Reciprocity. Sort of the same theory in which an antenna can both receive and transmit. What actually happens is that any semiconductor junction is light sensitive. Light impinging on a semiconductor junction adds energy to the atoms and causes electrons to cross the boundary between the P material and the N material. This constitutes a current flow. This current flow through the bulk resistance of the diode causes a voltage drop. The brighter the light, the more current flows, and the higher the voltage drop. At least to a point. Going the other way, current is applied to an LED which causes a current to flow. This current flow energizes the atoms near the junction. These atoms have some of their electrons raised to a higher energy level. This higher energy level lasts typically for just a fraction of a second. Then the electron will fall back to it's previous energy level. In doing so, it releases the energy added by the outside current in the form of a photon. There are several hundred thousand to several billions of these energy releases every second, so you see a steady stream of light. Laser diodes work basically the same way, but the ends of the die are cleaved in such a way as to provide a reflecting surface to the light. This basically constitutes a tuned cavity, which promotes light of one frequency, or more precisely one narrow frequency band. And in the case of a red emitting laser, you see red because that is the frequency that the cavity is tuned to. This explanation is rather simplified. There are a lot more things going on and it takes some heavy duty math to give a full explanation. I am neither inclined or able at the moment to give the mathematical explanation necessary to give full coverage of all the physical aspects of the operation of LED's and Lasers. Suffice it to say the above is a basic treatment of the physics involved. Hope this gives you an idea as to what is going on. Regards, Jim On Wed, 20 December 2000, 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. jim@jpes.com -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.