Long ago I built a light beam communication system. It was sophisticated in that it Frequency Modulated the carrier so that it was relatively immune to interference from light sources. The light emitters were small indicator grade LEDs. Even so, I was able to get it to work at a thousand feet (305M) with the help of optical concentrators (lenses). Since then I have been buying much more powerful LEDs such as the 1Watt Luxeon Star. So I decided to experiment with a more powerful emitter, the red 1W Luxeon Star.
I first connected the LED to a power supply with a 10 ohm current limiting resistor which also served as a current measuring point. I adjusted the PS until I measured 2 volts across the 10 ohm, which meant that the LED 200 mA was passing through the LED. It didn’t get excessively warm after a few minutes, so I figured I could go somewhat higher, considering that the white 1W Luxeon LEDs can handle 350 mA; the red LEDs are lower voltage and should be able to handle more current, probably 400 mA.
Under cover of darkness, I used the camera lens at first, then a lens from a magnifying glass. I could focus the LED onto a fence about fifty feet away. Then I tried focusing on a tall palm tree more than a hundred feet, but I couldn’t see it clearly, probably because I was holding the lens with my hand. I needed to mount it to something. But before that I needed to fabricate a modulator and detector.
For the detector I used a solar cell connected to a 150 ohm earphone from a telephone handset. The Amplitude Modulator I came up with was simple: just build a high current common emitter amplifier with a Darlington transistor pair, and put the LED in series with the collector. This gave me control of the current through the LED and it would bias the LED so that it was emitting light all the time.
I used a pocket radio for the audio source so I could just tune it in to a local station that would broadcast mostly music. I plugged the modulator into the headphone jack. It was a little ragged at first, so I put a 100 ohm resistor across the output to simulate a speaker load. With an adjustment of the volume control, the sound from the earphone sounded pretty good.
Now I need to work on the lenses. I had some Fresnel reading lenses but they’re packed in some box, I don’t know which or where. These are about the size of a page of paper, big enough to cover the whole page of a book. But they’re very flexible, so I may have to mount them in some kind of frame. So for now, I’m using a 3 inch magnifying glass to focus the LED.