diff --git a/enviro/__init__.py b/enviro/__init__.py
index f92753d..f4c4fda 100644
--- a/enviro/__init__.py
+++ b/enviro/__init__.py
@@ -10,9 +10,9 @@
 i2c = PimoroniI2C(I2C_SDA_PIN, I2C_SCL_PIN, 100000)
 i2c_devices = i2c.scan()
 model = None
-if 56 in i2c_devices: # 56 = colour / light sensor and only present on Indoor
+if I2C_ADDR_BH1745 in i2c_devices: # colour / light sensor and only present on Indoor
   model = "indoor"
-elif 35 in i2c_devices: # 35 = ltr-599 on grow & weather
+elif I2C_ADDR_LTR559 in i2c_devices: # ltr-599 on grow & weather
   pump3_pin = Pin(12, Pin.IN, Pin.PULL_UP)
   model = "grow" if pump3_pin.value() == False else "weather"    
   pump3_pin.init(pull=None)
@@ -30,7 +30,17 @@ def get_board():
   if model == "urban":
     import enviro.boards.urban as board
   return board
-  
+
+# return any additional sensors connected with Qw/ST
+def get_additional_sensors():
+  if I2C_ADDR_SCD41 in i2c_devices:
+    try:
+      import enviro.sensors.scd41 as scd41
+      yield scd41
+    except RuntimeError:
+      # Likely another device present on the SCD41 address
+      pass
+
 # set up the activity led
 # ===========================================================================
 from machine import PWM, Timer
@@ -411,9 +421,14 @@ def get_sensor_readings():
   readings = get_board().get_sensor_readings(seconds_since_last, vbus_present)
   # readings["voltage"] = 0.0 # battery_voltage #Temporarily removed until issue is fixed
 
+  # Add any additional sensor readings
+  for sensor in get_additional_sensors():
+      for key, value in sensor.get_sensor_readings(seconds_since_last).items():
+          readings[key] = value
+
   # write out the last time log
   with open("last_time.txt", "w") as timefile:
-    timefile.write(now_str)  
+    timefile.write(now_str)
 
   return readings
 
diff --git a/enviro/constants.py b/enviro/constants.py
index 78cfe8b..1cdf3fd 100644
--- a/enviro/constants.py
+++ b/enviro/constants.py
@@ -47,3 +47,8 @@
 WATER_VAPOR_SPECIFIC_GAS_CONSTANT = 461.5
 CRITICAL_WATER_TEMPERATURE = 647.096
 CRITICAL_WATER_PRESSURE = 22064000
+
+# I2C addresses
+I2C_ADDR_BH1745   = 0x38
+I2C_ADDR_LTR559   = 0x23
+I2C_ADDR_SCD41    = 0x62
diff --git a/enviro/sensors/scd41.py b/enviro/sensors/scd41.py
new file mode 100644
index 0000000..ae0f307
--- /dev/null
+++ b/enviro/sensors/scd41.py
@@ -0,0 +1,26 @@
+import time
+
+import breakout_scd41
+
+from enviro import i2c
+
+breakout_scd41.init(i2c)
+breakout_scd41.start()
+
+def get_sensor_readings(seconds_since_last):
+  retries = 25
+  while retries > 0 and not breakout_scd41.ready():
+    time.sleep(0.2)
+    retries -= 1
+
+  if retries == 0:
+    return {}
+
+  scd_co2, scd_temp, scd_humidity = breakout_scd41.measure()
+
+  from ucollections import OrderedDict
+  return OrderedDict({
+    "scd_co2": scd_co2,
+    "scd_temperature": scd_temp,
+    "scd_humidity": scd_humidity
+  })
\ No newline at end of file
diff --git a/main.py b/main.py
index 95eff3b..de938bb 100644
--- a/main.py
+++ b/main.py
@@ -73,9 +73,9 @@
   # here you can customise the sensor readings by adding extra information
   # or removing readings that you don't want, for example:
   # 
-  #   del readings["temperature"]        # remove the temperature reading
+  #   del reading["temperature"]        # remove the temperature reading
   #
-  #   readings["custom"] = my_reading()  # add my custom reading value
+  #   reading["custom"] = my_reading()  # add my custom reading value
 
   # is an upload destination set?
   if enviro.config.destination: