Added paho-mqtt dependency to Readme

README.md

@@ -11,7 +11,11 @@
 
        sudo git clone https://github.com/hotplot/enviroplus-mqtt /usr/src/enviroplus-mqtt
 
-5) Add a new file at `/etc/systemd/system/envlogger.service` with the following content:
+5) Install [Paho MQTT tools](https://www.eclipse.org/paho/):
+
+       pip3 install paho-mqtt
+
+6) Add a new file at `/etc/systemd/system/envlogger.service` with the following content:
 
        [Unit]
        Description=Enviro+ MQTT Logger
@@ -62,6 +66,10 @@
                             stabilise before starting to publish readings
         --use-pms5003         if set, PM readings will be taken from the PMS5003
                             sensor
+
+        -r, --retain  tell MQTT broker to retain the last message        
+
+
         --help                print this help message and exit
 
 ## Published Topics

src/logger.py

@@ -15,7 +15,7 @@
 
 
 class EnvLogger:
-    def __init__(self, client_id, host, port, username, password, prefix, use_pms5003, num_samples):
+    def __init__(self, client_id, host, port, username, password, prefix, use_pms5003, num_samples, retain):
         self.bme280 = BME280()
 
         self.prefix = prefix
@@ -33,6 +33,7 @@ def __init__(self, client_id, host, port, username, password, prefix, use_pms500
             self.pm_thread = threading.Thread(target=self.__read_pms_continuously)
             self.pm_thread.daemon = True
             self.pm_thread.start()
+        self.retain = retain
 
 
     def __on_connect(self, client, userdata, flags, rc):
@@ -89,9 +90,9 @@ def take_readings(self):
         return readings
 
 
-    def publish(self, topic, value):
+    def publish(self, topic, value, retain):
         topic = self.prefix.strip("/") + "/" + topic
-        self.client.publish(topic, str(value))
+        self.client.publish(topic, str(value), retain=retain)
 
 
     def update(self, publish_readings=True):
@@ -101,6 +102,6 @@ def update(self, publish_readings=True):
             for topic in self.samples[0].keys():
                 value_sum = sum([d[topic] for d in self.samples])
                 value_avg = value_sum / len(self.samples)
-                self.publish(topic, value_avg)
+                self.publish(topic, value_avg, retain=self.retain)
 
         self.client.loop()

src/main.py

@@ -14,6 +14,7 @@ def parse_args():
     ap.add_argument("--interval", type=int, default=5, help="the duration in seconds between updates")
     ap.add_argument("--delay", type=int, default=15, help="the duration in seconds to allow the sensors to stabilise before starting to publish readings")
     ap.add_argument("--use-pms5003", action="store_true", help="if set, PM readings will be taken from the PMS5003 sensor")
+    ap.add_argument("-r", "--retain", action='store_true', help="tell MQTT broker to retain the last message")
     ap.add_argument("--help", action="help", help="print this help message and exit")
     return vars(ap.parse_args())
 
@@ -30,7 +31,8 @@ def main():
         password=args["password"],
         prefix=args["prefix"],
         use_pms5003=args["use_pms5003"],
-        num_samples=args["interval"]
+        num_samples=args["interval"],
+        retain=args["retain"],
     )
 
     # Take readings without publishing them for the specified delay period,