Modulated Light

[Graham G3ZVT]

To compliment the thread on schoolboy "Ground-transmission 'radio' " experiments, I thought I would start another about optical communication.

My experiments took place in the '70s. A friend had a TV & radio repair business across the road from where I lived, and our light-beam telephone linked my bedroom with his workshop.

The two transceivers were conceived independently by each of us, but they were both simplex (HDX) in operation with a push-to talk switch.

Mine used the amplifier out of an old portable R-R tape recorder and a field telephone handset. A torch-bulb was mounted in front of an ORP12, at the focus of a 5 or 6 inch diameter condenser lens, borrowed from my photo enlarger.

Great fun.

[G6Tanuki]

I think we've probably all played around with such things in the past, with varying degrees of success. Rather than feeding the output of the amplifier direct to the bulb, some standing DC bias so the bulb was partially lit seemed to be essential.

The thermal 'lag' of filament bulbs being a big issue for homebrew visual-light transmitters.

Modern high brightness LEDs would no doubt do a lot better. Perhaps one of the focussable LED flashlights could be used?

I'm sure I also remember a transmitter design that rather than modulating the light-source for transmission used instead a mirror mounted on a loudspeaker voice-coil.

Laser assembly from a dead CD/DVD-player would be another modern option.

Dr. Cyclops of course reminds you:: "Do not look into laser with your remaining eyeball".

[John10b]

I recall, only just, playing around in the lunch break, with optical communications, unfortunately I cannot remember details except having difficulty aligning the optical path. I think I had a mirror to return beam to receiver. I would be interested to learn how you did the practical setup, thanks.
John

[JohnBHanson]

Use a toilet roll with silver foil as one end as a microphone. The silver foil
will vibrate. Shine a light on it, and the reflection will be modulated. A
simple receiver can be a solar cell connected to an a amplifier. I have seen this
working as part of a stem demonstration, was quite effective.

[m0cemdave]

It's not generally known that the German Army in WW2 had an amazingly advanced (for its time) light beam telephone.

Helge Fykse (LA6NCA) has restored a couple and got them working, looks like his kids have had some fun with them:

https://www.la6nca.net/tysk2/lispr80/index.htm

https://www.la6nca.net/tysk2/lispr/index.htm
https://www.la6nca.net/tysk2/lispr/lispr2.htm

https://www.youtube.com/watch?v=Qq_s...ature=youtu.be
https://www.youtube.com/watch?v=vR4N6MTx_vw

Reprint of old E&WW article here:
https://www.la6nca.net/tysk2/lispr/lispr.pdf

[G6Tanuki]

I also remember one transmitter design involving a NE-2 type Neon bulb wired via a capacitor to the anode of the output-stage of a 2-valve amplifier, with a suitable resistor to keep the eon dimly lit in the absence of any modulation.

And I'm sure we all ground the end off an OC71 transistor then washed-out the white grease from within to produce on-the-cheap phototransistors.

[dave walsh]

I thought this would turn up as a companion piece to the ground sourced transmissions. Sort of a next step up from Telegraphy earth returns perhaps There was a modulated light design in Practical wireless I believe, using a photo cell and a car headlight. I think some people were sending messages across London by light beam but not very far.

There was a bit of a panic on with the govenment re "Pirate" [Independent] radio at the time. Tony Benn was head of the Post Office and it's regulatory function. The background was the mid sixties and the "summer of love!" Mr Benn had been quite popular with young people and was seen as a modern relatively progressive person, so it was a bit of a disappointment when he sought to introduce a new Bill through Parliament for an Act restricting anything percieved as a threat.

It seemed a bit of an over-reaction. A bit like most forms of prohibition when things [eg alcohol] just go "underground" [no technical pun is intended!] To my recollection, they were so spooked by these innocent experiments that the law was changed to cover ALL types of broadcasting, not just conventional radio transmissionst.'

Dave W

[G6Tanuki]

Originally Posted by dave walsh

It seemed a bit of an over-reaction. A bit like most forms of prohibition when things [eg alcohol] just go "underground" [no technical pun is intended!] To my recollection, they were so spooked by these innocent experiments that the law was changed to cover ALL types of broadcasting, not just conventional radio transmissionst.'

Dave W

Surely point-to-point communications was not 'broadcasting' [which to me implies one-to-many]??

Of course if you were a humble experimenter faced with paying your own legal fees, as opposed to the Treasury Solicitor [whose essentially unlimited costs were paid out of everyone's taxes] you really wouldn't want to get engaged in the legal side of things.

[dave walsh]

You're quite right Tanuki but I think that anything at all outside the BBC monopoly, on top of a great period of social change taking place, certainly upset somebody. Conventional TX's off shore on towers or dodgy ships could Broadcast [in a limited way] but I think there may have been a rumour that the "alternative" community would be trying to get beyond "point to point" inland. That would have been very unlikely then of course but the automatic response was just to shut things down. They probably didn't understand it anyway!

Radio Luxemberg could get though on a fading signal [some times] but it was a business not an unknown quantity and no sort of threat. It was all a bit silly when you think that some people still take the view that the well known "yoof" movement of the time was known all around the UK but probably only about 400 people all in one part of London

Dave W

In later years I met a chap who claimed to have been the only Hippie in Doncaster

[Panrock]

Back in 2016, I was looking into piggy-backing a complete VHF analogue television channel (S +V) onto a light beam, using modulated light or infra-red from LEDs.

This indeed proved possible, though fast LEDs were required. I demonstrated working systems at two NBTVA Conventions in Loughborough. The first time, I used channel B1 (405-lines) as the modulating signal, the second - channel E4 (625-lines colour).

Even though the baseband modulation was AM, this was modulating VHF subcarriers which were what was actually modulating the light. So to the eye the light always looked steady, regardless of vision and sound content.

The page is here: https://www.radiocraft.co.uk/opto.htm and hasn't been updated since.

Steve

[RogerEvans]

When I was at Imperial College we had an undergrad project which was to reflect a CW laser from a glass panel in one of doors and demodulate the reflected beam which picks up sounds from the far side of the door. It originated in one of the Spycatcher stories of illuminating a room in the US embassy with microwaves and detecting the reflection from a metal ashtray which works as a passive microphone.

Happy days!

Roger

[Oldmadham]

Originally Posted by dave walsh

I thought this would turn up as a companion piece to the ground sourced transmissions. Sort of a next step up from Telegraphy earth returns perhaps There was a modulated light design in Practical wireless I believe, using a photo cell and a car headlight. I think some people were sending messages across London by light beam but not very far.

There was a bit of a panic on with the govenment re "Pirate" [Independent] radio at the time. Tony Benn was head of the Post Office and it's regulatory function. The background was the mid sixties and the "summer of love!" Mr Benn had been quite popular with young people and was seen as a modern relatively progressive person, so it was a bit of a disappointment when he sought to introduce a new Bill through Parliament for an Act restricting anything percieved as a threat.

It seemed a bit of an over-reaction. A bit like most forms of prohibition when things [eg alcohol] just go "underground" [no technical pun is intended!] To my recollection, they were so spooked by these innocent experiments that the law was changed to cover ALL types of broadcasting, not just conventional radio transmissionst.'

Dave W

I recall seeing an article in a local (Oz) publication about a proposed version to be used for (the proposers hoped) license free broadcasting in the UK .

An "artist's conception" showed a quasi-omni-directional setup which a "consortium" proposed to put (in the first instance) on top of a tall London building.
It was a rather "flying saucer" like contraption, using an array of the (then) latest fad------lasers!

The article was most concerned with the regulatory aspects of this, which a few moment's consideration revealed to be a somewhat "hare-brained" scheme.
It looks like the UK govt of the day took it seriously.

[DrStrangelove]

IIRC there was a Maplin video sender that used light.

The images seemed remarkably fuzzy.

[Vintage Engr]

There were also a number of professional video + audio links that used modulated I.R.
The were o.k. for a short hop distance, but easily degraded under foggy conditions.

David.

[kalee20]

Years back, I did play around with modulated light, using a neon tube in the anode of a small pentode fed with AF, and a BC107 with the top sawn off as a detector.

I only transmitted a few inches, but sound quality was very good!

Originally Posted by G6Tanuki

Surely point-to-point communications was not 'broadcasting' [which to me implies one-to-many]??

Of course if you were a humble experimenter faced with paying your own legal fees, as opposed to the Treasury Solicitor [whose essentially unlimited costs were paid out of everyone's taxes] you really wouldn't want to get engaged in the legal side of things.

Though occasionally individuals do challenge the State - use of detectors for speed-gun radar traps was challenged - and the accused was found not guilty of infringing the Wireless Telegraphy Act. He had 3 defences, first, he said his warning gizmo wasn't equipped to demodulate/decode messages, it only indicated the existence of a transmission not its content; second, he understood that the transmission didn't carry any official information anyway; and thirdly, he said as soon as he realised his equipment was receiving an official transmission, he switched it off immediately, thus not infringing the Act. (Legislation has of course now made it a specific offence).

Originally Posted by Panrock

Back in 2016, I was looking into piggy-backing a complete VHF analogue television channel (S +V) onto a light beam, using modulated light or infra-red from LEDs.

This indeed proved possible, though fast LEDs were required. I demonstrated working systems at two NBTVA Conventions in Loughborough. The first time, I used channel B1 (405-lines) as the modulating signal, the second - channel E4 (625-lines colour).

That was modulation by the VHF signal, so 45MHz-ish? Effectively, S & V amplitude modulated onto two carriers, which then amplitude modulated the light? It's some doing!

[Panrock]

Originally Posted by kalee20

That was modulation by the VHF signal, so 45MHz-ish? Effectively, S & V amplitude modulated onto two carriers, which then amplitude modulated the light? It's some doing!

Thanks. Channel E4 with PAL was higher still. The results were very good - just like direct RF. All done with simple circuitry - I can send to anyone who wants to have a go themselves.

Steve

[dave walsh]

Really interesting comments there and as in many aspects of early radio, development the speculative experiments [as described] might turn out to be the origin of something quite spectacular. Also, Modulated light now seems to be very much a major aspect of physics, more than six decades on. Perhaps it will return as a communications medium in a type of "broadcasting" that we can't even contemplate yet? [Marconi applied to the Post Office in England, before Mr Benn was in charge, because most people couldn't really grasp what he was up to

The current edition of Nexus magazine has an article about the Genome. "Light seems to be involved in in all biological processses and interference patterns carry life's information. In short, bioholograms are riding the wave nature of DNA." It's another form of transmission when you think about it A major figure in this field is Tsian Kanchzhen who built a "Biotron" device. That's, coincidentally, another edition to the recent 'Tron' thread here and the amazing number of words that incorporate the term!

Dave W

On a lighter, possibly ironic, note [OldMadhan post 12*] I'm not sure if you mean an Australian publication or the "underground" magzine OZ distributed in London during the mid sixties. It was set up by Richard Neville, Germaine Greer, Clive James and many others who all chose to leave their homeland for Earls Court at that time. [As far as I know, it was only the Manchester author Howard Jacobson who went the other way I recall being in Soho one night as a couple of hippies roared around on a sort of beach buggy type Jeep. They weren't hanging about and I got the sense that they were being harrassed by the police, even in that location! Another "broadcasting" over reaction I suspect!

[DrStrangelove]

Originally Posted by RogerEvans

When I was at Imperial College we had an undergrad project which was to reflect a CW laser from a glass panel in one of doors and demodulate the reflected beam which picks up sounds from the far side of the door. It originated in one of the Spycatcher stories of illuminating a room in the US embassy with microwaves and detecting the reflection from a metal ashtray which works as a passive microphone.

Happy days!

Roger

"The Thing" was a bit more complex than a metal ashtray, designed by Theremin.

A reconstruction featured in a BBC programme last year or the year before, I think with Hannah Fry presenting.

I still can't understand how it works.

Allegedly Peter Wright figured out how it worked before the Cousins did.

[m0cemdave]

Modulated infra-red systems have been around for decades. They have been used for playback of recorded information in art galleries, museums, etc. where visitors are issued with small headset receivers, but are rapidly being superseded by digital playback devices.

They are also used in simultaneous translation setups for conferences and similar events, The transmitter uses LED arrays, usually positioned on top of the sim-tran booths, and the signal is applied to the IR via a modulated RF carrier typically in the Long Wave region. Delegates are issued with small receivers and earpieces or headsets and several different channels can be made available with various languages.

It is easy to apply the same method to visible LEDs and the recent development of very high efficiency types should make it a worthwhile avenue for "amateur" experimentation.

Some years ago an acquaintance of mine, who works in the serious end of the TSCM business, showed me an interesting device that had been discovered somewhere. It was (apparently) a standard LED light bulb as now commonly used in domestic and office lighting. But this one had a tiny hole in the side, and behind it was a miniature microphone. The light was modulated in the same way as the IR systems described above. The receiver just needed a view of the light from the lamp, through a window or whatever.

[RogerEvans]

I am sure that making the original 'bug' actually work was far from simple but the operating principle is exactly the same as the 1930s observation that aircraft flying between a radio transmitter and receiver caused the received signal to fluctuate due to interference between the direct radio signal and the signal reflected from the aircraft.

In the spying setup you have a microwave transmitter and receiver on roughly opposite sides of a room. The received signal is the sum (taking into account phase differences) of the direct signal and the signal that bounces off various objects in the room. The clever part is to add an object that is designed to reflect signals from the transmit antenna to the receive antenna, changes its shape slightly when picking up sounds in the room and also looks innocent. The transmitter runs CW, the received signal intensity changes very slightly in response to the sound due to the second path via the 'thing', the receiver has a homodyne mixer driven by exactly the same signal that fed the transmitter and the output of the mixer is a passable copy of the sounds in the room.

Roger