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Accuracy of PIC A/D inputsjohn_becker25/07/07  11:04 pm
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poriet
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Post Number: 190
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Posted on Monday, 23 July, 2007 - 05:00 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hi,
I was thinking of constructing a home-brew light link to transmit the composite video signal across the garden.
I know you can get these GHz radio transmitters,
but I wont know whether it will work OK outside until I buy it, and if it doesn't work theres not much I can do with it. With a home-brew link at least I can fiddle with it.
It is possible in principle to use either a laser diode or an IR LED. I think I prefer using the LED approach. Does anyone have any practical experience of modulating an IR LED at 5-8 MHz and then re-amplifying it? At these frequencies the techniques are rather specialised I suspect ( or perhaps it's not that difficult? ) I have Googled for hours, but theres nothing out there that I can see.
P.
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nielsejner
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Posted on Monday, 23 July, 2007 - 06:12 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

How big is your garden ?

Google for "VIDEO SENDER LIFETEC" in 'picture' mode.

From time to time I use such a wireless system from nests in a remote birdhouse to my house.

Sender and camera, running on 12 v car accu. Reaches max. 8 meters with nothing between but glass, the system as such, however is not suited for outside use, but should be shielded against rain.

Sejner
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gajjer
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Posted on Monday, 23 July, 2007 - 06:15 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hi Poriet

don't have much experience of what you are trying to do but I suspect the problem will be with the receiver end. My experience is that the photo-transistor isn't that fast.

I would suggest you look at the configurations of opto isolators and look at the speeds they are capable of. The optics could be tricky too because you will need a narrow beam to prevent inverse square law problems and loss of signal. Narrow beam means that you will have to align it carefully and in the IR not in the visible ( if you use an IR diode ).

Sounds interesting though. I'd like to hear how you get on.

Cheers
gaj
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gajjer
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Posted on Monday, 23 July, 2007 - 06:34 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hi again
just found a circuit in an old TI optoelectronic book.
It talks of a GaAs diode at the transmitting end ( TIXL12 ) and a Si avalanche diode at the receiving end ( TIXL56 ) it doesn't talk about distances though. However it is operating at 50MHz.

The receiver is very simple. 170V yes 170V feeds a 1M resistor down to the cathode of the TIXL56. The anode goes to ground. At the junction of the 1M and the cathode a 120pF cap taps off and feeds a RF transformer. Primary 11 turns secondary 1 turn. The transformer secondary feeds the input to the TV. No details of the coils given I'm afraid.

Don't know if that helps.

cheers
gaj
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obiwan
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Posted on Monday, 23 July, 2007 - 07:32 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

You can easily purchase many different "output/input" devices. And they're not that expensive.

I was mostly involved in the fiber optic cable manufacturing, not so much the interface to it.

I'm not sure if you can purchase the quantity of cable you would require, but you can look into it.

That would be your best bet, not trying to send it through the air.

But as Sejner noted, the first question is just how far you're talking about.

I don't think the modulation of a LED is going to be that much of an issue. Coupling the LED and the receiver is going to be your biggest issue, then the distance you need to transverse.

The receiver will probably be your biggest problem. Recovering the HF will be pretty sensitive. For one, the signal will be quite small, even with a coupling medium such a fiber.

And as Gaj mentioned, the inverse law of squares will limit the distance without a medium. The fiber medium will limit this effect as all the light (almost all of it) bounces around inside the fiber off of the inside of the walls of the fiber (actually it's much more complicated than that, numeric indexes and so on...)

Glass fiber is best, but they have some very good plastic fibers out there now.

If you can't get a length that is suitable for you, then you'll have to try and make a custom length, and that will involve more than you probably want to get into.

Plastic "can" be easier than glass in that respect because you can "heat polish" the end, and a drop of indexing oil and you have a coupling.

With glass fiber, you need to nick the fiber (after stripping it of course, and even that is tough if it has a Kevlar sheath, as most do). Normally that's done with a diamond or carbide blade. Then you have to polish it with various (VERY) fine grades of sand paper and then a polish, and then you add a drop of indexing oil.

But no matter what, fiber will still be your best bet.

As you mentioned, you could use a LASER, they're good for such things as this. But that will be problematic still if you wish to do it without a fiber (again).

Most transmission LASER's are going to be oriented towards fibers, and won't have such a tight beam pattern.

Most of the LASER's for such transmission are geared towards high quality stability and purity of wavelength, not beam pattern because they're coupled into a fiber anyway.

But, even if you don't plan on using a fiber or LASER, check out a couple of catalogs like Digi-Key and maybe Mouser, and maybe Newark Electronics. Even if you don't purchase from them, they can provide a wealth of information on the devices.

In my opinion, I'd go with the RF link. I think in the longrun, you will come out cheaper. I'm sure you'll be able to find a use for it later if you decide it's not going to work out very well, or you should be able to return it.

Check out Sparkfun Electronics and see what they have, they normally have something that would work.

That's just my four cents worth....

(Oh, for the receiver, you'll probably want to use PIN photodiode, I have a couple if you decide you might want to play with one. Looks more like a tiny solar cell in a big gold transistor)

(Message edited by obiwan on 23 July, 2007)
Do Not Hit The Fly That Lands On The Tigers Head.
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zeitghost
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Posted on Tuesday, 24 July, 2007 - 09:02 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

What's the problem with the Video Sender?

I get 60m range out of the one in my garage...

One of those tiny cameras with the built in transmitter is good for 40 - 50m with a fresh PP3.

Maplin did an IR video sender way back... the screen shots were remarkably fuzzy.
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nielsejner
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Posted on Tuesday, 24 July, 2007 - 10:40 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

zeitghost, may I ask what system you are using, reaching 60 m. ?

Sejner
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sounded_simple
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Posted on Tuesday, 24 July, 2007 - 11:29 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Im all for building projects but why not just purchase the radio link?

IR will have loads of trouble and will me more expensive.

There is still plenty of project work even if you buy the modules.
Getting the aerial configuration right is an art in itself!!
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poriet
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Posted on Tuesday, 24 July, 2007 - 12:40 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

OK guys; thanks for the input.

A 'Richard Warwick' (IIRC) has a website in which he hacked a TV modulator and turned it into a transmitter. By so doing, and by placing a video camera inside a robot, he was able to run the robot around the house and he could see exactly what the robot saw. Very clever. I did likewise, ( without the robot ) and left the camera in my girl's room. Sneaky. The magistrate was very understanding.
However, when I tried it outside ( the garden is maybe 50 feet max ) AND with an outside aerial, I got nothing. It was this miserable experience which has put me off buying a radio link. These are intended for use inside a house, not across a garden. They might work, but I dont trust them.
Anyway, that was the reason I started to look to an optical link.
Zeitghost: I would also dearly like to know what system you have if it works OK across 60 feet.
'Sounded simple' obviously had trouble establishing a radio link. Not only that, but he stole my quote from a Richard Harris film:
"lads if twas easy, everyone would be doing it!"
Regards
P.
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zeitghost
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Posted on Tuesday, 24 July, 2007 - 03:07 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

60 feet?

60 metres... it's a bog standard 2.45GHz video sender, with those funny paddle like semi directional antennas on the top....

Open air propogation is no problem, it's going through walls that gives the difficulty.

Poriet, you haven't been a naughty boy & made one of those "special" shampoo bottles for the shower have you?

There was a lad in north Wales done for that recently...
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terry
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Posted on Tuesday, 24 July, 2007 - 05:32 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

I use a 2.4Ghz video link from my r/c plane, see here http://myweb.tiscali.co.uk/elevated/plane.htm
Get the modules here http://www.active-robots.com/products/radio-solutions/av-modules.shtml

Terry
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poriet
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Posted on Tuesday, 24 July, 2007 - 05:41 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Zeitghost: Theres nothing wrong with the odd bottle of shampoo in the bathroom. That guy in Wales was misunderstood. Personally, I prefer to hide the camera in the light-switch as otherwise you can get soap in the electronics.

Terry: That's a really interesting site ; I never knew r/c guys had this sort of gear.

I think I've just bought a 2.4GHz system off ebay. Time will tell if it works.
However, I still rather fancy making a home-brew optical link and if anyone has some more info, I'd like to hear it.
Regards
P.
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boris
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Posted on Tuesday, 24 July, 2007 - 07:20 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Optical TX. Simple LASER pointer with a current transformer in the power line. Modulate the power to the LASER by sending the amplified video signal to the primary of the transformer.
"What's red and invisible?"
"No tomatoes"
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boris
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Posted on Tuesday, 24 July, 2007 - 09:52 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

A variation on this: http://www.i-hacked.com/index.php?option=com_content&task=view&id=162&Itemid=44
"What's red and invisible?"
"No tomatoes"
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terry
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Posted on Tuesday, 24 July, 2007 - 11:00 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

I'm interested in sending video by laser diode myself. As far as I can tell the important thing is to bias the laser diode correctly so you get the sinc pulses at the low end and white at the top. It looks simple to do using a fibre but free air would be tricky.

Terry
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zeitghost
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Posted on Wednesday, 25 July, 2007 - 08:50 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

I'd have thought that frequency modulation would be better, though I've never tried it.

The Maplin thing used amplitude modulation of the IR diode.

With the record modulator out of a vcr and the playback demodulator out of same, might be an interesting experiment... must try it some time.

The Maplin thing used two large fresnel lenses, one at either end of the link.
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terry
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Posted on Wednesday, 25 July, 2007 - 02:41 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

I did read somewhere that a laser diode can be modulated up to 1Ghz so it should be possable to use it like an RF modulator.

Terry
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poriet
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Posted on Wednesday, 25 July, 2007 - 05:55 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

I reckon the IR LED is a better bet because, as an amateur, I cant get hold of naked diodes with any info. AFAIK modules wont respond quickly enough.
Personally I cant see why it shouldn't be straightforward to establish an optical link across the garden ; the odd op amp and a pair of Grannie's spectacles and the job is done. The hard parts are:
(a) modulating the beam at 8MHz but getting the biasing right so it retains its shape.
(b) re-amplifying the tiny signal available from the detector diode ( a fast PIN photodiode surely is fast enough ? ) back to a pukker 1V video signal.
Boris: thanks for the info. However, I have seen that site and, while it's OK for audio, I dont think it would be much cop with a video signal,
although I could be wrong as I have been wrong before.
I was hoping someone had actually done this before, but it looks like the radio approach is preferred.
P.
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terry
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Posted on Wednesday, 25 July, 2007 - 07:44 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

poriet,
You say the modules respond too slow,I agree. I cut the diode out of cheap laser pointers and don't connect the feedback pin, lightning fast then.

Terry

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