Half a croissant, on a plate, with a sign in front of it saying '50c'
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Crystal TV

Slow-scan TV implemented with a crystal receiver
  (+7, -1)
(+7, -1)
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This idea is inspired by a comment by [nineteenthly] on another idea, which I'll link below.

I'm using the exact same principles as the crystal radio, but this is going to output an image.

Quick summary of a "crystal set" radio: Antenna, coil, diode, earphone, ground. Attach antenna to a tap on the side of the coil, the diode to the end, earphone to the diode and to the other end of the coil, and from that point connect the ground. The coil/antenna pair selects the radio signal; the diode retrieves the audio signal that was Amplitude Modulated onto it. (AM radio, see?)

OK, now we make a change. We're going to take the diode out of the circuit, and replace it with an LED. It gets glued to the side of a finely threaded rod. Then a very fine tube (about the size of a 10ga hypodermic needle) is attached (epoxy might be good) to the LED in such a way that the emitted light can be seen down the tube. The LED should then be covered so no light escapes any other way.

Now take a nut which will fit on the threaded rod. Braze a frame onto it such that you can have a piece of PVC pipe sticking out one way, and a crank handle sticking out the other.

Next, a toothpick will go into the inside of the pipe. Lengthwise, and positioned so that the light tube points at it when you've got the crank handle in the farthest position from your body. The light tube has to be just short enough to not hit the toothpick when the crank is turned. (I would have said that earlier, but you would have said "What toothpick?")

Finally, you'll need to put a piece of photosensitive paper or film into the PVC pipe. This should be cut so that it is slightly shorter than the inner circumference of the pipe. (Did I mention that this will be done in a darkroom? At least the cutting & using, anyway.) When it's the right length, the edges of the paper or film will slip under the edges of the toothpick and be held in place.

Now, you get your local radio station to transmit a picture. (Exercise for the reader, obviously.) The picture will be transmitted as sound levels, lowest for bright areas, loudest for dark; preferable to have them transmit borders on the left and right. Each scan across the picture will then be very rhythmic, with a definite break in between scans.

So you will be standing in your darkroom, listening to the earphone as the content of the picture is described; you make the decision that you want to "download" this image. You grab the crank, and start turning it as you are given a 5-count to get your timing steady, and then crank-crank-crank to the rhythm of the pshshshst - pshshshst - pshshshst of the image bands.

Then, a dip in the Dektol, the stop bath, the fixer, the wash, and viola!

(or was that a lute? the neck is kind of bent, or did you not turn the crank evenly?...)

lurch, Jun 12 2009

Emergency Crystal Radio Emergency_20Crystal_20Radio
Where this came from... [lurch, Jun 12 2009]

SSTV http://www.electron...n-television-tv.php
Slow-Scan Television [Ian Tindale, Jun 13 2009]

Vinyl video http://www.vinylvideo.com/
If it didn't work you could just turn the record into a Nipkow disc. [nineteenthly, Jun 13 2009]


       Right thanks. I did post a somewhat similar idea years ago when i was someone else, but you weren't to know and it actually involved a very small LCD as mentioned in my anno on the other idea. Right now i've just scan-read this but i'll read it properly in a minute. Just wanted to raise one issue which is maybe more surmountable than you think.
//Now, you get your local radio station to transmit a picture//
This isn't as hard as you might think.
Firstly, as [Ian] has posted, there's slow-scan TV. This has been done by radio hams.
Secondly, have you heard of the Vinyl Video project? It's a recent attempt to store images on vinyl records, and if you either broadcast those or used a similar technique with maybe plugging the record player into the TV, you could get a picture that way.
Thirdly, since yesterday's foray into Wikipedia's entry on crystal sets, i'm currently most enthusiastic about using a spark gap to transmit signals, and i can't help thinking that it would be peculiarly appropriate to couple your telly with a mechanical TV camera with a Nipkow disc (interesting spelling there) and a spark gap.
I'll read your idea properly forthwith.
nineteenthly, Jun 13 2009

       OK, reddit. What do i think? Well, i think i need to know more about electronics, but i don't see why that shouldn't work. I am, however, concerned at the dimness. I wonder about using a candle flame and some kind of shutter, or maybe sunlight focussed to a point by a lens, then piped with fibre optics through some kind of filter which can be polarised with a weak electric field.
nineteenthly, Jun 13 2009

       // i'm currently most enthusiastic about using a spark gap to transmit signals//
You really, really shouldn't, unless you want the radio regulatory people down on you like a ton of bricks.

There was a project done, I think by the BBC a few years ago, to laser-scan some Baird 78 rpm video disks.
AbsintheWithoutLeave, Jun 13 2009

       So, in the event of a major emergency in a particular locale, to disseminate a message to the people we're now doing what? Distributing pictures of 78's?
Ian Tindale, Jun 13 2009

       //in the event of a major emergency in a particular locale// - yes, I think this would be particularly good for about the third consecutive year of living in a bomb shelter with absolutely nothing to do...
lurch, Jun 13 2009

       [Absinthe], yes, i did think about the sparky interference thing, but i also wondered if there was a way to narrow down the frequency of the radio waves emitted and if the current was sufficiently weak, it would presumably not transmit very far.
nineteenthly, Jun 13 2009

       //but i also wondered if there was a way to narrow down the frequency of the radio waves emitted //
No, sorry.
AbsintheWithoutLeave, Jun 13 2009

       [bigsleep] - the fascinating thing about the crystal radio is that it can be understood in full by a regular human. Concept, components, functionality, the whole bit.   

       "In case of emergency here's how to make a small voltage power generator to replace common battery combinations." - Why? I have a card of 5 LM723's in the drawer, and I'd just be attempting to re-invent one. Requires precision; precision is generally not half-bakeable.
lurch, Jun 13 2009

       Well this is obsolete, useless and absolutely insane, I LOVE IT!
Hirudinea, Jun 13 2009

       //jumping from...// exactly. I *understand* a crystal set. I can google up enough info to build a voltage regulator, but I don't really "grok" what it's doing, so I don't find it so fascinating.   

       If you've got an idea for a half-baked power supply, I'd love to see it.
lurch, Jun 13 2009


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