I am currently doing a university project looking into potentially energy saving ideas. The project has a 'novel' element to it, in that ideas don't have to be "new", but for example you can take an idea and apply it to a new environment. I am looking specifically into lighting (as most do) and am trying to build on the idea of "intelligent street lighting" (see http://www.tvilight.com). I was wondering if it would be possible to set up a photodiode/LED combination in the same unit, so that in one state it acts as photodiode, in the other as a light source. Major problem I soon encountered (if it to be possible) was getting it from light source back to photodiode. The converse I believe could be triggered when the light received by photodiode fell below a critical level. This would not be possible for light source back to photodiode without another additional application (perhaps IR sensor?). Any help is greatly appreciated.