External sensors (#153)

* proof of principle, a DS18B20 is read and its value is transferred via MQTT, also discovery message works

* added names for ext sensors

* bugfix for names: there was a problem with the topics & added SHT21 support

* catch exceptions if a problem during readout occurs, shrink comments in config-file

* include ext_sensors in README file

* work in progress

* bugfix, also for ID of ds18b20 sensor

* Revert "bugfix, also for ID of ds18b20 sensor"

This reverts commit 1353b840b0b6142670ec59438db4b33fab764b8e.

file was intended for another branch

* Revert "work in progress"

This reverts commit b3e884f79f5356b79f64931cb4d45a150bfce2c7.

these changes were intended for another branch

* bugfix for ID of ds18b20 sensor

ext_sensor_lib/ds18b20.py

@@ -0,0 +1,61 @@
+#!/usr/bin/python3
+import time
+import os
+"""
+This reads the temp value of a DS18B20 sensor
+"""
+
+
+def sensor_DS18B20(sensor_id, verbose=False):
+    """
+    :param sensor_id: Name of the sensor id, can be found in /sys/bus/w1/devices
+    :type sensor_id: string
+    """
+    # read the file
+    try:
+        file = open('/sys/bus/w1/devices/28-%s/w1_slave' % sensor_id)
+        filecontent = file.read()
+        file.close()
+        # read temperature value and convert it
+        stringvalue = filecontent.split("\n")[1].split(" ")[9]
+        temperature = float(stringvalue[2:]) / 1000
+    except IOError as e:
+        # if an error occurs, we return -300
+        temperature = float(-300)
+
+    # format the temperature
+    temp = '%6.1f' % temperature
+    temp = round(temperature, 1)
+    # if we set the verbose, we print the current temperature
+    if verbose:
+        print ("1-wire %s: temp=%.1f" % (sensor_id, temp))
+    return temp
+
+
+def get_available_sensors():
+    """Returns all available sensors"""
+    # https://github.com/rgbkrk/ds18b20/blob/master/ds18b20/__init__.py
+    sensors = []
+    for sensor in os.listdir("/sys/bus/w1/devices"):
+        if sensor.startswith("28-"):
+            # we put all sensor ids into the list (except the "28-" part)
+            sensors.append(sensor[3:])
+    return sensors
+
+
+if __name__ == "__main__":
+    try:
+        # call the function to get all sensor_ids as a list
+        sensors_ids = get_available_sensors()
+        while True:
+            for sensor_id in sensors_ids:
+                sensor_DS18B20(sensor_id=sensor_id, verbose=True)
+                #sensor_DS18B20("0014531448ff")
+            time.sleep(2)
+    except IOError as e:
+        errno, strerror = e.args
+        print("I/O error({0}): {1}".format(errno,strerror))
+    except KeyboardInterrupt:
+        print ("keyboard interrupt")
+
+