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:2.0.0" to the top of your device code
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 VDDA pin (or VDD pin, in the case of the imp001, as VDD and VDDA 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, imp003 and imp004m, only some pins can be configured this way, so check the appropriate Imp Pin Mux chart. The imp005 does not support analog input.
The Thermistor class takes three to five parameters. Three are required, two are optional:
| Parameter Name | Description | Optional/Required |
|---|---|---|
| pin | Imp pin object capable of analog input | 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 (~25°C) |
const b_therm = 3988;
const t0_therm = 298.15;
pin <- hardware.pinJ;
// Thermistor on bottom of divider
therm <- Thermistor(pin, b_therm, t0_therm, 10, false);The read() method returns the temperature in Celsius.
local celsius = therm.read();
server.log(celsius);The read() method returns the temperature in Kelvin.
local kelvin = therm.readK();
server.log(kelvin);Thermistor is licensed under MIT License.