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Saturday, November 28, 2015

Photophone! Modulating the Sun by G3ZPF (and Alexander Graham Bell, and Mr. A.C. Brown of London)


Yesterday David G3ZPF sent us another very interesting e-mail, this one about some very creative sunlight communication experimenting that he and his brother did many years ago.  It appears to me that David -- on his own -- came up with a version of Alexander Graham Bell's Photophone (pictured above). 

Wikipedia says that Bell's invention was the first ever wireless telephony device.   Bell credited Mr. A.C. Brown of London for the first demonstration of speech transmission by light (in 1878).  

Here is the Wikipedia article: https://en.wikipedia.org/wiki/Photophone 

I think young David's placement of the small mirror on the cone of the AF amplifier's loudspeaker was brilliant! 


Hello again Bill,

Just (literally) finished the book and a couple of surprises awaited me in the final chapters

SSDRA = $200 on Ebay
My flabber has never been so ghasted....and I have a lot of flabber. I will treat my copy with even more reverence now. Srtangely I hadn't heard about EMRFD so I'll need to look into that

Modulating light
Your story about using a laser pen in a receiver reminded me of my method of modulating light which I've never seen anyone else mention.

In the mid 60's my brother had an electronics constructor set for his birthday. After the initial fascination I probably played with it more than him. I remember reading about modulated light transmitters and (because this was before I was anywhere near getting a licence) I decided to build one. My brother was sufficiently curious to help.

We started with the receiver. I purchased an OCP71 and managed to find an old 12" headlight reflector from somewhere. The cork from a wine bottle fitted nicely into the hole at the centre, and the cork was easily drilled out to accommodate the photo-transistor.

My brother constructed the "high gain audio amplifier" project from his constructor kit and we put two legs of the photo transistor across the mic input. We were rewarded by a buzzing sound so loud our parents yelled at us from the other room. It took us a few seconds to realise we were 'receiving' the 50Hz signal off the ceiling light (the house lights were on). Waving the headlight reflector around confirmed this. I still recall the excitement we felt at our 'discovery'.

So far, so ordinary, but the TX side is where I wandered off into the outfield. Normally people modulated an incandescent bulb but this required a many watts of audio power & the 'inertia' of the filament could be a problem.

I cannot remember what prompted me to do this, but next day I pulled the speaker grill off my tiny little medium wave transistor radio and glued a small mirror (from my mothers old 'compact') to the cone of the loudspeaker.

Then we went outside into a field near our house. My brother went to the far end and I set up the lil radio on a camping stool. Moving it around until the sunlight reflected off the mirror hit the headlight reflector about 200 yards away.

Then I turned on the radio. Instantly my brother started jumping up and down excitedly. It worked. My 200mW AF amp was modulating the *SUN* !

All those guys on 160m with their 10w of AM...pah. I had GIGAwatts of power under my control :-)

Looking from the receive end it was possible to see the light from the mirror flickering & I guess the movement of the speaker cone did not move the mirror exactly in the plane of the reflected beam. The 'wobble' fooled the phototransistor into seeing an amplitude modulated beam.

The beauty of this was that only a tiny audio amp was needed. This made me wonder about such a system being used in undeveloped countries (ones with more sunshine) as a comms system, with batteries recharged by the sun.

For the UK I thought about using a slide projector to provide the illumination, instead of the sun. Again a very low power audio amp was all that was needed, and there were no 'inertia' issues to worry about it.

But I was soon to suffer a setback. A few days later the headlamp reflector, sitting on a desk in my bedroom, managed to find itself in a position to focus the suns rays onto the cork holding the photo-transistor. Cooking the transistor & setting fire to the cork. Luckily my mother smelt the burning cork before any collateral damage was caused but I had a face-chewing when I came home from school.

I'd long since forgotten about all this until reading the later chapters of your book.

regards

David G3ZPF










2 comments:

  1. @ Bill ... Thanks for the heads-up on the photo-phone. I'd never heard of it.

    ReplyDelete
    Replies
    1. You are in great company David. I had never heard of it either until I looked for a photo to accompany your e-mail. Fascinating stuff. Thanks again, Bill

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