add BH1750 light sensor platform doc (#2821)

* add BH1750 light sensor platform doc

* don't repeat errors!

* review fixes

* add `multiplier` parameter

source/_components/sensor.bh1750.markdown

@@ -0,0 +1,95 @@
+---
+layout: page
+title: "BH1750 Light sensor"
+description: "Instructions how to integrate a BH1750 light sensor in a Raspberry PI into Home Assistant."
+date: 2017-06-10 00:00
+sidebar: true
+comments: false
+sharing: true
+footer: true
+logo: raspberry-pi.png
+ha_category: Sensor
+ha_release: 0.47
+ha_iot_class: "Local Push"
+---
+
+
+The `bh1750` sensor platform allows you to read the ambient light level in lux from a [BH1750FVI sensor](http://cpre.kmutnb.ac.th/esl/learning/bh1750-light-sensor/bh1750fvi-e_datasheet.pdf) connected via [I2c](https://en.wikipedia.org/wiki/I²C) bus (SDA, SCL pins) to your [Raspberry Pi](https://www.raspberrypi.org/). It allows you to use all the resolution modes of the sensor described in its datasheet.
+
+To use your BH1750 sensor in your installation, add the following to your `configuration.yaml` file:
+
+```yaml
+# Example configuration.yaml entry
+sensor:
+  - platform: BH1750
+```
+
+Or, if you want to specify the resolution mode of the digital sensor or need to change the default I2c address (which is 0x23), add more details to the yaml config.
+
+The sensor can work with resolutions of 0.5 lx (high res mode 1), 1 lx (high res mode 2) or 4 lx (low res mode), and make measurements continuously or one time each call. To set any of these operation modes, select one of these combinations: `continuous_low_res_mode`, `continuous_high_res_mode_2`, `continuous_high_res_mode_1`, `one_time_high_res_mode_1`, `one_time_high_res_mode_2`, `one_time_low_res_mode`.
+
+```yaml
+# Example of customized configuration.yaml entry
+sensor:
+  - platform: bh1750
+    name: Ambient light
+    i2c_address: 0x5C
+    operation_mode: one_time_high_res_mode_1
+    measurement_delay_ms: 200
+    scan_interval: 25
+```
+
+Configuration variables:
+
+- **name** (*Optional*): The name of the sensor
+- **i2c_address** (*Optional*): I2c address of the sensor. It is 0x23 or 0x5C.
+- **i2c_bus** (*Optional*): I2c bus where the sensor is. Defaults to 1, for Raspberry Pi 2 and 3.
+- **operation_mode** (*Optional*): Working mode for the sensor. Defaults to `continuous_high_res_mode_1` for continuous measurement and 1 lx resolution.
+- **measurement_delay_ms** (*Optional*): Delay time in ms for waiting the sensor to get the measure. Defaults to 120 ms.
+- **multiplier** (*Optional*): Correction coefficient to multiply the measured light level. Value between 0.1 and 10. Default is 1.
+
+
+### Directions for installing smbus support on Raspberry Pi:
+
+Enable I2c interface with the Raspberry Pi config utility:
+```bash
+# pi user environment: Enable i2c interface
+sudo raspi-config
+```
+Select `Interfacing options->I2C` choose `<Yes>` and hit `Enter`, then go to `Finish` and you'll be prompted to reboot.
+
+Install dependencies for use the `smbus-cffi` module and enable your _homeassistant_ user to join the _i2c_ group:
+```bash
+# pi user environment: Install i2c dependencies and utilities
+sudo apt-get install build-essential libi2c-dev i2c-tools python-dev libffi-dev
+
+# pi user environment: Add homeassistant user to the i2c group
+sudo addgroup homeassistant i2c
+
+# pi user environment: Reboot Raspberry Pi to apply changes
+sudo reboot
+```
+
+###### Check the i2c address of the sensor
+
+After installing `i2c-tools`, a new utility is available to scan the addresses of the connected sensors:
+
+```bash
+/usr/sbin/i2cdetect -y 1
+```
+
+It will output a table like this:
+
+```text
+     0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f
+00:          -- -- -- -- -- -- -- -- -- -- -- -- --
+10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
+20: -- -- -- 23 -- -- -- -- -- -- -- -- -- -- -- --
+30: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
+40: 40 -- -- -- -- -- UU -- -- -- -- -- -- -- -- --
+50: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
+60: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
+70: -- -- -- -- -- -- -- 77
+```
+
+So you can see the sensor address what you are looking for is **0x23** (there are more i2c sensors in that Raspberry Pi).