// Define pin for 555 timer output
const int pulsePin = 2;  // Interrupt pins on Arduino Uno are pin 2 (INT0) and pin 3 (INT1)
const float knownCapacitance = 0.000001; // 1uF known capacitance
const float knownResistorR1 = 10000;      // 10kOhm known resistor (R1)

// Variables for timing
volatile unsigned long lastTime = 0;     // Last interrupt time
double period=0;
void setup() {
  // Start serial communication
  Serial.begin(9600);

  // Set the pulsePin as input
  pinMode(pulsePin, INPUT);

  // Attach interrupt to pulsePin (for both rising and falling edge detection)
  attachInterrupt(digitalPinToInterrupt(pulsePin), handlePulse, RISING);
}

void loop() {
    
    if (period>0)
    {

      float frequency = 1000000.0 / period;

      Serial.println(period);

      Serial.print("Frequency: ");
      Serial.print(frequency);
      Serial.println(" Hz"); 

      float unknownResistance = (0.72/(frequency*knownCapacitance))-(knownResistorR1/2);

      Serial.print("Unknown Resistance: ");
      Serial.print(unknownResistance);
      Serial.println(" ohms");
      period=0;

    }
    delay(1000);  // Wait a second before next reading
  }

// Interrupt Service Routine (ISR) to handle pulse changes
void handlePulse() {
  unsigned long currentTime = micros();  // Get the current time in microseconds
  period=currentTime-lastTime;
  lastTime=currentTime;
}