//parameters of sensor calibration
#define RLOAD 2.0
#define RZERO 546
#define PARA 116.6020682
#define PARB 2.769034857

int val = 0;
float ppm = 0;
void setup() {

  //Serial.begin(9600); // not for attiny
  
  //leds as outputs
  pinMode(7, OUTPUT);
  pinMode(8, OUTPUT);
  pinMode(9, OUTPUT);
}

void loop()
{
  val = analogRead(2);                    // Read sensor analog

  float val2 = (1023 / val * 5 - 1) * RLOAD;
  ppm = PARA * (val2 / RZERO) * (val2 / RZERO);  //Value in ppm

  if (ppm < 750) {                               // Geen led on
    digitalWrite(7, HIGH);
    digitalWrite(8, LOW);
    digitalWrite(9, LOW);
  }
  if (ppm >= 751 && ppm < 1200) {               //Yellow led on
    digitalWrite(7, LOW);
    digitalWrite(8, HIGH);
    digitalWrite(9, LOW);
  }
  if (ppm >= 1201) {                            // Red led on
    digitalWrite(7, LOW);
    digitalWrite(8, LOW);
    digitalWrite(9, HIGH);
  }

  //for confirmation (on arduino Uno)
  //Serial.print(" C02 value in ppm: ");
  //Serial.println(ppm);
}