frequency2temperature.comp

component frequency2temperature "Frequency to temperature. \
Used with a NE555 circuit that converts temperature measured using a \
100k thermistor to a square-wave of variable frequency. ";

pin in float frequency_in "frequency input";
pin out float temperature_out "temperature output (C)";

function _ fp;
license "GPLv2 or later";
;;

#include <rtapi_math.h> /* exp() and ln() */

extern double log(double);

/* see http://en.wikipedia.org/wiki/Thermistor
   The B-parameter equation is:
   1/T = 1/T0 + 1/B * ln(R/R0)
   for a typical 100k thermistor we have
   T0 = 298 K
   R0 = 100 000 ohm
   B  = 2000 K (?)
   
   we get:
   T = 1/ ( 1/T0 + 1/B * ln(R/R0) )
   R = R0 exp( B * (1/T - 1/T0) )
   
   The 555-circuit used with the thermistor is an "astable" cofiguration
   see: http://en.wikipedia.org/wiki/555_timer_IC
   
   the output frequency is:
   f = 1 / (ln(2)*(R1+2*R2)*C1)
   C1 = capacitance from NE555 pins 2 and 6 to ground
   R2 = resistor between NE555 pins 6 and 7. Place the thermistor here!
   R1 = resistor between NE555 pin 7 and supply voltage
   
   
   Given a frequency, we must first find the resistance and then the temperature:
   R2 = 1/2 * ( 1/(f*ln(2)*C1) - R1)
   and 
   T = 1/ ( 1/T0 + 1/B * ln(R/R0) )
*/



/* thermistor constants */
const double T0 = 298;     /* NOTE: Kelvin units! */
const double R0 = 100000;
const double B = 2000;
/* NE555 circuit constants */
const double C1 = 1e-6;
const double R1 = 3500;

float thermistor2temperature(float R) {
   float T;
   if (R<=0)
       R=0.001; // avoid log(0)
   T = 1.0/ ( 1.0/T0 + (1.0/B) * log(R/R0) );
   return T;
}

float frequency2temperature(double f) {
    float R, T;
    if (f<=0)
        f=1; 
    R = 0.5 * ( (1.0/(f*0.6931*C1)) - R1);

    T = thermistor2temperature(R);
    return T;
}

    
FUNCTION(_) {
   temperature_out = frequency2temperature( frequency_in ) - 273.15;
}