hlw8012: fix constants for BL0937

[ianchi]

With the current implementation it is impossible to calibrate all measurements at the same time for the BL0937 sensor as it has different constants factors for converting pulses.

If you set voltage_divider and current_resistor to correctly calibrate voltage and current, you get a reading for power that is completely off.

Add a configuration option sensor_model to handle constants on per sensor model basis.

While at it, avoid computing the same factor on every update and move to setup logic to do it only once.

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Other

Related issue or feature (if applicable): fixes BL0937 sensor readings

Pull request in esphome-docs with documentation (if applicable): esphome/esphome-docs#1282

Test Environment

• ESP32
• ESP8266

Example entry for config.yaml:

sensor:
  - platform: hlw8012
    sel_pin:
      number: GPIO12
      inverted: true
    cf_pin: GPIO4
    cf1_pin: GPIO5
    sensor_model: BL0937
    voltage_divider: 1685

    current:
      name: Current
    voltage:
      name: Voltage
    power:
      name: Power

Checklist:

• The code change is tested and works locally.
• Tests have been added to verify that the new code works (under tests/ folder).

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated in esphome-docs.

[jesserockz]

I cant help but think there is a reason for the / 1000000.0f in the final calculations. Perhaps precision is lost if the multiplier is a smaller number/more decimals before multiplying the cf_hz/cf1_hz?
(If I am spitting out garbage, just let me know.)

The only other comment I have is probably, use the existing CONF_MODEL. You don't need to make a new one and call everything ...SensorModel when Model will do fine.

[OttoWinter]

@jesserockz That depends a bit on the scale of those values, but iirc those constants were just there for easier code (because iirc the datasheet uses those units). If anything, I think it should be more accurate now, because IEEE754 floats have higher precision closer to 0.

[ianchi]

Fixed name of config option to model.
Changed docs PR accordingly.

[jesserockz]

Just 2 suggestions below to use the cv.enum helper