PID Library

 * PID RelayOutput Example
 * Same as basic example, except that this time, the output
 * is going to a digital pin which (we presume) is controlling
 * a relay.  The pid is designed to output an analog value,
 * but the relay can only be On/Off.
 *   To connect them together we use "time proportioning
 * control"  Tt's essentially a really slow version of PWM.
 * First we decide on a window size (5000mS say.) We then 
 * set the pid to adjust its output between 0 and that window
 * size.  Lastly, we add some logic that translates the PID
 * output into "Relay On Time" with the remainder of the 
 * window being "Relay Off Time"

#include <PID_v1.h>
#define RelayPin 6

//Define Variables we'll be connecting to
double Setpoint, Input, Output;

//Specify the links and initial tuning parameters
PID myPID(&Input, &Output, &Setpoint,2,5,1, DIRECT);

int WindowSize = 5000;
unsigned long windowStartTime;
void setup()
  windowStartTime = millis();

  //initialize the variables we're linked to
  Setpoint = 100;

  //tell the PID to range between 0 and the full window size
  myPID.SetOutputLimits(0, WindowSize);

  //turn the PID on

void loop()
  Input = analogRead(0);

   * turn the output pin on/off based on pid output
  unsigned long now = millis();
  if(now - windowStartTime>WindowSize)
  { //time to shift the Relay Window
    windowStartTime += WindowSize;
  if(Output > now - windowStartTime) digitalWrite(RelayPin,HIGH);
  else digitalWrite(RelayPin,LOW);