BH1750 python library

## bh1750.py
#!/usr/bin/python2
# vim: expandtab ts=4 sw=4
# Inspired by http://www.raspberrypi-spy.co.uk/2015/03/bh1750fvi-i2c-digital-light-intensity-sensor/


import smbus
import time

class BH1750():
    """ Implement BH1750 communication. """
    # Define some constants from the datasheet
    POWER_DOWN = 0x00 # No active state
    POWER_ON   = 0x01 # Power on
    RESET      = 0x07 # Reset data register value
    # Start measurement at 4lx resolution. Time typically 16ms.
    CONTINUOUS_LOW_RES_MODE = 0x13
    # Start measurement at 1lx resolution. Time typically 120ms
    CONTINUOUS_HIGH_RES_MODE_1 = 0x10
    # Start measurement at 0.5lx resolution. Time typically 120ms
    CONTINUOUS_HIGH_RES_MODE_2 = 0x11
    # Start measurement at 1lx resolution. Time typically 120ms
    # Device is automatically set to Power Down after measurement.
    ONE_TIME_HIGH_RES_MODE_1 = 0x20
    # Start measurement at 0.5lx resolution. Time typically 120ms
    # Device is automatically set to Power Down after measurement.
    ONE_TIME_HIGH_RES_MODE_2 = 0x21
    # Start measurement at 1lx resolution. Time typically 120ms
    # Device is automatically set to Power Down after measurement.
    ONE_TIME_LOW_RES_MODE = 0x23

    def __init__(self, bus, addr=0x23):
        self.bus = bus
        self.addr = addr
        self.power_down()
        self.set_sensitivity()

    def _set_mode(self, mode):
        self.mode = mode
        self.bus.write_byte(self.addr, self.mode)

    def power_down(self):
        self._set_mode(self.POWER_DOWN)

    def power_on(self):
        self._set_mode(self.POWER_ON)

    def reset(self):
        self.power_on() #It has to be powered on before resetting
        self._set_mode(self.RESET)

    def cont_low_res(self):
        self._set_mode(self.CONTINUOUS_LOW_RES_MODE)

    def cont_high_res(self):
        self._set_mode(self.CONTINUOUS_HIGH_RES_MODE_1)

    def cont_high_res2(self):
        self._set_mode(self.CONTINUOUS_HIGH_RES_MODE_2)

    def oneshot_low_res(self):
        self._set_mode(self.ONE_TIME_LOW_RES_MODE)

    def oneshot_high_res(self):
        self._set_mode(self.ONE_TIME_HIGH_RES_MODE_1)

    def oneshot_high_res2(self):
        self._set_mode(self.ONE_TIME_HIGH_RES_MODE_2)

    def set_sensitivity(self, sensitivity=69):
        """ Set the sensor sensitivity.
            Valid values are 31 (lowest) to 254 (highest), default is 69.
        """
        if sensitivity < 31:
            self.mtreg = 31
        elif sensitivity > 254:
            self.mtreg = 254
        else:
            self.mtreg = sensitivity
        self.power_on()
        self._set_mode(0x40 | (self.mtreg >> 5))
        self._set_mode(0x60 | (self.mtreg & 0x1f))
        self.power_down()

    def get_result(self):
        """ Return current measurement result in lx. """
        data = self.bus.read_word_data(self.addr, self.mode)
        count = data >> 8 | (data&0xff)<<8
        mode2coeff =  2 if (self.mode & 0x03) == 0x01 else 1
        ratio = 1/(1.2 * (self.mtreg/69.0) * mode2coeff)
        return ratio*count

    def wait_for_result(self, additional=0):
        basetime = 0.018 if (self.mode & 0x03) == 0x03 else 0.128
        time.sleep(basetime * (self.mtreg/69.0) + additional)

    def do_measurement(self, mode, additional_delay=0):
        """
        Perform complete measurement using command
        specified by parameter mode with additional
        delay specified in parameter additional_delay.
        Return output value in Lx.
        """
        self.reset()
        self._set_mode(mode)
        self.wait_for_result(additional=additional_delay)
        return self.get_result()

    def measure_low_res(self, additional_delay=0):
        return self.do_measurement(self.ONE_TIME_LOW_RES_MODE, additional_delay)

    def measure_high_res(self, additional_delay=0):
        return self.do_measurement(self.ONE_TIME_HIGH_RES_MODE_1, additional_delay)

    def measure_high_res2(self, additional_delay=0):
        return self.do_measurement(self.ONE_TIME_HIGH_RES_MODE_2, additional_delay)


def main():

    #bus = smbus.SMBus(0) # Rev 1 Pi uses 0
    bus = smbus.SMBus(1)  # Rev 2 Pi uses 1
    sensor = BH1750(bus)

    while True:
        print "Sensitivity: {:d}".format(sensor.mtreg)
        for measurefunc, name in [(sensor.measure_low_res, "Low Res "),
                                  (sensor.measure_high_res, "HighRes "),
                                  (sensor.measure_high_res2, "HighRes2")]:
            print "{} Light Level : {:3.2f} lx".format(name, measurefunc())
        print "--------"
        sensor.set_sensitivity((sensor.mtreg + 10) % 255)
        time.sleep(1)


if __name__=="__main__":
    main()