> -----Original Message----- > From: Mark Skeels [SMTP:meskeels@EARTHLINK.NET] > Sent: Wednesday, December 20, 2000 2:17 PM > To: PICLIST@MITVMA.MIT.EDU > Subject: Re: [PIC] Sssssssssssmokin' > > ----- 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 > Like the majority of semiconductors, LED's are sensitive to light. Most semi's however are nicely packaged in either opaque plastic or metal encapsulations, so this effect can't be put to use. An LED has a transmissive package by definition. I'm not entirely sure of the operation though, I presume that light shining on the junction produces a current, but perhaps you can use it like a PIN diode where the reverse bias leakage current varies with the light. Anyone know for sure? Mike -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.