Feb/March 2012. The Winter has seen me pacing the floor and wondering what else I could do to ease this ghastly existence. A nice project - but what? A friend of mine has been telling me of the resurgence of 'light beam communications' and a recent article in Radcom had pricked my interest a wee bit.
The last time I fiddled about with modulated light was at my Grammar school's open day in 1966, when me and a couple of like-minded loons from the Radio Club (four o' clock every Monday, don't forget) hammered some audio from an amplifier into a long-suffering bike lamp and received the result on an ORP12 at the other end of the bench.
Things have improved immeasurably since then, even if I haven't - so I thought I'd give it a go. On the interweb I found several pages about intrepid hill-toppers who were sending their voices 70 miles and more by using super bright LEDs and photodiodes so sensitive they can detect starlight. Time to send CPC an order!
While the parts were on their way, I looked around for some suitable large lenses to launch and gather the light. As usual, the pound shop came up trumps with some 100mm diameter glass magnifiers which later turned out to be an almost perfect fit in 110mm plastic waste pipe.
Once the CPC order arrived I set to work.
There are a few modulation formats: simple baseband AM, subcarrier FM and subcarrier SSB. I plumped for a simple AM system to start with.
Elementary, my dear Watson. I've found a use for a spare tomato puree tin I had lying around. It provides screening for the high gain, high impedance RX front end. Also in the picture is one of the Pound Shop magnifying glasses.
The signal from the front end goes along a length of screened cable to the main amplifier which rolls off frequencies below 300 Hz (to reduce the 50/100 Hz buzz from street lights, shop signs etc) and above 4 KHz to keep the hiss at bay. It can drive a loudspeaker or a set of headphones.
This is the emitter LED mounted on a heatsink. It was bright even on initial testing at only 30mA, here it is at 100mA. The modulator is a 741 driving a VN67 vmos device, which is biased 'a bit on' via a pot.
No need for anything fancy at this stage.
This last couple of nights I've been poking the RX optics around the shack and out of the window. Any light can cause a 'deafening' of the receiver, and I've been amazed at some of the things I've had to turn off. The shack lights, obviously. Then the landing light, followed by the graticule light on the scope which the receiver found very 'loud' indeed. I also ended up sticking tape over the neons in the mains distrib boards scattered around the shack!
While I was pointing at my neighbour's kitchen window, I began to hear a 'pop...pop...pop' noise, and then I spotted an aeroplane going over with its strobe flashing. I quickly turned the optics to point at it and the pops turned to BAMs. Gee, these things are sensitive, and I'm sure my modest setup lags a fair way behind the best.
The transceiver itself. The TX side is just a 741 mic amp and a mosfet LED driver. Receive is taken care of by a two-stage amplifier with a 300 to 5000 Hz bandpass response followed by a small power amp chip.
The RX and TX can be used at the same time for duplex comms or for looking for reflections.
The TX optical tube. RX is similar. Just a length of 110mm plastic pipe with a cheap Pound Shop magnifying glass in the end. At the focus sits the Luxeon LED. My goodness it's bright! Both tubes are now mounted on a wooden platform which clamps to the pan/tilt head of a tripod. The TX tube has a small amount of adjustment in both planes to facilitate alignment.
Aug 2012: Finally I've had a chance to get the system on the air. My friend Warren G6OXW phoned me a couple of weeks ago and just mentioned his interest in optical comms. I told him that I had a system ready to go and he didn't waste any time getting his stuff going.
So on Saturday 4th August at 20:30 hrs local time, we each made our way up to our local high spot. In Warren's case this was Werneth Low in Gee cross, Cheshire. I made for Harrop Edge, two miles to the North. Using 2m talkback, we established contact and decided I would transmit first. However, Warren couldn't see my light by eye and failed to detect anything on his receiver.
At this point our 2m talkback frequency came alive with our old mates. They're all grumpy old men now and they sat there predicting dismal failure and generally making disparaging remarks.
So Warren TX'd and through my binoculars I could see a dim red pinpoint of light and from that moment things began to work. I talked him in and when his beam was on me it was incredibly bright. The red spot of light up on the hill was significantly brighter than the street lights in the valley. His test tone was so loud it almost blew my head off.
Switching over to his microphone, Warren then chatted to me via light beam while I replied on 2m. Then it was time for me to TX, and of course by now my receiver was peaked in Warren's direction so only a small adjustment was necessary to the TX optics to get my beam on him.
Here's what Warren saw:
That's my 1-watt led shining at Warren. A bit of test tone confirmed all was OK and then the duplex QSO was repeated, this time with me on the photons, Warren on 2m.
We had a great night, proved our gear was working and had plenty of banter with the old friends who kept us company. Many thanks to all concerned.
11 August 2012. More photonic adventures, this time over a longer path of 6 miles (10km) from Werneth Low to Hartshead Pike. Start time was 20:30 hrs to make sure we had time to set up in daylight.
Bearing in mind the difficulty we had getting our initial 'beamings' last time, I had constructed a wide-beam beacon consisting of a single 1-watt LED shining into a short reflective cone. The LED was chopped on & off at approx 20 Hz to make it easily visible among the other light sources.
Talkback was again on 2m.
A headlight start was called for again, as the daylight was too much for my beacon. It was also quite misty and I could only just make out the masts on Werneth Low, 10km to the south. Warren put his TX LED on and straight away a pinpoint of red light was visible through the bins. A wee bit of peaking at Warren's end resulted in a bright light.
Careful peaking at my receiver found Warren's tone but it had a strange gurgling effect on it, and looking at the light with my naked eye I could see a slight 'twinkling' effect.
There was still quite a lot of daylight about so we waited for darkness to fall, and as it did the results improved significantly.
Roger G8ILD who had gone along with Warren to see what we were up to was suitably impressed and spoke to me over the optical link. With me on TX, a small (but very fiddly) adjustment to my TX beam heading got communication going both ways.
At this distance, the tiniest movement of the TX or RX optics causes a loss of contact, so we reckon we'll have to beef things up if we want to achieve longer paths reliably.
Above is a picture of my equipment in TX mode. Off-beam like this the light doesn't look much at all, but only a complete idiot (like me) would dare to look into the beam.
Above: The view from my end with warren G6OXW on transmit. It was still quite light when we established the link, and you can see how misty it was. The two PMR towers to the left of Warren's light are barely visible.
So there we go - another fun night, lots of banter and another success.