[NLRS] Ronja Project - Optical Communications

[Rob Jahnke]

https://ronja.twibright.com/

[image: image.png]

Take a look at this website when you get time.
As far as I can tell, they used the BPW43 sensor with a simple Bi-Fet
buffer directly into a differential video amplifier. They were working with
a 10Mbps signal, so they needed the video amplifier, but the front end
would work just as well for our audio signals. There does NOT appear to be
any optical filter in front of the BPW43 sensor. They used either a 625nm
Red LED or an 875nm IR LED as an option.

This is their sensor mounting scheme (from
https://ronja.twibright.com/receiver/building.php )
[image: image.png]

The box with the LED protruding contains all of the circuitry for the 10
Mbps link and as far as I can tell, the whole box is mounted inside their
"stove pipe" tube painted flat black on the inside. They put the lens at
the front of the tube and a long light shade beyond that. It appears to be
that simple and yet they are getting a 10 Mbps data rate with no errors at
1.4 km with a 5 inch 2X magnifying glass for a lens.

Their transmitter is a single LED that is less powerful than what we are
using now (one led from the tail-light of a car from the period - 20 years
ago) and the same size 5 inch 2X lens.

It is interesting that they put  4 resistors on the lens to prevent fogging
or condensation.

*This system was/is used as a data link DAY and NIGHT for several YEARS in
many locations. So it works!*
Note from one user:
Deinstalled after about 12 years of nice operation [since 2003]. It is due
to loss of a line of sight, building reconstruction added extra floor and
roof ... -- silvije2, Zagreb, Croatia
<http://images.twibright.com/tns/770.html>

That is pretty encouraging. With my single LED flashlight (no fresnel
lens), and my  KA7OEI receiver with the fresnel lens and a short light hood
on the sensor, I was able to go 635 meters with a 30 dB margin in full
daylight. As I explained, the flashlight was against a wall of trees in the
shadow on the north side of the trees, and the field of view of the
receiver only saw the flashlight and the trees. So the background and field
of view matter. But the Ronja unit appears to work without much concern for
the background.

I am looking forward to working on a "Daylight" version of our system. If
not "Daylight" then at least "Golden Hour" or "Twilight" capable. More
experimenters welcome :-)

Rob - K0XL / K0THZ
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