library/enviroplus/gas.py

   1 """Read the MICS6814 via an ads1015 ADC"""
   2
   3 import time
   4 import atexit
   5 import ads1015
   6 import RPi.GPIO as GPIO
   7
   8 MICS6814_HEATER_PIN = 24
   9 MICS6814_GAIN = 6.144
  10
  11 ads1015.I2C_ADDRESS_DEFAULT = ads1015.I2C_ADDRESS_ALTERNATE
  12 _is_setup = False
  13 _adc_enabled = False
  14 _adc_gain = 6.148
  15
  16
  17 class Mics6814Reading(object):
  18     __slots__ = 'oxidising', 'reducing', 'nh3', 'adc'
  19
  20     def __init__(self, ox, red, nh3, adc=None):
  21         self.oxidising = ox
  22         self.reducing = red
  23         self.nh3 = nh3
  24         self.adc = adc
  25
  26     def __repr__(self):
  27         fmt = """Oxidising: {ox:05.02f} Ohms
  28 Reducing: {red:05.02f} Ohms
  29 NH3: {nh3:05.02f} Ohms"""
  30         if self.adc is not None:
  31             fmt += """
  32 ADC: {adc:05.02f} Volts
  33 """
  34         return fmt.format(
  35             ox=self.oxidising,
  36             red=self.reducing,
  37             nh3=self.nh3,
  38             adc=self.adc)
  39
  40     __str__ = __repr__
  41
  42
  43 def setup():
  44     global adc, _is_setup
  45     if _is_setup:
  46         return
  47     _is_setup = True
  48
  49     adc = ads1015.ADS1015(i2c_addr=0x49)
  50     adc.set_mode('single')
  51     adc.set_programmable_gain(MICS6814_GAIN)
  52     adc.set_sample_rate(1600)
  53
  54     GPIO.setwarnings(False)
  55     GPIO.setmode(GPIO.BCM)
  56     GPIO.setup(MICS6814_HEATER_PIN, GPIO.OUT)
  57     GPIO.output(MICS6814_HEATER_PIN, 1)
  58     atexit.register(cleanup)
  59
  60
  61 def enable_adc(value=True):
  62     """Enable reading from the additional ADC pin."""
  63     global _adc_enabled
  64     _adc_enabled = value
  65
  66
  67 def set_adc_gain(value):
  68     """Set gain value for the additional ADC pin."""
  69     global _adc_gain
  70     _adc_gain = value
  71
  72
  73 def cleanup():
  74     GPIO.output(MICS6814_HEATER_PIN, 0)
  75
  76
  77 def read_all():
  78     """Return gas resistence for oxidising, reducing and NH3"""
  79     setup()
  80     ox = adc.get_voltage('in0/gnd')
  81     red = adc.get_voltage('in1/gnd')
  82     nh3 = adc.get_voltage('in2/gnd')
  83
  84     try:
  85         ox = (ox * 56000) / (3.3 - ox)
  86     except ZeroDivisionError:
  87         ox = 0
  88
  89     try:
  90         red = (red * 56000) / (3.3 - red)
  91     except ZeroDivisionError:
  92         red = 0
  93
  94     try:
  95         nh3 = (nh3 * 56000) / (3.3 - nh3)
  96     except ZeroDivisionError:
  97         nh3 = 0
  98
  99     analog = None
 100
 101     if _adc_enabled:
 102         if _adc_gain == MICS6814_GAIN:
 103             analog = adc.get_voltage('ref/gnd')
 104         else:
 105             adc.set_programmable_gain(_adc_gain)
 106             time.sleep(0.05)
 107             analog = adc.get_voltage('ref/gnd')
 108             adc.set_programmable_gain(MICS6814_GAIN)
 109
 110     return Mics6814Reading(ox, red, nh3, analog)
 111
 112
 113 def read_oxidising():
 114     """Return gas resistance for oxidising gases.
 115
 116     Eg chlorine, nitrous oxide
 117     """
 118     setup()
 119     return read_all().oxidising
 120
 121
 122 def read_reducing():
 123     """Return gas resistance for reducing gases.
 124
 125     Eg hydrogen, carbon monoxide
 126     """
 127     setup()
 128     return read_all().reducing
 129
 130
 131 def read_nh3():
 132     """Return gas resistance for nh3/ammonia"""
 133     setup()
 134     return read_all().nh3
 135
 136
 137 def read_adc():
 138     """Return spare ADC channel value"""
 139     setup()
 140     return read_all().adc