Baud rate with LED & Phototransistor (or diode)

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

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.

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

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

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

Many MCUs have uarts that support it

-Lasse

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

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.

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

Cheers

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?

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.

Maybe adapt this...

PhotoCoupler-Fast.pdf

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

something like this looks to be a few mA,

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

-Lasse

Customer wants no metallic connections.

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

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

Cheers

Phil Hobbs

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

It should double every 10 deg C.

I presume this scheme will be full duplex.

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

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.