the other day, which appeared to have a large diaphragm whose centre was
packed by carbon dust of about an inch in diameter with a connection in the
centre and the other via the case. Pass a voltage through it and have an old
fashioned headphone in series with that and you had a one way line. Of
course if it was damp it rustled and any kind of poor connection in the
conduction was making crackling noises. It was a wonder it worked at ll.
Brian

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"Brian Gaff" <brian1gaff@gmail.com> wrote in message
news:umbltm$311p8$1@dont-email.me...
> the other day, which appeared to have a large diaphragm whose centre was
> packed by carbon dust of about an inch in diameter with a connection in
> the centre and the other via the case. Pass a voltage through it and have
> an old fashioned headphone in series with that and you had a one way line.
> Of course if it was damp it rustled and any kind of poor connection in the
> conduction was making crackling noises. It was a wonder it worked at ll.

As a 10-year-old lad, I remember a GPO engineer giving me the old carbon
mike from my parents telephone when he had to replace it. In series with a
1.5 V battery, it gave a very usable signal at an amplifier or tape
recorder - albeit crackly and poor LF response: telephone quality.

I sellotaped it across the mouth of a big parabolic photographic light as a
parabolic reflector and could hear people's voices at the other end of the
garden.

Yes I used an old lampshade and had it facing the dish part This worked
well, but it was very noisy and restricted at both ends of the spectrum,
when you think of how crude they were though, they were pretty amazing they
worked at all.

Later on when I was a youth, I built a light transmitter and receiver using
a torch at one end and an old lens at the other with an ORP 12 at the focal
point.
It had a couple of drawbacks. Firstly, it needed to be a fairly bright lamp
to be on all the time to allow the modulation not to go into a very non
linear area where you could see the lamp brightness changing as I imagine
thermal effects in the filament created the distortion, and of course, if it
was foggy it stopped working!

Later on of course we could use LEDs and photo trransistors but these meant
you needed lenses at both ends due to the light only coming out the one
direction. These needed much less brighten stability, but I still used
visual range leds in order to line the two up.
All good fun at the time, with little use to mankind.
I wonder if you could modify a laser pointer for this today?
Brian

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"NY" <me@privacy.invalid> wrote in message
news:umf9gv$3j7qu$1@dont-email.me...
> "Brian Gaff" <brian1gaff@gmail.com> wrote in message
> news:umbltm$311p8$1@dont-email.me...
>> the other day, which appeared to have a large diaphragm whose centre was
>> packed by carbon dust of about an inch in diameter with a connection in
>> the centre and the other via the case. Pass a voltage through it and have
>> an old fashioned headphone in series with that and you had a one way
>> line. Of course if it was damp it rustled and any kind of poor connection
>> in the conduction was making crackling noises. It was a wonder it worked
>> at ll.
>
> As a 10-year-old lad, I remember a GPO engineer giving me the old carbon
> mike from my parents telephone when he had to replace it. In series with a
> 1.5 V battery, it gave a very usable signal at an amplifier or tape
> recorder - albeit crackly and poor LF response: telephone quality.
>
> I sellotaped it across the mouth of a big parabolic photographic light as
> a parabolic reflector and could hear people's voices at the other end of
> the garden.

"Brian Gaff" <brian1gaff@gmail.com> wrote in message
news:umh249$3tcoh$1@dont-email.me...
> Later on of course we could use LEDs and photo trransistors but these
> meant you needed lenses at both ends due to the light only coming out the
> one direction. These needed much less brighten stability, but I still used
> visual range leds in order to line the two up.
> All good fun at the time, with little use to mankind.
> I wonder if you could modify a laser pointer for this today?

How responsive are LCDs? Could one be turned on and off quickly enough to
modulate the brightness of a laser pointer that was pointed at a remote
sensor? Or are they too slow for that. Maybe a tiny mirror on a moving coil
and magnet to move the beam on and off the sensor depending on whether a
voltage was applied to the coil; that would have problems with a coil (and
hence the mirror) overshooting and returning, which would impose an upper
limit on the pulse rate. Moving parts are always a problem.

You've also got the the problem that a laser pointer has a very small
diameter so it would need to be pointed exceptionally accurately at the
sensor, and maybe vibrations from someone walking across the floor or
jogging the laser pointer could either turn off the signal altogether or
else impose noise on top of the signal, giving rise to false bits.

Thought experiments are the perfect thing to occupy the mind,
post-Christmas.

Of course lasers are used in devices to monitor for earthquakes as very
small changes can be detected due to angular changes between sites. I'm told
that in a magazine there was a laser intercom project, but as you
surmise, the pointing accuracy was the main issue. But it used digital comms
and some kind of CCD array at either end to receive the pulses. I doubt it
would work on a boat!

You of course do not need coherent light if all you want is to talk to
people.
Brian

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"NY" <me@privacy.invalid> wrote in message
news:umhjqe$3vr8h$1@dont-email.me...
> "Brian Gaff" <brian1gaff@gmail.com> wrote in message
> news:umh249$3tcoh$1@dont-email.me...
>> Later on of course we could use LEDs and photo trransistors but these
>> meant you needed lenses at both ends due to the light only coming out the
>> one direction. These needed much less brighten stability, but I still
>> used visual range leds in order to line the two up.
>> All good fun at the time, with little use to mankind.
>> I wonder if you could modify a laser pointer for this today?
>
> How responsive are LCDs? Could one be turned on and off quickly enough to
> modulate the brightness of a laser pointer that was pointed at a remote
> sensor? Or are they too slow for that. Maybe a tiny mirror on a moving
> coil and magnet to move the beam on and off the sensor depending on
> whether a voltage was applied to the coil; that would have problems with a
> coil (and hence the mirror) overshooting and returning, which would impose
> an upper limit on the pulse rate. Moving parts are always a problem.
>
> You've also got the the problem that a laser pointer has a very small
> diameter so it would need to be pointed exceptionally accurately at the
> sensor, and maybe vibrations from someone walking across the floor or
> jogging the laser pointer could either turn off the signal altogether or
> else impose noise on top of the signal, giving rise to false bits.
>
> Thought experiments are the perfect thing to occupy the mind,
> post-Christmas.

On 27/12/2023 16:23, NY wrote:
> How responsive are LCDs? Could one be turned on and off quickly enough
> to modulate the brightness of a laser pointer that was pointed at a
> remote sensor?

Traditionally you would have used a Kerr Cell for this, which has a
similar geometry to a liquid crystal display, but uses a pure liquid
medium. I can't find them in Google shopping, so they are presumably
either specialist or no longer used.

On 28/12/2023 10:37, Brian Gaff wrote:
> You of course do not need coherent light if all you want is to talk to
> people.

The reason you would use a semiconductor laser is that the source size
is extremely small, so the optics can form a very narrow beam, so you
get very high power densities, and a reasonable distance, for a
relatively low input power.

With gas discharge lasers, the optical cavity is so long, and the full
gain is achieved over many traverses, that there is only enough gain in
one direction, so, again, you get a narrow beam.

For driving high speed monomode fibres, coherence is important, because
a wide frequency spread would result in pulse being smeared out, as
different wavelengths have different refractive indexes.

Brian Gaff <brian1gaff@gmail.com> wrote:

> Of course lasers are used in devices to monitor for earthquakes as very
> small changes can be detected due to angular changes between sites. I'm told
> that in a magazine there was a laser intercom project, but as you
> surmise, the pointing accuracy was the main issue.

I tried to use a laser pointer (the type used for pointing at the screen
in slide shows) but it was almost impossible to aim it accurately
enough, even with a strong tripod.

--
~ Liz Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk

On 27/12/2023 16:23, NY wrote:
> How responsive are LCDs?

Probably not fast enough.

Much easier to hack the electronics in the laster pointer and modulate
it directly.

--
Brian Gregory (in England).

On 28/12/2023 11:49, David Woolley wrote:
> On 27/12/2023 16:23, NY wrote:
>> How responsive are LCDs? Could one be turned on and off quickly enough
>> to modulate the brightness of a laser pointer that was pointed at a
>> remote sensor?
>
> Traditionally you would have used a Kerr Cell for this, which has a
> similar geometry to a liquid crystal display, but uses a pure liquid
> medium.  I can't find them in Google shopping, so they are presumably
> either specialist or no longer used.

If you want massively modulated light, you will use LEDs for low power
and lasers for higher.

LCDs are far too slow. they have trouble at 60Hz!

--
New Socialism consists essentially in being seen to have your heart in
the right place whilst your head is in the clouds and your hand is in
someone else's pocket.

My thoughts entirely. I think it would need to be fm modulated or maybe
digital though, as Laser diodes tend to be very poor at brightness control,
and are made to work at a fixed current anyway. Does anyone remember those
gas lasers in the Laservision players. Now that was what i call a laser,
sadly it was beyond the seeing range of the eye. Later ones by Pioneer were
semiconductors though.
Brian

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"Brian Gregory" <void-invalid-dead-dontuse@email.invalid> wrote in message
news:kv5fuvFs83mU1@mid.individual.net...
> On 27/12/2023 16:23, NY wrote:
>> How responsive are LCDs?
>
> Probably not fast enough.
>
> Much easier to hack the electronics in the laster pointer and modulate it
> directly.
>
> --
> Brian Gregory (in England).
>

On 28/12/2023 14:44, Brian Gregory wrote:
> On 27/12/2023 16:23, NY wrote:
>> How responsive are LCDs?
>
> Probably not fast enough.
>
> Much easier to hack the electronics in the laster pointer and modulate
> it directly.
>

This has been done. There was a web site, which I now can't find, by
someone who gave details of how he did it. I remember it used a
subcarrier of about 15kHz to feed the laser diode, and that was
amplitude modulated with the sudio.

One of the magazines many years ago had a similar project using filament
lamps, but I would imagine frequency response would be a problem there.

--
rgds
LAurence
<><

I'd have a really cool .sig, but right now I can't even remember my own
name.
~~~ Random (signature) 1.6.1

On 30/12/2023 13:46, Laurence Taylor wrote:
> On 28/12/2023 14:44, Brian Gregory wrote:
>> On 27/12/2023 16:23, NY wrote:
>>> How responsive are LCDs?
>>
>> Probably not fast enough.
>>
>> Much easier to hack the electronics in the laster pointer and modulate
>> it directly.
>>
>
> This has been done. There was a web site, which I now can't find, by
> someone who gave details of how he did it. I remember it used a
> subcarrier of about 15kHz to feed the laser diode, and that was
> amplitude modulated with the sudio.
>
> One of the magazines many years ago had a similar project using filament
> lamps, but I would imagine frequency response would be a problem there.

What about the supposed bugging device used by spies, which shone a
light (laser?) at a window and detected audio-frequency vibrations of
the glass from people talking in the room? How did that work? Would a
suitably fast light sensor in the path of the reflected beam produce an
audio frequency signal? Or was there more to it that than?

Many of the phones at work were in unheated, exposed buildings and often
very noisy so we used have problems with carbon microphones though
usually cleared with good hard tap. We changed them all for noise
cancelling microphones - mainly to get rid of the carbon microphone.

Noise Cancelling Microphone (Kit 394A)

On 30/12/2023 20:47, NY wrote:
> On 30/12/2023 13:46, Laurence Taylor wrote:
>> On 28/12/2023 14:44, Brian Gregory wrote:
>>> On 27/12/2023 16:23, NY wrote:
>>>> How responsive are LCDs?
>>>
>>> Probably not fast enough.
>>>
>>> Much easier to hack the electronics in the laster pointer and modulate
>>> it directly.
>>>
>>
>> This has been done. There was a web site, which I now can't find, by
>> someone who gave details of how he did it. I remember it used a
>> subcarrier of about 15kHz to feed the laser diode, and that was
>> amplitude modulated with the sudio.
>>
>> One of the magazines many years ago had a similar project using filament
>> lamps, but I would imagine frequency response would be a problem there.
>
>
> What about the supposed bugging device used by spies, which shone a
> light (laser?) at a window and detected audio-frequency vibrations of
> the glass from people talking in the room? How did that work? Would a
> suitably fast light sensor in the path of the reflected beam produce an
> audio frequency signal? Or was there more to it that than?

Doppler lidar. Mix reflected and incoming frequencies and detect the
resultant amplitude modulation.
Same principle as speed camera

--
All political activity makes complete sense once the proposition that
all government is basically a self-legalising protection racket, is
fully understood.

On 31/12/2023 12:11, The Natural Philosopher wrote:
> Doppler lidar. Mix reflected and incoming frequencies and detect the
> resultant amplitude modulation.

frequency, not amplitude

On 31/12/2023 20:23, David Woolley wrote:
> On 31/12/2023 12:11, The Natural Philosopher wrote:
>> Doppler lidar. Mix reflected and incoming frequencies and detect the
>> resultant amplitude modulation.
>
> frequency, not amplitude
Thefrequency of the amplitude modulation will be the frequency of the
audio signal

--
"Fanaticism consists in redoubling your effort when you have
forgotten your aim."

George Santayana