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I've read in the Make magazine that LEDs can sense darkness. How should I use it in a breadboard circuit? Is it possible to read the ambient light level and send it to the computer? How does this work? Does that mean a speaker can be a microphone? In that case, how would you do that?

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    \$\begingroup\$ Yes, a speaker can be turned into a microphone. Google has lots to say on the subject so I won't elaborate. I haven't heard about the LED detecting darkness before so I'm going to check that out. \$\endgroup\$ – I. Wolfe Jan 9 '15 at 21:19
  • \$\begingroup\$ All P-N junctions are sensitive to light. Some are simply less sensitive than others. \$\endgroup\$ – Ignacio Vazquez-Abrams Jan 9 '15 at 21:20
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    \$\begingroup\$ Ah, your question is misleading OP. I read the article and the LED doesn't detect darkness, a phototransistor does and turns on an LED. That makes more sense. \$\endgroup\$ – I. Wolfe Jan 9 '15 at 21:26
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    \$\begingroup\$ @I.Wolfe: It is actually possible to use a LED to detect light. It's just not a very good detector. \$\endgroup\$ – Ignacio Vazquez-Abrams Jan 9 '15 at 21:28
  • \$\begingroup\$ @Ignacio Vazquez-Abrams: Ah ok yeah that makes sense. Thanks for explaining, my brain is slow on Fridays... \$\endgroup\$ – I. Wolfe Jan 9 '15 at 21:36
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A led acts a light detecting diode as well i.e you can sense the extent of darkness and brightness by a LED but it is not going to be as effective as the one's made for this purpose specifically.

http://www.instructables.com/id/LEDs-as-light-sensors/

You can make it on your own and this instructable will prove to be helpful.

And answering your second question, yes the speaker can also act as a microphone based on this concept only.

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Taking this question literally, I don't think you can sense complete visual darkness with an LED. A human eye can respond to less than 100 photons, so I think you'd need something like a photomultiplier tube, silicon photomultiplier or (maybe) a cooled avalanche photodiode to detect the presence of total darkness.

Sensing normal room ambient light levels is very easily done with a dedicated chip such as those from Taos (née Texas Instruments) which give you a digital output and has built-in filtering to knock out mains frequency-related brightness variations.

enter image description here

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