You need to have some basics to do this (what will your heater be? how much power? powered by mains? isolation/safety? can you wire stuff? do you know what SPI is and which pins of an Arduino are used? etc)
There are modules you can get do do sensor interfacing. For example, Adafruit's MAX31855-based thermocouple amp:
https://www.adafruit.com/product/269
You'd need that and a K-type thermocouple for this example (compiles...not tested...)
It's purposely vague on how to control the heater. I simply use a "relay" but you'd need to determine what the physical implementation is (keeping in mind safety, power etc)
#include <SPI.h>
#include "Adafruit_MAX31855.h"
#define HIGH_TEMP_LIMIT 30.5 //oC upper limit (heat-off)
#define LOW_TEMP_LIMIT 29.5 //oC lower limit (heat-on)
#define SAMPLE_INTERVAL 100ul //mS mS between temperature samples
#define ERROR_LED_BLINK 150ul //mS mS on or off time of LED for sensor error
#define RELAY_OFF HIGH //# may have to change depending on setup/wiring
#define RELAY_ON LOW
#define LED_OFF LOW //# level to turn off LED
#define LED_ON HIGH
const byte pinRelay = 2; //pin# relay coil control
const byte pinLED = LED_BUILTIN; //pin# built-in LED for heat/error indication
const byte pinTC_CS = 4; //pin# chip-sel for MAX31855
//create a thermocouple instance
Adafruit_MAX31855 thermoCouple( pinTC_CS );
float
fTemperature;
unsigned long
tNow,
tSample;
bool
bLEDState;
void setup()
{
pinMode( pinRelay, OUTPUT ); //set relay control to output
digitalWrite( pinRelay, RELAY_OFF ); //ensure it's off
pinMode( pinLED, OUTPUT ); //same with LED
digitalWrite( pinLED, LED_OFF );
bLEDState = false; //false = off, true = on
//
tSample = millis();
}//setup
//state names
#define SENSOR_FAILURE 0 //sensor failure detected state
#define HEATER_OFF 1 //heater off state
#define HEATER_ON 2 //heater on state
//
void loop()
{
static byte
stateHeater = HEATER_OFF;
tNow = millis();
//if sensor failure detected (nonsense returned)...
if( stateHeater == SENSOR_FAILURE )
{
//just blink the LED quickly
if( (tNow - tSample) >= ERROR_LED_BLINK )
{
tSample = tNow;
bLEDState ^= true;
digitalWrite( pinLED, bLEDState );
}//if
}
else
{
//is it time for a sample?
if( (tNow - tSample) < SAMPLE_INTERVAL )
return;
tSample = tNow;
//take a reading, make sure it's valid
fTemperature = thermoCouple.readCelsius();
if( isnan( fTemperature ) )
{
//reading is not valid; enter safe state (heater off, blinking LED)
stateHeater = SENSOR_FAILURE;
digitalWrite( pinRelay, RELAY_OFF );
digitalWrite( pinLED, LED_OFF );
bLEDState = false;
}//if
else
{
//is the heater on or off now?
switch( stateHeater )
{
case HEATER_OFF:
//off...has solution cooled below the lower limit?
if( fTemperature <= LOW_TEMP_LIMIT )
{
//yes; turn the heater and LED on
digitalWrite( pinRelay, HEATER_ON );
stateHeater = HEATER_ON; //move to "on" state
digitalWrite( pinLED, LED_ON );
bLEDState = true;
}//if
break;
case HEATER_ON:
//on now...has solution warmed to the upper limit?
if( fTemperature >= HIGH_TEMP_LIMIT )
{
//yes; turn off the heater and LED
digitalWrite( pinRelay, HEATER_OFF );
stateHeater = HEATER_OFF; //move to "off" state
digitalWrite( pinLED, LED_OFF );
bLEDState = false;
}//if
break;
}//switch
}//else
}//else
}//loop