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Forum: Analog Circuits LED to LED receiver


von Jocculator (Guest)


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Hello,
I have been trying to make a LED to LED transmission work with 2 LED's.
Now I do receive some signal but the problem is that the receiver 
diode's capacitance is apparently pretty high so it takes a pretty long 
time to discharge it, which makes it that the signal builds up and down 
over time.

The receiver is just built from a voltage follower with an opamp that 
has very high input impedance, a high pass to filter out the DC part of 
the ambient light and a non inverting amplifier.
Also I can use only positive supply voltage.
Now one of the reasons the signal even gets worse is that after the high 
pass the signal is partly negative which can make some of the 1's very 
hard to notice even after amplification.
Now I think I would be able to recover the signal if it strong enough 
but maybe some of you guys have an idea how to improve the signal.

I have attached a picture of the signal before(orange) and after(green) 
filtering.
Also I have a picture of the amplified signal.

von Lothar M. (lkmiller) (Moderator)


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Jocculator wrote:
> The receiver is just built from a voltage follower
Any kind of schematics available?

> is that after the high pass the signal is partly negative
How can this be?

: Edited by Moderator
von matzetronics (Guest)


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Without a circuit diagram there's no way we can help improve your 
project. What frequency ranges do you try to amplify?

von Jocculator (Guest)


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I am not trying to amplify a very high frequency signal.
Maybe I can get it up to 1kHz that would be great and sufficient 
already.
I have put a picture of the schematic.
The Voltage over a Capacitance is proportional to the integral of the 
current over time. Since the current sometimes is negative since the 
capacitance charges and discharges the voltage will also be sometimes 
negative.

von DeepThought (Guest)


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It would be helpful sense to provide the values of the resistores.

Without having tried it by myself: I would suggest to use a trans 
impedance Amplifier instead of a voltage follower for the left part of 
the circuit.

von Jocculator (Guest)


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R2 220k, R3 100k, R4 1k
Transimpedance converter doesn't work in this case since the current is 
way too low. I tried it and I just cannot get a voltage at the output.
I also tried transimpedance with a reverse bias voltage to increase 
photocurrent and it didn't work.

von Lothar M. (lkmiller) (Moderator)


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Jocculator wrote:
> since the current is way too low.
No way to increase it?
Because when your signal is kind of pure noise(**) it looks like in 
"Signal_before_and_after_filter.png", then you will not find any kind of 
circuit to "descramble" that.


(**) at least I don't recognize anything akin a binary signal therein.

von Jocculator (Guest)


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No I don't think I have an easy way to increase it. Since I have only a 
positive supply voltage I can't really bias the diode very easily.
I did try to bias it via a virtual ground but that didn't really improve 
the signal.
The Signal looks like that because the LED has a capacitive effect and I 
can't really change that.

von Lothar M. (lkmiller) (Moderator)


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Jocculator wrote:
> The Signal looks like
Whats the payload in it? What do you want to get out of it?

von Jocculator (Guest)


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What I want to get out of it? It's not really clear but most likely some 
error codes so not really much information at all. That's why I'm 
contempt with the low bit rates.

von Lothar M. (lkmiller) (Moderator)


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Jocculator wrote:
> What I want to get out of it?
What information (aka 0 and 1 bits) is hidden in the noise of your 
screenshots?

von Nils (Guest)


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Hi.
You use a LED as receiver ? Please specify the partnumbers of both LEDs.
Best regards

von Jürgen von der Müllkippe (Guest)


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You must the Fotodiode umdrehen and the Verstärkung from the Amplifier 
must be higher. Put a 1M Resistor in the Leitung from PIN 7 to 6 and 
überbrücke ihn for example with 10pF.

Ich weiß, mein Englisch ist nicht so gut, aber ich habe keine Lust, in 
meinem Alter nochmal zum Englischkurs zu gehen.

von Jocculator (Guest)


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I use OSRAM 6-lead in-line MULTILED LRTB GFTG. for both transmission and 
receiver I use the blue LED.
But I probably will from now on try out other LED's.
As far as I could see there is no built in extra capacitance.
So it must be the inherent capacitance of the LED itself.

von Nils (Guest)


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A LED as a receiver can work. But it depends on the type of LED.
Better use a dedicated photodiode or photoresistor instead.

von Jocculator (Guest)


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Hier in deutsch für dich :)
Als ich ausgetestet habe wo ich ein Signal mit der LED empfangen kann 
habe ich festgestellt, dass es bei einem Spannungsfolger keinen 
Unterschied macht, wie rum ich die LED schalte außer, dass sich das 
Vorzeichen der Spannung ändert. Da ich aber nur positive 
Versorgungsspannung benutzen kann muss ich die LED in Durchlassrichtung 
schalten sonst funktioniert der OPamp leider nicht.
Ok du willst quasi, dass ich nicht einfach so rückkoppele sondern mit 
einem 1M und 10pf parallel. Was für eine Schaltung wäre das dann und was 
würde es bewirken?

von Heinz (Guest)


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Have a look on this project. It could help you a lot:
https://hackaday.com/2019/12/22/optical-communication-using-leds-alone/

von Jocculator (Guest)


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Wow I just used other LED's and it worked perfectly without any 
filtering.
I am honestly so confused. Anybody know why those particular LED's have 
that capacitive behaviour? I have tried 2 different RGB LED's from OSRAM 
and they both behaved the same so its probably just how they work 
because I didn't see any capacitors in the datasheets.

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