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simple comm system using visible light link

simple comm system using visible light link

Hi smile

I want to build a simple system with the following specifications:

1. group of LEDs with high directivity as a transmitter and a photo-detector as a receiver (works efficiently within at least 1.5 m "5 ft").
2. the nature of the message to be sent is just a finite number of bits (maybe just 8 bits serially sent).
3. for the interface used in receiver, 7 segments LED display that shows a string of 8 bits whenever a new signal is received.
4. for the interface used in transmitter, 7 segments LED display that shows a string of 8 bits to be sent. Each bit has a simple specific switch that assigns the value of the bit (0 or 1). And a button that initiates the transmission of the string of bits.
5. high quality: error in detection is minimized as much as possible.

The problem is I don't know where to begin. I have a good background in the theory but I'm not familiar with the modules in the market.
What modules do you guys recommend to buy to build this system with the above specifications (would be convenient if it's available in Amazon)? What's the circuit diagram? any complicated programming to be expected (I want to avoid coding as much as possible)?

Thank you smile

RE: simple comm system using visible light link

There are a bunch of chips that support this. Look up "IR communications".

Unless you only want paltry one foot or less communications you will need to modulate the signals. Modulation increases, by several orders of magnitude, the signal-to-noise ratio so the receivers can actually distinguish the correct signals from the overwhelming background.

You would fair better if you use IR and use visible light to show only sent and received characters.

Keith Cress
kcress -

RE: simple comm system using visible light link

Haven't you basically described the comm link for a TV remote?

Error detection/correction is a function of the environment. What are your requirements?

Is this for school? Student posting is prohibited.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!
FAQ731-376: Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list

RE: simple comm system using visible light link

Circa 1977 (good grief, 40 years ago, sigh...) for a high school science project, I used an IR LED and a phototransistor to send audio signals across the room, by means of simple amplitude modulation of the IR. Distance was optically assisted with a flashlight reflector on the LED and a telescope on the receiver end. The driver and receiver circuits were simple analog opamps, nothing complicated.

RE: simple comm system using visible light link

A seven segment LED module can display a limited set of 16 characters, e.g. ASCII 0-9,a-f.
It cannot display a 'string' in the sense of a row of 8 bits.
For that you need a row of single LEDs, which used to be available in one-row packages with a common cathode or a common anode or maybe either. I haven't seen them for a while, so you might have to use 8 discrete LEDs in a row.

<grossly simplified>
... fed into a shift register at the transmitter end,
and a shift register at the receiver end,
both driven by the same clock,
_if_ you are allowed to run a wire between transmitter and receiver.

If you are not allowed to electrically connect transmitter and receiver, things get more interesting. The receiver has to sample the incoming hi/lo signals at some muliple of the transmitter bit rate, so that it can detect actual edges despite differences in the clock rates at each end.

There are, as indicated, IR communication chips that do a lot of the work for you.
I have never been able to make much sense of their documentation, or used them myself, but they demonstrably work.
It might be a better learning exercise to read datasheets and app notes for serial comm chips, e.g. 8251 family, 16550 family, and similar devices.

Which would work with one transmit LED and one receive phototransistor.

If you use an array of transmitter LEDs, and a single phototransistor reciver, as you suggest in (1), the transmitter circuitry gets slightly simpler because you don't need a shift register to multiplex the switch settings into a single transmitter, but you will need to gate the switch outputs so only one transmit LED is on at a time, and you will need to make sure that you get roughly the same level of output from the receiver for every transmit LED, then feed the receiver signal into a shift register, find the edges, etc., as hinted above.

That may not be what you want.

Go read up on serial communications, opto or wired, then revisit your specification as outlined in your first message, and figure out where you want to go with it.

Mike Halloran
Pembroke Pines, FL, USA

RE: simple comm system using visible light link

Actually, a typical 7-segment LED often includes a decimal point, so there would be 8 LEDs that could be used to display the state of an 8-bit message; just saying...

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!
FAQ731-376: Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list

RE: simple comm system using visible light link

I forgot to mention that the 7 segment LED can also display the worldwide trademark of Joseph Lucas, darkness.

Absolutely true, there is a decimal point that you could use.
I would go crazy trying to remember which segment maps to which bit; OP specified an inefficient mapping instead of ASCII.

Mike Halloran
Pembroke Pines, FL, USA

RE: simple comm system using visible light link

Did this myself just over 40 years ago for a high school science project.
Back then, LEDs were fairly new and something of an unknown quantity.
We started with a simple incandescent torch lamp as the emitter, and one of the old-type germanium transistors with the painted glass envelope scraped clean as the photodetector.
We found that if the emitter and detector were mounted in black cardboard tubes with a suitable lens on the front, then we could transmit telephone-quality audio across the school rugby field.
Surprisingly, best results were obtained with the lamp running at well below full brightness, more of a dull glow.
We did try out a visible-red LED, and this gave quite good results too iirc.

RE: simple comm system using visible light link

We tried it to read out rail car refrigerators using a Radio Shack Model 100 (dare I call it a laptop)?

Trying baseband we had absolutely abysmal results.
So we modulated, but the guy in charge did it at a lame frequency, half of what TV remotes used. He chose 27kHz IIRC.

Then we had to add reflectors so a lot of 6V lantern battery flashlights died for the cause.
All you needed was some direct sunlight to overwhelm the photo-diode.

Then we added a very narrow passband IR filter and that helped a lot.

Finally you could wear this plexiglass bib thingy that held the M100 at working height and the reader unit was a flashlight with a pistol grip, looking like some sort of exotic deathray.

We could read out 32k bytes in one try... um.. frequently.

Keith Cress
kcress -

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