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Thread: Noise in phototransistors/photodiodes/light
face picon face BY : Sean Breheny email (remove spam text)

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.

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?

Also, does light emitted by an incandescant bulb have a significant noise
due to the discrete nature of the photons (like shot noise in electric
current)? How would one go about calculating the approximate amount of
noise from such a source? I have been using flashlights to simulate intense
background light (like the receiver is trying to pick out the source with
the sun as a backdrop) and the amount of noise I have been seeing is much
larger than I would expect due to electrical effects. I was wondering if I
am seeing real fluctuations in the light intensity.



| Sean Breheny
| Amateur Radio Callsign: KA3YXM
| Electrical Engineering Student
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