Some links:

* links to some ESP32 libraries
  https://www.arduinolibraries.info/architectures/esp32
* the OLED library https://github.com/ThingPulse/esp8266-oled-ssd1306
* vague guide to the OLED library
  https://diyprojects.io/using-i2c-128x64-0-96-ssd1306-oled-display-arduino/
* the sensor library https://github.com/beegee-tokyo/DHTesp

* Ultra Low Power mode for ESP32:

  - not Arduino-style
    https://github.com/espressif/esp-idf/blob/master/examples/system/deep_sleep/main/deep_sleep_example_main.c
  - Arduino-style
    https://github.com/espressif/arduino-esp32/blob/master/libraries/ESP32/examples/DeepSleep/TimerWakeUp/TimerWakeUp.ino

* BLE examples
  http://www.iotsharing.com/2017/07/how-to-use-ble-in-arduino-esp32.html
  https://github.com/nkolban/ESP32_BLE_Arduino/blob/6bad7b42a96f0aa493323ef4821a8efb0e8815f2/examples/BLE_notify/BLE_notify.ino

  maybe we want to have the esp32 *write* to the main thermostat?

  - deep sleep
  - read temperature
  - BLE passive scan for thermostat
  - connect
  - write id + values to a property
  - disconnect
  - repeat

  example Python server:
  https://github.com/RadiusNetworks/bluez/blob/master/test/example-gatt-server

  C++ library: https://github.com/nettlep/gobbledegook

NOTE: the ``patchedBLE`` subtree comes from
https://github.com/nkolban/ESP32_BLE_Arduino.git

Upgrading my environment::

  cd ~/src/makeEspArduino
  git pull

  cd ~/HW/arduino-esp32
  git pull --recurse-submodules

  cd ~/HW/thermostat/
  git subtree pull --prefix=sensor/patchedBLE/ \
      https://github.com/nkolban/ESP32_BLE_Arduino.git master

New board is identical to https://www.adafruit.com/product/3405 "Adafruit HUZZAH32 – ESP32 Feather Board"

Possible batteries:

* https://www.aliexpress.com/item/3-7V-8000mAh-7566121-Lithium-Polymer-Li-Po-li-ion-Rechargeable-Battery-Lipo-cells-For-Tachograph/32919158535.html
* https://www.aliexpress.com/item/3-7V-8000mAh-126090-Lithium-Polymer-Li-Po-li-ion-Rechargeable-Battery-Lipo-cells-For-Recorder/32919162482.html
* https://www.aliexpress.com/item/Size-906090-3-7v-6000mAh-li-ion-Lipo-cells-Lithium-Li-Po-Polymer-Rechargeable-Battery-For/32919146776.html
* https://www.aliexpress.com/item/1-2-4-Pcs-3-7v-Voltage-Polymer-Lithium-Ion-Battery-6000mah-906090-Lipo-Rechargeable-Battery/32929375943.html