Generally PN junctions are light sensitive, and temperature sensitive. A solar cell, for instance, is a specialized PN junction. Our pics, in the JW package, can exhibit very peculiar behaviour if the glass is left uncovered while running. Many (especially in kits) thermometers use diodes or transisters to measure temperature (the relationship is significantly more linear than that of a thermister). IIRC, two things happen. Light can cause a pn junctipon to generate electricity, and it also allows current to flow 'backwards' (ie, completes the junction). Generally this is a very small amount of current and voltage, and now we have much more control over how sensitive these junctions are to light, but it is a basic characteristic of the device. -Adam Mark Skeels wrote: > > ----- Original Message ----- > From: "Thomas McGahee" > To: > Sent: Tuesday, December 19, 2000 2:40 PM > Subject: Re: [PIC] Sssssssssssmokin' > > > I have made quite sensitive smoke detectors using a RED LED as the > > light source and another RED LED as the detector. > > > > I was not aware that a LED could _detect_ light. How can this be? > > Mark > > -- > http://www.piclist.com hint: The PICList is archived three different > ways. See http://www.piclist.com/#archives for details. -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.