From: Watson 'Atto Parsec' Name
Subject: Re: Unknown component... PIN photodiode?
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Date: Fri, 11 Oct 2002 19:04:51 -0700
NNTP-Posting-Date: Fri, 11 Oct 2002 19:01:12 PDT
Organization: InReach Internet
In article , email@example.com talked
> "Watson 'Atto Parsec' Name" wrote in message
> > In article <firstname.lastname@example.org>,
> > email@example.com talked about...
> > > Hello all ,
> > [snip]
> > I have a question for those who may have dabbled in this before. I tried
> > using a LED as a sensor, and found that it generated a few tenths of a
> > when exposed to light, like the photodiode above. I used it in a circuit
> > that senses the voltage the LED generates across a 10 meg resistor, but as
> > one would expect, the current is miniscule because the junction area is so
> > small, hence the 10 meg res. Is it better to use the LED in the
> > photoconductive mode rather than as a photovoltaic generator?
> I'd go for the photoconductive mode myself - your signal is the
> photon-generated photo-electrons, and in the photovoltaic mode you are
> thowing most of them away to forward bias the junction.
> If you don't need speed, get a high input impedance amplifier - I'd go for
> a single-supply FET-input part like the Analog Devices AD820. (Burr-Brown
> have got a nice part with much lower input noise, but I can't remember the
> number - I've got it at work, so e-mail me if Monday isn't too late.) and
> connect the non-inverting input to ground.
> Your LED then goes between the inverting input and ground, and the 10M
> resistor from the amplifier output to the inverting input.
> The op amp forces all your photo-current to flow through the 10M resistor -
> bigger signal, much less shot noise.
> Bill Sloman, Nijmegen
Thank you for the information. I'll try to change the circuit to
photoconductive. It's just for sensing light, nothing fast or sensitive.
Right now the only complaint I have is that the LED's milky red epoxy
diffuses the light so much that a lot of the sensitivity is from other than
straight ahead, in front of the lens. So I put the LED in the body of a
ball point pen, which cut down the sensitivity a lot. But now it's a lot
more directional. I'm using a FET and I was hoping to stay away from
opamps. I figure a transistor after the FET to invert the signal and then
feedback a bit of positive signal to give it hysteresis.
My email address is whitelisted. *All* email sent to it
goes directly to the trash unless you put NOSPAM in the
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