Non-invasive LED watcher

Other Hardware Sensors
1

I want to watch the LED of a piece of equipment. I can't open it (implies on loosing warranty). I'm seeking solutions to read this LED in a non-invasive way such as using LDR, Photodiode, etc. I've got no experience on this so any ideas and experience on this is welcome.

Extra question: I already have a photo reflective sensor (like this http://www.robotshop.com/productinfo.aspx?pc=RB-See-191&lang=en-US), can its photodiode be used to sense any colored LED?

2

Gatis:
I want to watch the LED of a piece of equipment. I can't open it (implies on loosing warranty). I'm seeking solutions to read this LED in a non-invasive way such as using LDR, Photodiode, etc. I've got no experience on this so any ideas and experience on this is welcome.

Extra question: I already have a photo reflective sensor (like this http://www.robotshop.com/productinfo.aspx?pc=RB-See-191&lang=en-US), can its photodiode be used to sense any colored LED?

Yes, it can be used that way and that would be a good solution to your problem.

3

I've use a small LDR to track the state of an LED. Use something opaque to keep ambient light from triggering your LDR.

Here is some code that self-tunes. It keeps track of the max and min LDR values and uses the mid-point as the limit for ON vs OFF.

void CheckRecordingLight(boolean &isRecording, unsigned long &lastRecordLightTime,int APIN_RECORDING,  
unsigned int &minReading, unsigned int &maxReading, unsigned int faultFlag)
{
  int reading = analogRead(APIN_RECORDING);  // Read the LDR
  if (reading < minReading)
      minReading = reading;  // New minimum
  if (reading > maxReading)
      maxReading = reading;  // New maximum
  unsigned int avgReading = (minReading + maxReading) / 2;
  
  // Light is ON if reading is less than the midpoint (unless this is the first reading)
  boolean recordLight = (reading < avgReading) && (minReading != maxReading);

  if (recordLight)
  {
    lastRecordLightTime = millis();
    isRecording = true;
  }

  // If the light has been off for more than two seconds
  if (millis() - lastRecordLightTime > 2000)
  {
    isRecording = false;
  }
}
4

I tried using the photodiode of a Photo Interrupter but it did not work with colored LEDs. In fact, it only works with IR-LEDs.

I wonder about the pros and cons of using LDR vs phototransitor vs photodiode (one that senses non-IR LEDs).

5

Gatis:
I tried using the photodiode of a Photo Interrupter but it did not work with colored LEDs. In fact, it only works with IR-LEDs.

I find that surprising. Google image search for "photodiode wavelength" and you'll find graphs of their sensitivity to various wavelengths -- and they're pretty sensitive to a wide range.

For something else to try, see Arduino Playground - HomePage. Use an LED of the same color as you're detecting.