Not all LEDs behave as good as others so a preliminary evaluation of a set of different products is suggested:
- Position a couple of identical LEDs on a breadboard aiming at each other
- Connect one channel of the oscilloscope to the positive lead of one LED
- Power the connected LED with a 500Hz square wave
- Connect oscilloscope's remaining channel to the other LED's positive lead
- Connect all grounds together
If you don't have a square wave generator you can use an Arduino:
digitalWrite(12, HIGH);
delay(1);
digitalWrite(12, LOW);
delay(1);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.
The L-53SF4C instead is able to run flawlessly at MODE 3 (3773Bb or 471B/s):
The picture below shows a bidirectional exchange where both packet and acknowledgement are clearly visible:
