#include <PID_v2.h> // include the PID library

// define constants for the PID control loop
#define PID_KP 30.0
#define PID_KI 0.7
#define PID_KD 200
#define PID_SAMPLE_TIME 250 // update the PID control loop every 0.25 second

// define constants for the temperature sensor
#define TEMP_SENSOR_PIN A0 // analog input pin for the temperature sensor
#define TEMP_SENSOR_RESISTANCE 10000 // resistance of the NTC temperature sensor in ohms
#define TEMP_SENSOR_B_CONSTANT 3950 // B constant for the NTC temperature sensor
#define TEMP_SENSOR_NOMINAL_RESISTANCE 10000 // nominal resistance of the NTC temperature sensor at 25C

// define constants for the heating element
#define HEATER_PIN 9 // digital output pin for the heating element

// define variables for the PID control loop
double setpoint; // the target temperature
double input; // the current temperature
double output; // the output of the PID control loop, used to control the heating element

// create a PID object
PID pid(&input, &output, &setpoint, PID_KP, PID_KI, PID_KD, DIRECT);

void setup() {
  // initialize the serial communication
  Serial.begin(9600);

  // set the initial setpoint
  setpoint = 25.0; // set the initial setpoint to 25C

  // initialize the PID control loop
  pid.SetSampleTime(PID_SAMPLE_TIME);
  pid.SetOutputLimits(0, 255); // limit the output of the PID control loop to a value between 0 and 255
  pid.SetMode(AUTOMATIC);

  // initialize the heating element
  pinMode(HEATER_PIN, OUTPUT);
}

void loop() {
  // read the current temperature from the temperature sensor
  input = getTemperature();

  // run the PID control loop
  pid.Compute();

  // control the heating element based on the output of the PID control loop
  analogWrite(HEATER_PIN, output);

  // wait for the specified sample time before updating the control loop again
  delay(PID_SAMPLE_TIME);
}

// function to read the temperature from the NTC temperature sensor
double getTemperature() {
  // read the raw analog value from the temperature sensor
  int raw = analogRead(TEMP_SENSOR_PIN);

  // convert the raw value to resistance
  double resistance = TEMP_SENSOR_RESISTANCE / ((1024.0 / raw) - 1.0);

  // calculate the temperature based on the resistance of the temperature sensor
  double temperature = 1.0 / (log(resistance / TEMP_SENSOR_NOMINAL_RESISTANCE) / TEMP_SENSOR_B_CONSTANT + 1.0 / 298.15) - 273.15;

  return temperature;
}