I am a student and I have to design a visible light communications project. The requirements are 20 cm distance between the receiver and transmitter, 20 kbps/s data rate and it must work in an already light environment. I have made a schematic and set it up on a bread board.
It works and I could probably fulfill my requirements, but barely. I'm driving my leds with a 20kHz square wave and you can see the result in the picture. The upper oscillograms are 1V per division and 50us per division(20 kHz) and are taken when I turn the leds towards the receiver. The lower ones are 0.3V per division and 20 ms per devision(50 Hz) and are taken when the leds are turned off so you can see the room lightning interference.
So my questions are:
- How could I filter the 50Hz interference better? It is not showing too much when I transmit with the leds, but without them I have a lot of noise.
- Should I choose larger caps and smaller resistors for my filters or the other way around? And what should be a good filtering frequency? For now I just played around with available component values and chose a frequency well above 50 Hz.
- If you have any design advice I will be very thankful. I am a beginner at electronics so probably have some flaws.