#Thermistor Class

This class makes it simple for an imp to read an NTC (“Negative Temperature Coefficient”) thermistor and determine the temperature. Thermistors are essentially temperature-dependent resistors. To use as a thermometer, a thermistor is used as half of a resistive divider, where the voltage across the full divider is known. The Imp then reads the voltage at the center of the divider to determine the ratio of resistance of the thermistor and the bias resistor (also the nominal resistance of the thermistor), from which the temperature can be derived.

To add this library to your project, add #require "Thermistor.class.nut:1.0.0" to the top of your device code

Usage

Hardware

A resistive divider can be formed with the thermistor on the top or the bottom; this class allows for either configuration. The top of the divider should be connected to the same rail as an imp’s V_DDA pin (or V_DD pin, in the case of the imp001, as V_DD and V_DDA are internally connected). The bottom of the divider should be connected to ground.

The resistance of the bias resistor in the voltage divider should be equal to the nominal resistance of the thermistor (the resistance at T0). This simplifies the temperature calculation and allows the largest dynamic range.

The center of the divider must be connected to a pin capable of analog input. On the imp001, any pin can be used as an analog input. On the imp002 or imp003, only some pins can be configured this way, so check the Imp Pin Mux chart.

Software

The thermistor class takes three to five parameters (three required, two optional):

Parameter Name	Description	Optional/Required
pin	Imp pin object capable of ANALOG_IN	Required
b_therm	Thermistor ß parameter, from datasheet	Required
t0_therm	Thermistor T0 parameter, from datasheet	Required
points	number of readings to average when reading the thermistor	Optional, defaults to 10
high_side_therm	Set false to place thermistor on low side of divider	Optional, defaults to true

The ß and T0 parameters are all available on the thermistor datasheet:

Parameter	Meaning
ß	Characteristic of the thermistor. Most thermistors have many ß values listed, for various temperature ranges. Choose the value for the temperature range you will be operating in.
T0	Temperature at which the nominal resistance (R) of the thermistor is measured. Typically room temperature (~25ºC)

Constructor

const b_therm = 3988
const t0_therm = 298.15

pin <- hardware.pin9

// Thermistor on bottom of divider

myThermistor <- Thermistor(pin, b_therm, t0_therm, 10, false)

Class Methods

Reading a thermistor

The thermistor object provides three methods for reading the temperature: readK(), readC() and readF(). These return the temperature in Kelvin, Celsius and Fahrenheit, respectively.

local kelvin = therm.readK()
local celsius = therm.readC()
local fahrenheit = therm.readF()

License

Thermistor is licensed under MIT License.