Optical receiver head completed

481THz optical receiver with 100mm lens

I now have a 100mm lens set-up in some grey drainpipe with 4x sighting scope attached on a tripod together with a "simplified version 3" KA7OEI head (of sorts) with a BPW34 detector. In my version the cascode stage is just followed by a single common emitter stage driving a crystal earpiece, although I intend to add an emitter follower so I can connect to my PC (to use with Spectran/SDR) or a transverter.

A 5mm 20 degree red LEDusing just its own built-in lens was shone out of the stairway window aiming down the street in daylight with 1kHz modulation. It was taking around 20mA. I then walked with the kit to the far end of the street and the signal was solid (S9) in the RX earpiece at a distance of about 0.25km. I now need to find a more distant test range and see how far this can achieve. Depending on results I hope to go looking for GB3CAM next week.

Lightbeam receiver progress

Today I did the part of projects I hate most - drilling holes in aluminum boxes. I can never find the right size drill bits, they are blunt and so on. Despite all this, I managed to get a small metal box complete with BNC socket, phone jack and switch mounted on the end of my optics pipe and carried out some first tentative tests. I am using part of a simplified version 3 KA7OEI optical head based around a BPW34 PIN photo diode. In my case there are no op-amps with discrete transistors used for the amplifiers and buffers following the FET cascode stage.

In the next day or so I want to add the sighting scope and find a way of attaching the whole optical unit onto a stable tripod. At this point and can try some more serious tests for example looking for my own 1kHz beacon at greater distance and, hopefully, GB3CAM at 22km if the system works OK.

The output of the head can feed either a crystal earpiece (to listen to the baseband tones or speech received) or into a PC so that it can be used with an SDR to copy various modes up to around 22kHz.

More 481THz lightbeam progress

670nm receive head and converter to 80m

Today I did some further light beam experiments, this time using a 25kHz modulation signal on the light beam and receiving the signal on my FT817 with this head/converter unit above. It consists of a BPW34 photo-detector feeding a cascode FET/transistor stage into an emitter follower and SBL1 mixer to convert the signal to around 3.584MHz. With a current into the TX LED of just 10uA (a very dim glow from the red LED) the signal was 20dB S/N in 0.67Hz bandwidth on Spectran at a distance of 25cms without optics. If my calculations are correct, this means a range of around 100m could be obtained even with this miniscule power if 100mm lenses were added at each end to give some 27dB gain at each end. Using the same TX LED at 10mA (1000 times the current) then the range is already in the many km region, and this is without using power LEDs. This is encouraging progress. Tomorrow I want to repeat the test with the same LED as the TX as the detector.