Optical system going NLOS

Thanks to the interest of a colleague in my optical experiments, I sped up the work on my MCW light receiver.

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.


First of all I studied the environment: the incandescent bulb produced a clear set of harmonics every 50 Hz, with those at n*100 Hz being stronger ("1"). Then the 520 Hz line is definately the laptop TFT dimmed backlight ("4"), since at full brightness the signal disappears. Notice the result of a mixing between 488 Hz MCW signal and the strongest 100 Hz harmonic ("2"): this certainly occurs at electrical levels, not optical.

Next was a ca. 4.5m LOS test, with the LED light overloading the sensor. Even the cameraphone could see it:


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:

A quick test at 15 WPM revealed a perfectly readable signal. The shade effect of a hand obstructing the light beam is surprisingly strong. A wav recording is available, if anybody wants to hear it.

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:


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! :-)