/***********************************************************************        
 * Filename ResCap.c
 * This program demonstrates a way to simulate an analog read by measuring 
 * the time it takes to charge a capacitor through a variable resistor.
 * It uses a photocell and a force sensitive resistor but the technique will 
 * work to read a potentiometer or any resistive sensor.
 ***********************************************************************/

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

int photocellPin = 0;   // Photocell and capacitor connected to pin zero.
int fsrPin = 2;         // Force sensitive resistor and capacitor connected to pin two.
int ledPin = 1;         // The LED connected to pin one.
int toggle = 1;	        // On/off state of LED.

unsigned long time;     // Save the time to set the blink rate.

/***********************************************************************        
 * RCtime() - Function, uses a digital pin to measure a resistive sensor
 * by first discharging capacitor then measuring the time it takes to 
 * charge the capacitor through a variable resistor. When the voltage 
 * rises to Vcc/2 the pin will go high.
 ***********************************************************************/
int RCtime(int RCpin)
{
  int PinVal = 0;                     // Start with zero.
                                      
  pinMode(RCpin, OUTPUT);             // Set pin to output and pull to LOW. (ground)
  digitalWrite(RCpin, LOW);

  delay(2);                           // Allow time to let capacitor discharge.
                          
  pinMode(RCpin, INPUT);              // Now set the pin to an input and...
  while (digitalRead(RCpin) == LOW) 
  {                                   // Count how long it takes to rise up to HIGH.
    PinVal++;                         // Increment to keep track of time.
    if (PinVal == 30000) 
    {
      break;                          // if we got this far, the resistance is to high,
    }                                 // no input to sensor, leave the loop.
  }                                   
  PinVal = PinVal/25;                 // Divide by 25 for calibration.

  if(PinVal > 1023) PinVal = 1023;    // Cap The value at 1023.

  return PinVal;                      // Returns zero - 1023.
}

/**************************************************************************        
 * loop() - function runs in a continuous loop until program is stopped.
 *
 * More presure on the force sensitive resistor makes the LED blink faster.
 * More light on the photocell makes the light dimmer.
 **************************************************************************/
void loop(void)
{
  int photocellReading = RCtime(photocellPin); // Read photocell.
  int fsrReading = RCtime(fsrPin);             // Read force sensitive resistor.

  if(millis()-time > (fsrReading))
  {
    toggle = !toggle;          //If true not toggle 
    time = millis();           //and reset time.
  }
 
  if(toggle) pwmWrite(ledPin, photocellReading*4);
  else pwmWrite(ledPin, 0);

//  printf("photocell %d - fsr %d\n", photocellReading, fsrReading); // uncomment for debugging.
}

/***********************************************************************        
 * setup() - function is run by main() one time when the program starts.
 ***********************************************************************/
void setup(void)
{
  wiringPiSetup();   // Required.

  pinMode(ledPin, PWM_OUTPUT);

  time = millis();   // Save the time to set the blink rate.
}

/***********************************************************************        
 * main() - required
 ***********************************************************************/
int main(void)
{
  setup();

  while(1)
  {
    loop();
  }
}