This code will allow a Rasperry Pi to read temperatures from the Freetronics infrared temperature sensor.

irtemp.c

// irtemp.c
// Raspberry Pi code to read temperature from the Freetronics infrared temperature module.
// By Jeremy Bornstein <jeremy@jeremy.org> after the Arduino-specific code from Andy Gelme (@geekscape)
// See http://www.freetronics.com/irtemp for more information about the module.
// This code requires the wiringPi library to be installed. See http://wiringpi.com/ for more information.
//
// NOTE: On the author's system, isnan(FP_NAN) returns false.
// This is clearly an error which could impact this code; I haven't tracked it down yet.
//
// There is no makefile. This isn't polished up to be a library--just a demonstration.
// Compile with:
//      gcc -o irtemp irtemp.c -lwiringPi
// and then run:
//      ./irtemp
//      ./irtemp ambient
//      ./irtemp ir
//      ./irtemp junk



#include <wiringPi.h>
#include <stdio.h>
#include <stddef.h>
#include <stdlib.h>
#include <unistd.h>
#include <math.h>


typedef unsigned char bool;
typedef unsigned char byte;
#define false 0
#define true 1

static const long IRTEMP_TIMEOUT = 2000;  // milliseconds
static const long IRTEMP_DATA_AMBIENT = 0x66;
static const long IRTEMP_DATA_IR = 0x4C;
static const long IRTEMP_DATA_JUNK = 0x53;

// Customize these for your own pins. These are the wiringPi pin numbers.
const int ACQUIRE_PIN =	7;
const int DATA_PIN =	0;
const int CLOCK_PIN =	1;

#define DATA_SIZE 5
bool capturing = false;
volatile unsigned char data_bit = 8;
volatile unsigned char data_byte = 0;
volatile byte data[DATA_SIZE];

void clock_pin_falling (void) {
    if (!capturing) return;
    if (data_byte >= DATA_SIZE) return;

    data_bit--;
    byte value = digitalRead(DATA_PIN);
    data[data_byte] |= value << data_bit;

    if (data_bit == 0) {
        data_bit = 8;
        data_byte ++;
    }
}

void reset_data() {
    byte i;
    for (i = 0; i < DATA_SIZE;  i ++) data[i] = 0;
    data_bit = 8;
    data_byte = 0;
}

void sensor_enable(bool enabled) {
    capturing = enabled;
    digitalWrite(ACQUIRE_PIN, enabled ? LOW : HIGH);
}

float temperature_read() {
    int msb = data[1] << 8;
    int lsb = data[2];
    return((msb + lsb) / 16.0 - 273.15);
}

bool temperature_data_valid() {
    byte checksum = (data[0] + data[1] + data[2]) & 0xFF;
    return(data[3] == checksum && data[4] == 0x0d);
}

float temperature_retrieve(byte data_type) {
    float temperature = FP_NAN;
    bool temperature_valid = false;
    long time_start = millis();
    bool done = false;

    sensor_enable(true);

    while (!done) {
        reset_data();

        while (data_byte < DATA_SIZE) {
            if ((millis() - time_start) > IRTEMP_TIMEOUT) {
                printf("TIMEOUT\n");
                done = true;
                break;
            }
        }

        //printf("%x %x %x %x %x\n",data[0], data[1], data[2], data[3], data[4]);

        if (!done && data[0] == data_type) {
            if (temperature_data_valid()) {
                temperature = temperature_read();
                done = true;
            }
        }
    }

    sensor_enable(false);
    return(temperature);
}


void temperature_setup() {
    wiringPiSetup();	// NOTE: uses "wiringPi" pin numbering scheme
    pinMode(ACQUIRE_PIN, OUTPUT);
    pinMode(DATA_PIN, INPUT);
    pinMode(CLOCK_PIN, INPUT);

    digitalWrite (ACQUIRE_PIN, HIGH);
    digitalWrite (DATA_PIN, HIGH);
    digitalWrite (CLOCK_PIN, HIGH);
    wiringPiISR (CLOCK_PIN, INT_EDGE_FALLING, &clock_pin_falling);
}

void main(int argc, char *argv[]) {
    temperature_setup();

    byte temperature_type = IRTEMP_DATA_IR; // default

    if (argc > 2) {
        printf("Specify no argument, or exactly one of: ir, ambient, junk\n");
        exit(-1);
    }

    if (argc == 2) {
        if (!strcmp(argv[1], "ir"))
            temperature_type = IRTEMP_DATA_IR;
        else if (!strcmp(argv[1], "ambient"))
            temperature_type = IRTEMP_DATA_AMBIENT;
        else if (!strcmp(argv[1], "junk"))
            temperature_type = IRTEMP_DATA_JUNK;
        else {
            printf("Invalid temperature type. Valid values are: ir, ambient, junk\n");
            exit(-1);
        }
    }

    float temp = temperature_retrieve(temperature_type);
    if (isnan(temp)) {
        printf("N/A\n");
    } else {
        printf("%f\n", temp);
    }
}