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

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.


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.

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