thermistors support

cnc/sensors/ads111x.py

@@ -27,14 +27,13 @@ else:
 def measure(channel):
     """
     Measure voltage on chip input.
+    Raises OSError(Errno 121) "Remote I/O error" on reading error.
     :param channel: chip channel to use.
-    :return: Voltage in Volts, None if no device connected.
+    :return: Voltage in Volts.
     """
     global __i2c_dev
     if channel < 0 or channel > 3:
         raise ValueError("Wrong channel")
-    if __i2c_dev < 0:
-        return None
     # configure
     data = struct.pack(">BH",
                        0x01,  # config register

cnc/sensors/thermistor.py

@@ -0,0 +1,70 @@
+"""
+This module reads temperature from NTC thermistor connected to ads111x.
+Circuit diagram for this module should be like this:
+
+     Vcc
+     ---
+      |
+      |
+     .-.
+     | |
+     | |   R1
+     '-'
+      |
+      o----------------o------------> ads111x input
+      |                |
+      | |              |
+     .-./              |
+     | /             + |
+     |/|   R0 NTC    -----  10 uF
+     /-'             -----
+    / |                |
+      |                |
+     _|_              _|_
+     GND              GND
+
+Since ads111x uses internal reference voltage, 3.3V should be well regulated.
+"""
+
+from __future__ import division
+import math
+import time
+
+import ads111x as adc
+
+CELSIUS_TO_KELVIN = 273.15
+
+# Circuit parameters, resistance in Ohms, temperature in Celsius.
+# Beta is thermistor parameter:
+# https://en.wikipedia.org/wiki/Thermistor#B_or_.CE.B2_parameter_equation
+Vcc = 3.3
+R0 = 100000
+T0 = 25
+BETA = 4092
+R1 = 4700
+
+Rinf = R0 * math.exp(-BETA / (T0 + CELSIUS_TO_KELVIN))
+
+
+def get_temperature(channel):
+    """
+    Measure temperature on specified channel.
+    :param channel: ads111x channel.
+    :return: temperature in Celsius
+    """
+    v = adc.measure(channel)
+    if v >= Vcc:
+        raise IOError("Thermistor not connected")
+    if v <= 0:
+        raise IOError("Short circuit")
+    r = v * R1 / (Vcc - v)
+    return (BETA / math.log(r / Rinf)) - CELSIUS_TO_KELVIN
+
+
+# for test purpose
+if __name__ == "__main__":
+    while True:
+        for i in range(0, 4):
+            print("T{}={}".format(i, get_temperature(i)))
+        print("-----------------------------")
+        time.sleep(0.5)