Wednesday, July 2, 2014

Visible Light Communication Chronicles Part I - The Light

I've begun experimenting with Visible Light Communication recently and hope to design a complete system using it. VLC is a promising means of communication having it's own bunch of pros and cons which i will not detail out here and waste my time and yours. Google It !! Nevertheless it is worth a try to design it and share the data with everyone on this planet.

I purchased a TEMT 6000 ambient light sensor breakout from Sparkfun as my first choice for a light sensor and check out it's performance. From the datasheet it can be seen that it has a fairly decent response in the visible light spectrum from around 400nm to 800 nm with peak sensitivity around 570 nm. I hooked it up to a 3.3V power supply and fed the ouput signal into my scope.

Now let me mention that my room has a flourescent light source - A tubelight. The light from this source is incident on the TEMT 6000 sensor. As i had expected the output on my scope is shown below. The signal was around 100 mV with an average frequency of 100 Hz. This was the "default" lighting of my room, call it the base lighting value. This is the output of the TEMT 6000. When i turned the light OFF and observed the output, the frequency component was not present, just a very small DC voltage resulting from the light from my laptop display.

When i superimposed light from an LED source directly on the sensor the effect of the flourescent light became negligible. The LED that i am using is this one. It is very bright, runs on 12V and draws around 2A of current. My first step is to modulate the LED source using a MOSFET whose gate will be driven by a UART TX pin from an Arduino. I will also find a way to eliminate the low frequency component in the output signal and get a a clean DC signal at the output. Alternatively i might go for a totem pole gate driver for the MOSFET. Once that is done i shall post about it here soon. So stay tuned...

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