# Led as an input photodiode [Arduino AVR]

I’m trying to use a LED as a light sensor(input) connected to a Arduino Nano. I connected the led with the Anode at digital pin 8 (PB0) with a resistance of 56 Ohm and the Cathode at an analog pin A0 (PC0). The main idea is to measure the voltage across the parasite capacitance in the LED which was stored during a short 5V pulse(turning PC0 high and then low) in reverse bias. First example is very simple.

uint16_t sen_time = 0;    //Variable for counting
void setup() {
Serial.begin(9600);
// Setting pin 8 to OUTPUT and the rest to INPUT
DDRB |= (1 << PB0);   // Setting OUTPUT PB0
PORTB &=  ~(1 << PB0);   // Setting Anode LOW

TCCR1B |= (1 << CS10); // Setting timer, no prescaling(f = 16Mhz)
}

void loop() {
/* Reverse bias pulse */
DDRC |= (1 << PC0);     //Setting A0 to OUTPUT
PORTC |= (1 << PC0);    //Setting A0 to HIGH
/* Setting Threestate Cathode */
DDRC &=  ~(1 << PC0);    //Setting A0 to INPUT
PORTC &=  ~(1 << PC0);   //Setting A0 to LOW

TCNT1 = 0;  // Setting timer the moment we turn off
//Waiting for the led to discharge and counting
{
sen_time++;
}
Serial.print("TCNT:");
Serial.println(TCNT1);
Serial.print("sen_time:");
Serial.println(sen_time);    //Print the value
sen_time = 0;                //Setting the value to 0
}


The results are as follows. In normal conditions: In darkness: In heavy light:

The variable which is incremented sen_time is pretty much accurate, having high values for darkness and low values for heavy light. But I can’t understand why my timer has trash values all way around.

I’m trying to solve it with interrupts(Pin change interrupts) but getting the pin three-stated just triggers the interrupt all way around. Plus the TCNT got trush values whenever I read it. Getting pull-up resistors just keeps the pin high. Some advices about how to solve this problem preferably with interrupts (due to time restriction on future project). Thank you and I hope to solve it as soon as possible!