feat: add MH-Z19 sensor module (#365)

* feat: add MH-Z19 sensor module
This adds support for NDIR CO2 sensor MH-Z19. The sensor module supports
reading CO2 ppm value and configuring the ppm range over the UART/serial
interface.

config.example.yml

@@ -34,6 +34,11 @@ sensor_modules:
     type: DS18S20
     address: 000803702e49
 
+  - name: mhz19
+    module: mhz19
+    device: "/dev/ttyS1"
+    range: 5000
+
 digital_inputs:
   - name: button
     module: raspberrypi
@@ -76,3 +81,10 @@ sensor_inputs:
     module: ds18b22
     interval: 10
     digits: 2
+
+  - name: co2_mhz19
+    module: mhz19
+    interval: 30
+    ha_discovery:
+      name: CO2 MH-Z19
+      device_class: carbon_dioxide

mqtt_io/modules/sensor/mhz19.py

@@ -0,0 +1,63 @@
+"""
+MH-Z19 NDIR CO2 sensor
+"""
+
+from mqtt_io.modules.sensor import GenericSensor
+from mqtt_io.types import CerberusSchemaType, ConfigType, SensorValueType
+
+REQUIREMENTS = ("pyserial",)
+CONFIG_SCHEMA: CerberusSchemaType = {
+    "device": dict(type="string", required=True, empty=False),
+    "range": dict(type="integer", required=False, empty=False, default=5000,
+                  allowed=[2000, 5000, 10000]),
+}
+
+class Sensor(GenericSensor):
+    """
+    Implementation of Sensor class for the MH-Z19 CO2 sensor using UART/serial.
+    """
+
+    @staticmethod
+    def _calc_checksum(data: bytes) -> bytes:
+        value = sum(data[1:]) % 0x100
+        value = (0xff - value + 1) % 0x100
+        return value.to_bytes(1, "big")
+
+    @staticmethod
+    def _check_checksum(data: bytes) -> bool:
+        return Sensor._calc_checksum(data[:-1]) == data[-1:]
+
+    @staticmethod
+    def _add_checksum(data: bytes) -> bytes:
+        return data + Sensor._calc_checksum(data)
+
+    def setup_module(self) -> None:
+        # pylint: disable=import-error,import-outside-toplevel
+        import serial  # type: ignore
+
+        self.ser = serial.Serial(
+            port=self.config["device"],
+            baudrate=9600,
+            bytesize=serial.EIGHTBITS,
+            parity=serial.PARITY_NONE,
+            stopbits=serial.STOPBITS_ONE,
+        )
+
+        # setup detection range
+        cmd = Sensor._add_checksum(b"\xff\x01\x99\x00\x00\x00" +
+                                   self.config["range"].to_bytes(2, "big"))
+        self.ser.write(cmd)
+        # no response
+
+    def cleanup(self) -> None:
+        self.ser.close()
+
+    def get_value(self, sens_conf: ConfigType) -> SensorValueType:
+        self.ser.write(Sensor._add_checksum(b"\xff\x01\x86\x00\x00\x00\x00\x00"))
+        resp = self.ser.read(9)
+
+        if len(resp) == 9:
+            if resp[0:2] == b"\xff\x86" and Sensor._check_checksum(resp):
+                return int.from_bytes(resp[2:4], "big")
+
+        return None