VOC Index

README.rst

@@ -74,7 +74,7 @@ Usage Example
         print("")
         sleep(1)
 
-For humidity compensated raw gas readings, we'll need a secondary sensor such as the bme280
+For humidity compensated raw gas and voc index readings, we'll need a secondary sensor such as the bme280
 
 .. code-block:: python3
 
@@ -90,12 +90,22 @@ For humidity compensated raw gas readings, we'll need a secondary sensor such as
     while True:
         temperature = bme280.temperature
         humidity = bme280.relative_humidity
+
+        # For compensated raw gas readings
         compensated_raw_gas = sgp.measure_raw(temperature = temperature, relative_humidity = humidity)
+
+        time.sleep(1)
+
+        # For Compensated voc index readings
+        voc_index = sgp.measure_index(temperature = temperature, relative_humidity = humidity)
+
         print(compensated_raw_gas)
+        print(voc_index)
         print("")
         time.sleep(1)
 
 
+It may take several minutes for the VOC index to start changing as it calibrates the baseline readings.
 
 Contributing
 ============

adafruit_sgp40.py → adafruit_sgp40/__init__.py

@@ -31,7 +31,7 @@
 """
 from time import sleep
 from struct import unpack_from
-import adafruit_bus_device.i2c_device as i2c_device
+from adafruit_bus_device import i2c_device
 
 __version__ = "0.0.0-auto.0"
 __repo__ = "https://github.com/adafruit/Adafruit_CircuitPython_SGP40.git"
@@ -111,6 +111,7 @@ def __init__(self, i2c, address=0x59):
         self.i2c_device = i2c_device.I2CDevice(i2c, address)
         self._command_buffer = bytearray(2)
         self._measure_command = _READ_CMD
+        self._voc_algorithm = None
 
         self.initialize()
 
@@ -133,8 +134,6 @@ def initialize(self):
 
             raise RuntimeError("Feature set does not match: %s" % hex(featureset[0]))
 
-        # VocAlgorithm_init(&voc_algorithm_params)
-
         # Self Test
         self._command_buffer[0] = 0x28
         self._command_buffer[1] = 0x0E
@@ -226,6 +225,34 @@ def measure_raw(self, temperature=25, relative_humidity=50):
         self._measure_command = bytearray(_cmd)
         return self.raw
 
+    def measure_index(self, temperature=25, relative_humidity=50):
+        """Measure VOC index after humidity compensation
+        :param float temperature: The temperature in degrees Celsius, defaults to :const:`25`
+        :param float relative_humidity: The relative humidity in percentage, defaults to :const:`50`
+        :note  VOC index can indicate the quality of the air directly.
+        The larger the value, the worse the air quality.
+        :note 0-100, no need to ventilate, purify
+        :note 100-200, no need to ventilate, purify
+        :note 200-400, ventilate, purify
+        :note 00-500, ventilate, purify intensely
+        :return int The VOC index measured, ranged from 0 to 500
+        """
+        # import/setup algorithm only on use of index
+        # pylint: disable=import-outside-toplevel
+        from adafruit_sgp40.voc_algorithm import (
+            VOCAlgorithm,
+        )
+
+        if self._voc_algorithm is None:
+            self._voc_algorithm = VOCAlgorithm()
+            self._voc_algorithm.vocalgorithm_init()
+
+        raw = self.measure_raw(temperature, relative_humidity)
+        if raw < 0:
+            return -1
+        voc_index = self._voc_algorithm.vocalgorithm_process(raw)
+        return voc_index
+
     def _read_word_from_command(
         self,
         delay_ms=10,
@@ -288,7 +315,7 @@ def _generate_crc(crc_buffer):
                 if crc & 0x80:
                     crc = (
                         crc << 1
-                    ) ^ 0x31  #  0x31 is the Seed for SGP40's CRC polynomial
+                    ) ^ 0x31  # 0x31 is the Seed for SGP40's CRC polynomial
                 else:
                     crc = crc << 1
         return crc & 0xFF  # Returns only bottom 8 bits