Using a 10mm red LED at ca. 30mA and the OPT201 without any lens or post-amplification I could make some spectrograms with Spectrum Laboratory.
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Next was a ca. 4.5m LOS test, with the LED light overloading the sensor. Even the cameraphone could see it:
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I used my CW memory keyer to send a meaningful message (pre-recorded holiday CQ) at the lowest possible speed, about QRSS0.5 (0.5 seconds per dot).
In total darkness except the laptop screen, the spectrogram looked like this:
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Then I moved the TX further away in another room, for a total of 11.5 metres Non Line Of Sight (NLOS). Only after my eye had adjusted to darkness I could see some weak red reflections on objects in the intermediate room, but the spectrogram was faster:
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There it is! My QRSS CQ was on the screen, fortunately not masked by the laptop backlight. Remember the receiver had no focusing lens whatsoever.
I forgot to measure the response of OPT201 with a 3.3 Mohm series feedback resistor. Next test could be 300m LOS still without lens on the receiving side.
Stay tuned! Or better.... watch out! :-)
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