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[DPRG] Using an IR LED as IR photodiode
Subject: [DPRG] Using an IR LED as IR photodiode
From: DeltaGraph at aol.com
DeltaGraph at aol.com
Date: Tue Oct 24 02:18:45 CDT 2006
OK,
I am not a real analog designer, but this may give you some ideas:
I built a photo diode amplifier some months back using a similar amplifier,
but a different configuration where the photodiode was reverse biased running
in photo conductive mode versus photovoltaic mode (your configuration) and
the results were good. Your circuit and selection of LED versus photodiode
will limit your circuit to low frequency applications where rise and fall times
are on the order of milliseconds versus microseconds when using a photodiode
in photoconductive mode.
Example of photo diode circuit (you can use your LED)
Page19 of _http://www.national.com/ds/LM/LM158.pdf_ (http://www.national.com/d
s/LM/LM158.pdf)
Check out _http://focus.ti.com/lit/an/sboa061/sboa061.pdf_
(http://focus.ti.com/lit/an/sboa061/sboa061.pdf) for some good info.
I built your circuit and I think one problem is that as less light falls on
the diode it begins to look like an open circuit allowing the ouiji board
effect to take over the + input on your op amp. That is, it is floating when the
LED is in the dark and it picks up everything.
I noticed (when the IR source was blocked) as I waved my hand around it was
picking up AC hum... and doing weird stuff.
My solution was to put a 100K to 1M resistor across the diode, which I am
sure had some effect on attenuating its output, but it did clean up the
amplifier output when the diode was not being hit with IR from the IR LED.
The interesting thing is that when I looked at the input with my scope
without the resistor, it looked OK, until I remembered that the scope has an input
impedance of 1M Ohms.
Hope this is of some use.
At least the links should be helpful.
Ron Grant
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