LED as detector for high speed link

that's good to know. That migth be one reason some LEF lighting is so flickery and terrible looking, not counting bad CRI.

Ha

Hmm I just did the 'silly' measurement on the bench, some red led (through hole) +15V with 10 meg ohm resistor to ground. Sitting in the room lights I got about 0.4 V, pulling my desk light right on top of it I got 3-4 Volts (0.3 uA) (desk lamp is 60W incandescent)

50 uA seems like a lot.

George H.

Mbit speed if possible

or when the link is not active)

0R974.pdf

with small amplitude. However, I need a large amplitude, to be able to dete ct bit level with logic 3V. (so over 2.4V)

ite LED (to cover the entire spectrum of the RED LED detector), a big one, and driving it with a lot of current, to get sufficient photon level at the red LED to have both high speed and logic IO level

wavelength, so that is why I use a white LED to be sure wavelenghts overla p

ent low load resistor for the red detector LED to have high bandwidth (1MHz )

ere some kind of saturation effect?

That's my recollection too, which would seem to make sense band-gap-wise. So, I was surprised when last night's red LEDs did so poorly detecting a GaN green source.

Yep. A one-cent one-transistor common base amplifier would do wonders for speeding things up.

Cheers, James Arthur

The LEDs I used last night had lensed cases, excepting the green emitter (who lost her head in an unfortunate--for her--encounter with a Dremel tool).

So, nose-to-nose, I think there's a fair chance I coupled something like 25-50% of the photons.

If Klaus wants to transmit over any distance, he's going to have to compromise on cost or range and speed.

Cheers, James

1 Mbit speed if possible

ator when the link is not active)

%20R974.pdf

t with small amplitude. However, I need a large amplitude, to be able to de tect bit level with logic 3V. (so over 2.4V)

white LED (to cover the entire spectrum of the RED LED detector), a big one , and driving it with a lot of current, to get sufficient photon level at t he red LED to have both high speed and logic IO level

ty wavelength, so that is why I use a white LED to be sure wavelenghts over lap

cient low load resistor for the red detector LED to have high bandwidth (1M Hz)

there some kind of saturation effect?

Did the LED have a clear lens? I used a red led with a red lens and it worked best with an orange (amber) led... less well with yellow and green. Even when I sanded lens down to a few mil of the surface.

George H.

n

d 1 Mbit speed if possible

icator when the link is not active)

LH%20R974.pdf

Bit with small amplitude. However, I need a large amplitude, to be able to detect bit level with logic 3V. (so over 2.4V)

a white LED (to cover the entire spectrum of the RED LED detector), a big o ne, and driving it with a lot of current, to get sufficient photon level at the red LED to have both high speed and logic IO level

vity wavelength, so that is why I use a white LED to be sure wavelenghts ov erlap

ficient low load resistor for the red detector LED to have high bandwidth (

1MHz)

s there some kind of saturation effect?

All cases were water-clear. I'd hacked the green emitter's lens off to make it a better point-source in another experiment, but here I don't think it was a big loss, since that allowed me to press the (red) detector LED almost smack up against the emitter's die.

I sent John some of those magnificent Agilent GaN greens a decade or two ago, and he posted about them here. :-)

Cheers, James

I hear Wernher von Braun said the same when he started work on the Redstone program...

1 Mbit speed if possible

ator when the link is not active)

%20R974.pdf

t with small amplitude. However, I need a large amplitude, to be able to de tect bit level with logic 3V. (so over 2.4V)

white LED (to cover the entire spectrum of the RED LED detector), a big one , and driving it with a lot of current, to get sufficient photon level at t he red LED to have both high speed and logic IO level

ty wavelength, so that is why I use a white LED to be sure wavelenghts over lap

cient low load resistor for the red detector LED to have high bandwidth (1M Hz)

there some kind of saturation effect?

t

ke

h

That also explains the funny "white" emission curve. Thanks :-)

I cannot add significant cost, since this is only a link for debugging. Any add on would increase the product price

I am working on that also, in parallel with the LED interface

Cheers

Klaus

Mbit speed if possible

or when the link is not active)

0R974.pdf

with small amplitude. However, I need a large amplitude, to be able to dete ct bit level with logic 3V. (so over 2.4V)

ite LED (to cover the entire spectrum of the RED LED detector), a big one, and driving it with a lot of current, to get sufficient photon level at the red LED to have both high speed and logic IO level

wavelength, so that is why I use a white LED to be sure wavelenghts overla p

ent low load resistor for the red detector LED to have high bandwidth (1MHz )

ere some kind of saturation effect?

Yes, I saw that in a datasheet, that the emission wavelenght is not the sam e as the receptivity wavelength, and therefore other colors than red might be better

A BC847 may be acceptable :-)

Cheers

Klaus

1 Mbit speed if possible

ator when the link is not active)

%20R974.pdf

t with small amplitude. However, I need a large amplitude, to be able to de tect bit level with logic 3V. (so over 2.4V)

white LED (to cover the entire spectrum of the RED LED detector), a big one , and driving it with a lot of current, to get sufficient photon level at t he red LED to have both high speed and logic IO level

ty wavelength, so that is why I use a white LED to be sure wavelenghts over lap

cient low load resistor for the red detector LED to have high bandwidth (1M Hz)

there some kind of saturation effect?

nd

t

.

This is the LED as written in the OP:

4.pdf

It's cheap, but also a good brand (Osram)

Yes, if you want to do that with internal pull-ups you'd have to blast it with light. It must be focussed and the measly watt that comes out of the 60W incandescent bulb is wasted in all directions. The other 59W just heat the room.

Klaus is aware that this method is going to be inefficient, it just has to be cheap. 1Mbit/sec pretty much precludes resistor values above the typical 50-100k internal pull-up.

Time to consider switching to LED or at least CFL :-)

[...]

--
Regards, Joerg 

http://www.analogconsultants.com/

Jawohl, sehr gut :-)

However, I cannot get used to the German patent lingo

That is actually the way I was thinking it could be hooked up. I just need a low loading resistance to increase the bandwidth (combating capacitive effects)

Yes :-)

I know I need to be carefull. It's actually also written quite explicitly in the datasheet, keep away from eyes

Cheers

Klaus

Here's my guesses for SFH350 photodiode and also an SFH756 IR LED... unsure where I got the capacitance guess for that... maybe here. their datasheet specs a minimum (on) photocurrent of

1.6uA. That and the typical offset voltage of uC internal comparators motivated the choice of resistor. I wanted around 300mV out.

RC corner freq: detector SFH250 11+6pF emitter SFH756 30+6pf

180k series R

SFH250 fc=52kHz ( 24kbaud) 19200 SFH756 fc=24kHz ( 12kbaud) 9600

with 5V rev bias SFH250 3.5p+6p fc=93kHz (50kbaud) 38400 SFH756 9.6p+6p(guess) fc=56KHz (25kbaud) 19200

High speed it is not. Also, stronger signals reduce reverse bias so will worsen bandwidth.

Right, Just a silly test and Klaus will have to measure his LED photo- response. I was thinking that a laser, with maybe a diffusor right in front of the LED/detector, might give more light.

I've got an LED light, but I do like an incandescent lamp too. Nice broad-band black-body source. And it extends into the IR... which is good for checking some things.

George H.

it speed if possible

when the link is not active)

974.pdf

th small amplitude. However, I need a large amplitude, to be able to detect bit level with logic 3V. (so over 2.4V)

e LED (to cover the entire spectrum of the RED LED detector), a big one, an d driving it with a lot of current, to get sufficient photon level at the r ed LED to have both high speed and logic IO level

avelength, so that is why I use a white LED to be sure wavelenghts overlap

t low load resistor for the red detector LED to have high bandwidth (1MHz)

e some kind of saturation effect?

Just an update:

I got it working now at 10MBit, allthough only with a smaller amplitude (I will use a "free" comparator in the micro to detect the 100mV signal

Solution: Blasting a cheap Red LED with a big Red Led, with a resistor on t he detector side to decrease fall times

Cheers

Klaus

If I use the same dirt cheap LED as transmitter I can get about 10kBit with small amplitude. However, I need a large amplitude, to be able to detect bit level with logic 3V. (so over 2.4V)

Great! Thanks for the feedback. 100mV with a resistor small enough to allow 10Mbit/sec is a lot. Probably the big red LED has enough power to set a table cloth on fire.

--
Regards, Joerg 

http://www.analogconsultants.com/

And by the way, at 1kbit I have full 3V amplitude, no need for a comparator (Rl xxx times larger than at 10MBit)

Cheers

Klaus

It's a 700mA LED, it can perform even better if I use a tube to concentrate the light