LED as detector for high speed link

I suspect you'll run into problems. White LEDs use a phosphor to convert blue light to white. There will likely be a persistence issue, not unlike CRTs or florescent light bulbs. So, to be able to transmit data in the

1Mhz range, the rise and decay time of that phosphor they use better be really, really short. IR seems to be the norm for data transmission with LEDs.

Blue LEDs are pretty fast. I recently had reason to measure the temporal re sponse of a 3200 lumen, 4000K white LED. With a 470-nm shortpass filter to separate the LED from the phosphor, it has a reasonable-looking step respon se with a TC of about 50 ns. With no filter, it's a drooly 300 ns.

Cheers

Phil Hobbs

Why not amplify the LED current output, into a comparator maybe?

I tested some Cree white LEDs for speed, and they had nanosecond response. The phosphors seem to be very fast.

But I'd guess that the best driver of a red LED is a red LED.

--

John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com

I cannot add circuitry to the detector side, since needs to be low cost

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?

...

Why not just use an IeDA transmit/receive pair? Speed is good, parts are available. And, because it's a standard interface there are laptops and PDAs and laser printers that have it already built in. A 'LED' might be tecnically capable of receiving, but it isn't optimal (not a lot of receiver area to pick up light).

It's even easier to transformer-couple an interface; Ethernet magnetics are mass produced and can outperform your target throughput handily, at quite reasonable cost.

That's supposed to be ' IrDA', of course...

Then it's going to be slow. LEDs make rotten photodiodes. Functional, but just.

I tried it years ago. But rather than dig for my notes, I just measured two combinations.

LEDs nose-to-nose, clear cases, If = 11.2mA, Vr = 0V.

emitter 'detector' photocurrent

------- -------------- ------------ GaN green AlGaInP red 1.7uA AlGaInP red same as emitter 62uA

That's actually not too bad. If we assume I was lucky enough to couple 5mW optical, that's

12mA/W photocurrent, or about 2% as good as a PIN diode.

Cheers, James Arthur

If the price of the emitter isn't relevant why not use a big tri-color RGB LED. I'm assuming there is some kind of software on the receiver end too, to transmit data back.

Run an "auto tune" test data sequence on the emitter side that cycles around thru the area of the color palette where you figure the best responsivity of the red LED is and have the receiver side communicate back where the best data rate happens. Then save that for use with that particular LED.

Err...I don't think that would work. The colour palette is a human eye kind of thing. Having Red and Green LEDs on may look to the eye as yellow but it is still red and green photons and no yellow photons.

piglet

My experience was that Red LED responds very badly to red but much better to higher energy/shorter wavelength green or blue light. White phosphors will only lose you efficiency and worsen timings.

Not sure what your detector circuit will be but I cannot see how you can get 2.4V (and isn't that above the Vf?) output without some amplification even if only a penny BC847.

piglet

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 was my experience too. (But I was using it as a spad. Reverse biased ) Forrest Mims is an LED as detector 'expert'. I emailed with him once.

George h.

The target is awfully small, so most of the photons miss. I bet a lens would help a lot.

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics

All LEDs of a given part number will have about the same spectral response, so he only has to figure that out once.

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics

Dunno what the human eye response has to do with anything a red LED used as photodiode should be sensitive in some amount to wavelengths shorter than red, into the ultraviolet I suppose.

IR photodidoe has its junction designed to respond best in the IR but when using e.g. red LED junction in a clear envelope as a photo-diode are there any guarantees that it will suck the least in response to a single wavelength in the area of red as opposed to some combination of the lower energy photos and higher?

In the case of real bargain-basement LEDs I'm not sure there are any guarantees of that. When you buy a bag of cheap LEDs from China their characteristics are all over the place and stuff like forward voltage vs. current varies wildly.

I tried one time to test a bag full and there's no nice bell curve to be found on IV parameters of parts like that; all the good shit that can be parameter-matched has been pulled by the mfgr already to make e.g. those LED christmas lights strings where the LEDs can be run in parallel with a single current-limiting resistor for the string because all the LEDs in the set have matched forward voltages.

Perhaps spectral response would be the one thing that would be invariant regardless but doesn't seem to apply to anything else at that price point.

Some fancy-schmancy Osram red LED where the LED in question actually has a datasheet perhaps but at the "dirt cheap" end...datasheet? What's a datasheet?

Indeed this all sounds so familiar from somewhere....

tirsdag den 21. maj 2019 kl. 16.46.30 UTC+2 skrev John Larkin:

use a laser?