Driver for Engduino Thermistor

Files
file	EngduinoThermistor.cpp
file	EngduinoThermistor.h

Classes
class	EngduinoThermistorClass

Enumerations
enum	temperatureUnits { CELSIUS, KELVIN, FAHRENHEIT }

Functions
	EngduinoThermistorClass::EngduinoThermistorClass ()
	Constructor. More...
void	EngduinoThermistorClass::begin ()
	begin function - must be called before using other functions More...
void	EngduinoThermistorClass::end ()
	end function - switch off the LEDs More...
float	EngduinoThermistorClass::temperature (temperatureUnits units=CELSIUS)
	Get the temperature. More...
float	EngduinoThermistorClass::temperature (temperatureUnits units, float B, float T0, float R0, float R_Balance)
	Get the temperature. More...
uint16_t	EngduinoThermistorClass::temperatureRaw ()
	Get the raw potential divider reading. More...

Variables
EngduinoThermistorClass	EngduinoThermistor = EngduinoThermistorClass()
EngduinoThermistorClass	EngduinoThermistor

Detailed Description

This is the driver code for reading temperature. This can equally well be done in a sketch file, but it is also here for completeness

The Engduino has a Murata NCP18WF104J03RB thermistor attached to analogue input A5. This thermistor has a resistance of 100K at 25C.

Function Documentation

void EngduinoThermistorClass::begin

(

)

begin function - must be called before using other functions

Nothing to be done.

void EngduinoThermistorClass::end

(

)

end function - switch off the LEDs

Nothing to be done.

EngduinoThermistorClass::EngduinoThermistorClass

(

)

Constructor.

C++ constructor for this class. Empty.

float EngduinoThermistorClass::temperature

(

temperatureUnits

units = CELSIUS

)

Get the temperature.

Parameters

units

A choice between CELSIUS (default), KELVIN, or FAHRENHEIT

Returns

The temperature on the given scale.

This function makes a call to the function that actually works out what a particular resistance means.

For the Murata NCP18WF104J03RB, the thermistor has a resistance of 100K at 25C and a B value of 4250 and a 100K balance resistor.

float EngduinoThermistorClass::temperature	(	temperatureUnits	units,
		float	B,
		float	T0,
		float	R0,
		float	R_Balance
	)

Get the temperature.

Parameters

units	A choice between CELSIUS (default), KELVIN, or FAHRENHEIT
B	Parameter in the B equation - see data sheet
T0	Temperature in Kelvin at which resistance is R0 - see data sheet
R0	Resistance at temperature T0 - see data sheet
R_Balance	Value of the balance resistor in the potential divider (100K on Engduino v1.0)

Returns

The temperature on the given scale.

This function measures the voltage and, as a consequence, calculates the resistance of the thermistor. From this, we can figure out the actual temperature

NTC thermistors can be characterised with the B (or beta) parameter equation,

1/T = 1/T0 + 1/B*ln(R/R0)

Where the temperatures are in Kelvin and R0 is the resistance at temperature T0 (25 °C = 298.15 K).

So far as the resistance is concerned, we have a potential divider with a balance resistor of value R_Balance. Assuming that the resistance of the NTC is R and the voltage across it is V, then Ohms law gives: V/Vcc = R/(R + R_Balance)

Given that V/Vcc is just (analog in)/1024 for a 10 bit ADC, we can work out that: R = (R_Balance * analogIn) / (1024 - analogIn)

uint16_t EngduinoThermistorClass::temperatureRaw

(

)

Get the raw potential divider reading.

Returns

Raw potential divider reading.

Raw voltage reading from NTC potential divider circuit