From: "Bill Sloman"
Subject: Re: Unknown component... PIN photodiode?
Date: Sat, 12 Oct 2002 00:24:09 +0200
Organization: Planet Internet
NNTP-Posting-Date: 11 Oct 2002 22:24:07 GMT
X-Newsreader: Microsoft Outlook Express 6.00.2800.1106
"Watson 'Atto Parsec' Name" wrote in message
> In article <email@example.com>,
> firstname.lastname@example.org talked about...
> > Hello all ,
> 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