Datasheet

void loop(void) {

// read the resistor using the RCtime technique

photocellReading = RCtime(photocellPin);

if (photocellReading == 30000) {

// if we got 30000 that means we 'timed out'

Serial.println("Nothing connected!");

} else {

Serial.print("RCtime reading = ");

Serial.println(photocellReading); // the raw analog reading

// The brighter it is, the faster it blinks!

digitalWrite(ledPin, HIGH);

delay(photocellReading);

digitalWrite(ledPin, LOW);

delay(photocellReading);

}

delay(100);

}

// Uses a digital pin to measure a resistor (like an FSR or photocell!)

// We do this by having the resistor feed current into a capacitor and

// counting how long it takes to get to Vcc/2 (for most arduinos, thats 2.5V)

int RCtime(int RCpin) {

int reading = 0; // start with 0

// set the pin to an output and pull to LOW (ground)

pinMode(RCpin, OUTPUT);

digitalWrite(RCpin, LOW);

// Now set the pin to an input and...

pinMode(RCpin, INPUT);

while (digitalRead(RCpin) == LOW) { // count how long it takes to rise up to HIGH

reading++; // increment to keep track of time

if (reading == 30000) {

// if we got this far, the resistance is so high

// its likely that nothing is connected!

break; // leave the loop

}

// OK either we maxed out at 30000 or hopefully got a reading, return the count

return reading;

}