documenting BME280 usage

Author: simonrupf

.gitignore

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 *.pdf
 *.png
+*.jpg

sensors/README.md

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+Sensors
+=======
+
+Most of these sensors are easy to connect to an MCU on their own, often
+requiring one or more resistors or capacitors to adjust for the MCU power level
+or analog input capabilities. For easier use and less soldering it is therefore
+easier to use one of the many breakout boards available online. These will
+package the sensor with all the required components (usually in SMD formats) and
+leads out clearly labelled pins to solder it onto the MCU with a few short wires
+or to stick it to a breadboard for testing.

sensors/bme280/README.md

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+BME280
+======
+
+The Bosch BME280 sensor offers humidity, barometric pressure and temperature
+sensing in a single chip. Adafruit offers a breakout board to easily connect the
+BME280 to an MCU: <https://www.adafruit.com/product/2652>
+
+There are other similar breakout boards that may be used with the Adafruit
+libraries, as long as they offer either I2C or SPI interfaces. Using hardware
+SPI is preferred, if your MCU supports it.
+
+Use cases
+---------
+
+This sensor is ideal for a weather station as it includes the three measurements
+typically found in classic analog barometers.
+
+Limitations
+-----------
+
+The sensor caches the measurements, which is why they are requested twice to get
+an updated number. If using I2C reading all different values is quite slow, so
+(hardware) SPI is the preferred interface on ESP8266 MCUs. The Digispark is only
+supporting I2C, so do not expect to read out the values too frequently.
+
+Wiring
+------
+
+The following is an example for connecting the Adafruit BME280 breakout board to
+a NodeMCU using the ESP8266's hardware SPI pins. If the `D8` pin isn't pulled
+HIGH during boot, the sensor connected to it interferes with the startup of the
+ESP8266. This can be avoided by using a resistor of 2 - 10 kΩ between ground
+and `D8` (the illustration below shows a 3.3 kΩ resistor).
+
+![Adafruit BME280 breakout board connected to a NodeMCU using SPI](bme280 nodemcu.png)
+
+Here below is an example for connecting the Adafruit BME280 breakout board to
+a DigiSpark using I2C pins. Although it seems possible to get software SPI
+working on the DigiSpark, (software) I2C is better documented and therefore
+easier to use.
+
+![Adafruit BME280 breakout board connected to a DigiSpark using I2C](bme280 digispark.png)
+