Build Guitar Pedal Using Arduino UNO

// CC-by-www.Electrosmash.com
// Based on OpenMusicLabs previous works.
// pedalshield_uno_booster.ino: pressing the pushbutton_1 or 2 turns the volume up or down.
 
//defining harware resources.
  #define LED 13
  #define FOOTSWITCH 12
  #define TOGGLE 2
  #define PUSHBUTTON_1 A5
  #define PUSHBUTTON_2 A4
 
//defining the output PWM parameters
  #define PWM_FREQ 0x00FF // pwm frequency - 31.3KHz
  #define PWM_MODE 0 // Fast (1) or Phase Correct (0)
  #define PWM_QTY 2 // 2 PWMs in parallel
 
//other variables
  int input, vol_variable=512;
  int counter=0;
  unsigned int ADC_low, ADC_high;
 
  void setup() { //setup IO - inputs/outputs pins configurations and pull-ups
  pinMode(FOOTSWITCH, INPUT_PULLUP);
  pinMode(TOGGLE, INPUT_PULLUP);
  pinMode(PUSHBUTTON_1, INPUT_PULLUP);
  pinMode(PUSHBUTTON_2, INPUT_PULLUP);
  pinMode(LED, OUTPUT);
  
// setup ADC- configured to be reading automatically the hole time.
  ADMUX = 0x60; // left adjust, adc0, internal vcc
  ADCSRA = 0xe5; // turn on adc, ck/32, auto trigger
  ADCSRB = 0x07; // t1 capture for trigger
  DIDR0 = 0x01; // turn off digital inputs for adc0
 
// setup PWM - for more info about this config check the forum.
  TCCR1A = (((PWM_QTY - 1) << 5) | 0x80 | (PWM_MODE << 1)); //
  TCCR1B = ((PWM_MODE << 3) | 0x11); // ck/1
  TIMSK1 = 0x20; // interrupt on capture interrupt
  ICR1H = (PWM_FREQ >> 8);
  ICR1L = (PWM_FREQ & 0xff);
  DDRB |= ((PWM_QTY << 1) | 0x02); // turn on outputs
  sei(); // turn on interrupts - not really necessary with arduino
  }
 
void loop()
{
//Turn on the LED if the effect is ON.
  if (digitalRead(FOOTSWITCH)) digitalWrite(LED, HIGH);
     else  digitalWrite(LED, LOW);
//nothing more here, all happens in the Timer 1 interruption.
}
 
ISR(TIMER1_CAPT_vect) //Timer 1 interruption.
{
  // read the ADC input signal data: 2 bytes Low and High.
  ADC_low = ADCL; // Low byte need to be fetched first
  ADC_high = ADCH;
  //construct the input sumple summing the ADC low and high byte.
  input = ((ADC_high << 8) | ADC_low) + 0x8000; // make a signed 16b value
 
// The push-buttons are checked now:
counter++; //to save resources, the push-buttons are checked every 100 times.
if(counter==100)
{
counter=0;
if (!digitalRead(PUSHBUTTON_1)) {
  if (vol_variable<1024) vol_variable=vol_variable+1; //increase the vol
    }
if (!digitalRead(PUSHBUTTON_2)) {
  if (vol_variable>0) vol_variable=vol_variable-1; //decrease vol
    }
}
 
//the amplitude of the signal is modified following the vol_variableusing the Arduino map fucntion
  input = map(input, 0, 1024, 0, vol_variable);
 
//write the PWM output signal
  OCR1AL = ((input + 0x8000) >> 8); // convert to unsigned, send out high byte
  OCR1BL = input; // send out low byte
}