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+===========================
+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
+   :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
+.. _`The code for the thermometer:
+   https://www.thenautilus.net/cgit/thermostat/tree/sensor/thermostat.ino
+.. _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
+   :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 (I'll write more details about
+this part at some future time).
+
+Then I built a transmitter:
+
+.. figure:: rf-trasmitter-diagram.png
+   :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`:
+   http://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