1. Home
  2. Electronics Design
  3. Baud rate with LED & Phototransistor (or diode)

Baud rate with LED & Phototransistor (or diode)

I need to make a device that's totally sealed, but is powered by, and communicates with, the outside world.

For communications, I'm thinking along the lines of embedding IR LEDs and phototransistors in the boards, and talking that way.

Alignment in the z direction should be easy -- that can be achieved by two flat plates that touch. Alignment in the x and y directions would be super-simple to get down to 1mm of error, and 0.2mm should be doable if necessary.

I'm thinking of just aiming LEDs at photo-transistors in the opposite device, and using on-off keying. But I'm not sure what sort of data rates I can achieve, and at what current consumption. I'd like to aim for

115200 baud, using no more than 0.5 mA for both transmit and receive. Less current consumption would be absolutely wonderful.

So, my questions are:

  • Am I nuts? Is this too much to ask for from available parts?

  • Should I do something fancier than driving an LED with a resistor and a logic output? Do LEDs persist in glowing after the current goes to zero, and can you turn them off faster by actively sucking current out by the judicious application of some negative voltage?

  • Should I do something fancier than hanging a resistor off the collector of the photo-transistor and looking at the output?

Comments welcome.

--

Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com
Reply to
Tim Wescott
Loading thread data ...

How about IrDA? You can find ready made transceivers spec'd for up to

115kb/s

formatting link

And you don't need to implement the full IrDA protocol if you rooling both sides on your own. But even full IrDA is not a big deal. Took me about 2 days.

--
Reinhardt
Reply to
rbehm

Depends what you're trying to accomplish. Phototransistors are really really slow, but if your data rate is 10 kbaud or less, they'll work fine. By comparison the LED is effectively infinitely fast.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net
Reply to
Phil Hobbs

There were several push-pull and flip-flop LED data-link schemes shown here in the past. I'm trying to locate the message-ID's. ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| San Tan Valley, AZ 85142     Skype: skypeanalog  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.
Reply to
Jim Thompson

Den fredag den 24. oktober 2014 23.53.25 UTC+2 skrev Tim Wescott:

look at irda, upto 115.2kbaud is just your usual uart with a

3/16 bit wide pulse for a zero, plenty of transceivers available

Many MCUs have uarts that support it

-Lasse

Reply to
Lasse Langwadt Christensen

Thank you. I'll take a look. I don't need the long-range capability, but if the prices are right, who cares?

--

Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com
Reply to
Tim Wescott

Yup. I'm just finding that out on the bench. I could maybe make it work that slowly, but I'd rather have something quicker.

--

Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com
Reply to
Tim Wescott

Thers some IP68 Ethernet connectors that would work better. You could run data and power thru it.

Cheers

Reply to
Martin Riddle

LEDs are plenty fast for your data rate. They go on/off in 10s of ns with simple drive, like logic through a resistor.

OOK can be messed up by ambient light. If that's an issue, consider some sort of modulation.

What's the distance? Consider RF or magnetic coupling?

--

John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com
Reply to
John Larkin

I named 115200 baud as a target, but I could stand less, maybe (I'd actually have to -- gasp -- think about it). Less current consumption is better.

--

Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com
Reply to
Tim Wescott

Maybe adapt this...

PhotoCoupler-Fast.pdf

on the S.E.D/Schematic Page of my website, to separate LED and phototransistor components? ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| San Tan Valley, AZ 85142     Skype: skypeanalog  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.
Reply to
Jim Thompson

e
d

something like this looks to be a few mA,

formatting link
tfdu4301.pdf

and that is with an LED current set for 70cm range

-Lasse

Reply to
Lasse Langwadt Christensen

Customer wants no metallic connections.

--

Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com
Reply to
Tim Wescott

Might work, assuming I can find photodiodes with exposed bases. It's a bit complicated, but I suppose that's the price of speed.

--

Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com
Reply to
Tim Wescott

Or you could use a photodiode and some screamingly fast op amp like an LM324. ;)

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net
Reply to
Phil Hobbs

You can probably just use conventional photodiodes connected to the bases of say, 2N2369's. ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| San Tan Valley, AZ 85142     Skype: skypeanalog  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.
Reply to
Jim Thompson

Well, we had a race -- and you guys lost. Or maybe I'm rapidly leading myself down a garden path.

It looks like an LED (Fairchild QED123) with about 0.5mA working into a photodiode (Radio Shack no-name, half of 276-0142) will be plenty fast when loaded with 22k-ohms, and have enough voltage response (156mV) to work unambiguously with a cheap comparator.

So unless the dark current goes up too much with temperature (which I doubt, but will check on data sheets), I think I have a quick, cheap, reliable answer.

--

Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com
Reply to
Tim Wescott

It should double every 10 deg C.

I presume this scheme will be full duplex.

Reply to
miso

Yes, but the light paths will be totally separate. Thank goodness!

--

Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com
Reply to
Tim Wescott

Since your requirement is remote power, i.e. isolated, you could combine the power and data scheme by going FSK. FSK is a signal with constant power, so one problem down. The data rates you are going to achive with your LED scheme are on the same order as you would get with FSK over some magnetic interface.

Reply to
miso

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.