LED as detector for high speed link

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Hi

For a test setup I am toying with an isolated interface where I need 1 Mbit
 speed if possible

The receiver is a dirt cheap red LED: (which is also used as an indicator w
hen the link is not active)

https://dammedia.osram.info/media/resource/hires/osram-dam-2493888/LH%20R97
4.pdf

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 b
it level with logic 3V. (so over 2.4V)

The price of the emitter is not relevant. So I was thinking to use a 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 the red
 LED to have both high speed and logic IO level

An LED often have a different emission wavelength than the responsivity wav
elength, so that is why I use a white LED to be sure wavelenghts overlap
  
I will drive the emitter LED with high current, and then have a sufficient  
low load resistor for the red detector LED to have high bandwidth (1MHz)

But, is this a good idea? Will I be overdriving the detector LED, is there  
some kind of saturation effect?

For the setup, I could use Win Hills 200A pulser:

https://www.dropbox.com/sh/svr7q23agwuvtat/AABdQSJq4d-7Qye96NLCCb9pa?dl=0
&preview20%0A-LED-pulse.pdf

Cheers

Klaus

Re: LED as detector for high speed link
snipped-for-privacy@gmail.com wrote:
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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.



Re: LED as detector for high speed link
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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  

Re: LED as detector for high speed link
On Mon, 20 May 2019 23:33:33 +0000 (UTC), Cydrome Leader

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


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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  

We've slightly trimmed the long signature. Click to see the full one.
Re: LED as detector for high speed link
I cannot add circuitry to the detector side, since needs to be low cost  

Cheers

Klaus  

Re: LED as detector for high speed link
On Monday, May 20, 2019 at 8:09:25 PM UTC-4, Klaus Kragelund wrote:
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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

Re: LED as detector for high speed link
On Mon, 20 May 2019 22:53:42 -0700 (PDT), snipped-for-privacy@yahoo.com
wrote:

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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  


Re: LED as detector for high speed link
tirsdag den 21. maj 2019 kl. 16.46.30 UTC+2 skrev John Larkin:
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use a laser?


Re: LED as detector for high speed link
On Tuesday, May 21, 2019 at 10:46:30 AM UTC-4, John Larkin wrote:
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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

Re: LED as detector for high speed link
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that's good to know. That migth be one reason some LEF lighting is so  
flickery and terrible looking, not counting bad CRI.

  
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Ha


Re: LED as detector for high speed link
tirsdag den 21. maj 2019 kl. 01.33.38 UTC+2 skrev Cydrome Leader:
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Mbit speed if possible
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or when the link is not active)
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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)
Quoted text here. Click to load it
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
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 wavelength, so that is why I use a white LED to be sure wavelenghts overla
p
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ent low load resistor for the red detector LED to have high bandwidth (1MHz
)
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ere some kind of saturation effect?
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=0&preview20%0A-LED-pulse.pdf
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https://books.google.dk/books?id=xR8uDwAAQBAJ&pg=PA449&lpg=PA449&dq
=white+led+slow+phosfor&source=bl&ots=k7rVDUHoDo&sigAC%fU3U0oTbJQZw
hXZN90ghs5k0GcpPKmvA&hl=da&sa=X&ved=2ahUKEwi2_r_ynKviAhVPJlAKHUHjCF0Q
6AEwCHoECAcQAQ#v=onepage&q=white%20led%20slow%20phosfor&f=false

Re: LED as detector for high speed link
On Tuesday, 21 May 2019 02:09:42 UTC+2, Lasse Langwadt Christensen  wrote:
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1 Mbit speed if possible
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ator when the link is not active)
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%20R974.pdf
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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)
Quoted text here. Click to load it
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
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ty wavelength, so that is why I use a white LED to be sure wavelenghts over
lap
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cient low load resistor for the red detector LED to have high bandwidth (1M
Hz)
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there some kind of saturation effect?
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dl=0&preview20%0A-LED-pulse.pdf
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t  
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ke  
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h  
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=white+led+slow+phosfor&source=bl&ots=k7rVDUHoDo&sigAC%fU3U0oTbJQZw
hXZN90ghs5k0GcpPKmvA&hl=da&sa=X&ved=2ahUKEwi2_r_ynKviAhVPJlAKHUHjCF0Q
6AEwCHoECAcQAQ#v=onepage&q=white%20led%20slow%20phosfor&f=false

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

Re: LED as detector for high speed link
On Monday, May 20, 2019 at 2:48:13 PM UTC-7, snipped-for-privacy@gmail.com wrote:

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...

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.

Re: LED as detector for high speed link
On Monday, May 20, 2019 at 10:26:56 PM UTC-7, whit3rd wrote:
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That's supposed to be ' IrDA', of course...

Re: LED as detector for high speed link
On Tuesday, 21 May 2019 07:26:56 UTC+2, whit3rd  wrote:
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I cannot add significant cost, since this is only a link for debugging. Any add on would increase the product price

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

Cheers

Klaus

Re: LED as detector for high speed link
On 5/20/19 5:48 PM, snipped-for-privacy@gmail.com wrote:
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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.

Re: LED as detector for high speed link
On 21/05/2019 7:46 am, bitrex wrote:
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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


Re: LED as detector for high speed link
On 5/21/19 4:56 AM, piglet wrote:
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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?

Re: LED as detector for high speed link

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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  


Re: LED as detector for high speed link
On 5/21/19 10:48 AM, John Larkin wrote:
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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.

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