Add Sense-HAT pressure sensor support

Fixes #68

Change-type: patch
Signed-off-by: Chris Crocker-White <chriscw@balena.io>

sensor/scripts/lps25h.py

@@ -0,0 +1,42 @@
+# Credit: https://github.com/youkidearitai/rpi-sensors
+import smbus
+import time
+
+class Lps25hsensor:
+    address = 0x5c
+
+    PRESS_OUT_XL = 0x28
+    PRESS_OUT_L = 0x29
+    PRESS_OUT_H = 0x2A
+
+    def __init__(self, smbus_addr = 1):
+        self.bus = smbus.SMBus(smbus_addr)
+
+    def setup(self):
+        try:
+            self.bus.write_i2c_block_data(self.address, 0x00, [])
+        except OSError as e:
+            print(e)
+
+        time.sleep(0.1)
+
+    def read(self):
+        try:
+            self.bus.write_i2c_block_data(self.address, 0x20, [0x90])
+        except OSError as e:
+            print(e)
+
+        time.sleep(0.05)
+
+        pressure = (
+          self.read_i2c_block(self.PRESS_OUT_XL) << 0 |
+          self.read_i2c_block(self.PRESS_OUT_L) << 8 |
+          self.read_i2c_block(self.PRESS_OUT_H) << 16
+        )
+        pressure = pressure / 4096
+
+        return pressure
+
+    def read_i2c_block(self, register):
+        blocks = self.bus.read_i2c_block_data(self.address, register, 1)
+        return blocks[0]

sensor/scripts/sense_hat_air_quality.py

@@ -1,4 +1,8 @@
 def get_readings(sensor):
+    # As we have a sense HAT we can give an indication of the air quality on the LED matrix
+    from ledmatrix import LedMatrix
+    from lps25h import Lps25hsensor
+
     # The sense HAT does not include any way to obtain an air quality score via gas measurement,
     # so we can create one using the temperature and humidity reading based on distance from ideal values
     max_iaq = 500
@@ -32,8 +36,9 @@ def get_readings(sensor):
     air_quality_score = (current_humidity_variance * humidity_weighting) + (current_temperature_variance * (1 - humidity_weighting))
     air_quality_score = air_quality_score * 500
 
-    # As we have a sense HAT we can give an indication of the air quality on the LED matrix
-    from ledmatrix import LedMatrix
+    # Get a pressure reading from the LPS25H on the sense hat at 0x5c
+    pressure_sensor = Lps25hsensor()
+    current_pressure = pressure_sensor.read()
 
     display = LedMatrix()
     display.clear()
@@ -90,7 +95,7 @@ def get_readings(sensor):
             'measurement': 'balena-sense',
             'fields': {
                 'temperature': float(current_temperature),
-                # 'pressure': float(sensor.environ.pressure),
+                'pressure': float(current_pressure),
                 'humidity': float(current_humidity),
                 'air_quality_score': float(air_quality_score)
             }