off")
It took a while to realise that those saw-toothed waves were actually
square waves, mangled by all the filters that my soundcard has, and
that the heights were different only because of those same filters.
Measuring the number of samples between rising edge and falling edge
gave me the timing. Then I had to do the whole thing again when I
noticed that the "low" widths where not always equal to the preceding
"high" width: it's not a 50/50 duty cycle square wave.
Then I built a transmitter:
.. figure:: rf-trasmitter-diagram.png
:alt: diagram showing an Arduino UNO wired up to a 433MHz
transmitter
:align: center
Connections:
=============== ===========
microcontroller transmitter
=============== ===========
+3.3V VCC
GND GND
pin 7 DATA
=============== ===========
and wrote `code to send the correct pulse train`_. The receiver wired
to the boiler recognises the signals and turns on and off.
.. _`Steven Hale`:
http://www.stevenhale.co.uk/main/2013/08/home-automation-reverse-engineering-a-worcester-bosch-dt10rf-wireless-thermostat/
.. _`Dave Hope`:
https://damn.technology/controlling-british-gas-wr1-receiver-arduino
.. _`Ross Harper`:
https://rossharper.net/2015/11/decoding-a-siemens-rcr10433-thermostat-signal-to-control-a-boiler-from-a-raspberry-pi/
.. _Audacity: https://www.audacityteam.org/
.. _`code to send the correct pulse train`:
https://www.thenautilus.net/cgit/thermostat/tree/trasmitter/sender.ino
