library/enviroplus/gas.py

   1 """Read the MICS6814 via an ads1015 ADC"""
   2
   3 import atexit
   4 import ads1015
   5 import RPi.GPIO as GPIO
   6
   7 MICS6814_HEATER_PIN = 24
   8
   9
  10 ads1015.I2C_ADDRESS_DEFAULT = ads1015.I2C_ADDRESS_ALTERNATE
  11 _is_setup = False
  12
  13
  14 class Mics6814Reading(object):
  15     __slots__ = 'oxidising', 'reducing', 'nh3'
  16
  17     def __init__(self, ox, red, nh3):
  18         self.oxidising = ox
  19         self.reducing = red
  20         self.nh3 = nh3
  21
  22     def __repr__(self):
  23         return """Oxidising: {:05.02f} Ohms
  24 Reducing: {:05.02f} Ohms
  25 NH3: {:05.02f} Ohms
  26 """.format(self.oxidising, self.reducing, self.nh3)
  27
  28     __str__ = __repr__
  29
  30
  31 def setup():
  32     global adc, _is_setup
  33     if _is_setup:
  34         return
  35     _is_setup = True
  36
  37     adc = ads1015.ADS1015(i2c_addr=0x49)
  38     adc.set_mode('single')
  39     adc.set_programmable_gain(6.148)
  40     adc.set_sample_rate(1600)
  41
  42     GPIO.setwarnings(False)
  43     GPIO.setmode(GPIO.BCM)
  44     GPIO.setup(MICS6814_HEATER_PIN, GPIO.OUT)
  45     GPIO.output(MICS6814_HEATER_PIN, 1)
  46     atexit.register(cleanup)
  47
  48
  49 def cleanup():
  50     GPIO.output(MICS6814_HEATER_PIN, 0)
  51
  52
  53 def read_all():
  54     """Return gas resistence for oxidising, reducing and NH3"""
  55     setup()
  56     ox = adc.get_voltage('in0/gnd')
  57     red = adc.get_voltage('in1/gnd')
  58     nh3 = adc.get_voltage('in2/gnd')
  59
  60     ox = (ox * 56000) / (3.3 - ox)
  61     red = (red * 56000) / (3.3 - red)
  62     nh3 = (nh3 * 56000) / (3.3 - nh3)
  63
  64     return Mics6814Reading(ox, red, nh3)
  65
  66
  67 def read_oxidising():
  68     """Return gas resistance for oxidising gases.
  69
  70     Eg chlorine, nitrous oxide
  71     """
  72     setup()
  73     return read_all().oxidising
  74
  75
  76 def read_reducing():
  77     """Return gas resistance for reducing gases.
  78
  79     Eg hydrogen, carbon monoxide
  80     """
  81     setup()
  82     return read_all().reducing
  83
  84
  85 def read_nh3():
  86     """Return gas resistance for nh3/ammonia"""
  87     setup()
  88     return read_all().nh3