Very short range line-of-sight optical link

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I'm contemplating (again) a mechanical-scanning display. All homemade displays I've seen so far are fairly lame because they only show time or some pre-programmed text and stored animations. If I'm to make a display, it's going to be able to show full-motion video. Since the damn thing is going to spin at some 5000rpm, the primary problem is of course the data link. Power can be delivered by rails and filtered, filtering a data link is a bad idea.

I don't like brute force solutions like putting a wifi-enabled/mpeg4-playing linux router on the spinning part, even though it could be very funny.

I'm thinking about using a very short range optical link between the static and rotating parts. The transmitting side could flood a pretty large area, so even if the receiver photodiode has to be off-centre it will still be getting the stream. But I never did anything like that, never did any data transmission lines actually. What is the maximum possible BW with a led/photodiode combo? Modulation types recommended? Something like SPDIF, digital stream with self-clocking NRZ: should work but the receiver could be not very simple and, most importantly, I don't want to spend years on decoding/ERC code. Or I could actually just make an analog optical AM link for composite video: should be easy-peasy but if light source/receiver get misaligned I'll get funny patterns.. Analog FM-modulated composite perhaps then? What's a few MHz for an IR LED?

The Dark Boxes are coming.

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Have you seen this project: http://ronja.twibright.com/

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IRDA? Or cheaper yet: Use an sfh5110 or equivalent and modulate an infrared signal using the signal coming from and uart AND a 38kHz signal.

There are pointy haired bald people.
Time flies when you have a bad prescaler selected.

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daqq, I'm afraid IRDA and sfh5110 will only offer a small fraction of bitrate I need.

CountZero, thanks, I haven't seen Ronja before. I'm looking at their stuff. It seems to be very elaborate and way above my head, I hope I'll be able to learn something from their project.

The Dark Boxes are coming.

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:) Have been thinking about this non-sunday-afternoon-Svo-project for a while, and fast opto-coupling will be a pain ITA. Have a look at the datasheet of TOTX173: designed for plastic fiber, and fmax=6MHz. And that is a pretty fast device. Not being able to guarantee a continuous sight, it will be hard to meet the requirements.
So: how about a radio-link, making use of a transmittor as used in old uC-systems, or scrapped VCR-system, and for the receiving end a demodulator from another scrapped VCR ?
Antenna coupling, capacitive, on the edge of the spinning disc ?

As an alternative: hack a wireless transmission system: it holds everything you need. I have one if you need it ;)

Nard

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Plons wrote:
Have a look at the datasheet of TOTX173: designed for plastic fiber, and fmax=6MHz.
I have been puzzling over TOSLINK datasheets trying to figure out how to bitbang a red LED as a transmitter. I think that TOSLINK is filtered for a 6 (12?) MHz carrier; banging a red LED at 2 MHz only gets me fuzz out of a TOSLINK receiver. I only need a couple hundred bits per second so will go RC5 infrared, but a visible wavelength would be nicer.

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Transmitters: The TOTX147 goes from DC to 15Mb/s, according to the datasheet. TOTX173: DC to 6Mb/s

Receivers: The TORX147 handles 0.1 to 15 Mb/s: so the receiver seems to be the troublemaker. It's brother, the TORX173 says DC to 6Mb/s

So for your (=dak664) purpose, the 173's are the way to go

Nard

When will I ever grow up: I a too curious. Will therefor test the 173-combo tonight :)

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Hopefully never, Nard :D

The Dark Boxes are coming.

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I agree, ... it's too much fun to sort things out :)

@dak667: I tested with TOTX173 and TORX173, and even a few Hertz is still okay with them. The only thing I don't get is that the receiver *should* be TTL compatible, but the low level output = 1.5V instead of less than 0.4V as the datasheet says.

Oh well, don't know what is causing that. Make sure to use a scope to check it at your side of the pond.

Nard

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Yes, I get a good link from TOTX to TOTR (in my case very old RX174) but I suspect the TOTX is modulating my slow bitrate at a high frequency, one which I can't bit bang through an LED with an 8MHz CPU. I want to have 12 transmitters talking to one receiver through 10 cm of air, cheap if I can use red LEDs with one TOSLINK detector but way too expensive for multiple TOSLINK transmitters. It is sort of working with infrared now, using a detector scrounged from a cable TV box and bitbanging at 38 KHz, but I don't have enough IR diodes at present for a full test.

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The pictures I forgot to post yesterday: 1 mtr fiber between TOTX173 and TORX173, @ various frequencies, as low as a few Hertz

And that low level problem: my mistake; missed a pin ;)

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To round it off:

TOTX173 replaced with ordinary red led with 680Ohm in series @ 5V block: 11 Hz is no problem as you can see. Led shines into the fiber, and is very forgiving in terms of distance.

There is one important thing to realise though: the TORX173 is optically designed for a fiber-"nose" very close to its sensor. Shining with a red led from 10cm, the TORX has prblems detecting it as it is short-sighted as hell :lol:
The torx needs a contact lens ;)

I get back to work

Nard

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Ooh, I see you see the parts handy!

I remember from sound-linking experiments that both the sensor and emitter in TOSLINK are extremely shortsighted. Is there some decent LED/photodiode selection guide that lists rise/set times? Everywhere I go I see jigawatts and gigacandels, but no speed data. Ronja guys really did a nice job of finding the right stuff, but their stuff is too powerful for my matchbox-sized appliance.

The Dark Boxes are coming.

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It must be my RX174 receivers then, as that setup doesn't work for me. I epoxied a fiber into a hole drilled into an MV5023 LED so I know the signal is high enough. Oddly, I can't find any datasheet for the RX174 on the web. It is only about 15 years old :)

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@dak664: no, I can't find datasheet either for RX174

@Svo: transmitter is not the problem (me thinks), but the receiving end is.
I understand you want to investigate the optical link further before considering radio-solutions ?
Reason I came up with the TOTX and TORX is that these are designed for high speed, and when applicable for your project, it's just 6Mb/s for continuous link ! If continuous sight is not guaranteed, more speed is needed. Or multiple receivers of course.
I think you need to divert your rearch towards fiber-optic receivers as used in telecom. Expensive and probably hard to get stuff.

My test-setup is gone .... crap .... becuase I would like to know if an ordinary red led can do 6 Mb/s to a TORX173. How much speed do you need ?

Nard

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Why not only rotate mirrors ?

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Okay Svo, the TORX173 is a speedy device. And so is the ordinary red led: attached pictures. 4 MHz. Woohoo !

(mark the BW limit, and the effect on the signal. Breadboard huh?)

Nard

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Wouldn't a transformed based design work? E.g. just two coils, one in the base, and one in the rotating part? You probably could locate them very close together for low loss. Maybe you could even use it for power too. I don't know what the frequency limit would be though.

Last Edited: Tue. Sep 29, 2009 - 03:30 PM
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Feasible indeed. As capacitive coupling is. But it looks like Svo wants to use an optical link.

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A modulated higher powered laser that shines into a circular beam splitter that diffuses the light in a 360 degrees circle with an photodiode that looks onto it?

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Thanks Nard, your research is amazing! I checked out some IRDA transcievers and those up to 4mbps are readily available, although I'm not sure what exactly they mean by mbps there. I had no idea that IrDA is so fast these days and it seems to be a nice solution, as it doesn't require precise focusing. I'll keep on looking around for stuff that I can buy quickly instead of waiting for a month long delivery. Ronja project boosts confidence, too.

I plan on making the light link uninterrupted. By flooding or by mirrors ;)

re: transformer/radio: with optical link in confined space I can do baseband comms with relatively little effort. I'm not sure if I can do this with radio. It will spin like hell and relative position of antennae will change all of the time and the electrical noise from bad contact in power rails and a hundred of simultaneously switching LED's is expected to be rather high. Readily available modules like ZigBee's probably can work in such conditions but I need about 8mbps link speed, ideally.

The Dark Boxes are coming.

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Svo *loves* lasers 8)

Final speed test: 8MHz with just an ordinary led and a TORX173 :!:

Edit:

How about: disc-shaped pcb, spinning @ 5000 RPM, one TORX173, not necessarily on center, with glasses of course to compensate for its shortsight, and 4 (or more) low cost LED's on the stator surrounding the spinning disc. All LEDs are fed with the same signal. No multiplexing required.

One problem though: video is analog .... this transmission is 1 bit-digital. You need at least an extra bit for the syncpulses.

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There seems to be plenty of discrete photo devices available. It's hard to search because normally they're listed unsorted and two adjacent models can have rise times of 3us and 20ns. Of course I wanted to avoid extra circuitry but what's wrong with a transistor or two.

LED's:
BIR-BO0331 20ns 850nm
BIR-BO07J4G 20ns 850nm

PD's:
SFH213/SFH213FA - 5ns

Re: video, I'm counting in bits already. The best luck scenario would be: 45x2 LED's at 0.5 degree angular resolution: that's 64800 pixels. Suppose I want at least 16 greys, that's 64800*4 = 259200 bits per frame. 25 frames/sec => 64800000 bits/s. When this doesn't work out, I'll start reducing the bit depth or make it one field only or add smartness. Inner circles definitely don't need angular resolution as high as those on the rim. I might transmit in cartesian coordinates, even.

IIRC IrDA uses some very simple kind of modulation, like 0 = short pulse, 1 = no pulse. Can be self-clocking if bits are thoroughly scrambled. If that's true and it works for someone, I don't see why it shouldn't work for me. With LED's that can switch at 25MHz, hey!

The Dark Boxes are coming.

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Oh, awesome results you have on the scope! Your LED could be not as ordinary as you think it is ;) I just browsed through some datasheets and it seems that rise/fall times of 500-3000ns are considered normal for LEDs, if specified at all.

The Dark Boxes are coming.

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Svo wrote:

Quote:
.... but what's wrong with a transistor or two.
ROFL !

25 frames per sec ... is that interlaced ?
About the used led: it's chinese, does that give a clue ? Will try other leds as well. This time I didn't disassemble the test setup, btw. I see in my crystal ball it may be needed more often soon ...

Nard
< dinner: Sauerkraut mit Bratwurst >

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The plan is like so:

             circuitscircuitscircuits

     circuitscircuitscircuitscircuitscircuits


 circuitscircuitscircuitscircuitscircuitscircuits



--------(45 leds)-------########(45 leds)#########  



 circuitscircuitscircuitscircuitscircuitscircuits


     circuitscircuitscircuitscircuitscircuits

             circuitscircuitscircuits

One side is offset by half-led-width. So 25 frames is 50 fields interlaced. But as we say here, so far it's written with a fork on water. Everything is adjustable :)

Enjoy your Sauerkraut mit colourburst!

The Dark Boxes are coming.

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Plons wrote:
Okay Svo, the TORX173 is a speedy device.

Darn, putting together a Digikey order I see the TORX173 is listed as non-stock, minimum order 50 for $250, and Mouser says it is obsolete. Wonder what replaced it? I'm going to try removing the optical filter from a standard IR receiver and see if it works with red LEDs.

The IR link is working nicely though, 12 ATTiny85 in sequence send 12 bits each to one ATTiny85 receiver acting as a USB keyboard that types the cell voltage data into a laptop. Very reliable, no errors on 10 channels in ~100K transmissions. The other two channels get Vcc through 100 ohm resistors, and they have ~1 percent error rates. Old roller-ball mice have two IR leds that can be salvaged, and old cable boxes/VCRs have IR receivers.

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TORX141 goes from 0.1 to 15Mb/s. With Manchestercode it works from 0Hz :)
TORX111 from DC to 6Mb/s

How about going through Digikey's catalogue and have a spec look at all available TORX devices ?

I doubt if your trick with removing the IR filter will give you the result you need: these receivers want to see a carrier. They are more than just
Btw, I did also tests with IR diodes/transistors, but all I have in the box can't go higher than 50 kHz or so (all depends on pullup and IR-drive) And no typenumbers either :(

Nard

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Plons wrote:
TORX141 goes from 0.1 to 15Mb/s. With Manchestercode it works from 0Hz :)
TORX111 from DC to 6Mb/s

How about going through Digikey's catalogue and have a spec look at all available TORX devices ?


All the cheap ones are specd lower limit 0.1. But I was giving my old RX174s a square wave up to 1 MHz and got nothing but fuzz. I'll order some TORX177s for a test.
Quote:
I doubt if your trick with removing the IR filter will give you the result you need: these receivers want to see a carrier.
The Tiny's already put out the 38KHz carrier and RC5 Manchester code, so it's just a matter of trying a red LED against a bare detector. It might need a red filter/lens though.

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I got my TORX177s from Digikey and also their LED assortment 511-8001-KIT-ND. Testing with oscilloscope on output, input a 1 meter optical fiber with other end aimed at LED driven with ATTiny45 banging RC5 through 120 ohms to +5.

All the water-clear LEDs (red, orange, yellow, green) worked at 4 cm, but the red ones worked out to 10-15 cm (the 611nm 511-1217 going farther than the 630nm 511-1215, but it is also a narrower and 3x brighter cone). Transparent worked not so well, at 2 cm, and diffused not at all except for direct contact to the red case. That explains why I had no success with the old MV5023's which have a diffused lens and 1/10th the intensity.

Interestingly, water-clear 940nm infrared LEDs 160-1029-ND worked as well as the visible non-red colors. So I suspect the toslink receiver (or at least this one in particular) is an infrared detector with maybe an interference filter at the interface. With a clear lens it could probably illustrate both an infrared and visible link over ~100 cm.

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There are TOSLINK modules that can go up to 20 or 50 Mbps. Depending on the manufacturer, DC either passes or not. One device I have seen had minimum frequency of 100kHz or so, so any longer pause between pulses or pulse duration between pauses gets distorted.