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LED selection

Not all LEDs behave as good as others so a preliminary evaluation of a set of different products is suggested:

  1. Position a couple of identical LEDs on a breadboard aiming at each other
  2. Connect one channel of the oscilloscope to the positive lead of one LED
  3. Power the connected LED with a 500Hz square wave
  4. Connect oscilloscope's remaining channel to the other LED's positive lead
  5. Connect all grounds together

If you don't have a square wave generator you can use an Arduino:

digitalWrite(12, HIGH);
digitalWrite(12, LOW);

Looking at the 2 channels it should be observed:

  • The Transmitter's channel showing a crisp 5v signal
  • The Receiver's channel showing a lower voltage signal with transitions slopes

Testing different LEDs with the same conditions shows that some produce a higher or lower voltage and transitions that are steeper or slower and more gradual. To obtain the best performance it is required to find a LED with the following characteristics:

  • Highest voltage produced when hit by light
  • Fastest and steepest transitions between states

The picture below shows the KCL5587S that is evidently not the LED we are looking for.

AnalogSampling PJDLS bad LED

The L-53SF4C instead is able to run flawlessly at MODE 3 (3773Bb or 471B/s):

AnalogSampling PJDLS good LED

The picture below shows a bidirectional exchange where both packet and acknowledgement are clearly visible:

AnalogSampling PJDLS bidirectional exchange