title: Arduino: Using analogRead() on Photoresistor to read LED with PWM
body: <p>I have been working on a project with Arduino and have come across something that I find fascinating/confusing. So, I had to test something before constructing this project. I built a simple circuit that consists of just an LED and photoresistor. What I had to test was whether the photoresistor was capable of determining the brightness of an LED that was being dimmed through PWM. My initial expectation was that this would not work (the photoresistor would either read 1023 or 0 because PWM is achieved digitally). To my surprise, the photoresistor was able to accurately read the brightness of the LED (accurately to an extent -- this is simply based off of comparing the apparent brightness of the PWM LED with an LED placed in series with a certain resistor)! This is exactly what I wanted, but I am just curious as to why this works. I am not sure if my original doubt was due to a misunderstanding of photoresistors or PWM. Any help would be much appreciated. Thank you!</p>
  
  <p>Here is the code I am running (I am not using the analogWrite() function because the project I am working on requires me to have a certain level of control over the PWM):</p>
  
  <pre><code>const int LED_PIN = 9;
  const int PHOTO_PIN = 0;
  
  //These values have been altered and tested
  const int HIGH_TIME = 250;
  const int LOW_TIME = 2750;
  
  void setup()
  {
    pinMode(LED_PIN, OUTPUT);
    pinMode(PHOTO_PIN, INPUT);
  
    Serial.begin(9600);
  }
  
  void loop()
  {
   digitalWrite(LED_PIN, HIGH);
   delayMicroseconds(HIGH_TIME);
   digitalWrite(LED_PIN, LOW);
   delayMicroseconds(LOW_TIME);
  
   Serial.println(analogRead(PHOTO_PIN));
  }
  </code></pre>