piclist 2000\06\04\191043a >
Thread: Noise in phototransistors/photodiodes/light
face BY : rottosen@KILLspamidcomm.com

Sean Breheny wrote:
> Hi all,
> For a project that I am working on, I need to detect the output of a few IR
> leds at about 300 feet (abt. 100 meters) away. The output is pulsed, to
> help distinguish it from background light.

What is your pulse rate and what is the receiver bandwidth? Of course,
the narrower the bandwidth the better for noise.

> I have succeeded in doing this out to about 100 feet, but am having
> difficulty beyond that because of noise. I have found appnotes which
> explain photodiode noise to some extent, but nothing on phototransistor
> noise. Could someone point me to a good reference for both?

I have no references at hand but I suspect that a phototransistor is
noisier than the combination of a photodiode and a low noise transistor.
This would be because of the difficulty of optimizing both photo-gain
and noise in one device in the case of the phototransistor.

You may want to try making your own photodiode/transistor device for
comparison purposes. I think you would want to choose a transistor that
has good noise characteristics at very low currents. A 2N5089 is a part
that comes to mind. I expect that there are better ones.

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Again for comparison purposes, maybe try an LED light source to simulate
the background light. Maybe a whole bunch of white LED's ???

-- Rich

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<393AE199.602F671C@idcomm.com> 7bit

See also: www.piclist.com/techref/index.htm?key=noise+phototransistorsphotodiodeslight
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